Design and Planning Manual for Cost Effective Welding

DESIGN ANDPLANNING MANUAL For Cost-Effective Welding International Standard Book Number: 0-87171-605-4 American Welding Society, 550 N.W. LeJeune Road, Miami, FL 33126 © 1999 by American Welding Society. All rights reserved Printed in the United States of America NOTE: Although care was taken in choosing and presenting the data in this guide, AWS cannot guarantee that it is error free. Further, this guide is not intended to be an exhaustive treatment of the topic and therefore may not include all avail- able information, including with respect to safety and health issues. By publishing this guide, AWS does not insure any- one using the information it contains against any liability or injury to property or persons arising from that use. Photocopy Rights Authorization to photocopy items for internal, personal, or educational classroom use only, or the internal, personal, or educational classroom use only of specific clients, is granted by the American Welding Society (AWS) provided that the appropriate fee is paid to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, Tel: 978-750-8400; online: http://www.copyright.com ii Contents Section Page No. 1 Primary Concepts of Weldability..................................................................................................................1 2 Accepting Metal Fabrication Projects ..........................................................................................................5 3 Production Welding Cost Analysis................................................................................................................9 4 Modular Construction..................................................................................................................................17 5 Welding Process Selection............................................................................................................................23 6 Primary Concepts of Welding Design.........................................................................................................33 7 Fatigue Considerations.................................................................................................................................45 8 Welding Safety Considerations ...................................................................................................................51 9 Weld Joint Design Considerations ..............................................................................................................57 10 Weld Distortion and Control .......................................................................................................................63 11 Checklist for Sound Welding Decisions ......................................................................................................71 12 Defects and Discontinuities of Welding ......................................................................................................75 13 Nondestructive Examination .......................................................................................................................93 14 Information for the Welder........................................................................................................................103 15 Fitting Aids..................................................................................................................................................109 16 Welding Metallurgy: Practical Aspects ....................................................................................................119 17 Arc Stud Welding........................................................................................................................................129 18 Thermal Spray Fundamentals...................................................................................................................133 iii ........ 4 Weldability Can Be Determined by the Manufacturer ........................................................................................................................................................................................................................................................................................................................................... and Production Supervisor ......................................................................................... 2 Personnel Concerns about Fabrication Projects....... 3 Coordination of Expertise.................... 4 The Welder................... Technician................................... 4 Education.............................................................................................................................................................................................................................................................................................................................................................................................................................................................. 2 Fundamentals of Welding Decisions ......... 4 1 ................ 4 Bibliography/Recommended Reading List ........................................... 4 The 100% Quality Weld and Weld-Associated Failures ....................................................... SECTION 1 Primary Concepts of Weldability Contents Weldability ............................................................ and weldment aesthetics that determines weldability. a weld is a localized coalescence of metal that is used to join or repair metal components. it is the suitability for service.” critical base metal requirements that are necessary When the definition of weldability is reduced to for the intended service of the customer?” its lowest common denominator. All metals may be considered weldable by the very “Are there any inspections or requirements associated fact that they exist as metals.e. and welding procedures)?” delays may be avoided. weld size. “Are the fabrication requirements achievable from the standpoint of accessibility. stress relief. when in conflict. and schedule material size. distortion con- Personnel Concerns about trol.” • From the manufacturer: To ensure weldability. “Does the weld and its heat-affected zone meet the form the intended service. completed on time or fails in service?” erally. joint design. For a “How much money does it cost to make the weld (i. “Is the fabrication completed within cost and on Filler metal may or may not be added to the weld. The definition of weldability is: “The capacity of a • From the engineer: metal to be welded under the fabrication conditions im- posed into a suitably designed structure. clarified.SECTION 1—PRIMARY CONCEPTS OF WELDABILITY Section 1—Primary Concepts of Weldability Weldability • From the customer: Welding is used to make large and sometimes com. Gen. “Does the fabrication perform the intended service plex structures from smaller and simpler metal compo. or the difficulty of joining a material with a specific process. and. or the fabrica. • From the welder: tal understanding of welding and a knowledge of the re- “Do I have clearly defined instructions (such as draw- sources available. In general. The schedule?” capacity and ability to join these metal components with “Is it aesthetically pleasing?” available resources help define the term weldability. With a fundamen. position. the capacity of the fabrication to perform its “Do I have the proper equipment to accomplish the intended service. interaction of countless groups and individuals with specific needs may be involved. failures. the welder. resolved. these needs must what’s my profit)?” be identified. The “degree” of weldability can be defined in terms of: “Is there adequate environmental protection?” weld costs. inspection requirements and fabrica- Fabrication Projects tion sequence?” Major concerns about a fabrication project and its “Am I trained and qualified to perform the welding weldability are expressed by those involved as follows: required?” 2 . material type.. production failures. cost. In common usage the term “weldability” is somewhat “Is there adequate weld joint access?” ambiguous and about as varied as the people who use it. “lack of weldability” causing inservice ings that specify weld size. cost overruns. Depending on whether you with the weld that will prevent acceptance of my work?” are the customer. which will per. the inspector. this definition is “it. the ability of the weldment to meet fab- work?” rication acceptance standards. project to reach a successful conclusion. “Am I subject to any liabilities if my fabrication is not tor your definition of weldability may be different. and will it last as long as intended?” nents. solving most welding problems. the design engineers. important resource in the fabrication enterprise. the welds are often in areas where the structure quirement is necessary. and the heat-affected zone has undergone widely varying heating and cooling rates. it is obvious that require practical solutions as well. and made in most cases. It is for this reason the use of welding engineering techni- cians is quite common in larger companies. Welding problems project is repeated over and over again. Education is needed by the fabrica. Knowing the basics of lies in a fabricator’s ability to coordinate its available welding engineering can prevent a company from exten- expertise. The base material. For this reason. Each has different needs. The 100% Quality Weld and Weld- base metals. A balance of welding engineers and varying workload will find it difficult to make good welding engineering technicians is most desirable for re- welding decisions for all of the firm’s needs. Because of this. areas). These 3 . This is due to the fact that the base coordinate with other available expertise to make the best materials usually have undergone significant mechanical decisions for the company. However. Formation of this group may be the single most important action taken to ensure a successful and profit. and filler metal selection that are perceived as inherently weak or flawed. supervisor can never be overlooked. Technicians Fundamentals of Welding Decisions frequently are trained individuals with journeymen pro- One individual in a metals fabrication firm with a duction experience. This expertise will require a background in. Technician. Understanding the importance of fails. The resolution of many welding problems requires able completion for all parties involved. in effect. sections (commonly associated with highly stressed lowed unless there is an understanding of why the re. Since welds are usually located at changes of cross a specification or procedural requirement will not be fol. In many cases a weld Education joint lowers fatigue properties and lowers impact proper- ties. large or complex struc- volved in welding. the production supervisors. It metal it joins. and how they can affect the safety of personnel and the sects so many disciplines. including production as well as technical The Welder. The major error in this logic Coordination of Expertise is that the base metal and the weld metal do not perform independently. this and/or thermal treatments to optimize their properties. They handle the majority of the welding problems that will are. and welding processes. postheat treatments. These failures may be wrongly attributed to welds preheats. If an individual makes enough welding decisions. They are absolutely essential if a company expects to remain profitable. it is recommended that a preacceptance bid re- view group. However. Designers often mistakenly expect the welds they de- sign to have properties that equal or exceed the desired properties of the base metal. Often. this individual will find it necessary to within the weldment. suitability of the part for service creates a desire by all in- associated problems and the ultimate success of a project dividuals to follow requirements. expertise will be required beyond that of filler metals. that the solution to many weld. the large monolithic cast or wrought structures cannot be planners and estimators. and yield properties. since tion shop superintendents. the welding mechanics. since the weldability of a metal inter. The com. may require hiring a part-time consultant. If a company is small. and personnel. the weld usually performs its service in the as-cast condi- tion. One person normally does not have expertise in all of Rarely does the weld improve the properties of the the areas required to make sound welding decisions. and the weld’s will benefit a company to have a welding engineer to heat-affected zone are not homogeneous regions. be established to review projects prior to ac. tures must be welded with high joint efficiency. SECTION 1—PRIMARY CONCEPTS OF WELDABILITY It is obvious. in some cases a weld joint also lowers the tensile There is no substitute for education of personnel in. structural and metallurgical discontinuities arise. Unless the same much more than technical solutions. and integration of the technical Associated Failures fields. the weld. the one person cannot resolve all the issues necessary for the practical expertise of the journeyman mechanic and his project to be a success for all the participants. plexity of welding decisions is immense if the work var- ies. sive rework or a customer’s refusal to accept a manufac- There can be no question that people are still the most turer’s product. Production Supervisor ceptance. is determined by the Welding Handbook. The root cause of these types Often weld-associated failures are caused by lack of of failures is in the decision-making process and not in knowledge about welding and its effect on the base mate- the quality of the weld. • Properly utilize. 1. The manufacturer then controls most of the variables that determine weldability. Welding Technology manufacturer’s ability to: (WHB-1. The weldabil- List ity of a metal. the Manufacturer When a manufacturer designs and fabricates a weld- ment. and filler metals are frequently asso. coordinate. vol.SECTION 1—PRIMARY CONCEPTS OF WELDABILITY failures are more likely caused by poor decisions on the • Recognize the cause of existing and potential produc- part of engineers or company management due to a lack tion problems.8). These failures are wrongly viewed as resulting from inherent weaknesses of welds.: American Welding Society. 8th ed. of knowledge or training. with Weldability Can Be Determined by proper utilization of available expertise. welding processes. the failures could have been avoided. 4 . Improperly selected joint designs. In most cases. ciated with weld failures. to a large extent. Fla. Miami. and develop cooperation of locations of welds. it can truly be said that the manufacturer deter.. Bibliography/Recommended Reading mines weldability. available expertise to make sound welding decisions. rials. ............................................................................................. 6 Defining Special Projects ......................................................................................................................................................... 6 Establish a Detail Planning Group (Prior to Issuing Job Orders).................................................................................................................................................... SECTION 2 Accepting Metal Fabrication Projects Contents Background ................................................................. 6 Establish a Prebid Review Group (Before Accepting Special Projects) ................................................................................................................................ 7 Summary ................................................ 7 5 ... production • How and when to monitor the work in production? lead shop. The group should also Resolving these issues should be accomplished with determine the limitations and conditions that should be small review groups with specific tasks in mind. field work limits threaten their profit and the quality of their work. In addition. placed on the project for bid acceptance. and process • Should the company bid on the project? requirements of unusual work. Nearly all special projects require high special projects. They proba. Know. During the accep- tance of work.SECTION 2—ACCEPTING METAL FABRICATION PROJECTS Section 2—Accepting Metal Fabrication Projects Background Special project should be defined as follows: The critical question asked by a metals fabricator for a (1) Not a repetitive project. (2) Any type of field work that is away from the com- falls. nomical use of materials. paperwork. However. it is recommended that the member- • To what extent are production instructions required? ship of the preacceptance bid review group consist of • What are the costs to establish these instructions? comptroller. the fabricator. they can make allowances for them. quality. design. viewed by a small prebid review group before accepting nize a special project. There are increased costs in determining: son who plans and estimates jobs can assemble and un- derstand all the technical. quality. and the welder need a funda. mental understanding of the weldability of metals and the basics of sound welding decisions. and involves a significant number of what delivery or schedule times can be met. planner and estimator. They know what their minimum bid should be and pany’s home office. the (4) Unusual or complex work requirements (includ- engineer. The primary goal of the group should be equately meet project requirements? to assess the company’s limitations and capabilities in re- • What additional training/equipment is required to gard to technical. Establish a Prebid Review Group (Before Accepting Special Projects) Since a special project is a nonrepetitive project that may include multiple workshops having new or unusual Defining Special Projects requirements. man hours. They also know the pit. ing inspections. man hours and equipment. It cannot be expected that a single per- up-front costs. Field work limits the company’s flexibility for bly have experienced the normal day-to-day items that scheduling work accurately. material. many decisions that seem innocuous to welding end up jeopardizing the weldability of materials and the service suitability of the completed weldment. and special process require- meet project objectives? ments before accepting work. even though a company completed project to be considered successful is: Did the may have procedures that are applicable to the work. the company’s ability to combine work activities for eco- ing these pitfalls. project make a profit and was it completed on time? This includes work that the company has accomplished When large or small companies have repetitive work. the acceptance of such work should be re- The critical first step for a company is the ability to recog. (meaning a different type of work for the company). When the structure of a • What should the bid be? company permits. 6 . and records). • Are the company resources adequate? A comptroller or planner and estimator should over- • Do company equipment and personnel capabilities ad- see this group. they are usually quite successful. for a company to be successful when it (3) Any project that requires many different disci- accepts work that can be defined as a special project plines from the same company to accomplish its task. and quality assurance representatives. before but has not done for a long period of time. i. view group should also be responsible for material acqui- • Record requirements (degree of detail). Trying to re- cover from incorrect material after a job has commenced • Quality requirements (establishment of special proce- is almost impossible without the company experiencing dures. After the project has been accepted. • Records. that con- much it is willing to bid. and direct technical resolution to unexpected fabrication fore accepting special projects. if possible. and when practical. where material requisitions are made haphazardly. a prebid review group is nonrepetitive and has extensive quality and technical should examine all the job requirements. or at project. This detailed planning group should be determine whether the company wants to make a bid on headed up by a design engineer. • Specification (material and fabrication). Simple. should be made of workmanship. and records tion with shop supervision. for as work progresses. rately. and a quality assurance expert. After the production work starts. The referencing of general spec. a planning group is needed for detailing the job requirements to the produc- tion workforce. or not ac. a planning review tions and accountability. and production shop the project by the participants. If a project has been determined (by a company’s def- tential for noncompliance and error is when the project inition) to be a special project. concise. clear and concise job order instruc- control samples). This detailed planning effort is neces- sary. has authority to make and implement decisions. and may have different quality require- ments than the company normally uses. the prebid re- • Inspection requirements. It is crucial that this group requirements must be provided. Internal quality reviews or audits should be planned tations and conditions on the bid acceptance. to enable them to receive fast technical. Establish a Detail Planning Group • Inspection requirements. busy can cost you the company. If deemed necessary problems. Job orders should provide details are obscure and can be easily missed. (Prior to Issuing Job Orders) • Special process controls. early in the project. the project and what considerations and limitations Once a special project is accepted by a company. • Health and environmental considerations. 7 . should be placed with the bid. Mate- • Material and labor losses in production (rework). they should be listed as line items with specific direc. may involve Summary multiple trades. rial must be ordered properly and receipt inspected accu- • Cleanliness and shipping requirements. a production shop instructions should be clear. The group should consist resources must be expended in advance planning. • Special welding equipment and fixtures required. if a project is to progress within the preestablished • Special process requirements (mockup testing or schedules. Any project is slated for failure • Material requirements certification—receipt inspection. similar meetings should be held with the lead production tions and capabilities should be determined in regard to shop chairing the meeting. SECTION 2—ACCEPTING METAL FABRICATION PROJECTS Sometimes accepting a job just to keep the workforce chaired by engineering. The company’s limita. from this determination. a planner and estimator. The first review should take place cept the work. and special process requirements be. work surveillances discuss the impact of technical requirements on produc. inspection. Early in the production cycle. weekly or biweekly meetings should be held to least reviewed. The greatest po. • Fabrication documents. work instructions and During the planning and engineering time of the receipt inspection of material should be audited. group needs to be established to provide clear and con- ifications should be avoided where contract requirements cise work instructions.e. Establishing this in areas that would be expected to cause production and group early helps resolve many critical issues that make record problems. because the work is nonrepetitive. quality manual). the company should place limi. Details concerning the receipt of mate- a project a success.. During production. tions should be provided in the following areas: • Personnel availability and special qualifications. leader or project leader. quality. • Quality requirements (receipt inspection included). This group will requirements. quality assurance requirements. Work of a designer. These meetings should be to ensure work instruction compliance. It also establishes early ownership in rial. sition recommendations. large material and labor losses. Therefore. After a special project is accepted. This ensures that the work in- To determine whether the company should bid or how structions are adequate to accomplish the work. the prebid review group should tract requirements are clear to the workforce. and evaluate the following items: the material that is to be received is correct. ....... 13 Summary ............................................................................................................................................................ 10 Review of Cost Estimating ..................................................................................................................................................................................................................... 11 Joint Designs ....................... 14 9 .................................................................... 14 Bibliography/Recommended Reading List .......... SECTION 3 Production Welding Cost Analysis Contents Introduction ......................................................................................................................................................................................................................................................................................... 10 Welding Cost Variables .............................................................................................................................................................................................................................................................................................................................. 12 Records: Accountability for Accomplishment of Work and Inspections .................................................................................... 12 Automation/Mechanization of New or Existing Thermal Joining Processes ..................... 13 Cost Considerations of Nondestructive Examination (NDE) ........ 11 Fire Protection ........................................................ 11 How to Control Costs ................................................................................................................................................................................................. 11 Rework..................................................................................................................................................................... • Electrode deposition efficiency. electricity. sick leave. 10 . and capital invest- Review of Cost Estimating ments not tied to a specific customer. For purposes of this review. water. Definitions of these five ex- penses are listed below. or fix- apply to repair welding or new construction applications. many of the following terms can Fixturing costs are expenditures for tools. material. health insurance premiums. cess. or repair. fied to a productive effort/customer or do not result in direct productive work. direct labor. this includes any personnel assigned to a specific end use The application of welding as a major fabrication pro. head. over. Such examples include paid vacations.S. Material costs also include consumables • Joint design type. and piping. or other hardware) that ultimately become part of overhead. As an example. Prior to this time period. plicable to manufacturing costs.SECTION 3—PRODUCTION WELDING COST ANALYSIS Section 3—Production Welding Cost Analysis Introduction of specific projects for a customer. Welding Costs Material Costs As noted in the AWS Welding Handbook. tures that are directly utilized in the manufacturing pro- These cost estimates include material.. maintenance. labor. • Weld size. Direct Labor • Special weld procedure development or verification of Direct labor is a level of effort or services that can essential elements.e. Required re. in terms of welding costs. which can be traced or charged to an end product. However. not play a significant role in production applications. • Weld process. Its use was accelerated by the Overhead Costs military. ma- Army needed significant ordnance/weaponry equipment terials. there are other variables that need to a finished product. and welding. ceipt inspection and traceability documentation should • Weld type. fixturing. costs for welding basically include the same elements that are ap- Material costs involve those materials (i. the welding process did accomplishment. Welding was primarily utilized for small-scale fabrica- tion. building construction or maintenance. steel plate. Particular attention should be considered not to cess in United States industry began during the late exceed the estimated manhours/mandays for project 1930s. Navy required massive ship construc- tion for deployment during World War II and the U. The U. jigs. These individual components can be be considered: traced to the finished project. used during the manufacturing evolution. readily be charged or identified towards accomplishment • Heat and fire protection.S. Overhead costs are those costs of indirect services.. i.e. in terms of a unit of mea- sure expended. There are several fundamental variables (costs) that need to be considered prior to submitting a competitive Fixturing Costs bid for a specific project. or other expenses that cannot be specifically identi- that could only be practically constructed by welding. also be included. project. • Weld quality requirements. etc. For example. direction. or burning evolutions. “How can these evaluated prior to making decisions about utilization of a costs be controlled. 11 . shielding. transportation. • Nature of the material to be joined.) is numerous components. In many situations. ences exist in job preplanning through project comple. the existence or elimination of Another variable affecting welding reject rates/costs is potential rework costs deserves a high degree of attention. lists. it is preferable to accomplish the ma- jority of welding fabrication in the shop vs. Prior to welding. For example. material tardant material or cloth for protection from the welding. As an example of required etary impact in the overall manufacturing costs. the next obvious question is. the normal horizontal-fixed or rolled position welds found in shop welding. SECTION 3—PRODUCTION WELDING COST ANALYSIS Welding Cost Variables If at all possible. it is estimated that for every hour of actual Regardless of the location of the particular project. drawings. etc. and weld process. scaffold- in nature. welding conditions. There are a large number of welding variables that can For example. in the field. These should include an in-depth review of the of “firewatch” personnel and the installation of flame re. cal problems might include welding out-of-position. affect the manufacturing costs. and type of filler metals required. a similar amount of time is required for re. preheat or interpass tempera. The welder holds a higher than the shop welds! very key role. Other physi- tures. the amount of rework. the available skilled workforce. shop fabrication drawings are specific • Safety requirements: enclosures. Replacement of skilled personnel with inexperienced welders can be one Fire Protection source of higher reject rates/costs. in one case history involving Cres piping. shop supervisor. preplanning. or type. determine what type of variables need to be Now. the design the field reject rate was 420% over those joints welded in engineer should be concerned with joint design. These costs may include the use essential. tion. job preplanning. and inspection costs. vs. in solving these questions. and yet provide assurances for quality particular welding process for a specific job. Promotion of experienced personnel into management positions. work documentation requirements. can cause an increase in the reject rates. and parts fitup/ field fabrication. The AWS weldments?” Has the designer given adequate thought to Welding Handbook offers the following criteria for “fitness for use. applicable fabrication specifications. fications provide sectional views to show the location of • Location/orientation of fixturing (positioners. very tight space restrictions. weld size the shop. cessful job accomplishment. whereas drawings for field installation or modi. Since rework can be as high as 25% of the project costs. he/she must be familiar The disparity between shop and field reject rates can with the applicable weld procedure that lists variables be explained by reviewing the complexity behind field such as type of shielding or required flow rates. travel speed for auto. etc. Finally. The welding supervisor should and 107% higher for carbon steel. mirrors to complete the particular pipe weld. tolerances. thicknesses. Conduct meetings with the designer. or a difficult configuration that restricts the flow of The welder. • A clear definition of what physically can and cannot be done.. and schedules. the welder will encounter parameters (volts and amps). which may require the aid of mated/mechanized welding. welding time. Currently. Is preheating or How to Control Costs postweld heat treatment required? Many weldments involve a combination of shop and • Joint geometry. The average among be aware of the setup (field or shop). and scheduling are lated fire/heat protection. the same reject criteria was 189%. It is critical to understand that differ. It is project estimate regarding manufacturing costs and suc- very apparent why welding costs make a significant mon. Some additional negative conditions Rework may involve moisture or contamination in a piping sys- tem. retirements. definition of scope. production welding these three materials welded in the field was 238% time. finishing. For Monel®. has major control over the purging gas for piping welds. ing.” or has the component been overdesigned consideration: which increases costs? The remaining portion of this sec- tion will attempt to introduce answers/suggestions to aid • Type of welding operation to be performed. in many instances. A strong preplanning effort will provide a solid consumable costs. the company owner/management is concerned and other technical support staff to determine the proper with all of the above and additionally: utility/labor rates. important. and overhead costs. ” Automation/Mechanization of New or Existing Thermal Joining Processes Joint Designs Another aspect of controlling manufacturing/welding costs. Flux • Part programming is a critical time aspect.e. provides the welding foreman/welder with the essential • Economic requirements: initial investment and opera- welding elements including required gas shielding/flow tion costs. reduction in filler metal and distortion! serious consideration is given to automate (i. 12 . or maintenance. A careful re. sign selections. it is critical that fabrication projects the above factors. other variables need to be evaluated neer is very significant in controlling welding or manu. Lin. A welding procedure • Processing speed of parts per unit of time. another “tool” that consideration are: needs to be evaluated is the applicable welding proce- dure/process to support fabrication. not including associated increase the labor costs by a factor of 500%! This does operator training. as 100%) will equipment is a large capital cost. engineering support. to improve the electrode deposition rate. and equally so in costs. For example. Blodgett’s words. However. achieve the desired weld quality. a 45° double bevel groove joint design becomes ing the preplanning section regarding the utilization of the least expensive. and type of filler metals required.. etc. Very few people re- alize the great increase in weld metal and cost that results from a slight increase in weld size. an increase in fillet weld sizes by robots). has been shown that a decrease of 30° in joint design bev- is the introduction of automation or mechanization of els can result in a savings of almost 50%. and their proper storage is required here. • Location of the automated equipment should be as peratures. but as the plate thickness increases up to three In addition to the variables previously mentioned dur- inches. This will obviously avoid unneces. at one naval due to the additional welding. ordering filler coln Arc Welding Foundation). rates. it requirements are important with a new process and should be noted when purchasing bare filler metal higher production rates. involves the decision of when to utilize specific production applications were identified. The that for 1/2 in. When considering the filler metal selection. as there is Cored Arc Welding (FCAW). ing a new welding process (Flux Core—Twisted wire). Utiliza- fillet or groove welds. The responsibility of the design engi. automated welding equipment. (spools) for Gas Metal Arc Welding (GMAW). to 5/8 in. an increase in fillet weld exist to support its development. and increased welding deposition rate. the joint design. in sary down time to replace the spooled filler metal contain- the Design of Weldments (published by the James F. or joint de. after a successful development program involv- Another factor involving design decisions. A not include the potential problem of weldment distortion projected return must exist. Also. welding processes. amount of weld metal required. per foot. without violating heat input require. tion of this new process resulted in major labor savings ference between these designs. Additional criteria for Following a review of joint designs. should be unobstructed. Also. view of available procedures/processes will afford an • The path between the work area and control unit opportunity to determine costs. fillet welds are the cheapest design savings can be realized on future thick section weldments. over the previous welding method in reduced joint design Based on deposited filler metal costs alone. Purchase of complex sizes from l/4 in. welding current. FCAW). applicable preheat/interpass tem. prior to making decisions on adapting or purchasing facturing costs. before the welding time. coordination between the welding and actual opera- age be considered if filler metal types do not need to be tion of the automated equipment is important to frequently changed. • Efficient material handling and fixturing equipment ments. decreases the basic costs per pound. As an example. whether it be covered or bare the following: electrodes. thought should be given to using the largest diameter suitable for • Past experiences with similar automated equipment. The necessary (SAW) processes. or Submerged Arc Welding a need for a flexible software system. close to the prospective work as possible. shipyard. (using 1/4 in. Proper planning regarding future needs of specific electrode “The cost of welding is directly affected by the types. it the possibility of only long-term payback on investment. Blodgett states metal in large quantities. which includes new/existing thermal joining processes. or to mechanize specific welding processes a factor of 50% will require a significant increase in all (GMAW. There is obviously a significant dif. ers during the welding operation. Omer W. it is evident preparation. and payback period.SECTION 3—PRODUCTION WELDING COST ANALYSIS In his discussion about estimating welding costs. plate. that the largest available spool pound. use of In the inverse scenario. although it involves long-range preplanning with In a further representation of Mr. . tions (including backgouged roots). un- Several commercial code or military codes or specifi. (i. and the applicable inspections. depending on the frequency used. a brief overview of the six the investment? available nondestructive inspection/testing methods in- cludes the following: Visual Testing (VT). This method can be used on a wide range of electro con- • In-process inspections or data collection (measure. Records: Accountability for Accomplishment of Work Visual Testing and Inspections For all weldments. 