SABP-A-022

Best PracticeSABP-A-022 Stainless Steel Fabrication, Testing and Installation Document Responsibility: Materials & Corrosion Control Standards Committee 25 August 2008 Saudi Aramco DeskTop Standards Table of Contents 1 2 3 4 5 6 7 8 9 10 Scope and Purpose............................................ 2 Conflicts and Deviations..................................... 2 References......................................................... 2 Definitions and Abbreviations............................. 5 Introduction to Stainless Steels.......................... 6 Classes of Stainless Steel.................................. 6 Composition, Corrosion Resistance and Sensitization..................................... 9 Materials Selection........................................... 11 Castings .......................................................... 12 Fabrication....................................................... 13 10.1 Welding and Corrosion Resistance....... 13 10.2 Welding of Stainless Steels................... 14 10.3 Storage.................................................. 32 10.4 Shop Fabrication................................... 32 10.5 Pickling, Passivation & Iron Removal.... 33 10.6 Field Fabrication.................................... 34 Hydrostatic Testing.......................................... 36 Microbiologically Influenced Corrosion (MIC)... 37 Coating Stainless Steel.................................... 38 Summary.......................................................... 39 11 12 13 14 Previous Issue: New Next Planned Update: TBD Page 1 of 40 Primary contacts: Lobley, Graham R. on 966-3-8746678; Niemeyer, Dennis C. on 966-3-8736700 Copyright©Saudi Aramco 2008. All rights reserved. Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication, Testing and Installation 1 Scope and Purpose Stainless steels are more expensive than carbon and low alloy steels and are normally selected for their superior corrosion or heat resistance. For the austenitic stainless steels, good low temperature toughness is also a key mechanical property. However, improper fabrication and testing procedures can seriously degrade corrosion resistance and directly lead to premature failures. This SABP provides guidelines to optimize performance of stainless steels, particularly austenitic, duplex and martensitic grades. It identifies specific procedures and specifications to be followed, during manufacturing, fabrication and commissioning, in order to achieve optimum performance of stainless steel equipment and piping. This SABP is based on current industry experiences and is intended for internal Saudi Aramco operations and maintenance applications. It also provides guidance for inspection in fabrication facilities. 2 Conflicts and Deviations If there is a conflict between this Best Practice and a Saudi Aramco standard then the Standard shall govern. If there is a conflict between this Best Practice and an approved welding procedure then the approved welding procedure shall govern. If there is a conflict between this Best Practice and other standards and specifications, please contact the Coordinator of ME&CCD/CSD. 3 References 3.1 Saudi Aramco References Saudi Aramco Engineering Standards SAES-A-007 SAES-G-005 SAES-H-001 SAES-L-132 SAES-W-010 SAES-W-011 SAES-W-014 SAES-W-016 Hydrostatic Testing Fluids and Lay-Up Procedures Centrifugal Pumps Coating Selection & Application Requirements for Industrial Plants and Equipment Material Selection for Piping Systems Welding Requirements for Pressure Vessels Welding Requirements for On-Plot Piping Weld Overlays and welding of Clad Materials Welding of Special Corrosion Resistant Materials Page 2 of 40 Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication, Testing and Installation Saudi Aramco Standard Drawing AD-036821 Saudi Aramco Best Practice SABP-A-001 Polythionic Acid SCC Mitigation - Materials Selection and Effective Protection of Austenitic Stainless Steels and other Austenitic Alloys Material Guide for Centrifugal Pumps Saudi Aramco Technical Alert ALERT-93-011 3.2 Technical Alert Number 11, Avesta 254SMO Stainless, issued 11/21/93 Industry Codes and Standards American Petroleum Institute API TR 938-C Use of Duplex Stainless Steels in the Oil Refining Industry-First Edition, 2005 American Society for Testing and Materials ASTM A380 Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment, and Systems Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts Standard Test Methods for Chemical Analysis of Thermal Insulation Materials for Leachable Chloride, Fluoride, Silicate, and Sodium Ions ASTM A967 ASTM C871 American Welding Society AWS D10.18:2008 AWS D18.2:1999 Guide for Welding Ferritic/Austenitic Duplex Stainless Steel Piping and Tubing Guide to Weld Discoloration Levels on Inside of Austenitic Stainless Steel Tube International Standards Organization ISO 15156-3 Petroleum, Petrochemical and Natural Gas Industries Materials for Use in H2S-Containing Environments in Oil and Gas Production Part 3: Cracking-Resistant CRAs (CorrosionResistant Alloys) and Other Alloys Page 3 of 40 Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication, Testing and Installation National Association of Corrosion Engineers NACE MR103-2007 NACE RP0198 Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments The Control of Corrosion Under Thermal Insulation and Fireproofing Materials - A Systems Approach New Zealand Stainless Steel Development Association NZSSDA 3.3 Publications Optimising Stainless Steel Piping Fabrication Practice, Dr. Liane Smith, North Scottish Branch of the Welding & Joining Society, Aberdeen, 26th May 2005 NiDI Publication 11003: Nickel Stainless Steels for Marine Environments, Natural Waters and Brines (1987) Guidelines for Successful Use of Stainless Steel in Potable