13 . however. VT is one of the best and most economical inspections employed. It can be accom- Maintaining accountability for accomplishment of plished quickly by qualified inspectors to verify pre or work and inspection—nondestructive examination (NDE). a thorough review of the required records. ade- receipt inspections. easily ings. one of the most cru. shapes or forg. weld size.). cracks. UT. post welding criteria including. It is excel- VT). • Vendor certification of welding electrodes.200 in. Both types can only detect defects open to the cost which has dramatic impact. etc. or minute surface indications. thus in. but necessary overhead cost. • Verification of thermal joining surveillance inspections. lent for measuring plating thicknesses and can be used on painted surfaces. and finished weld surfaces. root welding passes. there can be a 35% de- records include: crease in associated NDE inspections. dercut. is necessary. rous materials is restricted to approximately 0. Flaws • Records for welder/brazer/NDE operator qualifications. Visual inspection of each sive quality control system for record origination/ weld layer can ensure quality.). Dye penetrant inspection is relatively economical. suitable for automation. and Ultrasonic Testing (UT).. • Records of casting fabrication. This is ET’s main advantage over MT. slag. and can be used on nonferrous and ferrous mate- testing has great potential for increasing costs. it rials. small electrical currents are induced in a material. provides assurance that the quality level for the product or cial decisions remains: is there a high degree of assurance weldment is being met for critical applications.). as it has been shown that if retention be established. Magnetic Particle (MT). quate joint designs. cleanliness. that utilization of the automated welding equipment or Specific definitions/applications of NDE are covered new processes will bring about a positive rate of return on in another section. though the depth of inspection on fer- ments. etc. is a tremendous. Eddy Current (ET). in-process. Utilization of this type of surface. The most significant disadvan- cations that might be referenced on drawings or be tage with this method is that it cannot detect subsurface included in the contract provisions. specified by the applicable fabrication MT can be accomplished only on ferrous materials. require that an exten. The volume of required docu. This method is reasonably economical. and serves as an mentation may involve hiring of additional staff. etc. SECTION 3—PRODUCTION WELDING COST ANALYSIS After review of the above criteria. ducting materials. plates. PT. however. to • Records of NDE inspections (ET.. RT. Dye Penetrant (PT). or test re- sults for compliance with the applicable specification/ Eddy Current Testing receipt inspection requirements. etc. Radio- graphic (RT). appearance (contour. Magnetic Particle Testing Before bidding on a project. Cost Considerations of Nondestructive Examination (NDE) Dye Penetrant Testing The requirement to incorporate nondestructive testing There are two types of dye penetrants: liquid and fluo- during the manufacturing or welding evolution is another rescent. are detected by the interruption of these currents. but not limited to. excellent tool in detecting indications on joint prepara- creasing overhead costs. Some examples of these required an in-depth VT is accomplished.100 in. The equipment for this technique is not expensive. or detection of surface flaws. and the labor costs are minimal. Magnetic particle is basically a surface and slightly subsurface inspection process. ET is an electromagnetic inspection method in which • Records that support test data for welding/NDE. documents.e. MT. personnel qualifications. 0. O. Welding Technology need not be evacuated. 8th ed. and structural butt welds. one individual can Procedure Handbook of Arc Welding.8). Davis. W. It • High cost of equipment and film.. inspection to be accomplished. inspection areas Welding Handbook. In regard to cost. Ultrasonic Inspection ASM International. Know is fairly complex. 1. ASM Handbook. Modern Welding Technology. Fla. signals. 1976. This makes UT considerably less expensive than RT. and Pandjiris. and requires the interpretative skills of costs then weld. ed. Summary Radiographic Testing With ever-shrinking budgets. This section has at- discontinuities.2 is a check-off sheet of costs to remem- • Required chemical processing of the exposed film. of internal flaws in components. 14 . N. to penetrate a specific object to reveal nondestructive testing is involved. rather a brief narrative • Stringent controls of the radiation source. Ohio. controlling costs in the public and private the most expensive. H. UT has several advantages including location Cliffs: Prentice-Hall. 1. fects or flaws. However. Welding Journal 47(7): 561–568. Because UT produces no radiation.. ber prior to the bid process. • Trained personnel to review/interpret the film. in terms of ease of operation. • Restriction of production efforts in the area of inspec- tion. Design of Weldments. trained technicians. One of the most versatile NDE methods is ultrasonic Cary.: American Welding Society. Fun- damentals of Quality Control and Quality Assurance. Ohio. PT manent record as the radiograph. 1968... is not intended to be all inclusive. 8th ed.. vol. As we have seen. Metals Park. 12. it Cooper. viewed. 1997. RT is widely utilized in inspection of pipe Lincoln Arc Welding Foundation.1 is a condensed review. This nondestructive method uses the industry is a major concern. improved equipment requires a better surface finish than MT for satisfactory provides computerized printouts for records. K. Table 3. The operate the machine and interpret the ultrasound wave Lincoln Electric Company. List vides a permanent record of most surface or internal de- Blodgett. Miami. due to the presence of harmful radiation. Englewood inspection. 1963. (WHB-1. and ability to be used on most materials. Boyer. The James F. radiographic inspection is from abroad. Cleveland. Table 3. Cleveland. there are penetrating radiation of X-rays. and pro. about some of the major subjects which should be re- • Skilled personnel capable of producing exposed film. or the gamma rays of a several variables to consider when welding and related radioactive source. vol. Ohio. 1973. sec. and strong competition Of the six NDE methods. A. H. J. W. It is very labor intensive for the following tempted to describe some of the welding variables that reasons: need to be considered. However. Bibliography/Recommended Reading This method can be used on most materials. Though unable to provide a per.SECTION 3—PRODUCTION WELDING COST ANALYSIS although more time consuming than MT. Careful considerations must be made prior to utilizing a particular welding process or mode. supervisor. If at all possible. and may in the long run actually reduce costs. The roles of the design engineer. 5. Job preplanning. The selection of automated equipment requires a close review of several variables. Weld procedure/personnel qualifications pose two problems. etc. 6. deposition rates. and accurate scheduling are three critical and necessary steps that must be accomplished prior to any project bid submittal or start of production. etc. Aside from raising the filler metal and labor costs. and can require considerable time to accom- plish. i. SECTION 3—PRODUCTION WELDING COST ANALYSIS Table 3. It has been estimated that for every hour of welding time. definition of scope.1 Review of Production Welding Costs 1.. 8. 12. 3. it is preferable to accomplish the majority of fabrication or welding in the shop vs. are all varied. There are several factors that affect the efficient utilization of the many welding processes. which will justify the capital expenditures. However. For example.e. Joint meetings to discuss particular attributes or suspected problem areas are necessary for uni- fied direction. 11. in the field. including long-term preplanning and costs. welding engineer. for thicknesses over 1-1/2 in. fixturing. Increasing fillet weld sizes beyond what is required can be detrimental. Fire/heat protection costs have significantly increased. with each being critical for success- ful project accomplishment. 9. Inexperienced welders or craftsman can affect the reject rates. the additional welding may contribute to serious distortion of the weldment. welder. 4. Choosing the correct joint design for the particular plate thickness is very important.. it ensures that a quality product is being produced. Nondestructive testing during the manufacturing process has potential for increasing costs. Early identification of specific needs is required. 7. the double bevel “T” joint design is more economical to employ than increasing the fillet weld size. 10. There must be a projected positive rate of return. joint geome- try.. Maintaining accountability of work accomplishments or inspections via the use of formal records is a necessary overhead cost that must be factored into the bid. Both are expensive. The results of these meetings will decrease costs and enhance the product quality. an equal amount of time is required for related fire/heat protection. 15 . 2. Maintaining a skilled workforce should be a high priority. ❒ Cost of origination/distribution of pertinent engineering instructions or drawings for prefabrication support. ❒ Supervision (labor) costs. ❒ Costs of origination/distribution of funding or work standard documents. ❒ Costs for preparation prior to welding. ❒ Estimated rework costs. etc.. nondestructive. ❒ Inspection costs: in-process. and retention. ❒ Possible post weld heat treatment costs. and shipping costs. etc. joint designs..SECTION 3—PRODUCTION WELDING COST ANALYSIS Table 3. ❒ Surveillance and audit costs. ❒ Weld procedure and welder performance qualification costs. i.. ❒ Receipt inspection costs. etc.e. i. ❒ Final cleaning. ❒ Distortion control costs during in-process welding. fitting. ❒ Fire/heat protection costs (including labor) associated with welding/burning. ❒ Formal record costs: origination. 16 . such as hydrostatic. casting or forging fabrications. issue instructions for base and filler metal inspections. i. etc. base metal cleaning. fixtures.e. etc. medical insurance.2 Checklist of Project/Welding Cost Variables ❒ Advance planning costs. ❒ Rigging or crane costs in the movement of the materials during and after the weldments are completed. ❒ Overhead costs (vacation. accomplishment of tasking actions.e. and other. ❒ Special equipment purchases: welding machines. machining. etc. meetings. tracking. electric power. ❒ Material and welding electrode costs. ❒ Labor costs associated with welding.) associated with labor and operation of the required equip- ment for the fabrication of weldments. preservation. .................................................................................................................................................................................................................................................................................................................................................................................................................................................................. 18 Productivity Value . 20 Designing for Zone and Preoutfitting Construction ............................................................................................... 21 Summary ................................................................. SECTION 4 Modular Construction Contents Introduction ................................... 18 Dimensional Accuracy (Process Control) .................................................................................................... 21 17 .................................................. 19 Preoutfitting—A Major Advantage ......................................... the ural evolution of large structures. prior to shifting to a new zone. This concept initiated the move- ment of the design and planning functions toward the • Time. This process should be system. machinery and Productivity Value electrical components could be packaged in a less expen. time. Also. joining of additional blocks. ated by trial and error.SECTION 4—MODULAR CONSTRUCTION Section 4—Modular Construction Introduction Optimizing Resources Modular construction or block construction was a nat. it requires function of the other two and therefore must be consid- the engineering design and production work scheduling ered together. re- and planners to rapidly define the production of an entire sources. Each of the variables is a more than just assembly and fabrication work. • Resources. planned. an entire pipe system would be that can be used to evaluate performance by comparison. it is an expression struction. The prior quantity of work to be accomplished within boundaries. material personnel. zone outfitting approach. package definition until the proper balance of time. as it establishes a baseline 18 . formulated mathematically. The structure and its systems are converted into zone function is complete. ods. designed. reducing “productivity value” which is a function of: the field assembly time. and scheduled for production without Each work package process and operation can be evalu- consideration of the other systems that would be fabri. These three variables must be balanced to obtain an Implementation of modular construction encompasses optimum productivity value. This method of construc. For example. These zones define work packages that define a uated in relation to resources. The tivity is determined. scheduling of people and material is optimized. plete a work package that may include portions of multi- tion offered new opportunities to the outfitting trades. As methods change or schedules are readjusted. process must be included. and quality is achieved. This may require changes in work systems method enables designers. Trial and Error Evaluation System Approach Productivity value is a unitless term that cannot be Traditional methods used a systems approach to con. and quality. repeated periodically to reevaluate the productivity value. The efficiency of a work package can be defined by a sive shop environment and landed on the block. Modular Construction the variable balance may be shifted and the optimum conditions may no longer be present. A crew of craftsmen can com- entire structure was completed. The geous for structures to be assembled in small blocks amount of time lost for material movement and relocation followed by joining these small blocks together until the of people is minimized. the blocks could be preoutfitted prior to erection and • Quality. The processes should be eval- zones. By designing and planning for the zone work. clude the prior and following operations after the specific ductivity. to be an integral part. In modular construction it is essential to change from A proper review of a specific zone project must in- the systems concept to a zone concept to optimize pro. Rather. The assemblies could be turned over so work was at a more convenient physical location and much of the weld- ing could be flat position work. until the optimum level of produc- cated during the same time and in the same location. ple systems within a zone. By implementing zone meth. It became advanta. customer-supplier relationship must be measured and variations in process parameters can be identified and the controlled to maintain an optimum productivity value. Variations in con. SECTION 4—MODULAR CONSTRUCTION condition for the measured operation. this will force the use of less than the tradi- product prompts the allocation of additional resources to tional tolerances.1—Typical Histogram 19 .. Basically. The data shown represent a series of actual measurements Figure 4. each operation Accuracy Control Program of the production process will provide a final product to the following operation. the following operation partment or ship centerline. causes evaluated. tural dimensional accuracy. Proper implementation of these advanced concepts requires all structural members (1) Histogram. In than-optimum conditions. available. tural block dimensional control. or when resources are not that a zone approach to outfitting is successful. electrical systems) in addition to the ing operation must result in a satisfactory consistent level structural members. An important tool for controlling process perfor- fiable responsibility and accountability to individuals mance in structural fabrication is an accuracy control completing incrementally completed segments. Process control is necessary to ensure meet schedule requirements. additional time is used to compensate for cor- rection of quality deficiencies.1 is a typical histogram.e. for dimensional accuracy to ensure proper alignment. system components are located by measurements from ditions from the prior process or operation will affect the the nearest structure in lieu of the more traditional com- productivity variables. forcing struc- will be impacted by optimizing the evaluated process. of quality being passed to the following operation. ods utilizes basic statistical analysis. In addition. Figure 4. Deviations from nominal values can become cumula- Changes in the quality variable will skew the balance of tive and the larger block connections must be monitored the three productivity value components and cause less. Two of the more common methods of displaying the data for accuracy control measurements are the histo- Dimensional Accuracy (Process Control) gram and the process control chart. This process provides identi. utive systems (i. Starting with a poor-quality some cases. With the use of statistical process controls. this now requires the The quality value is a good example of the effect of matching blocks to consider the alignment of the distrib- one operation on the other process steps. The and platforms to be within predetermined tolerances. Each process. This program. Each of these meth- Successful preoutfitting is greatly dependent on struc. Similarly. 2 is another tool that is used to measure the performance of a continuous process. thus redefining normal performance. the variations in the process can be on baseline performance can be determined. outfitting work. process per- Scheduling and Improved Work Environment formance can be measured numerically and decisions no Production defines several objectives of this planning longer need to be based solely on management impres.SECTION 4—MODULAR CONSTRUCTION taken over a period of time. monitoring process performance with statistical controls. (2) Process Control Charts. normal process inconsistencies can be properly align at the boundaries. ough planning and scheduling is required to optimize this ally move the upper and lower control limits under con. These methods are valuable tools for management deci- sions. welding operations can be scheduled to allow flat Figure 4. By utilizing that the systems continuing from one zone to another the bell curve. it is essential tion of predictable variations in a process. The variation between the upper and lower con.2—Process Control Chart 20 . The process control chart shown in Figure 4. collected and displayed to provide a meaningful analysis. Adjustments in the process can actu. Very thor- determine the cause. By collecting the data in the sure that productivity is being improved and the effects histogram format. concepts. much of the fitting and sions. The structure must be estimated and a process can be evaluated to determine accurate and within predefined tolerances to ensure that the frequency and magnitude of deviation from perfor. The Preoutfitting—A Major Advantage chart is used to monitor an operation by comparing mea- sured accuracy to the accuracy predicted by statistical One of the most important benefits of modular con- methods. effort. the matching systems in two zones are within dimen- mance to established tolerances. Excursions be. struction. When sequencing work by zones. the result may be the erection of empty steel assemblies. By implementing a control program.” The bell curve is a standard statistical representa. Changes in each process can be measured to en. When outfitting does not follow zone preoutfitting trolled conditions. which then require the more expensive con- Other similar types of charts can be used to assist in ventional methods of installing the distributive systems. struction methods is the opportunity to implement pre- trol limits are normal and are expected. The objective of this approach is to yond the limits are unusual and should be investigated to minimize the outfitting work after erection. effort. By advance planning. sional tolerances to allow for proper connections. An accuracy control program provides essential infor- The histogram can be used to approximate a “bell mation for successful implementation of modular con- curve. Advancements in the use of computer-aided design By early zone planning. to be performed at the To reap the benefits of this construction method. programming will enable a designer to define zones of a Another objective is to schedule work in uncongested structure and the applicable portions of each system will and more efficient work locations. systems arrangement drawings can automatic welding processes. and zone approach to design. 21 . SECTION 4—MODULAR CONSTRUCTION position welding and horizontal fillets. high or narrow efits of a zone design concept without a large amount of sites. structure installation. approach to design. design. This will gain the ben- erators perform their tasks in enclosed. system and in demonstrating the location and path of the (5) Less on-site cleaning and reduction in fire distributive systems. planning effort which incorporates important production personnel needs to accomplish their work efficiently. be broken down into these zones. As this position. Effective use of modular construction must incorpo. the optimum base of zone hazards. the quality support the zone construction method. This objective must consider work area space changes in many areas must be considered: allowances to ensure that craftsmen have sufficient room to perform their tasks without interfering with other • The design effort must support a zone approach. Additionally. the key variables (time. and quality). material lists can be generated to productivity will be increased. Design by system has value (3) Reduction in lost material. labor will be reduced and be eliminated. Implementation of preoutfitting will not be optimized If properly implemented. in defining the capability of each functional individual (4) Less on-site rigging and material handling. it is not effi. planning and work packages requires a zone approach to (6) Improved working conditions on the assemblies. The time span for a production zone work package can be reduced by scheduling multiple tasks. resources. compatible tasks can be scheduled to be • Planning must be directed toward a zone work pack- performed in unison. rather than having op. For example. there are differences between the systems and productivity. same time. Quality and productivity will be enhanced by this duplication of effort. rial and production planning. As stated in the construction will be cost effective and improve quality introduction. to ensure the proper balance of efforts must be discouraged. preoutfitting and modular without the corollary design package. Designing for Zone and Preoutfitting Some of the benefits of preoutfitting include: Construction (1) A higher percentage of shop fabrication and assembly. From this base. the CAD ciated with multiple-operation manufacturing. Similarly. adequately monitor the fabrication process to ensure that the blocks and distributive systems align properly. components can be identified (CAD) will soon permit a design to be made with the tra- so that different types of work can be scheduled and kept ditional systems approach and then be converted to a separate at the earliest manufacturing operation. This zone configuration. of the work will be enhanced. work. which are Summary not in conflict with each other. For example. Also. cient to have a blast or paint operation being performed • An accuracy control program must be implemented to in the same zone as a pipe-fitting operation. a system design methodology. However. This will maintain the advantages of will simplify the manning and scheduling problems asso. with a zone ing will allow greater use of mechanized and semi. and considerations of conflicting age that is producible. (2) Less congestion and worker interference for rate design efforts that enhance zone definition for mate. ... 28 SAW.... 30 Bibliography/Recommended Reading List .......................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... 26 GMAW .................................................................................................................... 24 Process Selection ...................................................................................................................................................................................................................... 24 SMAW................................. 27 FCAW .............................................................................................................................................................................................. SECTION 5 Welding Process Selection Contents Common Welding Processes ......................... 29 GTAW ............................................................................................................................ 32 23 ............................................ When the base metal and filler metal become molten. fore determining which is the best process for a specific vides many of the alloying elements to the weld. while all alloying elements Some processes are better suited to certain base mate- are supplied by the wire electrode alone. The electric arc is a very effective weld metal composition since many elements are diluted and portable heat source for melting most metals quickly. new equipment. which creates a gas shield and also pro. The maximum heat input must be 24 . wire and typically obtain additional shielding from an inert gas shield similar to the GMAW process. Each process can be ranked in terms of its dep- merged Arc Welding (SAW) process (see Figure 5. As a minimum. and typi- and acceptance by the shop personnel are accomplished. This can become a compli- fed into the joint. there are other factors that must be considered. replace it. Therefore. or process (see Figure 5.1–5. which automatically Productivity is usually a very important consideration feed a spooled electrode filler wire into the joint while on most jobs. and reinspect ing the welding process. Both Base Metal Type GMAW and GTAW utilize inert shielding gases that flow around the molten weld pool. an automatic process whereby both filler wire and travel However. For each pro. The Sub- this factor. Typical semi-automatic arc welding cated decision. which conducts the welding current and melts as it is continuously fed into the arc and deposited into the Process Selection joint. lar type of dry flux in a granular form that covers the but more importantly with the shop’s capabilities and the welding arc and molten pool. The Gas Tungsten Arc Welding (GTAW) familiar with a new welding process.3). It is difficult to predict involve arc welding. when crossing the welding arc. Weld metal com- The most common welding processes used in industry position varies with each process. speed are controlled by the machine which runs along a For example. and deposited in the joint. the following items also use a dry flux contained in the center of a tubular should be considered prior to selecting a welding process. Spooled FCAW electrodes job requirements.SECTION 5—WELDING PROCESS SELECTION Section 5—Welding Process Selection Common Welding Processes cess the weld metal composition is an alloy combination of filler metal and remelted base metal. SMAW electrodes have a thick it. who is very familiar not only with the various processes. piece. since a considerable amount of time is required must be shielded from oxidation and contamination dur. Most welding processes utilize a consumable elec- trode. The most common manual arc welding process is When selecting a welding process for a specific appli- the Shielded Metal Arc Welding (SMAW) process (see cation.5) uses a nonconsumable tungsten even a new brand of filler metal. several factors that affect productivity and weld Figure 5. a separate hand-held filler wire is melted in the arc the actual productivity rate will be less than the average. in which a fixed length of electrode is hand quality must be balanced. many factors must be taken into account be- dry flux coating. rials than others. Figures 5. Reject rates also have a significant impact on actual pro- The molten weld pool created with all arc processes ductivity.4) is osition rate in pounds of weld metal deposited per hour. (FCAW) process (see Figure 5. due to the number of conflicting advan- processes include the Gas Metal Arc Welding (GMAW) tages and disadvantages which each process possesses in process (see Figure 5.1).5 describe the most common welding they mix together then solidify to produce one solid processes and summarize their advantages and limitations. The deposition rate is a significant part of the welder manipulates the torch manually.2) and the Flux Cored Arc Welding each situation. Process selection must be made by someone SAW process obtains shielding and alloying with a simi. Until adequate training electrode to establish an arc in the weld joint. to remove defective weld metal. The application. cally. it usually takes time for a shop to become preset track. 6. use of high-productivity pro.. A high-productivity process such as SAW would avoid lack of fusion defects. forming. In practice. equipment required for the processes discussed. i. identify and accurately weigh all of the variables in- cess to the joint is limited. Studies have shown that re. The GMAW and FCAW processes are probably tered containments. welding. and other heat-treated or cold- worked materials. of shop employees toward a new process or procedure The four basic welding positions are: flat. Processes that derive their advantage Most large welding shops have access to the welding from high productivity and high heat input may be lim. Table 5. since the highest productiv. being exposed to the elements or fer across the arc with simple operator controls. Since input used to ensure that specific mechanical properties some processes require more access to the joint root to are met. The length of the weld must also Fabrication and Inspection Standards be taken into account. If possible. when new equipment must be evaluated to determine if the increased productivity or versatility would Joint Design and Thickness offset the initial cost and training.e. such as the attitude and perception cesses and filler materials is limited. since many processes are lim. While some of the when selecting a process.1 is a general comparison guide for the most Welding Position commonly used processes in the industry. dams. vertical. be taken into account when selecting the best welding plished in this position. It takes very little wind to disturb the full advantage of the wide variety of machines available gas shield that is critical for high-quality GMAW and today and all of the functions each one is capable of per- GTAW welding. as would be necessary with a small repair. aspects of the metal transfer across the arc. Whenever possible. This restricts their use outside of shel. the joint ison for many applications. shop prefabrication is rec. It can be seen from this chart that many variables must ity and weld quality are attained when welding is accom. As the section thickness increases. The SMAW and FCAW processes the most influenced by these advances. and also may limit the maximum heat pass. 25 . No “synergic” power sources that manipulate the waveform of process is tolerant of direct rain. the process selec- tion should reflect this. welding productiv- ity becomes more important. the selection of some pro- lose much of its advantage if the heat input were re- cesses may also require a joint design change. age-hardened. change. Availability of Equipment pered. it takes experience to on large weldments that cannot be repositioned and ac. stricted down to the range achievable by GMAW. should be in the flat position. ductivity and weld quality are greatly affected by many ommended where practical. these machines ject rates for field welding increased from 100% to offer a significant advantage over the previous generations greater than 400% over those for shop welding. in the form of can also be affected by wind. relative to The weld joint position plays a very important part some of the factors discussed above. Another factor complicating process selection is the rapid change brought on by the computer age. Since most repair work is done process for a given job. volved.e. Some factors. but to a lesser degree. SECTION 5—WELDING PROCESS SELECTION limited on some types of materials. which is Environmental Conditions taking place in the electrical power sources used for arc Wind and rain are the two field conditions that typi. of power sources. may be more difficult to analyze than others. as shown in Figure 5. Proper placement of the welding current through complex electrical circuits. tarps. horizontal. It takes a considerable amount of study to take cally affect welding. ratings are subjective. times. quenched. and overhead. since a higher-productivity process Most welding and inspection is governed by welding may not realize this advantage—particularly if the opera- standards that often limit the use of some processes on tor has to frequently stop the process to set up for the next certain materials. tem. or other temporary containments can rectify These advanced machines automatically adjust all of the this situation.. Due to the many problems associated with waveform parameters to optimize the mode of metal trans- welding outside (i. however. There are ited for these applications. Since pro- cramped inside of a structure). they do provide a general compar- ited to only a few positions. Many alloys are weldable. The process is fairly tolerant of environmental conditions. The 12-in. Poor usability for aluminum and most bronzes. repairs on thick material. fixed Thick sections are weldable. Electrode travel. Welding is frequently interrupted due to the short. Higher potential for weld contamination than for other processes. The flux coat- ing often provides alloying and deoxidizing elements. Foreign material exclusion for nuclear applications is poor.SECTION 5—WELDING PROCESS SELECTION SMAW Shielded metal arc welding is a manual arc welding process. is required. easy to set up. Not suitable for thin material but is suitable for thin-wall Good accessibility in space-restricted areas. low cost. Arc visibility is good. which helps to control heat input.1—Shielded Metal Arc Welding 26 . A covered electrode is used to deposit the filler metal and supply shielding to the molten weld pool. length of electrodes. Advantages Limitations All-position capability. The process creates spatter and smoke fumes and leaves a slag that must be removed before the next layer can be Equipment is simple. to 18-in. low maintenance. long electrode has a dry flux coating that decomposes in the arc to form a gas shield and a solid slag covering on the cooled weld deposit. Special electrode care is required since most flux coat- ings absorb moisture. and is deposited. Excellent for field and repair work. Severe arc blow can be a problem when using direct current (dc) Figure 5. Not used for thin metals except with pulsed spray and short-circuit transfer welding. pulsed spray. Most metals are weldable. A small-diameter. Handling gas bottles for field work can be expensive. Figure 5. Excessive weld spatter with CO2. shielding. continuous consumable electrode is melted in the arc and deposited in the joint. Gas shielding is easily disturbed during field work. and spray transfer. Minimal stub loss. Equipment is complicated and expensive. Restriction on the use of short circuiting transfer welding. Limited accessibility. four modes of metal transfer are possible: short cir- cuiting. SECTION 5—WELDING PROCESS SELECTION GMAW Gas metal arc welding is a semiautomatic or auto- matic process. No slag removal and minimum interpass cleaning. Depending on the amperage and wave form used. Flash burns easily obtained.2—Gas Metal Arc Welding 27 . All-position welding with some GMAW processes. globular. Spray transfer limited to the flat position. Good weld puddle visibility. High deposit rates (three times that of SMAW). Shielding is provided by a gas that flows through the weld torch. Easily adapted to robotics. Advantages Limitations High-quality welds. SECTION 5—WELDING PROCESS SELECTION FCAW Flux cored arc welding uses equipment similar to GMAW. Self-shielded process has lower-quality welds than exter- nal gas shielded. Equipment is complicated and expensive. Some electrodes provide ade- quate shielding with the flux core alone. Less joint preparation required than for SMAW and GTAW. but others require the addition of gas shielding. Weld spatter can clog nozzles. The main difference is that the electrode is a tubular wire with a flux core. Flash burns easily obtained. Large amounts of smoke and fumes. Figure 5. The flux core is susceptible to moisture pickup. and accessibility can be a problem.3—Flux Cored Arc Welding 28 . Good penetration with gas shielding. Self-shielded process is good for field work. which may cause porosity and worm tracks. Tolerant of mill scale. Slag coating must be removed between layers. All-position welding possible. Equipment is bulky. Advantages Limitations Large single-pass fillets possible. High deposition rates possible. The flux is fed through a hopper and a guide tube and de- posited in the joint ahead of the welding arc. Penetration is excellent. Maintenance relatively low. Figure 5. High toughness properties are hard to obtain. No flash burns. Best suited to long welds and thick sections. Less angular weld distortion. Advantages Limitations High deposition rates. Limited to ferrous metals and high-nickel alloys (no age- hardenable alloys). Welding positions limited to flat and horizontal. SECTION 5—WELDING PROCESS SELECTION SAW Submerged arc welding is very similar to GMAW except the weld pool is shielded. and alloyed with a granulated flux that melts in the arc.4—Submerged Arc Welding 29 . Excellent weld quality. Slag and unfused flux must be removed. Smoke and fume quantities are low. Joint fit-up and design are critical to penetration control. Two or more electrode wires can be used in tandem or metal powders can be added to in- crease deposition rate to obtain optimum me- chanical properties. Less weld joint preparation. deoxidized. ture easily. since it absorbs mois- Weld appearance excellent. Special care required for the flux. Both hands must be used to hold the torch and filler wire. NONCONSLE TNGSTEN ELECTRODE GS SIELD ILLER ETL OLTEN RC SOLIDIIED WELD ETL WELD ETL Advantages Limitations Excellent arc visibility. GMAW. The filler metal is added to TORC OD the weld pool separately. Lower deposition rates compared to SAW. COLLET OD togenous welding (no filler wire added) is possible with GS NOLE some materials. Au. Excellent weld quality on most metals. Wide range of thicknesses can be welded. High-amperage. spatter. Precise filler metal placement. Shielding gas is easily disturbed by gusts of wind. Figure 5. which limits accessibility. The process can be used either manual or automatic. or smoke. All-position welding possible. Filler metal is not always required. water-cooled torches are heavier and The best undercut control. Difficult to monitor heat input. Shielding of the weld pool and SIELDING GS IN electrode is provided by predominately inert gases.SECTION 5—WELDING PROCESS SELECTION GTAW DIRECTION O WELDING C CP Gas tungsten arc welding is a gas shielded manual or ELECTRODE automatic process that uses a nonconsumable tungsten LED electrode to establish the arc. Amperage can be controlled by remote foot or hand Very limited for outdoor field work. switches. Flash burns are easily obtained. bulkier than air-cooled torches.5—Gas Tungsten Arc Welding 30 . or SMAW. Very clean welds can be achieved with no significant slag. 6—Four Primary Welding Positions Table 5. SECTION 5—WELDING PROCESS SELECTION Figure 5.1 Process Comparison Chart Process SMAW GTAW GMAW FCAW SAW Quality Good Excellent Excellent Good Excellent Deposition Rate Fair Poor Good Good Excellent Field Work Excellent Poor Fair Excellent Poor Equipment Maintenance Low Low Medium Medium Medium Smoke/Fumes High Low Medium High Very Low Heat Input Control Excellent Poor Good Good Satisfactory Arc Visibility and Filler Metal Placement Good Excellent Satisfactory Satisfactory Poor Variety of Metals Weldable Satisfactory Excellent Good Good Fair 31 . cesses (WHB-2. Miami. Miami.5). ———.: List American Welding Society. ———.: American Welding Society. Welding Processes and Practices (WPP).6). (PHB-4).: American Welding can Welding Society. Recommended Practices for Gas Tungsten Arc ———. 2. Miami. Welding (C5. The Everyday Pocket Handbook for Shielded Metal Arc Welding (SMAW) (PHB-7). and Drilling (C7.2).: American Welding Society. 32 . Fla. Recommended Practices for Laser Beam Weld. Miami. Society.8).. ———. Fla. Welding Handbook. Cutting. Fla.SECTION 5—WELDING PROCESS SELECTION Bibliography/Recommended Reading ———. Fla.: Ameri. 8th ed. Fla. Miami. Welding Pro- ing. Recommended Practices for ———.: American Welding (GMAW) and Flux Cored Arc Welding (FCAW) Welding Society. Fla.: American Welding Society. American Welding Society. Fla. Miami. vol. Miami. The Everyday Pocket Handbook for Gas Metal Arc Gas Metal Arc Welding (C5. .......................................................................................................................................................................................... 34 Base Metal Selection.................................................................................................................................................................................................................................................................................................................................... SECTION 6 Primary Concepts of Welding Design Contents Introduction ......................................................................................................... 43 33 .................................... 35 Welds Create a Discontinuity in the Base Material...................................... 35 Minimize the Number of Welds........................................................ 35 Welds Rarely Improve upon Base Metal Properties ...................................................................................................................... Base Metal Properties ................................................................................................ 36 Full Penetration Welds ........................................................................................................ 40 Bibliography/Recommended Reading List ................................................................................................ 35 Standardize Component Welding Design....................................................................... 36 Joint Designs—Amount and Type of Weld ........................................................................................................... 40 Summary for Improved Fabrication Design .............. 34 Service Requirements............................ 35 Keep Weldment Designs Simple ..................................................................................................................................................................................................................................................................................................................................... 38 Fillet Welds...... 35 Location of Welds .............................................................................................................................................................................................................................................................. 36 Weld Deposit Mechanical Properties ..................................................................... 39 Residual Stress—Fact or Fiction? ....................................................................................................................... 34 What the Designer Should Know about Welding ............................................................................................................... 39 Design Considerations for Change of Cross Sections..................................................................... 35 Weld vs................................................................................................................................................................................................................. 36 Partial Penetration Welds ................................................................. material is used for its strength-to-weight ratio proper- ture. What appears to be ments. reading is provided at the end of this manual for more de- • Impact resistance (stress and temperature dependent). not related to the service. If a quenched and tempered and how and where the filler metal is placed in the struc. Recommended • Abrasion resistance. HAZ and base metal) suitable for the expected service. draftsman. and the weldment may be subject to premature. Some of the service requirements the designer may con- ments. machinery. Welding decisions will be less than fully ing two pieces of metal. but it requires special consider. it can clear in the designer’s mind before making welding de- be welded. any engineer. The designer should consider only the service require- The term “designer” is used in this section to address ments that make the weldment (the combination of weld. even though the properties are the filler metal type. Using complex. ing instructions regarding metal fabrication require. Often more than savings. and the heat-affected zone (HAZ) next to the weld will It is poor practice to seek to achieve certain properties affect the structure’s service. disaster when it hits the production floor if the fabrica. tailed information on welding design specifics. It is essential for the design engineer to review the ties. technician. • Fatigue resistance.SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN Section 6—Primary Concepts of Welding Design Introduction Service Requirements Inexperienced engineers frequently believe that if Service requirements are the first item that must be their design can be conveyed to paper as a drawing. appearance). pressure vessel fabrication. The designer must understand how the weld cific design. About Welding It is rare that a designer would be concerned with Welding design must not be based on isolating the more than a few of these service requirements for a spe- weld alone. corrosive environ- strength-to-weight savings may appear to provide cost ments. high pressures and stresses. and impact loading is not of concern. and should not impose requirements of impact properties just to review the finished product. because the material used has the ability to achieve high tical perspective to weld design that would be missed if impact resistance. • Creep strength. anodizing. adequate without clear definition of the service require- ation if it is to be done economically. than heavier section mild steel. not to provide detailed welding design information but to • Erosion resistance. The intent of this section is • Corrosion resistance. based upon the material’s ability to effect of the weld on a structure’s performance including achieve these properties. sign decisions. These individuals are engineering type employees sider are: with cognizance of structural. provide basic welding design concepts. and • Tensile. piping. the designer actual production site prior to and during fabrication. or other person issu. the welding process and parameters. • Temperature (high and low temperature). and shear stresses and strains. and fatigue and impact loading. compressive. Many factors influence the from a weldment. Weldments experience high and low tempera- quenched and tempered steel in a complex design for tures. a good design on a computer or drawing board may be a in-service failures. Service requirements may be very simple or quite tion requirements are not properly considered. What the Designer Should Know • Aesthetics (color match. This provides some prac. Welding is the most common means of join. If 300 Series stainless steel is used the designer were primarily an analytical type. However it may be more expensive to fabricate one of these conditions is involved simultaneously. because of its high-temperature yield and oxidation 34 . also reduces engineering analysis time and may include in fact. these Weldments that involve multiple crossing of stiffeners on steels offer little advantage for fatigue. welds will cause the expensive to purchase and to fabricate than the quenched weldment to have: and tempered steel alloys or the heat-treatable steels and • Lower fatigue resistance than the base metal. Mild steel is much less As compared to base metals. • Corrosion resistance. terms of fabrication or engineering time to analyze each structure. specialty items. ber size. Keep Weldment Designs Simple tigue stress ranges are not improved with these steels. quenched and tempered steels offer little advantage. Standardization base metal and they have different properties. Base Metal Properties Welds are used to join base metals because they are economical. and reliability Minimize the Number of Welds relative to other fabrication processes. they must be given special design consid. Standard design details will simplify fabrica- tion and reduce costs. pad eyes. varied mem- the base metal. Welds are. base metal during fabrication. • Leak tightness. heat. a discontinuity (a stress riser) in the structure— complete foundations. The weld is a discontinuity which is both a geometric trode care may also be required. SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN resistance. property differences. From a design prospective. Welds do not act the same as the and mass production of similar items. If complete reversal of service stress is expected. Special electrodes and elec. due to high residual stress and distortion. Com- mild steel. chemical. special consideration should be given to their use. and sometimes stress relief. In many cases. • Lower impact properties than the base metal. Since fabrication is and metallurgical notch that decreases the suitability for much more expensive with quenched and tempered service below that of the base metal alone. because they are typically used for higher plex designs are expensive to fabricate and can lose loads and will most likely require additional stiffening. or even the size and shape of structural members. Shaping plate by are: pressing or breaking is usually preferable to welding one 35 . ponents will operate in the sensitizing temperature range • Fatigue loading. Weld vs. Weldment design should be kept as simple as possible. and interpass temperature controls during welding. Base Metal Selection Welds Rarely Improve upon Weldments should be made from the most economical Base Metal Properties and readily weldable base metals available that are suit- able for in-service requirements. should not be concerned about weld sensitization of the • High-temperature service. in service anyway. Service areas Welding and its related processing is one of the most where welds can perform favorably toward base metals expensive operations in fabrication. These steels also the same plane. Because welds are different than Unique welding processes. and mechanical signs. multiple intersecting “I” beams. and different materials are not economical in eration. • Impact loading. Fa. Steels de. in some instances. as required in steam boilers. weld joint de- caused by their metallurgical. signed for special applications frequently require pre- • Higher residual stress than the base metal. will usually require considerably less rework. have high joint efficiencies. the designer • Allowable stress. stiffeners. or com- have lower buckling stability for similar cross sections of plex patterns of stress flow should be minimized. steels. boiler com. Welds Create a Discontinuity in the Base Material Standardize Component Welding The designer always must keep in mind that a weld Design does not create a monolithic structure that has uniform Standardization of design is necessary for automation properties throughout. Complex designs also limit welding access and may cause the inability to meet post-weld inspection requirements. only the maximum allowable fatigue stress or static loads. serviceability. The most economi. and the temper- the structure. premature failure of a component could occur due to unanticipated loads. the amount of weld deposit. make are predominately those where proper welding ac. stress risers. metal compatible with the design. or creep is a pri. system status. There are many other secondary factors that affect Designs should avoid welding across flanges under joint design selection. semiautomatic. it will suffice to say the primary reasons for se- cess has not been provided for by the designer. the ability to accommodate service (NDE) will usually be difficult. impact. etc. the welds should be placed in low-stressed • Increased costs. inspection requirements and the ser. how much it has been di- luted into the weld metal. such as available processes and tension as drastic reductions to allowable service loads skill levels. are imposed by most codes. then Nondestructive Examination requirements (i. loca- Avoid welds on edges of members subject to tensile tion of the weld. the deposit and the localized effects on the structure (espe.e. In most cases. ally experience higher loads than the rest of the member and. the designer should arrange for the welds to be made in the flat position. areas. since a weld acts as a stress riser. and the rate of weld metal cooling. Often D1. Joint Designs—Amount and Type of cal. as will any necessary fol. In general the de- cially for fatigue applications). position of the weld.SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN or more pieces of plate together. cost. The cooling rate can be drastically affected by base From a design point of view.. base metal thickness and properties.1). welds should be kept in the lowest stressed • Increased base metal cracking. the schedule of the project. The cooling rate of the weld can also affect the Location of Welds mechanical properties of the weld and the heat-affected zone. preheat and interpass temperature).e. However. fastest and highest-quality weld is achieved in the flat position with standard welding processes. When fatigue. • Decreased impact resistance. Examples of such controlling codes are the AWS duce a deposit that exceeds base metal properties. Edges of members subject to tensile loads usu.1. stress/strain and corrosion) and to accommodate welder low up repairs. areas as practical. Partial penetration welds can often The as-deposited mechanical properties of a weld in a be used for tension. The least processes at substantial cost and time savings. upon the base metal chemistry. If welding lecting a joint design are to provide the desired service access is difficult. amount of weld should be used to meet the necessary de- The location of welds should always provide the sign service requirements. • Increased amounts of residual stress. base metal HAZ. the carry service loads is only as good as the weakest part of technique used to deposit the filler metal. result in the weldment having: Welds should be located according to the cost of mak- ing the weldment. The most costly welds to lection is discussed by another section of this manual. When possible. Reducing the number of than the welding filler metal specification requirements. component parts by improving the design or purchasing In production the weld metal properties are dependent standard shapes is usually more economical than welding. vice requirements. aesthetics.. Designers will frequently use filler metals that pro- cated. practical to provide access for welding and NDE. in combination with the Institute of Steel Construction (AISC) Code. Structural Welding Code—Steel. Because welds represent a metallurgical notch and are • Increased distortion. a structure’s ability to metal type and thickness. The allowable loads that signer should use the lowest possible yield strength filler welds are permitted to carry are determined in many in. Specific weld joint design se- welder easy and open access. Partial Penetration Welds A partial penetration weld is a weld that intentionally Weld Deposit Mechanical Properties does not completely penetrate through the thickness of a joint (see Figure 6. the weakest link is the weld ature of the base metal prior to each weld pass (i. and shear stresses that fabrication should always be considered to be different act parallel to the longitudinal axis of a weld and for 36 . and the American these higher-strength deposits. Snipes or “rat holes” should be as large as accessibility. stances by a standard or code to which the item is fabri. mary concern. welds. or automatic welding will determine the type and amount of weld. environmental conditions. • Decreased fatigue resistance. Flat posi- Weld tion welding with simple designs allows high deposition The type and direction of loading of the weldment rate mechanized. compression. the number of fatigue. 1—Partial Joint Penetration with(1/2) Joint Geometry Optional 1/2 1 1/4 WELD CROSS SECTION SYMBOL (1/2) 1/2 3/4 WELD CROSS SECTION SYMBOL 1 1/8 (1/2) 1/2 (1/2) 1/2 WELD CROSS SECTION SYMBOL (1/2) 1/2 WELD CROSS SECTION SYMBOL Figure 6.1—Partial Joint Penetration with Joint Geometry Optional 37 . SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN (1/2) 1/2 Figure 6. 4). often cause the largest distor- shear while in service are much less critical than those tion and have high residual stress. high temperatures. Lamellar tearing and cracking Partial penetration welds are less expensive and cause of the base metal adjacent to the weld is a common result less distortion than full penetration welds. Services involving high shock. ous lack of fusion conditions at their roots. joint designs or lower-strength welds to be used. through thickness strains. They should not be used in corrosive environments cial joint design consideration to minimize residual stress unless the metal is resistant to crevice corrosion. Full penetra- Welding defects that are placed in compression or tion welds are expensive. thereby allowing partial penetration ments. In some cases. particularly ness of a joint (see Figure 6. However. Groove welds are used on on heavy sections. Often used only where required by the design service require- welds that are subject to shear stresses can be designed to be 50% efficient. In a practical sense. permitted the highest allowable stresses for design ser- partial penetration welds have crack starters and continu- vice by all codes. Mild steel structural plate has low ductility and they should not be used when the weld roots are sub.2). and pressure vessels often require full penetration welds.SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN compression loads normal to a weld (see Figure 6. These and the possibility of cracking and lamellar tearing (see joints have poor resistance to fatigue and impact loading. Open roots of partial penetration welds may be subject to crevice corro. corners. weld failures will be experienced dur- butts. Full penetration welds should be ing fabrication before service loads are imposed. and tees. Full penetration welds on heavy sections require spe- sion. they are placed in tension by service loads. Figure 6. Figure 6. of poor welding joint details on heavy structural steels.2—Compression and Tension Loads for Partial Penetration Welds 38 . axial or bending cyclic loading (fatigue).3). Full Penetration Welds Making full penetration welds on very thick material for these types of loads can cause higher residual stress than A full penetration weld penetrates through the thick- the anticipated service loads. through its thickness and it cannot accommodate high ject to transverse cyclic tension or bending tension loads. These varying conditions lead to and are used to join two relatively perpendicular sec. the depth of weld ratio is considered to be the most desirable. corrosion. those items that affect fa- tests.) above their calculated values to play it safe. It through gradual changes of the cross section. signers in not applying liberal safety factors and over- which greatly increases the cost of welding. Increasing fillet weld Changes of the cross section must be gradual in both the sizes by two sizes (1/8 in. welding time. A 5/16-in.1. How- tions. Structural Welding Code—Steel. the amount of weld reinforcement. However. some conservative positions by design engineers. Intermittent fillet welds and continuous fillet welds made from one side only should be avoided where fa. The various codes and standards each have differ. Fillet weld sizing formulas and tables can be found 1/8-in. fillet welds. and Fillet welds are generally triangular in cross section the actual failure path. or see impossible to produce. loads are experienced. in general. they are unlikely to propagate weld defects into failures except under fatigue service applications. SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN NOTE OVERLAP NOTE OVERLAP OF 1/8 1/4 3/8 1/4 1/4 3/8 1/4 3/8 3/8 Figure 6. It is preferable to have more than 100%. It should calculations that are backed up by extensive laboratory also be noted that. the actual mechanical properties of the weld deposit and HAZ. fillet weld can be readily failures unless unanticipated service fatigue or impact achieved in one pass. because a 1/16-in. a 4:1 the selection of the welding process. If possible. Laboratory tests and calculations can also vary by changes of cross section to be a 2:1 ratio.3—Full Penetration Groove Welds Fillet Welds penetration. Small fillet welds result in low residual stress and ever.5 and 6. AWS Design Handbook for Calculating Fillet Weld Size. Oversize fillet welds are very expen- is better to keep the fillet size down to one pass and sive and can result in excessive distortion. rily in shear. Cross Sections One of the common mistakes of engineers is that they One of the fundamental errors made by design engi- will frequently increase the fillet weld by one size neers is not allowing for stress and strain to take place (1/16 in. fillet weld is nearly in AWS D1. ent requirements based on different theories and assump.6). it sizing fillet welds. 39 . and possibly the need for flame straighten- The smallest typical production welds are generally ing. tigue life are also of concern if impact loading is expected. being given to visual inspection. and cost of filler metals by concern (see Figures 6. distortion. Design Considerations for Change of tigue. changes in the cross section take place in the base metal Fillet weld sizing is not strictly based on engineering instead of in the weld metal through a weld joint. Great care should be taken by de- Larger fillets will require a minimum of three passes. Since fillet welds are used prima. filler metal. it is extremely rare that fillet welds result in service weld distortion. or debris and contamination traps are a concern. extend the length of the weld to achieve joint efficiency. Failures also is not uncommon for production personnel to be commonly occur as a result of thermal and/or mechanical conservative and go oversize due to increased attention fatigue associated with rapid changes of the cross section.) has the potential of increasing weld metal and the base metal when fatigue is of primary welding time. Some common design practices allow tapered sections or tions. • Use the minimum weld size or the least amount of weld metal consistent with the design requirements. Residual Stress—Fact or Fiction? (7) The weld and heat-affected zone are usually the weakest link in a weldment design. • Peen the welds lightly during the welding process. keep welds in low-service stress stresses exist and do they affect welding design? Are re. • Avoid joint designs that cause high residual stress through the plate thickness. observing preheating and interpass tem- perature for a quenched and tempered metal or quench-hardened material will lower residual stresses. and fractured welds and base metal. (2) Welds are geometric and metallurgical notches in base metal. • Stress relieve the weldment. The weld is usu- Figure 6. • Prestress the weld joints prior to welding. Without stress relief. • Use low-yield strength filler metals with high ductility. (4) Welds can raise the residual stress in base metals to the yield point. areas. sharp corners. when practi- cal. • Avoid full penetration welds on heavy wall structural assemblies (jumbo sections). Use standard shape material or use castings. Summary for Improved Fabrication Design (1) Only consider service requirements that make the weldment suitable for service. Do welding residual (8) When practical. the inexpe. (3) Welds in base metal frequently cause the design- allowable stresses for fatigue to drop. should have the welds placed across the plate ends (through the plate thickness) on heavy sections. This metal fracturing can take place during fabrication or in service. The ef- fects of residual stress can be seen in distorted weld- ments. • Avoid the use of notches. rienced design engineer questions.SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN sidual stresses caused by welds? Yes! Residual stresses as high as the yield point do exist in weldments.4—Weld Configurations that ally easier and less expensive to make. Some items that can minimize residual stress and metal fracture: • Use low-yield strength base metals. • In general. One way to allow for this residual stress in the design is by placing welds in low-service stressed areas. balancing the weld around the neutral axis or welding toward rigidity. Reduce component parts or the number of plates. riser that is preloaded by residual stress. localized areas of yield point resid- ual stress are common and should be expected. or rapid changes of cross section (stress risers). • Sequencing the weld by back stepping. (5) Keep weldment design simple. 40 . May Cause Lamellar Tearing (6) Avoid welding when practical. Joint details. The weld is a stress What every welder knows as a fact of life. 41 SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN Figure 6.5—Design Considerations for a Change of Cross Section (Tubular) . SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN Figure 6.6—Design Considerations for a Change of Cross Section (Nontubular) 42 . Structural Welding Code—Stainless Steel (D1. loads. vol. to 3/8-in. 1. weld root is subject to cyclic tension or impact loads. Fla.: American Welding Society. (D1.: American Weld- (19) Balance welding about the neutral axis. American Welding Society. Design Handbook for Calcu- distortion. Welding Handbook.: Ameri- (15) Use the least amount of weld metal necessary to can Welding Society. maximum Fla. Welding they are to be used for fluid containment.: American Welding Society. all compression ———.3).: American Welding Society. Structural Welding Code—Reinforcing Steel (16) Use partial penetration welds in lieu of full pene. Some stan- dards require a two-layer minimum for leak tightness if ———. axial tension and compression loads. lating Fillet Weld Sizes (FWSH). List ible with the design.8). Fla. Miami. Fla. ing Society.. ———. Fla. A 1/8-in. (10) Use flat position welding. (17) Use full penetration welds only when required by ———. meet design requirements. (13) “As deposited” welds will have mechanical prop- erties that are different from their specifications due to base metal dilution. Bibliography/Recommended Reading (14) Use the lowest yield strength filler metal compat. Miami. Miami. tration when the design permits (i. leg sizes depending on the welding process). Fla. and cracking. (11) Avoid designs that require welding across tension (22) Base metal stiffness does not increase with in- flanges. in order to decrease plate or section thickness. This reduces costs. Additional stiffeners may need to (12) Avoid corner welds or edge welds that are subject be added to a structure which uses high-strength material to tensile fatigue. of the weld). Structural Welding Code—Steel (D1. Use single pass ———. tension and shear loads that act parallel to the axis Miami. Technology (WHB-1. (18) Use fillet welds when practical..: American Welding Society. the service requirements. 8th ed. SECTION 6—PRIMARY CONCEPTS OF WELDING DESIGN (9) Design for prefabricating the material in the shop (20) Use partial penetration welds for joints subject to when practical. creasing yield strength.6). fillet is typically the smallest practical fillet size. residual stress. Miami. Miami. It’s cheaper. faster and (21) Do not use partial penetration welds when the usually higher quality. Structural Welding Code—Sheet Steel (D1.1).e.4). 43 . fillet welds where possible (5/16-in. ............................................................................................................. 46 Loading ........................................................................... 48 Surface Finishing to Improve Fatigue Life .................................................................................................................................................................................. 48 Repair of Fatigue Cracks .......................................... 50 45 .............................................................................................................................................. 46 Inspection ........................................................... SECTION 7 Fatigue Considerations Contents Introduction ............................................................................................................................................................ 48 Peening............................................................................................. 49 Checklist for Fatigue Considerations..... 49 Base Metal and Weld Metal Changes in Cross Section............................................................................................................................................................................................................................ 48 Grinding .................................... 46 Stress Concentrations................................................................................... 48 Code Allowables and Actual In-Service Fatigue Life............................................... 46 Crack Initiation Sites ................................................................................................................................................................................................................. 50 Bibliography/Recommended Reading List .................................................................................................................................................................................................................................................................... 49 Thermal Fatigue .......................................................................................................................................................................................................................................................................................................................................................... 48 Surface Shape and its Effect on Fatigue ................................................................. Also. In bending fatigue crack growth from the roots of fillet welds. but under cyclic conditions tures. pressure fluctuations. that have been deposited for repair. tigue. centration factor. It is also uncommon that fatigue cracks initiate from in- This occurs in regions of high tensile residual stress ternal weld defects.1). A large percentage of failures are directly related to fatigue. cracking (unless the stress and/or the cyclic loading is tions. hard rough in pump turbines. by definition. a stress concentration. Many structures can sustain When they do occur in service on weld-fabricated struc- static service loads forever. The loads may be overload and/or brittle failure during their final stages of induced by cyclic mechanical forces. Varied flame-cut edges that have not been dressed by grinding loads of tension or tension and compression may cause are subject to fatigue. or cladding sion component. where the members have lost their ability to Loading carry stress because of loss of load-carrying cross sec- tion. buildup. In some cases. compressive in nature and still cause crack initiation. interface surfaces located along the toes of fillet welds or ating bending loads. or rotational bending as is common edges of butt welds (see Figure 7. are common locations for failures. Essen. partial penetration welds with insufficient throat surface. weld termination sites failure in fatigue in a matter of a few thousand cycles. Few welds experience well below a load that would cause yielding. a combination of these loads. they are usually associated with welds and their the structure may fail in a short period of time. Welds tial elements of fatigue are fluctuating loads with a ten. Progressed fatigue cracks have caused catastrophic failures. initiated. Failures are associated with a permanent be subject to fatigue failure. discontinuities and stress risers. members fail cataclysmically due to Fatigue loads are cyclic in nature. As soon as a fatigue crack is physical change in the metal at and near the stress con. Under some circumstances. and electrical motor shafts. temperature varia. thickness will typically fail through the center of the The fluctuating service fatigue load can be purely weld and the crack will not progress into the base metal. vibration from machinery or reduced). Welds and their heat-affected zones average load no greater than the static condition. with an heat-affected zones. and localized must be considered stress concentration areas that may plastic strain. The fluctuating fatigue loads must have a tension component but may reverse Crack Initiation Sites even into compression loading. corrosion and wear. propagate out of the tensile residual stress field. In addition. environmentally corrosive conditions may increase the fatigue crack propagation rate.SECTION 7—FATIGUE CONSIDERATIONS Section 7—Fatigue Considerations Introduction caused by welding. How- significant since the maximum tensile loading is at the ever. or and torsional fatigue. However. Welded structures that are subjected to repeated fluctuating loads (cyclic) Stress Concentrations are candidates for fatigue failure at strain levels well Fatigue cracks only initiate at stress concentrations. are. below their yield strength. surface discontinuities are most from the roots of partial penetration groove welds. the crack itself becomes the primary stress con- centration (crack tip). The residual stresses become redis- tributed under the influence of cyclic loading and cracks Component failures result predominately from fa. or a combination of fatigue and corrosion (corrosion fatigue). defects that are large in 46 . Crack initiation sites are typically at weld-base metal Loads may also be placed on a component by fluctu. SECTION 7—FATIGUE CONSIDERATIONS Figure 7.1—Cracks 47 . 1. or shafts with mill scale will have considerably better fa- trant (PT). Some stan- for high-strength steels in fatigue applications other than dards do not permit peening of the last layer of a weld permitting higher maximum allowables for static or dead because finished inspections may not be able to be prop- loads. Welds should be kept in low- likely to progress into cracks if undesirable loading is stressed areas. Peening of these surfaces is essential to improving fatigue life. Peen each weld layer to Surface Shape and its Effect on minimize tensile residual stress loads. an improved fatigue performance (number of cycles and mation. High-yield base metals have lit- typically progress from the edge of the weld and through tle advantage if reverse cyclic loading is experienced. Structural Welding Code—Steel. Repairs should com- pletely remove the fatigue crack. However. plates stage of crack growth. tigue life.) stresses are on the base and weld metal surfaces. open to or very close to a weld surface. All as-welded surface shapes result in stress jacent base metal area should be avoided. A reduction of component. or the number of cycles slag. it is necessary to know how surface conditioning can im- Inspection prove fatigue life. Fatigue crack growth from the toes of welds will sion) should be avoided. Code Allowables and Actual Peening In-Service Fatigue Life Light peening of a surface. Any major The surface shape of a component is critical to its fa- abrupt changes in the cross section in the weld or the ad. if accomplished properly. sion which will increase fatigue life. typically associated with lack of fusion. the repair of such cracks and their prognosis for success needs to be discussed. it will lower the residual stress in the weld joint.1. erly accomplished. and the direction of load. it can be determined where the failure initiation sites were. for fatigue applications it is mined by testing with controlled mockups. that desig. If necessary to facilitate subsequent inspections. Fatigue cracks most commonly progress from the complete airplane fuselages and wing assemblies to fa. such as the American Welding Soci. Finish welds Fatigue should be contour ground to a smooth finish. Reverse cyclic loading (tension-compres- present. It should be noted that some codes. To obtain actual fatigue life data. The original condition that initi. provide no advantage in the allowable stress ranges cycles can be achieved with proper peening. (The airline industry manufacturers subject passes. the location of the weld.SECTION 7—FATIGUE CONSIDERATIONS area. risers or stress concentrations. Weld rein- forcement should be kept to a minimum. There are codes. cracks or loading. nate certain allowable cyclic loads based on the type of and place the surface of the weld under light compres- weld. By proper examination of fatigue fracture surfaces. Since fatigue failures are common in machinery ser- vice. higher than the nominal cross-sectional stress of the ated the fatigue crack must be changed. Also. light con- Repair of Fatigue Cracks tour grinding of the peened surface may be accomplished to facilitate subsequent inspections. surfaces of components and peak service and residual tigue testing. Improvements as ing. the rate of loading. or the actual critical that peening not be limited to internal weld component. the heat-affected zone (HAZ) into the base metal at right angles to the fluctuating primary stress in the weldment. subject to surface pitting by corrosion. are most should be examined. just because of a change of shape. The final sur. pene. it must be deter. such as the AWS high as 20% in the fatigue stress range or the number of D1. That means that the stress face condition should be inspected and proven free of in the component in these as-welded surface areas is linear discontinuities. Surface Finishing to Improve Fatigue and the direction and amount of primary stress. and eddy current (ET) tests would be the best tigue resistance than the same base metal that has been inspection processes for discovering fatigue cracks. Visual inspection is of limited value during this stress limit) over base metal with mill scale. The magnetic particle (MT). 48 . Life Since surface shapes can directly affect fatigue life. A simple example of this is that a Fatigue cracks are difficult to find during the early finely finished smooth machined plate or shaft will have stage of growth since they are tight and exhibit no defor. will provide a smoother surface than the as-welded con- ety’s D1. dition. However.2—Surface Geometry Effects difference in thermal conductivity between materials on Fatigue Life used in a given joint configuration. thermal strains can be induced as a result of the Figure 7. Grinding that is properly accomplished can im- being addressed here is the shape of the surface and not prove fatigue performance by at least 20%. Contour not. weld defects and sharp weld re-entrant an. or where surface of a plate it will reduce the fatigue life of the known service problems exist. lief on components. if simulated undercut is machined into the be accomplished for critical applications. Since con- forcement or increasing re-entrant angles or machined in touring of welds is very expensive. During the cool- down cycle. and consequently. Also. to be improved over the as-welded condition by proper It is important to realize that the only thing that is grinding. residual stress or different mechanical properties. Improper change in cross section is one of the most weld ripples. or improperly installed pipe hangers that were used to support service loads and dampen the number of fatigue cycles. is more expensive than making the weld. the fatigue life will be decreased. Fur- ther. removal of weld reinforcement to nearly zero with- tion in fatigue life due to these surface conditions would out creating a noticeable angle between the reinforce- be present regardless of whether a weld were present or ment and the base metal improves fatigue life. Base Metal and Weld Metal Changes Grinding in Cross Section Contour grinding of welds and the adjacent base metal to remove undercut.2). If all base metal fatigue samples are machined into grinding of welds should be performed prior to stress re- different shapes. it should only For example. such as pressure vessels. causing high ther- mal strains. Fatigue is often experi- enced due to differential thermal expansion or vibration of structural and machinery components. plate even though a weld is not present. but will be re- strained by the thicker cooler sections. The hot thin sections must thermally expand. high thermal strains can easily develop at abrupt changes of the cross section without considering other external loads. Thermal Fatigue When a component is comprised of thick and thin sec- tions and is subject to transient changes in temperatures. such as grinding should not leave gouges or heavy grind marks. Reduc. SECTION 7—FATIGUE CONSIDERATIONS The following weld geometry attributes are stress ris. excessive weld reinforcement. such as increasing amounts of rein. Abrupt changes in the cross section (stress concentrations) must be avoided or minimized for improved fatigue resistances. again due to differences in temperature. lower fatigue life. These localized stresses and strains can be induced in the component by different sources. 49 . not allowing systems to freely expand during heating periods. In every case. Poor transitions in thicknesses cause higher local cyclic fatigue stresses. Piping system fatigue failures are often caused by vibration of nearby machinery components. the thinner sections will cool-down much more rapidly than the thick sections. differences due to a weld deposit being present. fatigue life should be expected 7. and in some cases it simulated weld ripples. gles is very beneficial for improving the fatigue life of a ers that directly lead to a lower fatigue life (see Figure component. common causes of reduced fatigue life. During the heat-up cycle the thin material will heat up much more rapidly than the thick sections. England: (12) For dissimilar metal welds involving thermal Abington Publishing.) fatigue considerations are critical.: American Welding Society. Contour grind the weld and in. sliding foundations or bends in piping List systems to allow gradual strain distributions over larger areas. ners or plate edges when practical for critical fatigue (4) Avoid rapid or abrupt changes in cross-sectional applications. weld joints. vol. as this may result in fatigue cracks adjacent (8) Reverse thermal strains are common in piping and to welds during service. is completely removed. 50 . thicknesses especially in weld-associated areas. Miami. (16) Avoid welding transverse to primary tension fa- (5) Keep weld reinforcement and re-entrant angles to tigue stress (i. ———. Structural Welding Code— (9) When repairing a fatigue crack. (Available (11) Avoid the use of doubler or cover plates where through AWS. (1) Keep welds in low-stressed areas. Miami. 8th ed. two base metals selected. the Welding Society. Fla. Fatigue Strength of Welded Structures. Fla. England: Abington Publishing. Miami.e. re- paired fatigue cracks will redevelop (usually at a more Fatigue Assessment of Welded Joints Using Local Ap- rapid rate). across tension flanges of beams). (14) Full penetration welds have better fatigue perfor- (2) Keep the maximum stress and the number of mance than partial penetration welds. Bibliography/Recommended Reading flexible supports. be sure the crack Steel (D1.. These internal loads can be re- duced by providing gradual changes in cross sections. Welding spect the weld for critical applications.) fatigue applications..1). (17) Flame-cut edges on heat-treatable base metal (6) Contour grinding or peening will improve fatigue should be ground to a smooth surface to remove peak performance. Technology (WHB-1. machinery components where abrupt changes of thick- ness are present or components’ piping systems are not allowed to freely expand. American Welding Society. 2nd ed. it improves fatigue (18) Avoid forced alignment of flanged joints and performance and minimizes surface corrosion. the rate of loading. hardness residual/stresses and stress concentrations. (15) Avoid the use of welds on external cor- (3) Avoid reverse cycles (tension and compression). cycles as low as is practical.8). Fla. (Available through AWS. a minimum.SECTION 7—FATIGUE CONSIDERATIONS Checklist for Fatigue Considerations (13) Continuous fillet welds have better fatigue perfor- mance than intermittent fillet welds.: American (10) Unless the stress pattern. proaches. or the basic design is changed.: American Welding Society. (7) Keep components painted. 1. number of cycles. Welding Handbook. the filler metal should have a ther- mal expansion rate approximately midway between the Proceedings of the International Conference on Fatigue. ............................................................ 53 Use of Inert Gases .......................................................................................................................................................................................................................................... SECTION 8 Welding Safety Considerations Contents Introduction ............... 54 Bibliography/Recommended Reading List ..................................................................................................................................................................................................... 52 Production ................................................................................... 54 Planning for Safety ................... 52 Design........................................................ 53 Respiratory Protection and Ventilation .................................................................................................................................................................................................................................................................................................................................................................................................... 55 51 .... it can be seen that several of the items will It can be said. such as those of the Occupational Safety and Health Administra- Design tion (OSHA). equate and safe welder accessibility. e. cover industry-associated health and safety hazards. more than safety. This reference guide is intended to oxygen? This resulting from the plan-specified weld pro- stimulate the imagination of everyone. present and overlooked or not properly considered. All too often. ard into consideration before starting work. sometimes resulting in loss weigh production techniques against worker safety re- of life. serious collateral damage. production superintendent. equipment. jeop- sequent safety precautions necessary to resolve every po. While these items do not directly ad- product. and work environments. dress safety.. Safety is usually consider prior to issue of plans and instructions: limited to general areas of consideration. related to production capabilities and costs. ning. Each area has specific AWS Z49. unplanned occurrences.1. For example. planner. and formulated plan. planner. from the designer cess. or mechanic forming the specified task. shielding gas. “gas metal arc” welding which incorporates an inert to the mechanic. or mechanic with insights into potentially • Fabrication processes. such as: (2) Design assemblies so that fabrication welds can • Can the component be handled or positioned for be performed in areas where adequate ventilation can be fabrication? provided. vidually in this section.. and production fabrication. component configuration consistent with the intended ing pages is not intended to address all of the potential weld process. cesses. to look for potential hazards and elimi. There are many industry publications available that sidered from three specific points of view: design. greatly impact the safety of the production personnel per- signer. and more often (1) Design and lay out welds which will allow for ad- than not. These and other industry reference materials can be valu- able sources of information if your organization is not fabricating under some other safety standard. but unfortunately are often overlooked. or the sub. It cannot be assumed by the designer that nate them before an accident occurs and it becomes a production personnel will take this type of potential haz- costly lesson learned the hard way. must be con. and Allied Pro- responsibilities and concerns and will be looked at indi. by defini. ardizing a welder’s safety by asphyxiation due to lack of tential problem. Safety in Welding. plan.g. i. • Joint design as related to accessibility. with reasonable certainty. that no de. hazardous conditions are • Process safety. will a joint’s backside accessibility be hazards associated with welding fabrication. The designer must use knowledge and experience to tion. Safety during the design phase is usually not a field The following are specific areas that designers should that is given independent consideration. quirements. as related to welding fabrication. in a tank. 52 .SECTION 8—WELDING SAFETY CONSIDERATIONS Section 8—Welding Safety Considerations Introduction • Can the assembled unit be lifted and transported to the installation site? The purpose of this section is to provide the designer. hazardous conditions that commonly exist in production • Materials. or both. and AWS Effects of Welding on Health I–X. Accidents are. Safety.e. ever plans to have an accident. Cutting. This All of the above are important aspects of a well- results in unnecessary risk to personnel. with a single access at the top of the tank. are the weld joint design and The information provided and discussed in the follow. wherever possible. accidents and losses will be greatly reduced. whether related • Do not weld in a tank or void without adequate venti- specifically to welding or some other trade. pro. greatly increased. i. A welder should avoid the gases. • The nature of any toxic materials to which a welder is • Do not weld in areas that are not fire safe. carbon dioxide. personnel. tanks which portable equipment. serious injury. the welder himself. CAUTION • Do not weld or oxyfuel weld/cut in an oxygen- enriched environment. • The single most important factor influencing the • Do not weld while standing in water without properly amount of fumes inhaled by a welder is governed by insulated footwear. Never store leaky cylinders indoors. NOTE: All inert gases will displace air. While safety is everyone’s concern. Without a thorough understanding bulkheads or walls without stationing a fire watch or of the risks and the initiative to eliminate potentially haz. SECTION 8—WELDING SAFETY CONSIDERATIONS (3) Provide direction on plans and instructions where found that the majority of fires have been caused by known fire and safety hazards exist. The following is a listing of many common hazardous conditions and practices that have caused and will con. will occur.e. how to spot unsafe conditions under various circumstances. the ultimate re- sponsibility for a welder’s safety is the welder who is • Do not oxyfuel cut. As the safe working environment ethic becomes gas tungsten and gas metal arc welding that employ intrinsic to all workers. and death. hazardous material and air-tight compartments or voids • Do not weld on tanks that contain or have contained which have been closed for long periods of time. and natural gas are heavier than air and will flood a space if stored inside a room or compartment. tentially harmful gases. Whether standing on wet floors and never touch the bare metal respiratory damage occurs during welding will de- parts of an electrode holder with any exposed skin or pend on the use of precautionary measures that are in- wet covering on their body. of respiratory damage can be eliminated. Production • Do not weld with equipment that is improperly insu- lated or has broken electrical insulation. The danger of electrical dicated by an evaluation of the hazards involved. planners and design inert gases. Equipment and machinery in the fellow worker’s safety.. combustible material or liquids unless they have been (4) Design assemblies to utilize automated systems inerted according to IAW local instructions. With shock is particularly present in hot weather where the only a few. pane. protection. the work can be moved to a safe location. presence of contamination on 53 . supervisors. the chance welder may be wet from perspiration. taking appropriate precautions to protect against fire ardous conditions in the interest of their own and their on the opposite side. Always store in a well-ventilated area. • Do not enter a previously sealed void without check- Training that instructs the welder on the types of ing the atmosphere for the presence of oxygen and po- safety hazards which exist in his/her particular work en. • Do not weld when material or personnel are improp- erly supported or staged. made fire safe. mapp. It has been filler and base metal. relatively simple precautions. especially if exposed will depend on the type of welding process. • The respiratory health hazards associated with weld- • Do not assume that because power sources are low ing operations evolve largely from the inhalation of voltage they are not dangerous. Permanent weld stations are have contained or presently contain flammable or health. by the position of his head with • Do not transport arc welding cables coiled around the respect to the plume of the fumes. must be pro. injurious and costly accidents area must also be protected. as the potential fire hazard is • Do not forget that argon. shoulders when the conductors are carrying power. Respiratory Protection and • Do not dip a hot electrode holder in water to cool it Ventilation off. lation and especially when using processes such as vided. dusts and metal fumes produced. or carbon arc gouge on performing the work. • Do not weld or grind without appropriate eye and skin tinue to cause lost time. vironment and even more important. weld. cape.SECTION 8—WELDING SAFETY CONSIDERATIONS the base metal or the presence of volatile solvents in Use of Inert Gases the air. Not properly moved may introduce a serious hazard by volatilization identifying and subsequently funding safety require- or decomposition of the residue. Due to the serious nature of the potential hazards as- • Zinc and magnesium oxide inhalation. must be traveled to reach fresh air. may produce a harm- ful exposure. in their efforts to true when halogenated materials or plating solutions are complete a job in a timely. buildup. within 24 sonnel hazard. — Toxicity symptoms similar to zinc and magnesium Inert gases are used in many production methods in except it is more severe. creates a potential per- — Toxicity disappears almost invariably. content of the atmosphere below that required to support human life. nize the lack of oxygen until he is too confused and weak to save himself. This can be a dangerous should be undertaken until complete control of fumes has approach to doing business. This is particularly ments may cause production personnel. Any such Each opening from which the inert gas is flowing must coatings should be removed prior to welding. materials such as high-lead bronze. placed the oxygen is odorless. Planning for Safety fessional safety advice should be consulted prior to It is often the case during the planning and funding welding. To minimize the hazards.. mercury or paints contain. cobalt (0. additional emphasis is warranted. to reduce this hazard.1 mg per cubic meter) are: ide (CO 2 ) for inerting tanks containing flammables. Pro. and mercury (0. cost-effective manner. Positive exhaust systems must be in fume fever although inhalation of very large amounts operation to reduce the possibility of an inert gas of iron fumes for a period of years may cause a condi. depending largely on the amount of such material present and the amount of metal melted. attention. Personnel using and/or having access to inert gas should receive additional training about its potential hazards and Welding on materials containing lead. sociated with the use of inert and other gas mixtures re- quired during welding. with toxic materi. Low areas tion known as siderosis. should accidental leakage occur. — Toxicity produces chills. particularly if the gas that has dis- results. even accidentally.1 mg per cubic helium (He) for some types of welding and carbon diox- meter). argon (Ar) for purging cipitated by less exposure. Safe working 54 . nausea four to The use of nonflammable (inert) gases within structural eight hours after exposure. etc. enclosures. overlook serious safety considerations. the building and repair of ships. This attitude or lack of been provided. fevers. or other toxic the appropriate methods for handling and working with it. pipelines or containers of any sort workers places an unnecessary burden on those produc- from which the contents have not been completely re. nitrogen (N 2 ) for drying air piping. — Strongly suspected of causing a chemical pneu. phase of a project. An atmosphere deficient in oxygen (O2) usually gives monitis which may have severe or even fatal little or no warning. No such welding pense of completing a project. where heavier gases tend to accumulate require special • Materials coated. that safety is assumed to be a normal Welding on materials containing Beryllium may pro. especially when a considerable distance — They are extremely dangerous. • Nickel (1 mg per cubic meter). work practice and is just another unaccounted for ex- duce an extremely dangerous condition. piping systems. gases is that sufficient quantities may be released into an occupied compartment or space to reduce the oxygen • Copper fumes toxicity. longer lasting and is pre. In addition to general ventilation requirements als such as lead. greatest health hazard in welding or cutting. or rigid have individual ventilation. cadmium. the use of inert gas must be • Iron and aluminum apparently do not cause a metal closely controlled. a deposit of iron in the lungs. the average individual may fail to recog- — These oxides produce severe fume pneumonitis. piping systems. A trained observer may notice an increase in pulse or breathing rate in time to es- • Cadmium oxide and beryllium oxide. to involved. awareness of what is required to ensure the safety of the Welding on tanks. tion people trying to meet the schedules. however. to name a few. local exhaust ventilation should be ing toxic materials produce what is probably the installed in such a manner as to exhaust the inert gas. The inherent danger in the use of inert hours. ventilation control measures should be established. Fla. Safe Practices (SP). 55 . The same ———. planning. and Allied Processes ment (F1. Miami. ———.: American Welding Society. Miami. Fla. tion. and production personnel. Safety and Health Fact Sheets (SHF). a costly lesson to be learned would be in the ———. American Welding Society. Miami.: American Welding Society. eye.3). Fla.1).: American Welding Society. and fire protection add consider- able cost to a job and must not be overlooked. Safety in Welding. Arc Welding Safely (AWS). ———. Fla. Fire Safety in Welding and Cutting (FSW). Fla. SECTION 8—WELDING SAFETY CONSIDERATIONS conditions for your personnel are not an incidental part ———. Bibliography/Recommended Reading ———. Miami. ear. Miami. Without careful coordination between design. can Welding Society. Miami. can be said for thorough planning and funding of that Fla. Areas that require special safety considerations can Welding Society.2). 2nd ed. (Z49.: Ameri- or expense of doing business.: American Welding Society.: American Welding Society.: American Welding Society. A Sampling Strategy Guide for Evaluating Contaminants in the Welding Environ. task prior to the start of a complex job. I-X (EWH). respiration. Fla. Effects of Welding on Health. ventila. Staging. Lens Shade Selector (F2. skin. Cutting. List Miami. vols. requires thoughtful coordination of assets and per- sonnel—usually from a variety of trades. resulting in a potential loss of both human and material resources.: Ameri- making. Successful completion of any complicated task Miami. may be overlooked. ———. Fla. .......................... 60 Pipe Welding Considerations........................................................................................... 60 Base/Filler Material........................................................................................... 58 Design................................................................................................. 58 Accessibility........................................................................................................................ 59 Environmental Conditions ............................................................................................................................................................................................................................................................. SECTION 9 Weld Joint Design Considerations Contents Introduction ............................................ 58 Weld Joint Design Considerations .................................................................................................................................................................................................................. 60 Summary ............................................ 61 57 ................................................................................................................................................................................................................. 60 Temperature Control......................................................... 61 Bibliography/Recommended Reading List ........................................................................................................................................................................................................................................................................ SECTION 9—WELD JOINT DESIGN CONSIDERATIONS Section 9—Weld Joint Design Considerations Introduction Weld Joint Design Considerations Manufacture/assembly of most products requires When selecting welded joint designs. who will be involved with the work. heat treated). operating condition/envi. cost. met. machinery components. Cost tion from related engineering and production personnel The designer should keep in mind that variables in in the following areas: joint design. joint accessibility and position. required nondestruc- standard/code being invoked. (3) Review of assembly sequence including weld (8) Nondestructive examination methods and the ap. (5) Thermal effects of welding. ments including weld type and size. meetings (2) Weldability of metals and the effect of their should be held to review the projected fabrication with strengthening mechanisms (alloyed. Standards/codes have specific sections that define/clarify structural welding. As in any as- sembly process. nondestruc- (1) Mechanical and physical properties of base met. Items to discuss at these meet- ings should include: (4) Preparation and fabrication of welded joints. etc. information on material properties and welding. pressure vessels. and nondestructive (6) Effects of restraint. funding. When welding is used for items need to be considered. material type. What is the function of the weldment? Generally all fabrication is covered by a standard or code that is in- voked by the customer. tive tests. (5) Welder and procedure qualification requirements. plicable acceptance standards. and fabrication sequence have an effect on als and weld metals. joining. The AWS Welding Handbooks are a good source of (8) Staffing and equipment requirements. prevention and cause. 58 . (2) Review of possible base materials. minimize fabrication cost. (6) Post weld stress relief and machining. (9) Documentation of job performance. the following joining of component parts. the design en. the basic objective of a weldment design Application is to perform the intended function of the product with minimum fabrication costs. and (1) Review of the fabrication standard/code require- grinding. and production personnel hardened. During the initial design/planning stage. (7) Distortion control. stress concentrations and testing. re. material selection. piping Design systems. gouging. and the acceptance standards that have to be ronment. age related engineering. bine components. welding process. processes with consideration to mechanized processes. (7) Alternate fabrication methods. The design engineer should have basic knowledge or seek consulta. (11) The proper use of welding symbols and terms. tive testing. the assembly is called a weldment. tur- During the conceptual design phase. quired inspections and maintenance/repair. (4) Review of possible weld joint designs and welding (9) Applicable welding and safety codes/standards. Preplanning helps (3) Welding processes (advantages and limitations). assembly techniques. residual stress. cold worked. cutting. Information is also given for gineer needs to review items such as the fabrication allowable joint designs/limitations. (10) Assembly and erection methods. impact failure. If weld deposition rates are of concern in the field tion of a load. to a permits. tigue life. is largely dependent on factors including weld metal (7) Prestress weld joints to off set weld shrinkage. Smoothly ground weld reinforce. During impact loading. Welds joints can be put into two groups. properties and stress transfer. possible service temperature. shrinkage. Will the life. be considered. To increase fatigue life. determine what type of weld joints are (3) Balance welding around the neutral axis. sharp corners should be replaced with smooth transitions. Joint access and position may deter. Use needed. Designs that are should be considered rather then Shielded Metal Arc susceptible to fatigue failures are often also subject to Welding (SMAW). Weld reinforcement When selecting a weld joint design. Sub. joint has fusion through the entire cross section of the (4) Weld toward restraint. but rather on piping or sheet metal. With preplanning. (11) Use subassemblies to accommodate distortion rection along the path of stress flow will reduce the fa. Joint accessibility for the welder needs to be taken ment will also increase the fatigue life. into consideration when issuing design instructions and ties such as excessive reinforcement. and changes in cross section should include a minimum 4:1 taper into Accessibility or out of a weld joint. A full-penetration by welding alternately on either side of the plate. (8) Peen welds. A partial-penetration joint allows an unfused area (5) Do not use excessive force to align joints for fit within the joint cross section. but date weld joint shrinkage. drawings for assembly in the shop or field. (10) Use the least amount of total weld metal consis- When designing a weld joint. up. Is the job new 59 . Weld joint efficiency requirements will. SECTION 9—WELD JOINT DESIGN CONSIDERATIONS Welding Processes lack of penetration or weld surface roughness should all be avoided to maximize fatigue life. overlap. large extent. Impact loading may result from any sudden applica- ment. (1) Minimize welding by using standard shapes. Design factors of distor- Joint Efficiency tion control include: Joint efficiency is the ability of the weld joint to trans. fatigue resistance must tent with adequate design. Any abrupt change in cross section or di. undercut. pletely penetrate the joint thickness. acidic fluids. This in turn de. (9) Use processes that use the minimum total heat Fatigue Resistance input to fabricate the joint. full double “V” or “U” joints so that welds can be balanced penetration and partial penetration. be used in the above-mentioned service without specific Distortion is primarily due to restricted expansion when consideration. etc. Welds are often designed as full penetration and fil- welding be accomplished in the field or in the shop? let welds designed as continuous instead of intermittent Each welding process has its limitations. weld. Keep in mind that (6) Preplace or preposition weld joints to accommo- joint efficiency is not totally equated with weld size.) will the Distortion Control joint experience? Weld joints with backing strips or backing rings and partial penetration joints with open There are a number of distortion origins during the roots will be subject to crevice corrosion and should not manufacture of structural sections and piping systems.” What type of service (operating media such as high- temperature steam. fer stresses between the members being welded and the (2) Use partial-penetration joints when design weldment. ductile and have suitable impact resistance at the lowest termines the weld joint design. seawater. simple butt joints should be used instead of lap or “T” joints. the heat of welding is applied. a member is re- then the Flux Cored Arc Welding (FCAW) process quired to absorb energy rapidly. The weld and base material should be mine which welding process can be used. it is necessary to sized to the members being joined will increase fatigue consider what welding processes are to be used. distor- tion can be kept to a minimum. Gas Tungsten in order to maximize fatigue life. Arc Welding (GTAW) should not be used on heavy struc- tural sections. Weld joints subject to im- pact loading will gain increased resistance by design in- Corrosion Resistance corporating many of the guidelines used for “Fatigue Resistance. Weld discontinui. and the weld does not com. Impact Resistance merged Arc Welding (SAW) will only be used in the flat or horizontal position preferably in a shop-type environ. (10) Butt weld fittings may be joined directly to one another. Pipe Joint Design Design criteria for welded pipe joints are set forth in Temperature Control specifications. cooling (air or water). to consider when designing new weld joints.). number of weld passes. base material paint. Components that will be weld repaired or replaced lection of weld process and associated joint design. Windy conditions re- quire additional sheltering if a gas-shielded welding Pipe Welding Considerations process is selected. (11) A length of pipe between two socket fittings re- Base/Filler Material quires two weld joints. These criteria are dependent upon the Heat transfer from the welding process into the weld piping system. O-ring seating surfaces located near weld repair areas (7) Availability of fittings. and may affect the final selection such as: must therefore be accessible for repair. during (1) Contractual requirements. considered? cessibility by permitting plates to be turned. Base metal and filler material selection can determine A piping designer should take into consideration that what weld joints can be used. socket. often should be designed for ease of welder access and verse weather conditions and contaminates such as water. temperature.. throughout their life span. Ad. requiring only one joint. or cast)? cally eliminates a source of possible future failure. ble backside of a component that requires weld repair. (4) Piping wall thickness. the design engineer should visit and review the metals are extremely crack sensitive depending on the job site prior to issuing work instructions. size of (9) Butt weld joints are more difficult and generally welding electrode. When welding a component with finished ma. Material thickness and maximum temperature the paint (5) Specifications. backing ring. When choosing base/filler any joint in any piping system could be subject to failure materials the designer has to be aware of conditions that under a given combination of adverse circumstances. The designer should keep Environmental Conditions this in mind for weldability and welding access in the A job site and work condition review will assist in se. Consequently. ease of weldability (i. (3) Piping material. or repair? and not under ideal conditions. rework. 60 . a welded joint can be eliminated in design. factors: chined surfaces control of temperatures before. consumable insert. space.. and piping (5) What material types and thicknesses are available? to be rolled. whenever (2) What is the form of the base material (wrought. can withstand have to be known before welding.e. erosion. A (6) Classification of joint (i. or square butt). field. etc. particular joint design by one or more of the following nents. corro- tortion of those surfaces to an acceptable level. (2) System application (pressure. This (3) What service environment will the material be could be done by designing bends instead of welded subjected to? joints into a given piping system. tions. Some (8) Welding accessibility for butt or socket joints vary considerations for controlling heat to a desired area can with location. may be paint that needs to be protected on the inaccessi. or specify- ing repairs and welding processes. and forced more expensive to make than socket welds. There sion. joint design. and after welding has to be addressed so as to keep dis. may have to be weld repaired. base metal/filler metal combination. and joint design). process to be used. limiting length of weld bead. Butt welds usually require greater working be done by preheat and interpass temperature restric. To assist in making sound design/planning (6) Are dissimilar metals to be joined? (Some filler decisions.SECTION 9—WELD JOINT DESIGN CONSIDERATIONS construction or repair? Can component subassemblies be (4) What physical and chemical properties need to be accomplished in the shop? Shop assembly increases ac.e. it automati- forged.) Fabricated components. require heat input control for distortion protection. and oil in or near an existing weld joint are factors type. mock-up of the job may have to be done. The designer is guided in selection of a and the adjacent base metal can cause damage to compo. Designers must also consider that repairs may be necessary in the field (1) Is the project new construction. Miami. ing component parts. 61 . vol. Modern Welding Technology. and the fun. Welding Handbook. The minimum amount of weld Metals Handbook. W. O. Brazing. 8th ed. Lin- to the specific weld joint design that is selected for join. vol. coln Arc Welding Foundation. Welding. American Society for Metals.. 9th ed. The James F. and Nondestructive Examination the service requirements of the weldments. Welding Engineers designing welded components need to Technology (WHB-1. Cleveland. (A2. 6. the design engineer American Welding Society. Park. Fla. Fla. SECTION 9—WELD JOINT DESIGN CONSIDERATIONS Summary Bibliography/Recommended Reading List When designing weldments. and metal should be used while maintaining weld joint Soldering. (Available through AWS.4). The calculations need direct application Blodgett. Design of Weldments.) ner member being joined. ing. 1. Miami. damentals of the fabrication processes.8). Inc. and their effects. H. Cary.: American Welding Society. ———. Standard Symbols for Weld- needs to know the properties of the materials selected. Englewood Weldment design should always be based on the thin. Brazing.: American know basic mathematical formulas for calculating forces Welding Society. 1997. Cliffs: Prentice-Hall.. Ohio. Metals strength requirements. Ohio. ............................................................................................................................................................... 65 Considerations for Minimization of Weld Distortion.......................................................................................................... SECTION 10 Weld Distortion and Control Contents Introduction .. 66 Bibliography/Recommended Reading List ............................................. 64 Theoretical Shortening under Ideal Conditions ................................................................................................................................................................. 64 Mechanisms of Weld Distortion ............................... 65 Metal Properties Affecting Distortion ................................................................................................................................................................................................................ 69 63 ............................... high- strength steel. Because of their different mechanical and CONTRACTING metallurgical properties. ARC IS EXTINGUISHED The metallurgical and mechanical properties of the base metal greatly influence the degree of distortion ex- perienced. It is readily visible on thin plate and sheet metal when welded. • Filler metal with higher yield strengths than their base metals. ARC IS STRUCK assembly parts together to complete a fabrication. WITH RESIDUAL STRESS Mechanisms of Weld Distortion We l d M e t a l S o l i d i fi c a t i o n S h r i n k a g e Figure 10. The weld that is deposited in the plate will de. If the solidified Distortion of Metal Adjacent to a Weld Due weld metal could be removed from the base metal it to Heat Input would have a shorter length than the base metal length it was deposited upon. crease in volume about two percent during solidification and cooling to ambient temperature. mild steel. copper-nickel and 300 RESOLIDIFIED Series stainless steel. Fabrication distortion is caused by: HEATED PORTION • Volumetric shrinkage of molten metal solidifying to EXPANDS ambient temperature. different base metals exhibit SHAPE WITH varying amounts of distortion for the same amount of COOLING heat input. The following base metals are listed in order of increasing distortion problems: HY-100.SECTION 10—WELD DISTORTION AND CONTROL Section 10—Weld Distortion and Control Introduction UNWELDED PLATE Control of distortion during fabrication is essential. distortion can make it extremely difficult to fit sub. Since the yield strength of the weld Another mechanism by which weld distortion oper- deposit is invariably higher than the base metal. have high residual stress at ambient temperature. Weld distortion can be extremely expensive to correct and it is easier to prevent than to correct for critical applications. HY-80.1—Shrinkage in a Weld Caused (Shortening) by Expansion and Contraction Figure 10. • Thermal shortening of metal adjacent to a weld. Monel®.1 exhibits the primary distortion mecha- nism involved when a weld is deposited in a grooved will cause the adjacent base material to distort and to plate. Weld distortion is evident on practically all weldments. the weld ates is that the metal (which includes previously depos- 64 . In addition to lowering stress-carrying capabili- ties. and Monel® all have high thermal expansion rates width and length and increased thickness is high elastic and this. The end result is that the cally) to accommodate this increased energy. This results in weld distortion and high residual stress. nent deformation (shortening) of the one-inch length of strains thermal expansion in the plane of the plate. increased while that of the adjacent cooler base metal remains information can be gained as to what temperatures are stronger due to its lower temperature. The metal is only the temperature increases. The reason for this is Conditions that when weld metal is deposited. the thermally upset base and weld more expansion and contraction is predicted for a given metal now have a shorter length and shorter width in the temperature change. As the temperature is increased. it must thermally expand (volumetri- dicular to the weld surface). Each metal’s properties increased thickening or deformation that was formed (thermal expansion. 65 . The heated area adjacent to the weld metal yield points and to cause plastic flow. the periphery area (this may be of manufacturing stresses and resultant weld distortion. Castings area that is plastically deformed is always at a lower tem. and shorter in length than (ideal). compressive strain. metal is heated. This base metal after it has been subjected to temperatures places the heated area adjacent to the weld under high higher than approximately 180°F. difference in temperature is created in a narrow zone. and plates are stressed by perature than the center of the weld. As one inch of base through thickness dimension of the base metal (perpen. the strain to cause yielding free to thermally expand and plastically deform in the (permanent deformation) decreases. and since it nonuniformly heated area adjacent to a weld is generally cannot thermally expand in its length because of restraint thicker. Restricted Expansion during Heating Cycle the compressive strains become so high that they reach When base metal is heated in a plate by a single pass the compressive yield strain of the base metal.1). and any of weld metal. during the heating cycle in this peripheral weld area. adjacent to the weld metal. It should be noted that as restricted expansion (compression). have solidification stresses. Metal Properties Affecting Distortion Cooling Cycle Each component brings its own manufacturing history As the heated area adjacent to the weld cools off. the resulting curve represents the perma- heated area is much cooler. As the adjacent necessary to initiate deformation under ideal conditions metal continues to be heated to higher temperatures as a with single axial restraint. it small changes of temperature are required to reach base reaches yield point. Thermal Expansion perature. base metal outside the heated area (see Figure 10. At low temperatures these strains are elastic (no per- manent deformation). The cooler base metal re. The average thickness at ambient temperature of this plastically flowed area is greater than it was prior The coefficient of thermal expansion is used to ex- to heating. elasticity. 300 Series stainless steel. Under ideal conditions very result of thermal conduction from the deposited weld. The higher the coefficient. The yield strain of the base metal By developing these curves for different base metals being heated decreases with increasing temperature from published data and then comparing them. in conjunction with relatively low yield strains. In other words. narrower in width. The end result of this shortening in nickel. SECTION 10—WELD DISTORTION AND CONTROL ited weld metal) adjacent to the weld is heated under cation can result in distortion. the metal. thermal conductivity. the base metal being heated adjacent to increased temperature results in plastic deformation.1). and plastic deformation of the weld and the base metal makes them subject to considerable distortion. yields (or plastically flows) perpendicular to the plate or weld surfaces (see Figure 10. copper- plane of the plate. This thickening can only be accommodated press the amount of expansion and contraction of metal by shortening of dimensions in the plane of the base with temperature change. Thermal Conductivity Metals with low thermal conductivity will experience Theoretical Shortening under Ideal increased distortion due to a greater temperature gradient between the weld and base metal. By the weld thermally expands as the temperature rises subtracting the yield strain curve from the thermal ex- while the base metal a short distance away from this pansion curve. the of residual stress to the fabrication assembly. This causes retained rolling and cooling operations. the base metal cannot The nonuniform heating and cooling of the weld conduct the heat of welding away rapidly and a large metal and the surrounding base metal during weld fabri. previously deposited weld metal or the base metal) has a higher yield strength than the weld. yield strength. due to its lower tem. the metal experiences compressive strains. its structural shape) influence the severity During the cooling cycle. SECTION 10—WELD DISTORTION AND CONTROL causing large thermal expansion in this localized area. A material with a higher modulus is more resistant tical (see Figure 10. The cooling and short- Neutral Axis ening of the flanges place the webs under compression. yet provide adequate joint access for good fusion for the welding process used. Because of different cooling pat. due to its ing process and operator must be provided. the flanges of rolled I-beams are typically under ten. Components have their own residual stress patterns that can affect the resultant distortion of the finished product after welding. it is much more susceptible to distortion than mild steel. • Use smallest weld size possible to meet the strength requirements.3). Yield strength or yield strain is an indication of the Distortion is best controlled by welding both sides simul- metal’s resistance to permanent deformation. Conversely.2). 66 . the resultant shrinkage of the second weld will not equal the distortion of the first weld. The structural • A “J” or “U” groove requires less welding than a tacks need to be thick enough so that they cannot be bevel or “V” groove and will result in less distortion fused through by subsequent passes. Position weld joints at a neutral axis to provide less leverage for shrinkage forces. it should be noted that the Balance Welding mild steel and most alloy steels have nearly the same modulus of elasticity.4). Weld in a sequence that offsets one shrinkage stress against a previously deposited weld on the opposite side Yield Strength/Strain (see Figure 10. use castings or break/roll material into ally a minimum of two inches long and are often two lay- different shapes rather than welding several pieces ers thick. or balance welding around Considerations for Minimization of the neutral axis. It • Minimize root openings and bevel angles of full pene. mal conductivity than mild steel and this is partially why • Use partial penetration joints whenever possible. re. Stainless steel has much lower ther. plate. weld toward the sion. imum restraint. some of their effectiveness. For exam. gle “V” in the same plate thickness and will help bal- sulting in distortion. base metals with high yield strain are less susceptible to distortion due to their in. Structural tacks are usu- rolled sections. to distortion. closing during welding and will help force the root weld pass to deform instead of pulling the plates together. because centroid or the center of gravity when practical. Oversized welds increase distortion. or to other assemblies. Joint designs need Manufacturing Residual Stresses to be selected that will balance welding (see Figure 10. Modulus of Elasticity • Use flanged corners instead of welded corners. low yield strength at high temperatures. Use standard shapes or than distorting the base metal. bevel angles. and shapes have varying residual Reduce distortion by restraining assemblies in fixtures stress patterns that directly affect distortion. When practical weld toward max- ple. since increased stress is required to strain or stretch the material. yield strength filler metal accommodates some of the dy- namic weld joint stresses by stretching rather than creat- Joint Design ing distortion. Modulus of elasticity is a measure of the stiffness of a • Use intermittent welds to reduce welding where prac- metal. placed on hot weld metal will be very effective. weld away from edges. castings. ance angular distortion. Joint designs should have minimum root openings and creased strength. However. • A double “V” groove requires less welding than a sin- Plastic flow occurs in this restricted expansion area. However. Restraint terns. A lower taneously such as double fillet welds where practical. adequate access for the weld. during manufacture the thinner cross section of the webs cool first followed by the flanges. These tacks will restrain the root opening from together. Weld Distortion Structural Tacks Minimize Welding Structural tacks help force the welds to deform rather Avoid welding when practical. as this will reduce of thicker metals. However. while the webs are under compression. should be kept in mind that any restraint that can be tration joints. 2—Fillet Weld Applications 67 . SECTION 10—WELD DISTORTION AND CONTROL Figure 10. The two interference fit examples show how it may be used to maintain critical inside dimensions and circularity when adding length to or repair welding a finished machine part. cause less distortion than continuous which stretches the weld in the area peened. The result is less weld distortion. since each subassembly can be adjusted for distortion before being matched to another subassembly. amperage. tortion control. Peening is welding (see Figure 10. Backstep Welding Backstep welding uses the restraint of a previously Subassemblies deposited weld to minimize the tendency of the two Fabrication of subassemblies provides additional dis- plates from pulling together as the welding progresses. distributed along the Weld shrinkage may be counteracted by peening. Weld process se- lection should consider weld deposition. Proper edge preparation with joint alignment and minimum root openings decrease welding and result in Prefabrication less weld distortion. Excessive distortion can result from oversizing small fil- β let welds by one size on thin material. and travel speed. A blunt tool should be used to plastically deform (stretch) the weld after slag is removed. Weld Process Selection Figure 10. Welds made in short segments. Use mockup test assemblies to estimate amount of preset necessary as the amount of preset will vary with each welding process and the parameters used. Controlled Wandering (Backstep) Weld Peening A backstep weld is used in wandering sequence. a root bead should only Prestressing and Interference Fits be peened lightly or not at all because of the danger of Prestressing surfaces to be welded by bending or by causing a root crack.4—Joint Designs The greater the total heat input by conventional weld- to Minimize Distortion ing processes the greater the distortion. However. Use of largest suitable electrode is fre- quently recommended as transverse weld stresses are somewhat cumulative and distortion can be increased by Fit Up multiple passes. Do not oversize fillet welds.3—Sequence Welds Alignment of the joint is set up out of position to allow for anticipated shrinkage during welding to bring the assembly to the desired position (see Figure 10. most effective at higher temperatures. Peening marks should be removed interference fits develops tensile stresses on surfaces to be from the final weld surface if they are expected to inter- 68 . α Overwelding Weld convexity increases shrinkage but does not in- crease strength. weld processes with less joint preparation. Do not use excessive force to align Welds made in the flat position or with joint rotation joints for welding. This will counteract weld and base-metal short- ening and reduce weld distortion. length of the joint.SECTION 10—WELD DISTORTION AND CONTROL welded. Use intermittent welds or partial penetra- tion welds where practical.5).6). High residual stress will be induced allow use of larger electrodes and deeper-penetrating into the weld joint resulting in weld distortion. Preset Figure 10. the component will “walk” or move during the machining of the weld deposit (removing the weld de- posit changes the residual stress pattern) and the part may be lost as scrap when finished. properly accom- plished. These stresses Fla. Peening marks may act as stress risers if peening is not accomplished properly.: Stress Relief American Welding Society.8). Welding Handbook.5—Controlled Wandering (Backstep) Weld added to the load stress may exceed the material’s yield strength. Figure 10. Welding Technology (WHB-1. Fla.: American Welding Society..6—Presetting for Fillet and Butt Welds Bibliography/Recommended Reading List fere with final inspection. Stress relief after welding reduces residual stress of fabrication. vol. Welding Processes and Practices (WPP). 1. is essential for components that are to receive in- tricate machining to close tolerances. Miami. 69 . weld metal and surrounding base metal. The shrinkage of a cooling weld applies stress to the ———. American Welding Society. 8th ed. Stress relief. SECTION 10—WELD DISTORTION AND CONTROL OGRESS N OF WELD PR 4 DIRECTIO 3 2 1 Figure 10. Miami. Without stress relief. ..................................................................... 73 71 ......................................................................................................... 72 Fabrication Conditions Imposed ................................................................................................................................. SECTION 11 Checklist for Sound Welding Decisions Contents The Intended Service Requirements............................................... 72 Base and Weld Metal Properties ......................................... i.or low-temperature physical • The base metal and weld metal properties. Will the weld and heat-affected zone (HAZ) provide ❒ Grain size. ❒ Homogeneity/soundness (coring. embrittlement. ❒ Refractory oxide formation. welding engineer. Design Cognizance ❒ Sensitization to corrosion and corrosion cracking. ❒ Fatigue resistance (thermal and mechanical). and lamellar tearing). However. As an aid. a Sound Welding Decisions Checklist is provided in this section. ❒ Aesthetics (color match. ❒ Erosion resistance. ❒ Diluted weld metal properties. and The Intended Service Requirements cracking). It should metallurgical notches and drastic changes in base be noted that: metal properties. mary importance. • The production supervisor and mechanic are most ❒ Quench hardenability properties. The effects on the HAZ and weld metal properties Once a problem or potential problem area is identi. • The fabrication conditions imposed. welding engineer. and the rate of change of temperature are of pri- son having technical cognizance. it should be referred to or coordinated with the per.e. the weld metal. ❒ Elongation/ductility. shop supervisor. • The intended service requirements. properties).. They can result in weld defects. ❒ Temperature (high. ❒ Age-hardening properties. tions (environment).SECTION 11—CHECKLIST FOR SOUND WELDING DECISIONS Section 11—Checklist for Sound Welding Decisions No item has a greater effect on weldability than ❒ Tensile and compressive stresses and strains (axial/ the common everyday decisions made by the designer. or the production supervisor. 72 . • The design engineer is most knowledgeable about the ❒ Base metal chemical composition. due to the maximum welding temperature reached fied. the desired service? Consider only those service re. loose structure. ❒ Creep resistance. quirements that make the weldment fit for use. intended service of requirements. anodizing. knowledgeable about the imposed fabrication condi. there are three ❒ Abrasion resistance. bending). and appearance). ❒ Viscosity or fluidity. ❒ Work hardening properties. • The welding engineer is most knowledgeable about ❒ Tensile/yield strength. basic areas that should receive consideration: ❒ Impact resistance (stress and temperature dependent). ❒ Conductivity (thermal). ❒ Susceptibility to hot cracking. mechanic. Before making a welding decision. it is a list of spe- cific areas that deserve attention in avoiding the majority Welding Engineering Cognizance of serious welding problems. the designer. and the journeyman ❒ Corrosion resistance. to make sure you have checked items that can affect weldability. ❒ Solubility of gases (porosity. This list is not all Base and Weld Metal Properties inclusive and may be added to depending upon the indi- vidual’s specific experience. ❒ Schedule. structural tacks. Fabrication Conditions Imposed material selection. ❒ Weather or atmosphere. back setting. and cost to meet NDE). ❒ Weld shrinkage. ❒ Quality control evaluations (defects vs. and facilities. ❒ Skilled. prestress. 73 . field). ❒ Weld location (accessibility. materials. and equipment. temperature ❒ Number of welds. shop vs. ❒ Weld procedure (various welding parameters). weldment geometry. restrictions. ❒ Purging requirements. SECTION 11—CHECKLIST FOR SOUND WELDING DECISIONS ❒ Thermal expansion (yield strength vs. of a welding project. position. failures ❒ Available equipment. qualified mechanics (experience and good ❒ Thermal stress developed Restraint Safety of men morale). ❒ Weldment design (joint design. and restraint). and peening. weld sizes. sources will often determine the success or failure ❒ Weld sequence (shop controlled). ❒ Welding process characteristics. and equipment maintenance). ❒ Machinability. Production Supervisor’s Cognizance ❒ Cleanliness. ❒ Distortion control. The actual welding environment and welding re. ................................................................................................................................................................................... 76 The Cause of Welding Defects ....................................................... 91 Melt-Through......................................................................................................................................................................... 88 Arc Strikes.................................... 77 Delayed Cracking—Hydrogen Embrittlement .................................................................................... 85 Lack of Fusion or Incomplete Fusion .............................................................................................................................................................................................................................................................. 87 Undercut (Including Root Undercut) ............................. 77 Cracking .................................................................................................................................... 90 Burn-Through ...................................................................... 91 Root Centerline Crease ............................................................................... SECTION 12 Defects and Discontinuities of Welding Contents Introduction ...... 89 Spatter ................................................. 91 Root Concavity.................................................................................................................................... 85 Slag Inclusions ................................................................................... 92 75 ................................................................................................................................................................................................................................................................... 91 Incomplete Insert Melting .................................... 91 Oxidation ........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... 90 Crater Pit....................................................................................................................................................... 92 Bibliography/Recommended Reading List .............. 76 Weld Acceptance Standards .............................. 76 Weld Discontinuities ..................................................................................................................................................................... 84 Summary of Rules to Minimize Hydrogen Embrittlement Cracking .................................................................................................................................................................................................................................................................................................................................................................................................................................................................. 85 Lack of Penetration .............................................................................................................................................................................................................................. 83 Techniques for Allowing Hydrogen to Escape from Welds........................................................ 88 Tungsten Inclusions ........................................................................................................................ 86 Porosity ............................................................................................................................................................................................................................ 84 Minimizing the Available Amount of Hydrogen to the Weld Deposit................................................ 84 Welding Processes and their Available Hydrogen .................................. 91 Root Convexity.................................................... pore of porosity that does not meet the acceptance stan. or in some cases useless. Weld Acceptance Standards • GTAW—Tungsten inclusions. service are call defects. are called Each welding process has specific characteristics that discontinuities. and has been replaced with weld metal). • Mild steel is very susceptible to lamellar tearing in corrosion resistance properties are lowered. The greatest influence is whether the 76 . have: a nonuniform metallurgical structure (cast metal • Inconel ® alloys are subject to oxide inclusions. heavy sections with high through thickness stresses. high residual stress. be better left as found rather than attempting repair. weld discontinuities are expected to be detrimental to the service. En- gineering evaluation and judgement may be necessary to The specific properties of a base metal can make it prevent unnecessary expenditures of resources to repair a much more prone than other base metals to certain weld defect that would not affect the service of a weldment. • Tin bronzes are susceptible to intergranular cracking dard and is deep within a casting wall would frequently (hot shortness). All unintentional weld Welding Process Characteristics conditions that could possibly affect the weld’s service. • GMAW—(short arc)—Lack of fusion. centerline cracking on concave bead shapes. layed cracking of the weld and the heat-affected zone ponent to become less useful. micro shrinkage in the adja- • Aluminum alloys are susceptible to porosity and in- cent cast metal may develop into micro casting tears. lower fatigue performance. rious distortion problems may develop. Weld acceptance standards are usually written for the Base Metal Selection and Properties worst case. cific materials and processes can greatly reduce weld re- able conditions in the weld or the adjacent base metal. The Cause of Welding Defects Welding Environment There are many specific causes of welding discontinu. • SMAW—Starting porosity. Those discontinuities that do not meet make it more susceptible to a certain type of weld dis- the acceptance standards imposed by the owner/engineer continuity such as: or are reasonably expected to have detrimental effects on • SAW—Solidification centerline cracking. defects such as: The designer/owner that imposes acceptance standards • Quench and tempered and heat-treatable steels are should always keep in mind that in some cases a weld re. If ject rates. very susceptible to hydrogen embrittlement and de- pair just to meet acceptance standards may cause a com. however they can be generally categorized in the fluence on the total number of weld rejects than any following groups: other condition. They may not be reasonable in all cases. se- complete fusion. they must be evaluated. and that are caused by the welding process.SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING Section 12—Defects and Discontinuities of Welding Introduction Personnel Skill and Experience Weld discontinuities are unintentional conditions that The operator’s manual skill and knowledge with spe- occur during the welding process and result in undesir. The • Copper-nickel alloys are prone to incomplete fusion reason for this is that when the casting is repaired. A (HAZ). and frequently. The welding environment probably has a greater in- ities. it will and porosity. see Table 12. cold. phosphorous. evaluated for suitability based upon orientation. per colors. likely be higher than the rest of the weld in low melting ing the full length of a weld. • Lamellar tears. in elements such as lead. of crystalline metal. If fracture surfaces are exposed to • Underbead cracks. of solidifying weld metal. • Mirror welds. or exists does not mean that the weldment is unsuitable • Face cracks. Welds that experience galvanize (zinc) contamina- For the location of the above-listed discontinuities in tion or are made on free machining metals are also weld joints and the associated base metal. brass alloys. This cracking occurs at elevated temperatures and • Toe cracks. etc. The last metal to freeze will most very short (microscopic) to very long indications extend. because a crack is present • Crater cracks. they close off a finite amount of liquid metal to accommodate weld metal shrinkage or contrac- Usually a tight linear separation of metal that can be tion (see Figure 12. lead.6. When grain growths from each side of a weld are nearly parallel to each other. When cracks are left in service they must be • Heat-affected zone cracks. since it usually requires the least amount of energy to progress into a failure. location. which also increases its crack susceptibility. as in deep Shape penetrating welds. contracting volume tip. constituents like sulfur. Hot Cracking • Root surface cracks. air during hot cracking they frequently will exhibit tem- • Weld interface cracks. Location Crater Cracking Cracking takes place in the weld. SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING weld is made in the field or the shop. Service Impact • Wet systems to purge. weld. Hot cracking is most common in bronze and • Weld metal cracks. plastically flow without fracturing. or zinc. It has a is generally caused by insufficient liquid weld metal nec- high length-to-width ratio and is characterized by a sharp essary to accommodate a restrained. for service.7 weld metal is the most rapidly cooled metal of the entire exhibits potential crack locations. This crack several inches away from the weld. • Wind. copper-nickels and base metals that are high • Lamination. tin. zinc. loading and the base metal properties.9). its ductility and does not have the ability to elastically or • Restricted access welds. Cause mental conditions that increase weld rejects are: Cracking occurs when the metal under stress exhausts • Out of position welding. unless the base material is welds are: specifically designed for energy absorption under these types of conditions. and sul- fur. rain. • Throat cracks.8). • Longitudinal cracks. flux cored arc and electroslag welding. Solidification Cracking This cracking mechanism is commonly witnessed in Cracking deep penetrating welds which may have long solidifying surfaces that have a chevron shape as is often seen with Definition submerged arc.1 subject to hot cracking. It A linear rupture of base and/or weld metal. Some field environ. phosphorous. tin. the weld-base metal fusion line or the weld heat-affected zone (HAZ). and Figures 12. Any cracking that exists transverse Weld Discontinuities to primary stress in fatigue or is subject to impact loading The common discontinuities found on completed must be considered hazardous.1–12. generally travels in grain boundaries between the grains • Transverse cracks. Cracking is one on the most serious discontinuities. Weld. Figure 12. • Root cracks. It is being cooled in all directions after the welding 77 . The craters that appear at weld bead termination sites ments of age hardened or brittle metal may sometimes are common areas of cracking (see Figure 12. However. 12. 12.4. 12.3. 12.1.1.1. 12. at point where arc is terminated Throat 12d 12.2.5.5. 12.1. 12.6 W Weld only as discussed herein Inclusions Slag 2a 12. HAZ—weld heat-affected zone 78 .6 W Incomplete fusion 3 12.4.1.5.4. HAZ.5.2.1. 12. 12.2. 12. 12.2.3. 12.6 W Weld only as discussed herein Piping 1d 12. 12. 12. 12.2. 12. 12. 12.6 W Weld metal. 12.6 W Weld axis Toe 12e 12.2. 12. 12.4.6 W/HAZ Junction of weld and base metal at surface Laminations 8 12.SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING Table 12.6 W Weld.3. 12.3. 12.6 BM Base metal.2. generally near midthickness of section Seams and laps 10 12. 12. 12. 12. 12. 12. 12.6 W Weld only as discussed herein Cluster 1b 12.4.3. 12.5 BM Base metal. 12. 12. 12. 12. at root Underbead and 12g 12. Weld may propagate into HAZ and base BM metal Crater 12c 12. 12.1.4.5 W Root of weld preparation penetration Undercut 5 12. 12. 12. 12. 12. 12.2. 12. 12.6 W.6 BM Base metal. 12.5. 12.2. 12.3.3.4.2.5.2. 12.2.3.1.1—Fusion Weld Discontinuity Types Discontinuity Type of Discontinuity Identification Appearing in Figure Number Location* Remarks Porosity Uniformly scattered 1a 12. BM—base metal.1.5.1.5. 12. near weld HAZ Cracks (includes hot cracks and cold cracks) Longitudinal 12a 12. 12.3. 12.5 W At joint boundaries or between passes Inadequate joint 4 12. 12.2.4.6 W. 12.2. 12. 12.4. 12. 12.1. HAZ.4.2.1.4. 12.6 W Weld only as discussed herein Linear 1c 12.5. Weld or base metal adjacent to weld BM fusion boundary Transverse 12b 12. 12. 12. 12. generally near midthickness of section Delamination 9 12.3.5. 12. 12.4.4. 12.3. 12. 12.2.6 BM Base metal surface. 12.4. in HAZ heat-affected zone Fissures W Weld metal *W—weld. almost always longitudinal Lamellar tears 11 12.5.5.6 HAZ Base metal. 12. 12. 12. 12. 12.4.5 W Outer surface of joint preparation Overlap 7 12.3. 12. 12.1.1. 12. 12.5.2. 12.1.2.3. 12.1. 12.3. 12.6 HAZ Junction between face of weld and base metal Root 12f 12. 12.6 HAZ Junction of weld and base metal at surface Underfill 6 12.5.1.3.3. 12. 12.1. 1) 79 .1—Double-V-Groove Weld in Butt Joint (See Table 12.1) Figure 12.2—Single-Bevel-Groove Weld in Butt Joint (See Table 12. SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING Figure 12. SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING Figure 12.4—Double Fillet Weld in Lap Joint (See Table 12.1) 80 .1) Figure 12.3—Welds in Corner Joint (See Table 12. SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING Figure 12.5—Combined Groove and Fillet Welds in T-Joint (See Table 12.6—Single Pass Fillet Welds in T-Joint (See Table 12.1) Figure 12.1) 81 . 10). Cracking Mechanism Thin.10—Typical Centerline Cracks Figure 12. The weld crater solidifies from its perimeter toward its center.8—Solidification crater size that must solidify is much reduced in diameter. or are subject to low melt- ing temperature segregates. Monel® . For Gas Tungsten Arc Welding (GTAW). Aluminum. welding current decay should be used to help reduce the shrinkage stress while continuing to fill the crater to create a flat or slightly convex bead termination site.9—Typical Crater Cracks Due to Thin Concave Bead Shapes 82 . Weld bead termination sites are usually concave in shape (a high stress profile) and are very prone to cracking. and AISI 4140 are quite susceptible to crater cracks. By reducing (de- caying) the current before extinguishing the arc the actual Figure 12. Metals most likely to crater crack have wide solidification ranges. which is a form of hot crack- ing. copper-nickel. In- conel® .SECTION 12— DEFECTS AND DISCONTINUITIES OF WELDING LEGEND O OT SUF ACE C AC 1 C ATE C AC T O AT C AC 2 1 1 2 FACE C AC TOE C AC 2 1 E ATA FFECTED 1 T ANS E SE C AC O NE C AC 11 UNDE EAD C AC LAE LLA TEA 12 WELD INTEF ACE C AC LONGITUDINAL C AC 1 WELD E TAL C AC 1 O OT C AC 1 11 12 1 1 2 1 Figure 12. AISI 4130. low ductility at high temperatures. Figure 12. concave welds (like weld craters) have high suscep- tibility to centerline cracking (see Figure 12.7—Potential Weld Crack Locations arc is withdrawn and does not have the arc as a source of heat to slow its cooling rate. making all of its shrinkage strains away from the crater center. Corner welds that involve welding across the end of both members and butt welded joints. cracking oc- curs by shearing from one inclusion in the plate to an- other inclusion within the base metal. some of these inclusions are rolled out into thin. (3) Keep weld sizes down. SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING Cold Cracking Cold cracking generally occurs below 400°F. This low ductility is the result of non-metallic inclusions that developed during the initial pouring of the ingot to make the material. mini- mize through-thickness strains and are most desirable be- cause in some applications they place the metal under through-thickness compression. Figure 12. and AISI-4340. AISI-4130. Cold cracks are predominately transgranular. (7) Select fine-grain steels with stabilized inclusions than cannot be rolled out into lamellar inclusions. while hot cracking usually occurs between grains or interdendritically. (2) Peen all weld layers. (4) Use low-strength filler metals. or weld metal immediately or several days after the weld HY-100. occurring across metal grains. (6) Provide for slight root openings under “T” welds. 83 . These inclu- sions are in the form of oxides. silicates. When the plates are rolled. (5) Use thin plate (less than one half inch). The was completed.11). Thus comes the term delayed cracking. lamellar plate inclusions lying paral- lel to the plate surface at different depths throughout the thickness of the material.11—Examples of Lamellar Tearing Delayed Cracking—Hydrogen Embrittlement The presence of hydrogen can develop tremendous presence and the effects of hydrogen during welding can stress when present in welds and their heat-affected cause cracking of the base metal (under-bead cracking) zones in low-alloy steels (heat treatable) such as HY-80. Lamellar Tearing Some mild steel base metals are subject to cracking adjacent to welds due to their inability to withstand high through-thickness stresses. TEE welds are examples of unavoidable through-thickness stresses. because their through-thick- ness ductility is low. When high through-thickness stresses are placed on the plate by welding. etc. This cracking pattern has a stair-step appearance in the base metal (see Figure 12. Recommendations to prevent lamellar tearing are: (1) Select weld joints that minimize through-thickness stresses. the most successful remedy is clad welding the plate in the area of the groove’s reinforcement fillet or fillet weld or peen each weld pass. In these cases. sulfides. AISI-8630. Marten- sitic base metals are subject to high stresses and have a microstructure that is susceptible to low-temperature cracking. SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING The presence of hydrogen causes a drastically localized 200°F minimum preheats and interpass temperatures are loss of ductility in ferritic welds and their heat-affected not suitable for all base metals. zones. Classical cracks are transverse across the weld de- posit. Hydrogen may also contribute to other types of Hold the Weld at Interpass Temperature cracks such as longitudinal cracks and localized cluster cracks depending on the restraint and environmental con- after Welding ditions. Techniques and procedures that a welder must The weld and the adjacent base metal should be held use to minimize cracks are: near the maximum interpass temperature for at least one (1) Minimize the amount of hydrogen available to the hour after welding; and on heavy sections, 24 hours is molten weld metal. highly desirable to permit hydrogen to diffuse from the weld without developing cracks. (2) Allow hydrogen to escape from the weld and the HAZ. Minimizing the Available Amount of Techniques for Allowing Hydrogen to Hydrogen to the Weld Deposit Escape from Welds Welds should be made with the lowest hydrogen con- tent materials available. Weld electrodes must be kept Deposit Thin Beads clean, dry and properly stored in a heated, vented oven when not in use. Welds should be made only on clean Thin weld passes allow hydrogen to escape much and dry base metals. All sources of hydrogen should be more rapidly than thick weld beads, because the hydro- minimized. gen has less distance to travel. The time required for hy- drogen gas diffusion is a square relationship of the weld (1) Water, rain water, condensed water from torches, pass thickness. Therefore doubling the weld pass thick- and the atmosphere are all sources of hydrogen. ness quadruples the time for hydrogen to escape from the (2) Combined water in electrode coatings and fluxes weld. are sources of hydrogen. Only baking will drive off com- bined water. Many coatings are hydroscopic and will au- Time Delay between Deposition of Succes- tomatically absorb moisture out of the air. sive Weld Passes (3) Paint, grease pencils, oils, wood, paper, shop dirt, and tape are all potential sources of hydrogen. Hydrogen escapes most rapidly immediately after a weld is deposited, probably because of the amount of hy- drogen present and the elevated temperature of the weld deposit. It is much more effective to provide hydrogen Welding Processes and their Available diffusion time between each weld pass than to deposit Hydrogen several layers of weld metal and then allow time for hy- drogen to escape. Hydrogen escapes much slower from Of the commonly used welding processes, gas metal heavy weld buildups. arc welding (GMAW) has a lower amount of hydrogen available than do the shielded metal arc, flux cored arc or Closely Control the Preheat and Interpass submerged arc welding. Since SMAW consumes its hy- droscopic coating during the welding process, higher Temperature amounts of hydrogen are available to the molten weld Maintaining the preheat and interpass temperatures puddle. Flux cored arc welding, like SMAW, has a flux increases the diffusion of hydrogen from the weld. It is that is consumed in the welding process that may intro- essential that the preheat soaks completely through the duce hydrogen to the molten weld puddle. Of the arc base metal thickness. Rapid cooling of the weld deposit welding processes that utilize flux, submerged arc weld- makes it much more susceptible to cracking. It has been ing requires the greatest amount of concern because of its reported that at temperatures above 200°F hydrogen em- high flux usage. SAW consumes approximately a pound brittlement cracking is unlikely to occur. This means that and a half of flux for each pound of weld metal depos- a minimum preheat temperature of 200°F during welding ited. Also, its weld passes are generally thicker than the will minimize the possibility of hydrogen-induced crack- other processes, which makes it more difficult for hydro- ing and also accelerate the removal of hydrogen. However, gen to escape. 84 SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING Summary of Rules to Minimize ing, paper, wood, grease, and paint will all contribute to slag formation if they are allowed to contaminate the Hydrogen Embrittlement Cracking weld joint. (1) Good housekeeping and proper handling of weld- ing materials. Cause (2) Use thin weld beads. (3) Allow time for hydrogen to escape between weld By far the most common reason for slag inclusions is passes. incomplete de-slagging of the previous weld layer, leav- (4) Maintain interpass temperature for up to 24 hours ing slag at sharp re-entrant angles and at the edges of after welding. weld beads. When an SMAW deposit is made, slag may (5) Use welding processes that will minimize the tightly adhere to both the start and stop areas, leading to amount of available hydrogen. slag inclusions. Slag entrapments can be caused by a number of reasons: lack of operator skill; amperage too low; improperly prepared weld joints; improper back gouging; erratic progression; weave too wide; electrode Slag Inclusions too large; and improper bead placement. There is also a common misconception by welders that tightly adhering Definition slag, which is difficult to remove, can be burned out by A slag inclusion is a non-metallic solid inclusion the next pass by turning up the amperage. This practice is trapped within weld metal, between weld passes, or be- extremely risky, unreliable, and should not be attempted. tween the weld metal and the base metal. Service Impact Shape Most acceptance standards allow some slag to be con- Slag inclusions are usually elongated globular shapes. tained within weld deposits but it is limited in size and However, slag can be found in stringers and in thin or frequency. It is not considered nearly as severe a defect very thick layers. Slag inclusions can be microscopic in as a crack, since slag is usually globular in shape. It is size or very much larger, extending through the entire more closely allied to porosity in its effects on service, thickness of a weld or across a complete layer of weld (as except that slag, when its area becomes significant or its can be found in submerged arc welds). shape is not globular, will probably affect fatigue perfor- mance to a greater degree than porosity. Location The most common area for slag entrapment is in the Lack of Fusion or Incomplete Fusion first layer and between the root layer and the second layer. Slag is also found at sharp re-entrant angles be- tween weld passes and between weld passes and the base Definition metal. Overhead welding is most prone to slag inclusions, This condition exists when there is incomplete fusion due to the quick freezing slag of smaller weld beads. between the weld metal and a preceding weld deposit or the weld metal and base metal (fusion that is less than Sources of Slag complete). Frequently, incomplete fusion appears as metal-to-metal contact with a thin line between beads The major source of slag is the flux and electrode with no metallurgical bonding. coating used in SMAW, FCAW, and SAW processes. However, GMAW and GTAW processes may contain slag inclusions due to metal and silicon oxides. As the Shape amount of oxygen increases in GMAW shielding gases, On weld cross sections, lack of fusion is usually a the amount of slag produced increases. Slag can also curved line. On a radiographic image, lack of fusion will occur with GTAW process, when the molten end of the frequently appear to have a straight portion with a curved filler metal is removed from the shielding gas and oxides end (a tail). are formed. The oxides are introduced into the weld pud- dle when the filler metal is added. Another source of slag Location is the base metals themselves, since nonmetallic oxides and nitrides are available to form slag from these materi- Lack of fusion occurs between weld passes, or be- als. Shop dirt in the form of oxides of burning and goug- tween weld passes and the base metal (see Figure 12.12). 85 SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING INCO LETE FUSION INCO LETE FUSION INCO LETE FUSION INCO LETE FUSION INCO LETE FUSION C Figure 12.12—Lack of Fusion Locations Cause Table 12.2—Melting Temperature The most common cause of incomplete fusion is inad- Comparison Chart—Base Metal equate welding heat (amperage/current). The heat input vs. Refractory Oxide from welding must be great enough to melt the base metal and to allow sufficient time for thin surface oxides Melting Melting to be removed. Other causes of incomplete fusion are im- Temperature Temperature Metal (°F) Oxide (°F) proper travel speeds (too fast or too slow), insufficient electrode size, improper joint design selection, improper Aluminum 1100 Al2O3 3659 electrode or torch manipulation, improper surface clean- ing, and improper removal of refractory oxides. Monel® 2400 NiO 3614 Refractory oxides melt at much higher temperatures Inconel® 2525 Cr2O3 4109 than their base metals and are a major cause of incom- plete fusion, particularly in high-nickel or aluminum al- Titanium 3137 TiO2 3344 loys. The only oxide that melts at a lower temperature than its metal constituent is iron oxide. Also some filler metals are alloyed with elements that form refractory ox- ides, such as: titanium, aluminum, nickel, magnesium any lack of fusion. Lack of fusion is considered a serious and chromium. Base metals that readily form refractory defect when it is open to the surface of a joint. In general, oxides are: aluminum, Inconel®, copper-nickel, Monel®, lack of fusion is considered a higher stress concentration and K-Monel® (see Table 12.2). factor than slag, due to its linear shape and sharper edges. Removal of Oxides Wire brushing has little effect on the removal of refractory oxides for most metals except aluminum. In Lack of Penetration most instances grinding, pickling, or machining is used Definition to remove oxides. When welding K-Monel® light grind- ing is frequently required between each layer or every Unintentional, incomplete penetration of a weld other layer of weld metal. through the thickness of a weld joint. Service Impact Shape and Location Some acceptance standards permit lack of fusion in Lack of penetration is the unfused or unpenetrated welds. However, other acceptance standards do not permit portion of a weld joint (see Figure 12.13). 86 or the use of improper welding parameters such as: welding amperage too low. and welding pro- cess used. centage of service failures involving ship hulls. soluble in molten aluminum than in solid aluminum. a one-sided joint weld). cation of a porosity-free weld. improper filler metal manipulation.SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING INCO LETE O INT E NET ATION INCO LETE O INT E NET ATION Figure 12. Porosity can be formed by any vol- Porosity atile material being trapped within molten metal.. improper po- larity. The gas is entrapped within the weld during the so. Figure 12. The liquid metal cannot keep the element in The most common shape is spherical.e. Lack of penetration is a very se- rious discontinuity when fatigue is involved (particularly when the axis of the discontinuity is transverse to primary Location stress). The welding arc penetration and the weld pud- dle fluidity depend on these conditions. gaseous el- lidification process (see Figure 12.13—Incomplete Penetration Cause Lack of penetration is most commonly caused by im- proper weld joint selection for the weld process used. partial penetration joints welded from one side are desirable joint designs. Porosity is caused when there is not enough time for the gas bubble to escape the molten weld metal before the puddle solidifies. Another error is sometimes just forgetting to weld the opposite side of a weld joint. Hydrogen is 20 times more another.14). In some metals like aluminum. so the element ity can be elongated and extend from one weld pass into collects into a gas pocket. chemical reactions that create sulfur Porosity is a void or gas pocket contained within a dioxide or carbon monoxide gas. piping Lack of visual porosity on a weld surface is not an indi- systems. shielding gas. improper travel speeds. 87 . and entrapment of weld. filler metal. however.14—Locations of Porosity Service Impact Few codes permit lack of penetration when a full pen- etration weld is required. However. Unlike mild steel. solution as the puddle cools and solidifies. ements (most likely hydrogen) will come out of solution near the solidifying dendrites growing into the liquid Shape metal. when Cause properly selected for service. high-nickel and copper alloys do not spread or wet easily. Lack of penetration has resulted in a large per- Porosity is always located within the weld deposit. low-alloy steels. and stainless steels. pressure vessels. Weld joint designs must be properly selected for the base metal. Lack of fusion that is totally enclosed within a weld is not nearly as hazardous as lack of penetration open to a joint surface (i. and machinery components. gas evolving out of the weld from elements that were in solu- Definition tion at a higher temperature. improper filler metal selection. or improper backgouging. poros. the reduction of metal ox- ides or nitrides. some standards allow undercut to the weld puddle. The inclusions Definition. linear ruptures in the whichever is less. They most frequently occur at arc starts. metal. deep or 10% of the thickness of the base metal. this technique rarely works. it is very rare to find porosity leading to service metal surface deposited from the electrode used in the failures. • Inadequate purging of equipment that uses external shielding gases. acceptance standard limits. It is rare stickout too long. Undercut is always a groove that is left Location below the adjacent base metal surface. touching the tungsten with the filler 1/32 in. UNDERCUT • Shop dirt contamination of the weld joint. etc. Service Impact Cause Undercut has a direct effect on weld performance in fatigue loading of critical applications. Undercut may be on either or both faces of a weld. on the weld surface. The thin- There is a common misconception among welders ner the base metal. or on the adjacent base metal surface. Porosity can come from so many sources that it is sometimes difficult to identify the contributing source that undercut would contribute to a weld-associated fail- and resolve the problem. • Poorly maintained equipment. Although welds have Definition been removed from service with extensive amounts of Particles of tungsten in a weld or in the adjacent base porosity. that has been trode diameter. • Excessive welding arc lengths that aspirate atmo- spheric gas.SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING Sources of porosity include: • Moisture in electrode coatings or fluxes. particularly if the amount of porosity is within gas tungsten arc welding process (see Figure 12. improperly ground tungsten. the more likely it is that undercut may that porosity can be burned out by the following pass. however. • Free machining constituents of metals such as sulfur OVERLAP OVERLAP or selenium. Shape Undercut (Including Root Undercut) The tungsten inclusions may appear as globular shapes or as splintered or flaked pieces. even if the weld was Inclusions can be located completely within the weld made from only one side (see Figure 12.. Service Impact Tungsten Inclusions Nearly every acceptance standard permits some po- rosity to be present within a weld. Figure 12. Acceptance stan. Shape and Location may occur in clusters or widely distributed particles. UNDERCUT • Rust (oxides and nitrides). dipping the tungsten in 1/64 in. The size of the inclusions may range from several thou- Undercut is an intermittent or continuous narrow sandths of an inch to pieces larger than the tungsten elec- groove. affect the service of the weldment. however. tungsten welds are to be reasonably free from undercut. UNDERCUT • Oil. ure other than for critical fatigue applications.16). Tungsten inclusions may be caused by excessive am- dards typically specify that undercut shall not exceed perage for the size of tungsten. Other codes may state only that the tungsten as received from the manufacturer.15). 88 . deposit. • Paint.15—Undercut • Travel speeds that are too high or too low. • Excessive amperage that will disrupt the shielding gas. or inadequate shielding gas flow. immediately adjacent to a weld toe. melted into the base metal and not subsequently filled by weld metal. Note: Tungsten inclusions cannot be burned out of a Location weld by increasing welding current and rewelding over the area.17). Tungsten inclusions are usually accepted to the same standard as porosity. even where the amounts on tungsten exceeded the acceptance standard. Arc Strikes Definition An arc strike is caused by unintentional melting or heating of a finished weld or base metal surface by an electric current. Convexity and recommended for a-c welding than is used for the same undercutting may exist with the arc strikes. usually of alternating current.001 in. Service Impact Tungsten is considered to have a similar effect on ser- vice as porosity.17—Arc Strikes 89 . Small fis- amperage range with dc. It appears as a localized disturbance of the metal sur- crease of 0.SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING Figure 12. It results in a localized weld and heat- affected zone (HAZ). sures (cracks) and porosity may also exist.16—Tungsten Inclusions Alternating current (ac) requires larger tungsten diam. Service failures associated with tungsten are rare. Arc strikes made in weld joints or on inte- common metals. A tungsten diameter in.25 amps face and exists as a single spot or multiple spots. Tungsten inclusions must be removed by rior weld passes that will be welded over are usually not grinding or machining. which is more than Arc strikes may be found on the finished weld or base 3000°F higher than the melting temperatures of most metal surface. Tungsten melts at 6120°F. Figure 12. Usually one size larger tungsten is with some alignment (see Figure 12. is recommended for every 1. Shape eter than does direct current (dc) straight polarity because of higher electrode heat input. It is usually the result of abruptly remov- The most frequent causes of weld spatter include: ex. mal with most welding processes except submerged arc or adding additional filler metal at the termination site of welding and gas tungsten arc welding. or short cir. Weld spatter may or may not attach or weld itself to the existing weld or the adjacent base metal (see Figure 12. 90 . Service Impact Arc strikes are critical on heat-treatable steels. Weld spatter does not form an inten- tional part of the weld. and low amperage that causes sticking and dragging of the welding elec- trode. if improperly used. and Location A crater pit is usually a circular-shaped depression. ing the welding arc or not using current decay with cessive welding current. the travel speed. light grinding of the surface may be advisable. loose ground clamps. Cause Arc strikes may be caused by improper manipulation of the electrode. Some spatter is nor.18—Spatter rotating shafts particularly if they are made of quench- hardenable steel. arc length too long. It is a cavity that extends down into a weld at its termina- Location tion site (weld stop). However. Definition.SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING of concern. It may be minimized by gradually reducing the cuiting the welding arc causing a high surge of welding weld current (current decay) at the weld stop. GTAW welds. Crater Pit Shape Spatter is usually spherical or globular in shape. if the arc strikes are extensive. Spatter should be considered similar to arc strikes for highly hardenable heat-treatable metals that are subject to severe Definition fatigue loads. Arc strike protection of the base metal adjacent to the weld joint is a good practice and should be maintained for critical applications. gradually withdrawing the welding arc. Cause Crater pits are caused by a volumetric contraction of Cause molten metal during solidification and lack of filler metal to fill the void. such as high-speed rotating shafts. Some Spatter standards exclude the presence of weld spatter. Globular metal particles that are ejected from the welding arc area. Even magnetic particle inspection prods can cause arc strikes. Shape. Some standards may require that all arc strikes and associated HAZ be removed by grinding. An arc strike area creates a hardened weld and HAZ due to the extremely rapid quenching caused by the surrounding metal. increasing current that will expel weld spatter.18). GTAW. Service Impact Excessive spatter could mask a defect during weld in- spections and in those areas it must be removed. Arc strikes should not be permitted on high-speed Figure 12. It is controlled by using the proper joint design.20). Root Concavity A root surface depression or concavity that may be ORIGINAL INSERT due to gravity (weld segment in the overhead position). Burn-Through Melt-Through A root surface or base metal irregularity on a consum- A void or hole fused and extending into the surface on able insert or closed root full penetration joint resulting the back side of a consumable insert weld. (centerline crease) concavity formed by distortion or up- Crater pits are not addressed by most acceptance standards. This condition may be in the form of a slight oxidation. Incomplete Insert Melting Oxidation Incomplete melting of the consumable insert either Root surface oxidation results from partial or com- with or without the fusion between the insert and the plete lack of purge of atmospheric gases and moisture base metal along one or both sides of the consumable in. distortion caused by subse- quent weld passes can cause concavity of the root pass. setting of the root surface by subsequent weld passes. from heated or molten weld metal on the root surface of a sert (see Figure 12. a heavy black scale. or an extremely rough crinkled appearance. NOT MELTED surface tension. or too high of an internal purge pressure on the molten metal. or open root weld. weld joint (see Figure 12. Similar conditions result on the weld face when drafts of air displace the arc shielding gas and permit oxidation. they are not expected to cause in-service failures. a backing in fusion completely through a localized region without strap weld.19—Incomplete Melting Figure 12. The melt-through may stick out from this burn-through area. the correct INSERT NOT COMPLETELY MELTED WELD JOINT INSERT COMPLETELY MELTED Figure 12. Droplets of metal may development of a void or opening. SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING Service Impact Root Centerline Crease Unless crater pits have cracks or other associated weld An intermittent or continuous shallow linear groove defects.20—Oxidation 91 .19). Also. or may not be associated with the weld root. . Welding Inspection (WI-80). and proper welding parameters (i. the correct size consum- able insert.: Amer- American Welding Society. Fla. Miami.: American Weld- ing Society.e. Practical Reference Guide for Radiographic In- spection Acceptance Criteria (PRG). Miami.8).10).0). and proper welding parameters (i. Miami. Fla. ———. travel speed.21—Convexity References/Recommended Reading List American Welding Society. Technology (WHB-1. Fla. ———.e. Welds (B4. Fla.. travel speed. Miami. distortion caused by subsequent weld passes can cause convexity of the root pass. The Everyday Pocket Handbook for Visual In- spection of Welds (B1. It is controlled by using the proper joint design. 92 . 1.: American spection and Weld Discontinuities (PHB-2).: American Welding Society. and purge pressure).: American Welding Society. 8th ed. Guide for Visual Inspection of Welds (B1. ican Welding Society. Figure 12. Root Convexity Root weld reinforcement (convexity) that is beyond the base metal surface which may be due to gravity (weld segment in the flat position). Also. Fla.SECTION 12—DEFECTS AND DISCONTINUITIES OF WELDING size consumable insert. Miami. ———. Fla. and purge pressure). vol.: American Welding Society.11). Miami. Welding Miami. or low in- ternal purge pressure effects on molten metal (see Figure 12..: ———.21). Guide for Nondestructive In. ———. ———. Standard Methods for Mechanical Testing of ———. amper- age. Fla. Miami.: American Welding Society. Welding Society. Fla. Welding Inspection Technology (WIT). amperage. Welding Handbook. surface tension. ............................................................................................................................ 94 Nondestructive Examination Methods ... 94 When Should NDE Be Performed?.............................................................................................................................................................. 94 Bibliography/Recommended Reading List .......................................................................................................................................................................................... SECTION 13 Nondestructive Examination Contents Introduction .................. 102 93 ........................................ More than one may be used on the same weldment. observation and attention to requirements as the primary savings of time and money can be immense. test of the repair area and surrounding material can be vides methods to measure or detect soundness criteria used to determine the extent of defects and ensure part placed on material by design requirements. 94 . and other indicators of spection ensures that quality is built into an assembly and potential inferior quality. and money. weld bead contour and rough- signs. Maintenance inspection can be scheduled to meet the service needs of a product. soundness before repair welding begins. and specific inspection standards are re- cations that involve weld buildup for corrosion or wear quired to effectively ensure quality. weld preparations. inspection area. When to perform an inspection can be as important as selecting an NDE method. it becomes the fabricator’s responsibility There is a wide variety of NDE methods available to- to meet those requirements. residual slag and oxide films. rather than during fabrication or in service. dercut. VT uses trained this stage. Various NDE methods often provide a quick and relatively inexpensive Visual Inspection Testing (VT) way to verify the material’s properties or identification. bead placement. The proper selection and application of NDE to ance of the finished product. ma- terial. un- provides evidence of a quality product. joint misalignment. The various test methods do not necessarily compete with each other. visual inspection of the joint de. the term nondestructive testing. With the proper selection and application of various NDE can ensure that the material has acquired quality at. Adequate lighting. Although visual inspection is exclu- sively a surface application. as well as provide assurance that de- In-Service Inspection sign requirements are met. hence. After a designer has com- pleted the selection of materials and determined an as. Monitoring of known and suspected wear and stress areas with NDE can determine When Should NDE be Performed? the frequency of service required. cleaning of can reduce final inspection requirements.1. The following sec- Raw materials that meet specification requirements tions present a brief overview of major NDE methods. and other repair welding should be inspected prior to welding. key phases of fabrication or assembly can save time.SECTION 13—NONDESTRUCTIVE EXAMINATION Section 13—Nondestructive Examination Introduction repair. are essential for high-quality fabrication. In-process in. NDE methods. For detailed military requirements see Receipt Inspection MIL-STD-271 and MIL-STD-278. however. Nondestructive Test Methods sembly design. A nondestructive The field of Nondestructive Examination (NDE) pro. base metal defect repair. inspection mile day. inspection criteria. in such a way as to not adversely affect the serviceability of the Finished Product Inspection material. NDE can provide overall quality assur- tributes. To do this. experience and training can In-Process Inspection reveal discontinuities such as surface porosity and some During fabrication. and the deposited weld metal ness. markers are essential. Quality cannot be inspected into a material. Welding appli. VT provides important information about weld con- When material problems or discrepancies are caught at formance and practical quality control. See Table 13. types of surface cracks. NDE gathers this information through testing. requirements. Results are obtained immediately. developer is required.1). acceptance only where specified in the inspection rounded indications. terial composition and characteristics. or as required by the fabrication standard. Color and written codes help identify ma. the condition of materials that cannot be discontinuities open to the test the weld crater. highly portable. low cost. immediately. Welders and fitters should continuously com- Advantages Disadvantages pare their product to the fabrication requirements. Precleaning Does not interfere with other work The test surface must be cleaned (usually with isopro- in the area. When the excess penetrant is removed and a 95 . lack of bond. If open to the inspection sur- bead placement. the weld. The surface being inspected must be sufficiently Best method for detecting weld Limited to surface and smooth to allow removal of excess dye without over bead contour and roughness. are available. It is often beneficial to use mag- Capable of detecting very must be removed prior to nification as an aid to VT. up to Background lighting should be supplemented with a por. specimen size. the dye that bleeds out of the dis- Table 13. VT inspection acceptance criteria must first be de- fined. shrinkage areas. and joint misalignment. Coatings such as paint and scale (2) Magnification. pyl alcohol) to remove contaminants to allow the pene- Results are obtained immediately. and weldments of most solid. cold shuts. Results are obtained Some penetrants are flammable Poor surface finish may mask surface discontinuities. nonporous mate- rials. cluding cracks. and temperature penetrants. cleaning the test surface. and some require special disposal. laps. The following tests are often helpful aids to VT: Personnel are easy to train. weld alignment attributes. which penetrates any discontinuities open to false indications. it should be used for small (0.2. Visual Inspection Testing PT is typically used to inspect forgings. edge lamination. No easily recordable permanent record. progresses. No practical limit to inspection). Inspection process is time consuming (typically one hour per (1) Visual. beyond required. corrosive to sensitive base metals. weld bead contour and placement. SECTION 13—NONDESTRUCTIVE EXAMINATION developer is applied. See Table 13. the surface. 500°F. The light should be moved as a pointer to focus attention as the inspection Test results are easy to Post cleaning of penetrant and interpret. surface.2 diverse applications and can be used in all phases of fab. Areas that are questionable and need addi- tional attention should be marked and inspected again. easy to use. and inspected with MT. ex- Advantages Disadvantages trusions. Penetrant Testing (PT) Rough and porous surfaces can A liquid penetrant (dye) is applied to the surface being hide actual indications and create inspected. all weld and base metal defects can be detected in- undercut. VT provides a simple method of examination with Table 13. weld size. cracks. castings. trant to penetrate surface discontinuities (see Figure 13. the Well suited to nonmagnetic Detection is limited to presence of cracks and surface porosity. However. No easily recordable permanent Does not interfere with record of actual surface other work in the area. Penetrant Testing rication. The portability Cannot reliably detect of PT materials make them an ideal tool for field inspec- Very fast and economical to use. residual slag. Inspection materials/ Temperatures are restricted In addition to a clean inspection area. face. indications. equipment are relatively (typically 50 to 100°F) but high- face finish are important factors for visual inspection. table light directed at the inspection area.1 continuities provides a visible indication of the flaw. and open porosity. lighting and sur. some types of surface tion. It is recommended that paint be removed and the inspec- Complex equipment not Some inspection materials can be tion preparation be extended a minimum of 1 in.003) linear and inspection. In- process observations should include joint alignment. Minimal equipment required. Removing Excess Penetrant The excess penetrant not remaining in the discontinui- ties is removed (see Figure 13. Both color contrast dye. Postcleaning After the test surface has been evaluated. which requires solvents to remove the penetrant. and fluorescent dye. which is removed with water. which can only be seen with ultraviolet light.4—Visible Indication after and any remaining penetrant.3). a liquid penetrant is applied and allowed time to penetrate discontinuities by capillary action (see Figure 13. The test surface is then examined with visible light. when color contrast dyes are used. depending on the specific application.1—Cleaned Test Surface Figure 13.2—Penetrant on Test Surface and in Crack Applying Penetrant After the cleaners have evaporated.3—Excess Penetrant Removed after the emulsifier has been applied. which becomes water soluble Figure 13. or with ultraviolet light. There are three basic categories of penetrants based on the method of their re- moval: post emulsifier. which can be seen with visible light.SECTION 13—NONDESTRUCTIVE EXAMINATION Figure 13. and washable. are commonly used. solvent washable. Applying Developer The developer is evenly applied on the test surface. it is cleaned (usually with isopropyl alcohol) to remove the developer Figure 13.2). when fluorescent dyes are used. Application of Developer 96 . Adequate time is allowed for it to act as a “blotter” for the discontinuity (see Figure 13.4). slag. Conversely. The (Longitudinal Magnetism) 97 . even coat of dry iron oxide particles are sprinkled or dusted onto the sur- face.5—U-Shaped Magnet in When the field is oriented at 45 degrees or greater from perpendicular. The stronger the flux leakage. it will attract the iron par- ticles. extrusions. (2) Wet Method—in which iron oxide particles are suspended in water. See Table 13. Flo- rescent coatings are usually added to the particles to en- hance their visibility. MT is typically used to inspect ferromagnetic castings.7—Direct Method magnetic field within the part being inspected. weld- ments. Fillet Weld Root Applying the Magnetic Particles The iron oxide particles are applied to the surface of the test area while the electrical current is still producing the magnetic field.6). Figure 13. where there is no disruption of the field. the field must be Containing a Discontinuity oriented in at least two directions (angled at from 30 to 45 degrees from each other) to ensure adequate flux leak- age. The iron particles must be allowed to migrate freely across the in- Figure 13. For that reason. If a discontinuity causes a sufficiently strong disruption (flux leakage) of the magnetic field. There are two widely used methods of doing this: (1) Dry Method—in which a thin. Establishing the Magnetic Field There are four primary methods used to establish the Figure 13. There are a choice of colors available to provide a color contrast on the test surface. SECTION 13—NONDESTRUCTIVE EXAMINATION Magnetic Particle Testing (MT) A magnetic field (flux) is applied to the inspection area (see Figure 13. Precleaning Usually grinding or “needle gunning” to remove paint.3.7).6—Throat Crack in a spection surface. and scale is sufficient precleaning. forgings.5). the stronger the forces of magnetic attraction (see Figure 13. The greatest flux leakage occurs when the major dimension of the discontinuity is perpendicular to the direction of the magnetic lines of force. The field is maintained while an iron oxide powder is evenly applied to the surface. kerosene or a petroleum distillate. the flux leakage is undependable for dis- Contact with a Ferromagnetic Material continuity detection. The liquid is allowed to flow onto the test surface. The MT method is limited to the inspection of ferro- magnetic materials and can detect surface and (under certain conditions) near-surface discontinuities. the particles are easily removed by blowing them off with regulated air (when using dry MT method) or by letting them flow off with a liquid me- dium (when using the wet MT method) (see Figure 13. and components subject to cracking. lished as shown in Figure 13. it around the conductor and within the part. nonrelevant indications may result Other work in the area from changes in: is not disturbed. If done a hollow cylindrical part. Multiple inspections (angled 30 to Results are obtained 45 degrees from each other) may be immediately. This method is best suited to flat the yoke ends are most easily detected. A magnetic field can be estab- (1) Direct Method. interpret. highly portable. netic field.SECTION 13—NONDESTRUCTIVE EXAMINATION Table 13. required in order to detect all linear Figure 13. Defects oriented in line with the length of the part cannot be detected with this geometry of the part and defect orientation usually deter. 98 . As the current flows through properly. and equipment are relatively low cost. a longitudinal • Magnetic writing. the magnetic field in the yoke induces a corre- in contact with the part. A magnetic field pressure air (when the dry method is used) or they are al- can be established by placing a central conductor within lowed to run off (when the wet method is used). (3) Wrapped Coil Method. to the length of the central conductor cannot be detected No practical limit to with this method since the direction of the magnetic field specimen size. Linear discontinuities lying perpendicular to the axis of the part are most easily detected. The magnetic flux lines within sponding field in the part. since the direction of the magnetic field cannot mine which method or methods are used to inspect a be maneuvered beyond that as shown. method. Linear dis- parallel with a line connecting the two electrodes are continuities lying perpendicular to the line connecting most easily detected. and required. Linear discon. A magnetic field can be • Heat treatment. Linear discontinuities lying part flow between the two ends of the yoke. is easily cleaned. After the test surface has been evaluated. (especially for machinery applications). An electrical power source is easy to use. a cylindrical magnetic field is set up will remain.3 Magnetic Particle Testing Advantages Disadvantages Inspection materials Limited to ferromagnetic materials. Inaccurate interpretation of cannot be maneuvered beyond that as shown. heat-affected zones. part. No easily recordable permanent Test results are easy to record. field is set up around the conductor and within the part. usually by brushing or wiping.8 by passing an a-c or d-c tact with the flat surface of the part. No current from the yoke the part run perpendicular to the current path that flows flows into the part. to re- tinuities lying perpendicular with the central conductor move any remaining particles. Removing the Excess Magnetic Particles trodes while the current is flowing. The magnetic flux lines within the between the two electrodes. only the particles attracted by the flux leakage the conductor. surfaces. Care must be taken when testing certain materi- als with high hardenability to avoid moving the elec.9 by placing a yoke in con- lished as shown in Figure 13. the excess particles are blown off with low- (2) Central Conductor Method.8—Central Conductor discontinuities. (4) Yoke Method. given part. This can result in arc strikes which result in very small. As the current flows through the coil. Does not reliably detect rounded are most easily detected. Inspection time is relatively short. A magnetic field can be estab. When coupled with current directly into the test area with electrodes placed the part. Defects oriented perpendicular discontinuities. • Sectional geometry. established by placing a wire coil around a longitudinal • Magnetic permeability. and potentially very While the electrical current is still producing the mag- hard. (Circular Magnetism) Personnel are easy and economical to Degaussing may be required train. and other materials from when a pulsed electrical current is used to “shock” a one side only. and their shape. glass. locate. and determine liquid levels. therefore. surface discontinuities in ferromagnetic materials.4. and in some cases describe the shape of discontinuities. voids. Longitudinal waves are nated to receive) the reflected pulses which have been al. A rapid succession of pulses (creating a beam) are received and displayed on a cathode ray tube (CRT) (for thickness measurements. It can do so by passing through Pulses of ultrahigh-frequency sound are produced plastic. These CRT presenta- tions in turn are compared to known physical standards. determine bond quality. propagated into the test material at an angle normal to its 99 . a digital display Figure 13.9—Yoke Method may be used) as spikes (blips). Discontinuity Detection Like miniature sonar. SECTION 13—NONDESTRUCTIVE EXAMINATION Figure 13. used for ultrasonic inspection. The pulses are dissipated rapidly in air. loss of thickness.10—Sound Deflection from a Discontinuity tered by internal conditions and the geometric shape of the test piece. UT can be used to accurately measure thickness. couplant medium. MT may be used to detect many surface and near. The blips (or lack of blips) are evaluated as to their horizontal distance on the CRT. they Ultrasonic Wave Types must be coupled onto the test piece through a liquid. Applications tions. mentioned below. See Table 13.10). inves- Ultrasonic Testing (UT) tigate attachment location. piezoelectric transducer (search unit) (see Figure 13. inclusions and lamina. their height. As these discontinuities become closer to the sur- face and sharper (less rounded) they become easier to In addition to being able to detect the discontinuities detect. ceramic. UT can be used to detect. metal. These include all types of cracks. The transducer is then electrically Both longitudinal and shear waves are commonly switched to receive (or another search unit can be desig. determine soundness. Shear wave frequency is much greater than that of longitudinal frequencies. alternating magnetic field which portable. The eddy currents. inserted into a com- Results are obtained immediately. Ultrasonic longitudinal waves are able to detect: • Laminations. produces an alter- Advantages Disadvantages nating magnetic field which is introduced into the test ma- Modern equipment is High initial cost for equipment terial (see Figure 13. used as a refer- fragile. the bridge electrically “compares” both coils. Figure 13. eddy current density changes the inductive reactance of the primary coil.e. For that reason. surface. the proper orientation are If the eddy current field moves within range of a disconti- easily detected. As the primary coil is moved across the Internal discontinuities in interpret the results. record. The field induces alternating highly sensitive. Discontinuities whose major dimension is oriented parallel to the ultrasound path are difficult to detect with both wave types. • Nonmetallic inclusions. difficult to inspect. • Nonmetallic inclusions.. Linear discontinuities lying or variations in the test material. Ultrasonic shear waves are able to detect: • Internal cracks and surface cracks on far side only. for rapid inspection of The depth of effective penetration is determined by the pri- plate and bar stock. resolution. Equipment is complex and opposes the primary field. in programmable and turn. the coil Process can be automated readily detected. the magnitude and conform to the search unit Accurate location of the contact surface. cal instrumentation. is electrically “bridged” (i.. their resolving power is also much greater. Linear discontinuities lying perpendicular to the Large grain metal castings plate surface are readily (i. currents (eddy currents) in the metal. • Porosity. ence. These variations are registered on an out- Access is required from Poor surface and near-surface put device and. They are mainly used for thickness and sound- ness examination. Shear waves are introduced into the test material at an angle offset from normal to the sur- face.e. can be used to locate discontinuities Excellent sensitivity. A second coil. See Table 13. This change of discontinuity. Considerable operator skill and mercial Wheatstone induction bridge configuration) with experience is required to the primary coil. • Porosity. Very thin materi- als are also difficult to examine with longitudinal and shear waves. Test Object 100 . grinding and direction of the eddy currents are altered. • Cold shuts.11). 300 series stainless) can be detected.5. test material. is limited to detecting surface and near-surface parameters.11—Induced Eddy Currents in • Voids.SECTION 13—NONDESTRUCTIVE EXAMINATION Table 13. No easily recordable permanent mary coil power output and frequency. produce a weaker. depending on the sensitivity of the electri- one side only.4 Eddy Current Testing (ET) Ultrasonic Testing ET is a NDE method whereby a coil. The test surface must physically nuity or change in material properties. paint removal may be necessary. • Voids. energized with high-frequency alternating current. Due to the “skin effect” parallel to the plate surface are of both the eddy currents and the primary current. and training. Figure 13.12—Orientation of Radiation mine the accuracy of the results. from the surface. (depending on the frequency of test material on film (see Figure 13. comparing them to a standard. the rays expose the film. Mul- tiple shots at various angles can be used to detect most Advantages The primary advantage of ET over other methods is that it can be completely automated and produce accurate results at high speeds without ever contacting the part being tested. and section thickness. Thickness measurements the joint configuration.6. ET is. size.5 than 1/4 in. PT. Eddy Current Testing Advantages Disadvantages Radiographic Testing (RT) A permanent record can be The depth of the inspection area RT uses the penetrating power of X-rays or gamma produced with special com. interpretation. One of the primary limitations of RT is also be detected which may be related to welding or its difficulty in detecting cracks or other closed disconti- other fabrication processes. RT is most sensitive to discontinuities whose primary coil is waterproof. Test Plate. is limited to about 1/4 inch rays to produce an image of the interior conditions of the puter techniques. oily. especially well suited to continuous production lines. positioned between the source of radiation and a piece of Equipment is low to moderate cost and portable. some of which may not be detectable inspection surface to be smooth and clean to keep the wet. Limitations Applications The application of RT for weld inspection is largely ET can be used to identify and classify materials by dependent on the location of the joint in the weldment. See Table 13.12). The major limitation of ET is that the primary coil design must be compatible with the part geometry and defect or attribute type being investi- gated. After going through the test material. Another limitation is the Source. and Radiographic Film 101 . and position of the discontinuity. and degree of variation in film density is used to deter- Some techniques/ procedures allow the paint The final evaluation is mine the type. The film is developed. or greasy if the test coil at a consistent distance with UT. expensive and complex. and also most subsurface Some techniques allow the The surface must be sufficiently discontinuities. Surface and subsurface discontinuities can another method. whereas UT is most sensitive to discontinuities whose major dimension is perpendicular to the sound beam. Limitations Surface cleanliness is important for accurate results. Some types of equipment are unexposed film. which is typically no more Table 13. Some techniques require of shades of gray (film density) of the film. RT is capable of detecting all of the discontinuities detectable with VT. These fac- of thin sheet materials and nonconductive coatings are tors may combine to limit the best use of RT and favor also common. SECTION 13—NONDESTRUCTIVE EXAMINATION practical depth of detection. and to remain on the test dependent on inspector ultimately determine its acceptability. nuities which are not aligned with the source beam. Another advantage is that the signal generated by ET is often proportional to the size of the discontinuity detected. revealing Some techniques are easily any change in thickness of the material by the variation automated. major dimension is aligned with the source beam. The material is the primary coil). therefore. Care must also be used when calibrating the ET equipment with a reference standard as this will deter. surface. since any magnetic particles on the surface may cause nonrelevant indications. The pattern expensive training. MT. SECTION 13—NONDESTRUCTIVE EXAMINATION cially true for field work. For these reasons, RT should Table 13.6 only be required where absolutely necessary. When prop- Radiographic Testing erly trained personnel use well-maintained equipment Advantages Disadvantages according to approved procedures, RT is a safe and via- ble NDE method that plays a valuable role in assuring Permanent records easily High initial cost for equipment that the required quality is obtained. obtained. and training. Internal discontinuities in An electrical power source is the proper orientation are required (except gamma). References/Recommended Reading easily detected. High hazard potential. List Excellent sensitivity. American Welding Society. Guide for Nondestructive In- Laminations and other linear spection of Welds (B1.10). Miami Fla.: American Wide variety of materials discontinuities lying parallel to may be tested. the plate surface are difficult to Welding Society. detect. ———. Guide for Visual Inspection of Welds (B1.11). Linear discontinuities lying perpendicular to the plate Considerable time required Miami, Fla.: American Welding Society. surface are easily detected. for setup, exposure, and ———. Practical Reference Guide for Radiographic In- interpretation of results. spection Acceptance Criteria (PRG). Miami, Fla.: American Welding Society. ———. Standard Methods for Mechanical Testing of Welds (B4.0). Miami, Fla.: American Welding Society. discontinuities; however, due to limited accessibility, multiple shots are often not possible. When cracks are ———. The Everyday Pocket Handbook for Visual In- suspected to occur in a critical applications, another spection and Weld Discontinuities—Causes and Rem- NDE method such as UT may be a better choice. edies (PHB-2). Miami, Fla.: American Welding Society. Radiation Hazard ———. Welding Handbook, 8th ed., vol. 1, Welding Technology (WHB-1.8). Miami, Fla.: American Weld- One unique disadvantage with RT is that radiation ex- ing Society. posure to humans can result in permanent injury and ———. Welding Inspection (WI-80). Miami, Fla.: Amer- death, without the individual ever knowing they were ex- ican Welding Society. posed. The necessary precautions that must be taken to protect the health of workers in the area accounts for ———. Welding Inspection Technology (WIT). Miami, much of the expense associated with RT. This is espe- Fla.: American Welding Society. 102 SECTION 14 Information for the Welder Contents Introduction ................................................................................................................................................................... 104 Material .......................................................................................................................................................................... 104 Filler Material ................................................................................................................................................................ 104 Welding Process ............................................................................................................................................................. 104 Quality Control and Quality Assurance ...................................................................................................................... 105 Weld Size and Penetration ............................................................................................................................................ 105 Distortion Control.......................................................................................................................................................... 105 Standards and Specifications........................................................................................................................................ 105 Welding Department Required Information Facilitated by the Design Engineer/Technical Support .................. 105 Bibliography/Recommended Reading List ................................................................................................................. 107 103 SECTION 14—INFORMATION FOR THE WELDER Section 14—Information for the Welder Introduction welding on metals strengthened by cold rolling or age hardening will cause substantial loss in strength in the For the welding department to join components into a HAZ. The chemistry determines the electrode type and product that will perform in service, the engineer must heat treatment required to achieve the desired mechanical communicate to the welder the exact requirements of the properties. Although specific information is usually pro- design and quality control necessary to ensure that the vided by a welding engineer, many machine-shop weld part is built as designed. To perform this function, the en- repairs require material background by shop supervision gineer may call on other experts to assist him. The weld- beyond that of the average welder. This additional infor- ing department would provide information on equipment mation is helpful in making proper weld repairs. If the and welding skills available to perform the work. The base material is low-alloy steel and is being welded in metallurgist would provide information on material se- the final heat-treated condition, care must be taken so lections and their behavior, and provide procedures for that post-weld heat treatment (PWHT) does not exceed heat treating to achieve the desired mechanical proper- the tempering temperature of the base material. If the ties. The welding engineer would provide information on item is being heat treated after welding, a welding mate- welding and provide the welding procedures. Welding rial that will respond similarly to the base material must procedures can be written to cover a variety of situations be selected. Improper heat treatment can cause loss of and base material grades. Because of this, details of the mechanical and corrosion-resistant properties. procedure should be clarified to the welding department by the design engineer. Documentation requirements should be detailed on the engineering instruction with clear direction to access forms or with copies of forms to Filler Material be filled out and attached. In most organizations, the The filler material should be specified by the welding quality assurance requirements are buried deep in corpo- procedure or the engineering instruction. The welding rate procedures and difficult for the welder to uncover. procedure should be reviewed to ensure that it is clear in The procedures should be interpreted by the engineer, selecting the correct filler material. Some welding proce- and details provided to the welder. The term “welder” as dures are written to cover a variety of materials and situ- used in this section refers to the actual production welder ations and need to be clarified for the welder at time of or his immediate supervisor. use. Material Welding Process A welder must know the base material to determine a Each organization assigns the responsibility of pro- correct welding procedure. Providing material specifica- cess selection to different groups at different levels of tions to the welder ensures that he has this information. management. In today’s world of CAD/CAM, productiv- In some special cases, the production welding supervisor ity improvements, and quality demands, the welding or welder must know the base material chemistry, along equipment may be preselected by engineering and speci- with the mechanical requirements. Some specifications fied by the work document. In other cases, the selection only specify physical requirements, i.e., tensile and yield is left to the production shop foreman or the welder per- strengths. These types of specifications allow a variety of forming the work. Quality, weld appearance, productiv- materials to be used for the end product. In other cases, ity, qualifications, welder skills, equipment, and 104 has caused serious rework in many instances. Weld buildup.1. all can influence the process selec. and (5) Details of selected process/equipment. through design and fabrication methods. which is caused by shrinkage of the weld nugget during solidification. The quality assurance requirements are amount of shrinkage. The welder should (8) Base material specification. Engineer/Technical Support ficiency (i. wise to spell out the category the work fits. and quality assurance requirements. determine how much metal to apply to achieve the finish (9) Base material chemistry (special cases). However. Standard Symbols for Welding. work inside a nu. Because of this. dimension required. When hardfacing materials or tion and determine the end product cost. Fillet and partial penetration welds are the Coordinate Decisions with Fabrication Shop most economical and are frequently used. e. corrosion-resistance materials are being applied to im- quire cleanliness levels to the point that flux and other prove service performance. SECTION 14—INFORMATION FOR THE WELDER available electrode.e. Weld sequencing and restraints can be used to control the shrinkage in a desired direction. welds are required and not clearly identified. so it is welder to understand. Prior to Issuing Instructions tural fabrication shops.g. Also a smaller joint the welder gathers quality control information mostly by design cross section reduces shrinkage. and welded with the submerged arc reviewing documents to determine the quality control process would produce the least amount of shrinkage. This error (2) Drawing numbers. 105 .e. Proper and (3) Volume of work. help the job progress and avoid costly rework. the less erenced on the engineering instructions. (6) Welding procedure. consistent use of welding symbols is imperative and (4) Is process preselected? By whom? AWS A2. the process can be improved a great Standards and Specifications deal by providing this information via the work instruc. Weld- ing shrinkage cannot be eliminated. specifications. Brazing. with no bevel. and what documents need to be completed will solving problems and answering unanticipated questions. but slower welding process can sometimes be more productive than the faster weld- ing process that requires rework. ness should be specified.4. i. Nondestructive Examination. the welder is given a sheet of specific instruc. high pres- Preferably. such as pressure vessel category. but it can be con- trolled. The welding instruction Distortion Control should specify the welding process or specify that the process is the choice of the welding department. the routine of using partial pene- tration welds can cause errors when full penetration (1) Description of work. sure and temperature piping or low temperature and pres- tions to accomplish the desired work. buildup thickness. A high-quality. is a national standard. The welder should sure piping. dards to perform the work required. this system works quite well. In most struc. The welder will usu- ally seek the information from a person that he knows is familiar with the work. also. clear plant. A square butt word of mouth.. providing phone numbers of persons or departments ity control and quality assurance. the better the job will progress. the more detailed the that have technical authority helps to resolve problems information to the specific task and the easier it is for a quickly. It is rare that a welder will spend his time joint. and/or weld ef. shrinkage and distortion will occur. should be (7) Status of welding procedure? Qualified? specified in final finish dimensions. In general. full penetration) should be clearly specified to the welder. Specifying standards. The Quality Control and Quality assembly must be designed to allow for the weld shrink- Assurance age that will occur.. 14. For qual. Welding Department Required Weld Size and Penetration Information Facilitated by the Design Joint configuration. Quality control and quality assurance can be confus- Different welding processes will produce a different ing to the welder. All welding creates distortion. the minimum material thick- leftover items cannot be tolerated. Information as to what inspections are required and revisions on the engineering instructions can be helpful in when. Most standards cover a variety of work. Surprisingly. the faster the welding usually specified in some corporate document that is ref- process and the more weld produced per pass. See Figure also know whether he is qualified to the appropriate stan.. and applicable tion. as in machinery repair work. The job may re. 1—Typical WPS 106 .SECTION 14—INFORMATION FOR THE WELDER Figure 14. welding engineer. Miami. Fla. The Everyday Pocket Handbook for Shielded Bibliography/Recommended Reading Metal Arc Welding (SMAW) (PHB-7). Prequalified Joint Details in AWS D1.: ———. Miami. Fla. Miami. ———. ———. Fla. and job specialist. Structural Welding Code— ican Welding Society.: American Welding Society. (12) Filler material. Miami. Standard for Welding Procedure and Perfor- mance Qualification (B2. Steel (PHB-1). Fla. Miami. ———.: List American Welding Society. WELDPERFECT: The Easy ———. Miami. and who is Society.: Ameri. The Everyday Pocket Handbook for Arc Welding (15) Acceptance criteria. The Everyday Pocket Handbook on Metric Prac- can Welding Society. The Everyday Pocket Handbook for Gas Metal (17) Specify the responsible office of the design engi.: American Welding (16) Documentation (record) requirements.: American Welding Society. American Welding Society. Fla. Miami. Miami. Fla. can Welding Society. required to record and/or certify. ———. The Everyday Pocket Handbook on Welded Joint ican Welding Society. (19) Whom to contact when questions arise. The Everyday Pocket Handbook for Visual In- Guide to Perfect Welding (WPERF).1). Fla. Miami. Fla. Fla. Miami.: spection and Weld Discontinuities—Causes and Rem- American Welding Society. Suggested Preheat Temperatures for Welded (13) Joint type and weld penetration requirement. ———. Miami. Fla. 107 . SECTION 14—INFORMATION FOR THE WELDER (10) Base material mechanical requirements (special ———. Fla.1.: American Welding Society. tural Steels (chart). Structural Steel Materials (chart). Details for Structural Applications (PHB-3). Miami. Society. Fla. Miami. ———. Steel. ———.: American Welding Society. Fla. to be performed.: Amer. Suggested Filler Materials for Welding Struc- cases).: Amer. ———.: Ameri- Welding Society.2). Struc. (FCAW) (PHB-4). spection of AWS D1. Miami. Lens Shade Selector (F2. Fabrication and Welding Requirements (PHB-6). Miami.: American Welding (11) Base material post weld heat treat requirements. Society. American Welding Society. tural Welding Code—Steel (chart). Joint Weld Terminology and Standard Welding ———. The Everyday Pocket Handbook for Visual In- Symbol Interpretation (textbook). Welding Symbols (chart).: American Welding (18) Standards applicable to this job. The Everyday Pocket Handbook for Gas Metal Arc Welding (GMAW) of Aluminum (PHB-8).: Ameri- (14) Inspection requirements and when inspection is can Welding Society. ———. Fla.1. edies (PB-2). Miami. ———.: American ———. Arc Welding (GMAW) and Flux Cored Arc Welding neer. Fla. Fla. tices for the Welding Industry (PHB-5). ........................................................... 117 109 ................................................................................................ SECTION 15 Fitting Aids Contents Background ..................................................................................................................... 111 Bibliography/Recommended Reading List ......................................................................... 110 Fitting Aid Categories . 110 Current Problems in Fabrication................................................................................................................................................................................................................ and castings can have a deleterious effect on sur- manifest themselves during the fabrication process as face condition and dimensional accuracy if done improp- distortions of one sort or another. Current technology. The then. be monitored and offset. Fitting and Welding gins before the material enters the plant. Currently.SECTION 15—FITTING AIDS Section 15—Fitting Aids Background problems during the fabrication process. Stresses induced by fitting restraints. materials and ized stress and distortion. in turn. They may also be used to correct Burning or cutting operations can greatly increase the distortion while assembling components of structures. fabrication methods have changed dramatically. All of these Storage and handling of such materials as plate. deformation results. Non-ferrous metals. Mill toler- be an integral part of fabrication. and stresses that will shapes. and overall fabrication. misused while in storage. It is important to have a knowledge of the available Forming and Shaping aids so that an intelligent choice may be made for any When material is mechanically formed on rolls or simi- particular application. while increasing material and any heat treatments. Since cation. composites. The ideal situation would cutting may cause shrinkage and other distortions that must be to use the minimum number of aids possible. This causes stresses heating. specifications or fitting and welding time will be increased. Use of such technol- by the heat of welding or cutting. the methods used for fitting and aligning have cumulative effect of this may result in excessive local- changed. if not found before the material is used. Edge condition must be held to fortunately. Current Problems in Fabrication The problem of inaccuracy is many-faceted and be. reducing the amount of weld- Stresses from rolling mills and heat treatments can cause ing and sequencing welding to minimize distortion. Over the past one hundred years. can dramatically increase the accuracy of parts. 110 . and wood are used in a small portion of the total fabrication. and welding are well known. ting aids. large gaps. materials have flaws. there is little that can be done to eliminate or reduce stresses set in by mechanical forming. by obtaining precise fitting. It may be said that a fitting aid is a method or device Burning and Cutting used to hold or align (or both) two or more parts in a pre- determined location. and the need to erly. Edges and sur- face conditions are not always as they should be and need The use of fitting aids has been and will continue to to be checked when materials enter the plant. cause distortion when stresses are relieved assemblies. accuracy of assemblies if performed correctly. Other than forming parts productivity. Storage and Handling loys. inaccuracies. inherent flaws in the materials and other in. but un. Fitting aids were in use ances are often excessive and can add up during fabri- when the ancients built the first wooden ships. pipe. such as line lar equipment. fluencing factors seem to prohibit this. building materials are mainly ferrous al. These stresses result in distortion Foundry and Mill and deviation from design. that will. removal of portions of the ogy should decrease the need for fitting. by line heating. These problems can be reduced Problems start with the way many materials are made. Problems occur when material is handled roughly or hold and align components. This. The heat of and the structures themselves. create the basic need for fit. All opera- tions must be closely watched to ensure accuracy. 15). a piece of hard material.12–15.11). • Pneumatic devices (see Figure 15.1–15.2—Weld-On Saddle • Hydraulic devices (see Figures 15. Weld-On Saddle. Figure 15. • Jigs.4—Pulldown Threaded Devices (Figures 15.3—Yoke and Pin used for holding or backing the force applied by a wedge or other tool.20–15.1—Wedge and Step-Cut Dog or swivel. in length. stays. mocks.4). • Magnetic devices (see Figures 15. SECTION 15—FITTING AIDS Fitting Aid Categories The fitting aid devices documented herein are classi- fied by their primary method of mechanical advantage in their design. 111 . ta- pering from a thick board to a thin edge that can be driven or forced into a narrow opening. This device is attached by welding.19).14–15. a metal device welded or mechanically fastened to the part at one end and slotted at the other. Figure 15. also known as dog.13). as wood or metal.or “L”-shaped metal device used in con- junction with a wedge to straddle and hold one part to another. Pulldown. Wedge Devices (Figures 15. • Threaded devices (see Figures 15. a metal device Figure 15. also known as steamboat jack and ratchet jack.5–15. a “U”. Configurations are usually made from one-inch-thick steel plate and are typically 12 in.16). The effective length of the device can be changed by rotating the sleeve Figure 15.4) Wedge. used in conjunction with wedges or bull pins to align edges of plate. Some of the aids shown use a combination of methods for force application. and cables (see Figures 15.17–15.8) Push-Pull Jack.5–15. Yoke and Pin.10). Step-Cut Dog.22). and 17 in. • Gear-pulley devices (see Figures 15. a device having a ratcheting sleeve with op- posite internal threads at each end or with an internal thread at one end and a swivel at the other. a flat metal plate with an oblong hole. and fixtures (see Figures 15.8).9–15. • Wedge devices (see Figures 15. also known as “U” dog yoke and hairpin. also known as clip and wedge. used in conjunction with a wedge and anchor clip to pull one part toward another.1–15. • Strongbacks (see Figure 15. • Padeyes. any number of devices which are booked or welded having a screw at one end to apply force for aligning. pushing and holding parts together.or double-acting cylindrical pis- jack.6—Turnbuckle other. Jacking Clamp. junction with other devices for fitting. ton. Figure 15. and used to pull parts toward each Figure 15. Portable Hydraulic Ram. a device consisting of an angle support and a headless bolt. used for lifting.9–15. a hydraulic device having an oil Hydraulic Jack.SECTION 15—FITTING AIDS Figure 15.7—Jacking Clamp 112 .5—Push-Pull Jack Turnbuckle. The angle support and bolt can be welded or me- chanically fastened. a hydraulic and geared device having a single. a device having a metal loop or sleeve with opposite internal threads at each end or with an internal thread at one end and a swivel at the other. Clip and Bolt.10) power and Enerpack. or double-acting cylindrical piston used for hoisting or Used where short reach or stroke is required and in con- lifting. The effective length of the device can be changed by the sleeve or swivel. also known as Porta- Hydraulic Devices (Figures 15. also known as buda jack and bottle reservoir and a single. 13) Chainfall.9—Hydraulic Jack mechanical advantage in lifting or pulling.or “L”- shaped metal structure for straddling and holding parts Magnetic Saddle. also known as magnetic jacking together. also known as chain hoist. a device having gears and pulley(s) and operated by chain to obtain Figure 15.14–15. Come-Along.or “L”-shaped metal 113 .12–15. Vacuum Saddle.10—Portable Hydraulic Ram Figure 15. an air-operated device having suction pads for Magnetic Devices (Figures 15.11—Vacuum Saddle Gear-Pulley Devices (Figures 15. manent magnetic field(s) and a “U”.15) gripping relatively smooth surfaces and a “U”.11) chain for lifting or pulling. also known as vacuum jacking clamp. a device having a ratcheting gear- pulley arrangement to change the effective length of a Pneumatic Devices (Figure 15. This device is used in conjunction with a screw clamp. a device employing an electrically induced or per- and thread or hydraulic ram for applying pushing force.8—Clip and Bolt Figure 15. SECTION 15—FITTING AIDS Figure 15. 16) together.13—Come-Along Figure 15.15—Fitting Magnet structure for straddling and holding ferrous metal parts Strongbacks (Figure 15.14—Magnetic Saddle Figure 15. many other tools for applying forces to parts. any number of devices used to restrain applied forces and/or hold alignment. be welded or mechanically fastened and are used with duced magnetic field(s) for drawing ferrous metals together. These devices may Fitting Magnet. a device employing an electrically in. 114 . Strongback.12—Chainfall Figure 15.SECTION 15—FITTING AIDS Figure 15. This device is used in conjunction with a screw and thread or hydraulic ram for applying pushing force. SECTION 15—FITTING AIDS Figure 15. Jigs are often incorporated into fixtures. also known as a doughnut. a template. a wire bundle or rope with means for attaching ends. This device can be welded or me- chanically fastened.17—Padeye 115 . Fixture. Figure 15. Mocks. a device which imitates the shape of an object for reference or support. used for lifting. prop. Fixtures. and Jigs (Figures 15. pulling and holding parts. support and/or connector for lifting and applying force. a device used to guide a tool. Stays. and/or support parts.17–15.22) Mock.19) Padeye.16—Strongback Padeyes. Stay.20–15. and Cables (Figures 15. a metal device for use as an anchor. This device can be welded. and/or align a workpiece for an operation or process. This de- vice is normally attached by mechanical means to other fitting aids. position. a device used to hold. Jig. Cable. clamped or mechanically fastened. a strip of stiffening material used to hold. SECTION 15—FITTING AIDS Figure 15.20—Mock Figure 15.22—Jig 116 .18—Stay Figure 15.19—Cable Figure 15.21—Fixture Figure 15. stiffener attachment and welding station. and/or gear-pulley equipment and tack weld. American Welding Society. two-side butt conveyors. and final welding station. matic clamping rams. of a heavy structural beak and/or gantry. American Welding Society. welding station. a system that consists of tachment and welding station.. Welding Handbook. Welding Processes (WHB-1. a system that consists of a List heavy structural beam and/or gantry. a system that consists attachment and final welding station. Bibliography/Recommended Reading Web Positioning System. vol. plate positioning and/or feeding Panel Line with Tacking Station. weld backing bar and welding equipment. and bulkhead attachment a heavy structural frame hydraulic magnetic or pneu. hydraulic magnetic. hydraulic. Fla. SECTION 15—FITTING AIDS Specialized Devices Panel Line with One-Side Butt Welding Station. 8th ed. stiffener attachment and welding station.: ing equipment. pneumatic. and bulkhead Stiffener Positioning System. web frame attachment and welding station.8). 1. web frame at- One-Side Welding System. 117 . mag- netic and/or pneumatic stiffener clamping and position- ing equipment and tack welding equipment. Miami. ..................................... 127 119 .......................................... 120 Bibliography/Recommended Reading List .......... SECTION 16 Welding Metallurgy: Practical Aspects Contents Introduction ........................................................................................................................................................................... 120 Primary Base Metal Groups .......................................................................................................................................................... (9) Aluminum. HSLA. 8630. Mild steel’s chemis- (3) Low-alloy steel with quench hardening and tem. the effect that welding and filler metal composition have (8) Inconel® (NiCrFe. ASTM A 36. Mild Steel and High-Strength Low-Alloy Primary Base Metal Groups Structural Steel (OS. A 588. 17-4 ph stainless steel. HS of MIL-S-22698. A 441. microstructure of pearlite and ferrite.. has a relatively low hardenability and is not typically (2) High-strength. A 441. AISI-4130. • How welding affects these properties. ASTM A 514). service of a weldment. strengthened by heat treatment. Steel (1) Mild steel (ferrous alloys not strengthened by heat Mild steel. tain base and weld metal properties. with residuals of phosphorous and sulfur. Designers should have an understanding of the pri. Base Metal Strengthening Mechanisms of Mild their day-to-day work.e.e. low-alloy.30%. resistance. (11) Metals strengthened by age hardening (K-Monel®.. ASTM A 242.. ordinary steel (OS). man- pering heat treatment before welding (i. sion resistance. 316. and Grade 50) The following list covers the majority of base metals and their strengthening mechanisms used by designers in 1. ASTM A 36]. (8) Detrimental elements that reduce weldability. ganese up to approximately 1. From this list the first seven groups will be reviewed (4) The effect welding has on base metal mechanical as to: properties. sired properties. 120 . silicon. resistant properties. Monel®). HY-80. sile strength properties are primarily developed from its A 572.5%. HS. chanical properties. with typically well less than 0. 347). on the metallurgy and service life of the weldment. Its primary yield and ten- ized) (i.29% carbon. 600 and 625). and A 588 Grade 50). HY-130.SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS Section 16—Welding Metallurgy: Practical Aspects Introduction (4) Low-alloy steel hardened by quenching and tem- pering (heat treatment after welding) (i. The designer should be aware of: (10) Metals strengthened by cold work (mild steel and (1) The specification requirements that are invoked. structural steel (normal. (6) How filler metals obtain their strength and corro.e. A 242. obtain and maintain their mechanical and corrosion. aluminum 6061-T-6. 4340). 8620. (2) How metals obtain their strength and other me. (5) The effect welding has on base metal corrosion • How these metals develop their mechanical and resistance. A 572. • What metallurgical effects put the weldability of the metal at risk. This should include how metals 321. mary concepts of welding metallurgy as it relates to the (5) 300 Series austenitic stainless steel (304.. 4140.e. 70Cu:30Ni). try is basically carbon up to approximately 0. • What special processes are required to maintain de- (7) Special treatments which are necessary to main. corrosion-resistant properties. and a small amount of HY-100. There should also be an awareness of (7) Nickel-copper (70Ni:30Cu). (6) Copper-nickel (90Cu:10Ni. treatment) [i. T-l. 356- (3) How metals obtain and maintain their corrosion T6). Postheat treatment is rarely accomplished after welding. Welding Effects on Mild Steels Base Metal HY-100. This is primarily due to the presence of man. causes a tightly adhering oxide scale that retards the nor- mal rate of corrosion. post-weld heat treatment. They require similar protection as the base metal proved weldability). etc. and toughness at low levels of carbon content (im- als. These additions may cause porosity and/or develop low melting temperature constit- 2. silicon. silicon and other deoxidizers in the filler metals comprised of elements such as manganese. structure of tempered bainite and/or martensite. as HS. Mild tility and high toughness at low temperatures without steel is the most readily welded of all the steels. or are painted or galvanized. automobiles. Welding Effects on Base Metal Corrosion Resistance plished. These steels receive their There is no loss of corrosion resistance of the base primary yield and tensile strength from the micro- metal due to welding. Typical applications are ships. function of these elements is to increase strength. nickel. There is increased hard- ness in the heat-affected zone (HAZ) but the depth of Quenched and tempered steels such as HY-80 have hardening is not found to the extent that is present in the moderately high hardenability. towers. Alloying base metals. They are alloyed to pro- quenched and tempered low-alloy steels. well defined mechanical properties. How Filler Metals Obtain Metal Strengthening proximately 1550°F and tempering (reheating) at 1100°F and Corrosion Resistance or higher.12 to 0. harden- cantly better corrosion resistance compared to base met. Detrimental Elements that Reduce Weldability more. which are typically created by quenching the material from ap- 5. Typical applications 121 . molybdenum. tanks. but if corrosion unless they are thermal sprayed with aluminum welding must be accomplished. those steels that contain a grain-refining element such as vanadium will have yield strengths of 50 ksi or 7. bridges. This process consists the range of 150°F to 200°F is sometimes advisable. prove machining operations. HY-130. thermal spraying. The carbon content is kept low to improve weldability and eliminate the need for post-weld heat Recommended weld metals have higher tensile and treatment. ing dimensional stability or when required by fabrication Mild steels such A-36 or OS typically will have yield codes for pressure vessels or other designs where cyclic strengths below 50 ksi. or painting). The and weld quality. preheating in receive normalizing heat treatment. chro- which are used to improve the welding characteristics mium. These steels are frequently called free machining steels. maximum carbon con- yield strengths than mild steel or high-strength structural tents are usually in the range of 0. boilers. vanadium and boron. Sometimes these shielded metal arc filler metals may reduce cracking and steels are alloyed with a small amount of copper which porosity. appliances. of heating the steel to approximately 1550°F and air These steels are typically only stress relieved for machin- cooling. The HAZ’s duce weldments with high yield strengths with good duc- maximum hardness is limited by its carbon content. and the high-strength steels such fatigue is a major concern. elements are typically less than 6 percent and may be ganese. Depending on the alloy. Due dium. Corrosion Resistance of Mild Steel uents that cause hot short cracking. Welding is not rec- All of these steels have relatively poor resistance to ommended and should be avoided if at all practical. 3. When postheat is accom- 4. These weld metals do not have signifi. chromium. buildings. HY-80. Base Metal Strengthening Mechanisms of Welding has very little effect on the mechanical prop- Quenched and Tempered Steel erties of OS and HS base metals. it is usually for dimensional stability and re- quires special procedures. and ASTM A 514) Mechanical Properties 1. Each base metal specification has (usually galvanizing. or molybdenum and frequently to the rapid cooling rate of heavy sections. nickel. Some mild steels have lead or sulfur additions to im- home utensils. ability. pressure vessels. Steels alloyed with copper may be Low-Alloy Steel with Quench Hardening used without painting. use of low-hydrogen or zinc. and Tempering Heat Treatment before Welding (High-Strength Steels T-l.23%. What Special Treatments or Actions are Necessary structures and chemistries as mild steel and they are to Maintain Base and Weld Metal Properties strengthened by a fine-grained (small) crystal structure There are no special routine treatments of these steels created from small additions of elements such as vana- other than when heavy sections are cut and welded. SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS High-strength low-alloy steels have similar micro. 6. and buildings. chromium. These effects may include temperatures. They are frequently Stress relief as previously mentioned is generally not alloyed with elements such as nickel. similar to those of the base metal. gen embrittlement cracking). The heat-affected zone is the unmelted base Heat input that is too high is detrimental for good im- metal area next to the weld having any noticeable effects pact resistance. This can cause a drastic loss in the ability to arrest Welding can have drastic effects on the mechanical crack propagation during impact loading particularly at properties of the base metal and their heat-affected zone low temperatures. and slow travel speeds can changes in mechanical properties. it is sometimes accomplished denum and manganese. Welding Travel Speed (ipm) with excessive preheat and interpass temperatures can re- The maximum heat inputs should be in accordance sult in a loss of impact toughness. (HAZ). cracking of these metals while welding is lack of ade- sion resistance of these steels comes from the small quate preheat. Probably the most frequent cause of tinely painted for corrosion protection. These steels are some. high interpass from the heat of welding. It is important to protect the weld metal 5. cause a retarded cooling rate. Welding heavy sections without preheat and/or low interpass temperatures can Heat Input (J/in. How Welds Develop their Mechanical Properties properties. since this reduces the heat input into the base metal and previously depos- ited weld metal. Excessively high preheats. with the applicable code. They are rou. they should be used since they are easier and to Maintain Base and Weld Metal Properties? more economical to fabricate. What Effects Does Welding have on the Base Metal energy without cracking can result if excessive heat in- Corrosion Resistance? puts are permitted. Preheat is frequently required to pre- vanized probably due to their susceptibility to hydrogen vent restraint cracking on heavier sections. One of the most limiting factors in welding Filler metals typically have chemical compositions these quenched and tempered steels is the marginal abil- similar to base metals except they are not micro alloy ity of weld metal to match base metal toughness.12%. heats can be verified by measuring the preheat on the opposite (back side) of the weld joint. impact resistance and duc. Improper welding techniques peratures. The manufacturer’s or code 122 . 3. Soaking pre- embrittlement. soaking preheats. However. Base Metal Corrosion Resistance of Quenched and properties of these base metals are to be preserved. Pre- Tempered Steels before Welding heat and interpass temperatures that are too low during The quenched and tempered steels have only slightly the welding process can result in base metal and weld better corrosion resistance than mild steel. penstocks. All in-process welding what difficult to weld and special precautions must be conditions causing excessive heat input result in the same observed if the yield strength.SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS are pressure vessels.) = Volts × Amps × 60 result in extensive HAZ/fusion line cracking. bridges. molyb. pact loading. 6. Stress relief should only be the weld metal goes through an extremely rapid quench accomplished if permitted by the manufacturer or the and would be susceptible to cracking at carbon contents governing code or standard. standard or qualified welding procedure. Welding Effects on the Base Metal Properties of Preheats that are too high will produce undesirable Steels that are Quenched and Tempered before nonuniform microstructure. The designer should keep in mind that if mild steel or grain-refined mild steel will meet the service require. ballistic protection. metal cracking. This is because close tolerance machining. recommended. high amperage. The carbon content is kept much for dimensional stability of weldments that are to receive lower than the base metal carbon content. What Special Treatments or Actions are Necessary ments. contents are typically less than 0. Retarded cooling rates cause a heterogeneous can result in hidden cracks and cracking that may not microstructure that is more susceptible to fracture on im- occur until many hours after welding is complete (hydro. A serious loss of the metal’s ability to absorb 4. Stringer beads (straight-line progres- Welding has little effect on the corrosion resistance of sion) having higher travel speeds than weaving beads the base metal. outcome: a retarded cooling rate from peak welding tem- tility are to be maintained. Filler metal carbon mining. Preheat and interpass tem- Welding peratures that are too high will result in low impact prop- erties. Strict adherence to the manufacturer’s recommenda- tions or code requirements is essential if the desired 2. Improved corro. ships. When preheats are required they should be amount of alloying elements present. (oscillating progression) are preferred. hardened by elements such as boron. They are rarely gal. Depending on the alloy. Desired Corrosion Resistance? mechanical properties can be selected by varying the Welding typically has no noticeable effect on the cor- tempering temperature. if known. tinely painted for corrosion protection. heavy sections with no preheat and low interpass temper- erties. compositions as their base metals. that will form low melting temperature resistance of these steels comes from the small amount of constituents. molybde- vanadium. etc. they should always be used in lieu of these post- weld heat treatable steels. The primary function of these elements is to num. is also the most hazardous to welding quenched and tempered 2. silicon. These volves stress relieving the part 50°F below the tempering metals often do not have mechanical property require- temperature. oil Tempered Steels after Welding and rust are broken down in the arc. or to serious embrittlement by improper heat treatment. Also. ductility. What Effects Does Welding have on the Base Metal and tempering (reheating) at 1100°F or higher.43%.05% vanadium or the weld will be subject to seri. Postheat treatment is accom. high-carbon bainite and/or martensite which are created by quenching the material from approximately 1550°F 4. Welding Effects on the Base Metal Properties of Steels that are Quenched and Tempered after and Tempering Heat Treatment after Weld. phosphorous. hidden cracks and cracking tance. Since these alloys are subject and fasteners for increased wear. How Welds Develop their Mechanical Properties mum carbon contents are usually in the range of 0. -4140. The most common element on earth. sometimes for improved fatigue resistance. zinc. maxi- 5. Usually this in. and alloyed with elements such as nickel. -8620.. improve hardenability (the depth of hardening). with the exception that the carbon 123 . -8630. post-weld stress relief should be under the direction of a The designer should keep in mind that if mild steel. SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS requirements should be strictly followed..e. Procedures and tech- niques to avoid hydrogen embrittlement cracking are dis. These steels are usually se. These steels primarily receive their yield and atures can result in extensive HAZ/fusion line and weld tensile strength from the microstructure of tempered metal cracking. Other hazardous better corrosion resistance than mild steel. tensile and yield strength. They are frequently nese. and manganese. tin. 4130. hydrogen. These steels are typically very difficult to weld and strengths and may have limited ductility and toughness special precautions must be observed or serious cracking even at room temperatures. Welding plished after welding to obtain desired mechanical prop. Alloying elements are typically less than 6 per. it is essential that the weld metal contain less are achieved by varying the heat treatment. increased hardness. shafts. molybdenum. and sometimes it requires forced ments in their applicable base metal specification. nickel. They are alloyed to produce high tensile and yield ties. immediate gross cracking. loss of such as AISI-8620. tensile strength. complete (hydrogen embrittlement cracking). chrome. Low-Alloy Steel with Quench Hardening 3. or 4340. Detrimental Elements that Reduce Weldability ments. Hydrogen is commonly produced when water. etc. -4340. These effects may include changes in Steels that are quenched and tempered after welding. ous temper embrittlement. The quenched and tempered steels have only slightly cussed in Section 12 of this manual. or welding engineer or a metallurgist. Improved corrosion sulfur.23 to 0. Improper welding techniques can result in lected for their high strengths or hardness for wear resis. alloying elements. high which lowers the weldability and requires special procedure requirements. Base Metal Strengthening Mechanisms of Steel next to the weld having any noticeable affects from the Quenched and Tempered after Welding heat of welding. mechanical and/or corrosion-resistant proper- ity. even the steels that are quenched and tempered before welding (i. These alloys that 0. They are rou- elements are those such as lead. HY-80/100) will meet the service require- 7. Filler metals may or may not have similar chemical cent and may be comprised of elements such as manga. They are not typically galvanized. The carbon contents are typically rosion resistance of the base metal. Welding ing (Post Weld Heat Treatable) (AISI-1040. Vary- cooling to minimize the loss of fabrication impact prop. The heat-affected zone is the unmelted base metal area 1. Base Metal Corrosion Resistance of Quenched and steels. are used for machinery applications such as gears. chromium. -4130.) Welding can drastically affect the mechanical proper- ties of the base metal in the heat-affected zone (HAZ). ing mechanical properties can be selected by the user and erties. may result. Commonly these metals are welded in a soft that may not occur until many hours after welding is condition called annealed. have high hardenabil. sult in base metal and weld metal cracks. 310. stress relieving is usually accom. Preheats as Depending on the alloy type and application. Probably the most frequent cause of their primary phase and structure do not change with cracks while welding these metals is lack of adequate temperature as with the quenched and tempered steel al- preheat. steels are used in combination with a dissimilar metal for treated condition. niobium. strengths of 35 ksi at room temperature. 310) are commonly used where high- immediately stress relieved. or copper son for this is the mechanical properties are achieved with residuals of unwanted sulfur and phosphorous. tin. When preheats are required. they loss of strength even at relatively high temperatures. close-tolerance machined components. When components are welded in the heat. The surface areas of the dissimilar met- 124 . What Special Treatments or Actions are Necessary Resistant Steel (CRES)] to Maintain Base and Weld Metal Properties? 1. approximate those attainable by the weld. 321. In most cases. Preheat and interpass tempera. oxidation resistance for both high. Typically carbon is less may require soaking times after welding at preheat tem. since fabrication process. there are no welding heat input restrictions Austenitic stainless steels containing nickel. Soaking preheats can be verified by are developed from what is called substitutional and in- measuring the preheat on the opposite (back side) of the terstitial alloy strengthening. Other hazardous elements However. and on these metals like those required on metals that are manganese with small amounts of silicon are sometimes quenched and tempered metals before welding. Detrimental Elements that Reduce Weldability special circumstances base metal or filler metals that One of the most common elements on earth. with the remaining balance of approximately 50 to 60% tures that are too low during the welding process can re. The microstructure is auste- weld joint.and low-temperature applications. zinc. chrome. alloyed with molybdenum. Generally. Steel Base Metal special developed procedures.08% for corrosion resistance. might be needed for machining). 347 Corrosion- 6. When any of these 300 series stainless followed. Ten- are not usually stress relieved unless they are welded in sile strengths of 75–80 ksi are common with ductility of the heat-treated condition or following manufacturing 30–35%. or code requirements is Typical austenitic stainless steels used in industry absolutely essential if the desirable properties of these have 18% or more chromium and 8% or more nickel. lected so that the resultant base metal properties nearly that will form low melting temperature constituents. These steels are nonmagnetic. These steels are typically used for their high- processes makes it necessary to dimensionally stabilize temperature yield strength.SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS content is usually kept much lower than the base metal. than 0. Their primary yield and tensile strength properties soaking preheats. Strengthening Mechanisms of Austenitic Stainless Strict adherence to manufacturer’s recommendations. Aus- through heat treatment after welding and the weld and tenitic 300 Series stainless steels typically have yield HAZ are recrystallized during heat treatment. 316. plished 50°F below the tempering temperature. un- to avoid hydrogen embrittlement cracking are discussed less unique and specialized procedures are developed.. Procedures and techniques welds match the base metal mechanical properties. post-weld heat treatment (PWHT) can be se- are those such as lead. sulfur. and excellent the metal or to reduce the hardness of the material (as toughness. Under 7. and are sometimes called the 18-8 stainless steels. sea-water service the galvanic series should be examined. The rea. the carbon high as 600°F may be required. metals are to be achieved. The manufacturer’s rec- ommendations or code requirements should be strictly Galvanic Series. frequently cause unacceptable distortion and scaling of This is because the weld metal goes through an ex. they should be loys. etc. titanium.20% and less. tremely rapid quench and is very susceptible to cracking at carbon contents approaching the base metal. in Section 12 of this manual. if post heat treatment for temperature strength is required. phosphorous. is also the most hazardous to welding these common to use undermatching filler metal and rarely do quenched hardenable steels. nealed) [304. these metals content will be 0. hydro- nearly match the base metal are used for welding. It should This will help determine if accelerated corrosion can be always be kept in mind that the temperatures reached expected due to the different metal being brought used in during the heat treatment and the quenching process will the same system. Austenitic stainless mechanical properties will not occur until later in the steels are not strengthened by quench hardening. nitic at all temperatures with small amounts of carbides. iron. Higher carbon con- peratures to minimize cracking or they may require to be tent metals (309. designers must take into account that the welds will typi- 300 Series Austenitic Stainless Steel (An- cally undermatch the base metal properties. Typically. with very little Since these metals are heat treated after welding. It is gen. SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS als can also greatly affect the amount and extent of corro- carbides, leaving the chrome available to form its protec- sion. In general austenitic stainless steels are not tive oxide film. Stressed stainless alloys that have exten- recommended for sea-water service environments. sive depletion of chrome by sensitization and are in a corrosive halogen product environment, such as salt wa- 2. Corrosion Resistance of Austenitic Stainless Steel ter, may be subject to stress corrosion cracking. Stress corrosion cracking can be extensive, penetrating com- Austenitic stainless steels have excellent atmospheric pletely through the base material thickness in a short pe- corrosion resistance and resist corrosion very well at riod of time. high temperatures. When alloyed with copper (320) it has good corrosion and erosion resistance in sea water. The iron in these austenitic stainless steels is protected 5. How Welds Develop their Mechanical Properties from corrosion or oxidation by the formation of a protec- and Corrosion Resistance tive stable refractory chrome-oxide film. This tenacious Stainless steel filler metals have chemistries similar to film develops when the surface of the metal is exposed to the base metals they are intended to weld. They develop an oxidizing atmosphere and once formed prevents con- their mechanical properties and corrosion resistance due tinuing corrosion or oxidation. The continued presence to their similar base metal chemistry and metallurgy. of this oxide film is imperative to maintain corrosion There is no significant improved corrosion resistance of resistance. If service conditions cause the loss of this film, the material will be susceptible to corrosion. Also, the stainless steel weld metals over the base metal. the effects of welding heat can cause a loss of the metal’s Some stainless steel filler metals have slight chemistry ability to form these protective oxides. These metals are modifications to permit the development of delta ferrite frequently used without painting. in the weld deposit. Fully austenitic welds have a ten- dency to fissure (develop small cracks). When delta fer- 3. Welding Effects on Stainless Steel Base Metal rite is present in the weld deposit the tendency for Mechanical Properties fissuring is reduced. Stainless steel filler metals such as 308, 309, 316, all may contain small amounts (usually Welding has very little effect on the mechanical prop- less than 10%) of delta ferrite when deposited. The stain- erties of austenitic stainless steel base metals. Since less filler metal 312 typically has 40% ferrite and is very austenitic stainless steels are not quench hardenable, the crack resistant. However, it should not be used for high- HAZ does not exhibit increased hardness as a result temperature applications, as the soft and ductile delta fer- of welding. These steels are readily weldable but do re- rite will change to a hard and brittle phase called sigma quire some special controls to maintain their corrosion ferrite that is extremely crack sensitive. resistance. If the stainless steel has been cold worked prior to weld- 6. What Special Treatments or Actions are Necessary ing to increase its yield strength, the effects of welding to Maintain Base and Weld Metal Properties may cause a drastic loss of the base metal’s yield strength in the HAZ to approximately its annealed condition. Austenitic stainless steel welds are somewhat subject to high-temperature fissuring. They are also subject to re- 4. Welding Effects on Base Metal Corrosion duced corrosion resistance with increased potential of Resistance stress corrosion cracking when sensitized. Sensitization The effects of welding can cause a serious loss of cor- by chromium carbide precipitation takes place at 900°F rosion resistance of the base metal adjacent to the weld. to 1500°F and is time dependent. The more time spent in For some service applications, the presence of carbon the sensitizing temperature range the greater the degree and the heat of welding can result in reduced corrosion of chromium carbide precipitation. To minimize fissuring resistance of some stainless steel alloys by the preferen- and sensitization, minimum preheats are used and inter- tial formation of chromium carbides in lieu of the protec- pass temperatures are kept low (350°F maximum). By tive chrome-oxide films. This depletion of available keeping the preheat and interpass temperature low, less chromium for oxidation resistance by the formation of time is spent at sensitizing temperatures and the amount chromium carbides is called sensitization. Therefore, for of the base metal and degree of sensitization will be re- special applications the carbon content may be held to duced. Avoiding high heat input welding processes will 0.04% or less (extra low carbon 308L or 316L) or addi- also lower the amount of time at sensitizing temperatures tions of titanium (321) or niobium (347) may be made to and the amount of base metal subject to sensitization. The the base metal. The stabilizing elements of titanium and use of stringer beads is recommended instead of weave niobium will form carbides, preferentially chromium beads, which minimizes heat input and sensitization. 125 SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS 7. Detrimental Elements that Reduce Weldability 70:30 CuNi (70% copper-30% nickel) wrought metals typically have yield strengths of 18,000–20,000 psi, and Some stainless steels have selenium, phosphorous, or tensile strength of 45,000–50,000 psi with elongation of sulfur additions to improve machining operations. These 20–35%. It is considered a hot short material having re- steels are frequently called free machining steels (303-Se, duced ductility at temperatures above 1100–1400°F and 316F, 347F). These additions may cause porosity and/or is subject to high-temperature cracking while under develop low melting temperature constituents that result stress. in hot short cracking during welding. Welding of these al- NiCu (70% nickel-30% copper alloy), commonly loys is not recommended. The contamination of weld called Monel®, has the highest tensile and yield strength joints by low melting constituents can cause serious weld- of any ratio of nickel-to-copper contents. Its minimum associated cracking. Lead contamination from radioactive tensile strength is 70–85 ksi and its yield strength is 23– shielding or from machinists’ hammers, sulfur from sulfu- 25 ksi with 35% minimum ductility. rized cutting fluids, lead and tin from soft solder contami- nation, and cadmium from silver brazing alloys, etc., are 2. Corrosion Resistance of Copper and Nickel Alloys examples of potential sources of contamination. Carbon contamination can have a serious effect on the These alloys have excellent atmospheric and corro- corrosion resistance of stainless steels; therefore, it must sion resistance. These metals are frequently used without be considered a detrimental contaminate. Its potential painting. sources are shop dirt, grease, oil, paint, shellac, antispatter compounds, and carbon arcing slag. CuNi–90:10 It is typically used for water corrosion resistance with Copper-Nickel and Nickel-Copper Base good antifouling resistance. This alloy has a higher iron Metals [90:10 CuNi, 70: 30 CuNi , and content than the 70:30CuNi alloys, which improves its strength and corrosion protection. This alloy performs 70Ni:30Cu (Monel®)] better in stagnant water (pitting/crevice corrosion) than either 70:30 CuNi or Monel® (70%Ni-30%Cu). It is also 1. Strengthening Mechanisms of Copper-Nickel and superior in its antifouling resistance. Nickel-Copper Base Metal CuNi–70:30 These alloys are not strengthened by quench harden- ing, since their primary phase or crystal structure does It is used primarily for water corrosion and erosion re- not change with temperature as it does with quenched sistance where increased strength and hardness is re- and tempered steel alloys. The primary yield and tensile quired with less antifouling resistance than the higher strength properties are developed from what is called copper content 90:10 CuNi alloy. It has better erosion re- substitutional alloy strengthening (the mixing of copper sistance in flowing water but has less pitting resistance and nickel atoms in a geometric pattern). Its structure is (crevice corrosion) than 90:10 CuNi. austenitic at all temperatures. These austenitic copper and nickel alloys have copper and nickel as their major Monel®–70NI:30Cu alloying elements, with small additions of elements such iron for yield strength and erosion resistance, manganese It is used primarily for water corrosion and erosion re- to minimize hot cracking, silicon for strength and manu- sistance where increased strength and hardness is re- facturing fluidity (high percentages silicon 4% for hard- quired (with less antifouling resistance) than the higher ness), and niobium for increased weldability with copper content copper-nickel alloys. It has better perfor- residuals of unwanted sulfur and phosphorous. mance for erosion resistance than either of the copper- nickel alloys, but has inferior performance for crevice 90:10 CuNi (90% copper-10% nickel) wrought metals corrosion and antifouling resistance. typically have minimum yield strengths of 15,000– 18,000 psi, and minimum tensile strengths of 38,000 psi Galvanic Series with minimum elongations of 25–35%. It is considered a hot short material having reduced ductility at tempera- When any of these copper and nickel alloys are used tures above 500°F to 1400°F and subject to cracking in combination with a dissimilar metal, for water service, while under stress. This metal is also less expensive than the galvanic series should be examined. This will help the higher nickel content copper-nickel alloys. It is more determine if accelerated corrosion can be expected, due difficult to weld than 70:30 CuNi and Monel® (70%Ni- to the different metal being used in the same system. The 30%Cu) metals. surface areas of the dissimilar metals can also greatly 126 SECTION 16—WELDING METALLURGY: PRACTICAL ASPECTS affect the amount and extent of corrosion. These alloys ations. These are frequently called free machining alloys. are frequently recommended and used for water-service These additions may cause porosity and/or develop low environments. melting temperature constituents that jeopardize welding due to hot short cracking. Welding of these alloys is not 3. Welding Effects on Base Metal Mechanical recommended. Properties The contamination of a weld joint by low melting Welding has very little effect on the mechanical prop- constituents can cause serious weld-associated cracking. erties of nickel alloys and copper base metals. Since Lead contamination from radioactive shielding or from these alloys are not quench hardenable the HAZ does not machinists’ hammers, sulfur from sulfurized cutting flu- exhibit increased hardness as a result of welding. These ids, lead, tin or zinc from soft solder contamination, and alloys are readily weldable but do require some special cadmium from silver solder, etc., are all potential sources controls to minimize cracking and weld porosity. If any of contamination. of these alloys have been cold worked prior to welding to increase its yield strength, the effects of welding may cause a drastic loss of the base metal’s yield strength in Castings the HAZ, approximating its annealed condition. Copper and nickel alloy components that are castings 4. Welding Effects on Base Metal Corrosion and are not made by weld fabrication should always have Resistance suspected poor weldability. Because of the iron contents permitted in these castings and the higher silicon con- The effects of welding have little effect on the corro- tents to facilitate pouring operations, some of these cast- sion resistance of the adjacent base metal. ings have extremely limited weldability. Unless properly alloyed for weldability, they may experience extensive 5. How Welds Obtain Metal Strengthening and fusion line cracking. Corrosion Resistance For Monel® castings, iron and silicon contents must Filler metals for welding alloys of nickel and copper be held low, preferably at 1.7%. Niobium is also required have chemistries/compositions similar to the base metals to minimize the harmful effects that silicon has on the they are intended to weld. They develop their mechanical weldability of Monel®. Niobium-to-silicon ratios of 1.5 properties and corrosion resistance due to their similar to 1.8 are believed to be adequate to minimize weld base metal metallurgy. There is no significant improved cracking caused by silicon. A Nb/Si ratio of 1.7 is con- corrosion resistance of weld metals over the base metal. sidered optimum. Iron content above 2% is a major con- In the case of Monel® filler metal (70% Ni/30% Cu), it is tributor to both weld metal and heat-affected zone found that under some wet conditions, these filler metals cracking. Also, shielded metal arc welding (SMAW) has will preferentially corrode over similar composition base slight advantages over gas tungsten arc welding (GTAW) metals. Monel ® welds on wetted surfaces of Monel® for casting repairs. castings will often preferentially corrode in lieu of the base metal. 6. What Special Treatments or Actions are Necessary Bibliography/Recommended Reading to Maintain Base and Weld Metal Properties List Since copper and nickel alloys are somewhat hot short American Welding Society. Practical Reference Guide for and subject to high-temperature fissuring, welding inter- Welding Metallurgy: Fabrication and Repair (PRGM). pass temperatures are maintained relatively low. Typi- Miami, Fla.: American Welding Society. cally, welding preheat and interpass temperatures are 350°F maximum. ———. Proceedings of the U.S.–Japan Symposium on Advances in Welding Metallurgy. Miami, Fla.: Ameri- 7. Detrimental Elements that Reduce Weldability can Welding Society. Some copper and nickel alloys have selenium, phos- ———. Welding Metallurgy, 4th ed., vol. 1, Fundamen- phorous, or sulfur additions to improve Machining oper- tals (WM1.4). Miami, Fla.: American Welding Society. 127 ....... 131 Operating Instructions ................................................................. SECTION 17 Arc Stud Welding Contents Introduction ........................................................................................................................... 132 How to Handle the Stud Welding Gun ............... 130 Elements of a Stud Weld ............................................................................................................................................................................................................ 131 Setting Up to Weld...................................................................................... 132 129 ............................... 130 Testing and Judging Welds .......................................................................................................................................................................................................................................................................................... 130 How a Stud is Welded........................................................................................................................................................................................................ an arc is drawn between the stud and the The stud. Stud welding may be performed in tight space re- strictions and with minimal clearance. The process accomplishes fusion at the stud/base metal interface using only the melted electrode as filler metal. and concrete shear pins. During stud welding. stainless steel. Stud weld- ing produces only localized heating and has low heat input. small Stud welding differs from manual arc welding. The process consists of two steps: Figure 17. inserted in the welding end (chuck) of the base material forming a molten puddle.1) less operator skill than other welding processes. How a Stud is Welded (D) (d) (e) (E) (f) (F) The process of stud welding has some similarities to the manual arc welding processes. The stud is then gun. which is around the stud. and zinc alloys. insulation pins. mag- nesium. to the welding cycle is controlled by equipment adjustments. The stud welding equipment is adjusted rule. (A) (a) (B) (b) (C) (c) The stud attachment surface must be clean and pre- pared for arc welding. The stud welding process is lim- ited by the stud size and the stud shape.1—Stud Welding Cycle 130 . arc time and the distance the electrode is plunged into the molten metal. other compatible base metals. Only one end of the stud can be welded with this process.. is essentially automatically controlled. Stud welded fasteners are attached with a full penetra- tion weld. The opera- plunged into the puddle and as the molten metal cools a tor then presses the gun to the work plate until the fer- weld is formed. They are available in a variety of shapes and sizes. economi- cal. and aluminum alloys. etc. Studs can be welded in most positions to carbon and alloy steels. pipe hangers. used to weld threaded connectors.SECTION 17— ARC STUD WELDING Section 17— Arc Stud Welding Introduction (1) Welding heat is developed by drawing an electric arc between the stud and the base metal. titanium. Additional special purpose applications are available for some copper. The welded studs may be torque or load tested for production quality assurance. is positioned against the work plate (A). It is very fast. Arc stud welding is an arc welding process that is (2) The two pieces are fused together by the weld. in that brackets. zirconium. and preproduc- tion mock-up testing is usually accomplished for process control. and requires Elements of a Stud Weld (See Figure 17. is flat against the plate to control the arc initiation. If there is any binding. which determines the arc length. and tap lightly with a hammer until it is held firmly in the taper. and tested whenever new operations start. SECTION 17— ARC STUD WELDING The trigger switch is depressed. Adjust the legs so the stud extends from 1/8 in. and Setting up the Gun at the beginning of each day’s work. Improper welds can result if there is any binding or friction between the stud and ferrule dur- ing the welding cycle. (3) Fasten the foot to the legs with screws and wash- (1) Positioning—Perpendicular to the base plate. (1) Chuck—The chuck holds or grips the stud in the After the arcing period. The gun is removed from the completed stud and holds the ferrule in position for welding. The stud is drawn away from the work plate. weld and the ferrule is broken away (F). This is vital.2) (C). (3) Foot—The foot is used to secure and align the ferrule grip. plunging the stud into the molten diameter of stud. (1) Insert a chuck that is of proper size for the stud. NOTE: The lift height. beyond the end of the ferrule. tighten leg screws so that the assem- bly is stationary. ferrule grip. to 3/16 in. Test Action of the Gun Work the lower assembly of the gun in and out to make sure that the stud moves freely through the ferrule. Weld a few studs to a piece of metal that is the same composition and thick. Inspect the sample stud welds for: (2) Put the legs in place and tighten. ness as the actual workpiece. Production studs that require proof testing should Adjustment of Plunge be load or torque tested to the design requirements. (3) Length—Approximately 1/8 in. When the stud stickout is correct. Centering of Ferrule on Stud Adjust the foot so that the stud is centered in the fer- rule. tightening set screws in foot. (2) Weld Quality—Adequate fusion and fillet. weld (4) Insert the ferrule grip into the foot and secure by quality acceptable. starting the welding Operating Instructions cycle (B). the main spring in the gun is gun for welding. readjust foot assembly until free Figure 17. weld pool of the plate to form a weld within the ferrule (2) Ferrule Grip—The ferrule grip fits into the foot hollow (E). ers provided.2—Gun Accessories action is obtained. inspected. A different chuck must be used for each automatically released. 131 . of the stud (5) Insert the stud in the chuck and the ferrule in the length is consumed by weld. creating an electric arc between the stud and the plate Gun Accessories (see Figure 17. (4) Legs—The legs are used to hold and provide po- sitioning of the foot and ferrule grip assembly. It is recommended that a few sample welds are made. has been preset at the factory and will automatically compensate for small changes in stud length. The leg Testing and Judging Welds length must be adjusted for different lengths of studs. Test the sample stud welds to the production require- ments. A molten weld pool is created when a portion of the stud and the plate are melted by the arc (D). Note that the power supply may not be calibrated to indi- Turn power OFF when making connections. cate actual welding current. is available and is connected for the required polarity. Welding Handbook. Secure the c-clamp to the work plate. Avoid pressing the trigger (5) Connect the combination cable (control and weld. vol. Excessive cable lengths adversely affect stud welding. American Welding Society. The settings provided are ap. (6) Make sure the power supply is set for required po- larity.: American setting of the power supply. 1.3—Stud Welding Setup 132 .8).3) proximate and will vary with the type of application. Welding (8) Adjust the time on the control unit and current Technology (WHB-1. This completes the electrical path needed to (7) After welding the stud. work plate. List stant voltage (CV) type and should have a minimum open circuit voltage of 65 volts dc. nal of the power supply and to the connector marked Pressing the trigger a second time will damage the chuck “power supply” on the control unit. Figure 17. Make sure both (3) Hold the gun square/perpendicular to the work.4). must be determined by the weld procedure test data). workpiece. The power supply is typically a con. How to Handle the Stud Welding Gun (2) Connect the ground cable to the terminal of the (1) Hold the gun firmly with your hand placed so that power supply (positive or negative ground orientation you can readily press the trigger. 8th ed. or stud threads. The Stud Welding (C5. Miami. away from the workpiece.. (2) Keep your hand off the side cable.SECTION 17— ARC STUD WELDING Setting Up to Weld (See Figure 17. (1) Position the control unit and the power supply as near the work area as possible.: American Welding light on the control unit should light indicating that power Society. connections are tight and eradicate any paint or rust at (4) Be sure the ferrule is seated firmly against the the connection points. again. ing cable) to the control unit and the gun. and that the input cable and the ground cable is Bibliography/Recommended Reading properly connected. Fla. (5) Press the gun’s trigger only once and release. (3) Connect the timer input cable to the other termi. Fla. Recommended Practices for (7) Turn on the power supply and the control unit. (4) Connect the control unit ground clamp to the (6) Do not move the gun during the welding action. Welding Society. draw the gun straight back supply power to the control unit. Miami. ———. ..................................................... 134 Thermal Spray Processes................................................................................................................................................................................................................................................................................................................................................................................. 135 Quality Assurance .......................................................................................................................................................................................................................................................... 137 133 ............................................................. 134 Machinery Component Applications ...................... 134 Corrosion Control Applications .......................................................................................................... 137 Advantages .................................................................... SECTION 18 Thermal Spray Fundamentals Contents Why Use the Thermal Spray Process? ........ 134 Finishing Thermal Sprayed Coatings ................................................................................................................................... 134 Fundamentals............. 137 Future Applications ..................................................................................................................................................................................................................... 137 Bibliography/Recommended Reading List .............................................................................................................................................................................................................................. 137 Limitations ............................... Examples of a substrate to form a coating. More than 20 years of corrosion protection of steel and iron can be obtained by applying thermal sprayed aluminum or zinc coatings. chrome plating. The heating zone of the arc wire process is produced plied to machinery components. age to the packing material and provide better operation of the component. the gas into a plasma state. der gun between a tungsten electrode and the nozzle. This will also cause less dam. tures. to reasons: build up worn areas) using a thermal spray process. nonmetallic material. and chemi- cal attack better than the substrate material can be ap. powder process. in a heating zone to a molten or tems. Thermal sprayed metallic coatings Arc Wire that will resist wear.1). and then propelling that material onto coating either in the shop or in the field. or instead of manufacturing of a than can be obtained with just oxygen fuel flame mix- new component. arc wire process. erosion. thermal spray materials are used as Corrosion Control Applications bond coats to create a mechanical interaction between the substrate and the coating. This excites appropriate. of machinery components. Standard practice is first to clean (1) Corrosion control.SECTION 18—THERMAL SPRAY FUNDAMENTALS Section 18—Thermal Spray Fundamentals Why Use the Thermal Spray Process? Fundamentals The thermal spray process is used primarily for two Before a machinery component is repaired (i. many components can receive a corrosion-preventing semi-molten state. creating temperatures higher pair. by creating an arc between two continuously fed metallic 134 . heat. A cellent repair method and should be considered. and flame wire process. A powder is fed into the of many machinery components. when gas or gas mixture is passed through the arc. preventing or minimizing corrosion of steel and iron substrates. the repair area is undercut. the heating zone is The thermal spray process can be a cost-effective produced by an arc that is created inside the plasma pow- method for dimensional restoration.. aluminum oxide grit blast to create an anchor tooth pat- tern for the coating to lock into. such as weld re. melted. flame powder process. Ceramics can be flame. longer-lasting surface powder process is used primarily for the refurbishment than the substrate material. The gas or gas mixture exits the nozzle. and propelled to the substrate by the force sprayed on packing areas of valve stems and pump shafts of the gas or gas mixture (see Figure 18. By definition. in lieu of other processes.e. and then use an (2) Machinery component refurbishment. these Thermal Spray Processes coatings can serve as an expendable anode. Plasma Powder Machinery Component Applications With the plasma powder process. There are four primary some of the items that have received this corrosion control spray processes used by fabricators at this time: plasma method are valves and piping. thermal spray is heating a metallic or a Because of the portability of arc and flame spray sys. corrosion. the component to be thermal sprayed. The process is an ex. In a wet environment. When repairing machin- ery components. forcing the Thermal spray is also used to enhance the service life plasma flame outside the gun. The plasma to give those areas a smoother. A wire is then continuously fed into the heating zone where it is melted and then pro- Finishing Thermal Sprayed Coatings pelled onto the substrate by the force of the burning gas- ses and compressed air (see Figure 18.4).2—Arc Wire Process 135 .2). The flame Flame Wire powder process is used primarily for machinery refur- bishment applications. Powder is fed into the heating wire process can be used for corrosion control and ma. and propelled to the substrate by the force of the burning gasses and compressed air (see Figure 18. zone and melted. The arc to create the heating zone. The flame wire The structure of a thermal sprayed coating may be process can be used for corrosion control and machinery different than the structure of that same material in refurbishment applications. Sometimes this different structure creates Figure 18. The flame wire process uses an oxygen fuel flame to create the heating zone. SECTION 18—THERMAL SPRAY FUNDAMENTALS Figure 18.3).1—Plasma Powder Process wires. The heating zone melts the wires and the molten Flame Powder material is atomized and propelled to the substrate by The flame powder process uses an oxygen fuel flame compressed air or an inert gas (see Figure 18. wrought form. The molten material is then atomized chinery refurbishment applications. 3—Flame Wire Process Figure 18. (100 TO 250 mm) Figure 18.SECTION 18—THERMAL SPRAY FUNDAMENTALS 4 TO 10 in.4—Flame Powder Process 136 . Plasma Flame Process. N. thermal spray equipment and consumables will produce Thermal spraying a highly corrosion-resistant mate- quality coatings. Ingham. Then there are the high-velocity oxyfuel system and the high-velocity plasma systems. how- ever. Coatings (metallic and ceramic) can cation (TSS). or to American Welding Society. and Shepard. industry uses the thermal spray process for numerous applications. most processes can apply high-quality coatings at angles up to 45 degrees from perpendicular. terial perform equal to Monel®. Guide for the Protection of equipment that will provide superior coatings for many Steel with Thermal Sprayed Coatings of Aluminum future applications are also being developed by equip. Miami.: American Welding Society. If the undercut certified procedures and personnel should be used to depth will reduce the strength of the component enough spray production components. no all the thermal spray processes required. Abradable coatings wear when contact is made bury.16). Thermal spray coatings can also be used for clearance Flame Spray Handbook. spray gun perpendicular to the substrate surface. These applications Bibliography/Recommended Reading include electromagnetic shielding. West- control. corrosion.Y. A. they should follow the thermal precise seal. thermal barrier coatings. Advantages Thermal sprayed coating properties can be tailored to ———. spray equipment and material manufacturer’s recommen. such as ball bearings or roller bearings without lab- ical applications. New processes and American Welding Society. 137 . improve the resistance to abrasion. ———. Miami. III. Fla. Thermal Spraying: Practice. ment and thermal spray material manufacturers. Future Applications Thermal sprayed coatings should not be used when the area to be repaired would see a point load or line There are processes that can accomplish the most crit- load. Thermal Spray Manual (TSM). Guide for Thermal Spray Operator Qualifica- tion (C2. Thermal spray facilities must also have rial (Monel®) over a carbon steel will not make that ma- personnel capable of making sound application and coat. Metco Inc. ing selection decisions or have an organization that can Thermal spray coatings are ideally applied with the help them do so. operators must receive thorough training and be It should be remembered that when refurbishing ma- tested to determine whether they have the ability to use chinery components with the thermal spray process. Miami. then thermal spray must personnel and the methods to determine whether their not be considered as a repair option. warpage or heat-affected zone is created.18). or high temperatures. and Zinc and Their Alloys and Composites (C2. If thermal sprayed coatings are properly finished mal spray process is generally considered a metallurgi- and they still separate or flake off during finishing the cally cold process.: American Welding Society. vol. be applied to restore or attain desired dimensions. Facilities must have the to make it unusable for service. While the substrate is frequently preheated. to pro.. and the thermal spraying of babbitt bearings. SECTION 18—THERMAL SPRAY FUNDAMENTALS problems in finishing. Theory. clearance control List (abradable coatings). and Appli- suit the application. This means in most applications no spraying operation should be reviewed. Quality Assurance Limitations To help ensure the quality of thermal sprayed coat- ings. Fla. Throughout the world. If personnel are unfamiliar with with the mating part. Fla. Tested and/or strength is added to that component. Miami.. H. ———. the ther- dations. Fla. This makes it possible to form a the finishing of coatings. Vacuum spray systems perform spray- oratory service-proven results.: vide electrical or thermal shielding (or conduction). ing on highly critical components for the airline industry.: American Welding Society. Mr. of the Ship Production Committee of the Society of Naval Architects and Marine Engineers. This project was performed by Puget Sound Naval Shipyard for the Welding Panel. SP-7. and hands-on professionals to improve scheduling and lessen rework. iv .S. the Lincoln Electric Foundation. The Manual was released to the public in 1992. Navy. Omer Blodgett. and New- port News Shipbuilding. Frank Gatto. Puget Sound representative to SP-7. was singled out for apprecia- tion. planners. In addition. Recognized contributors to the original project included the American Welding Society. Acknowledgment The original Manual was the result of a contract between Ingalls Shipbuilding and the Maritime Administration with support from the U. The Manual is meant to serve as a practical guide for engineers. Mr.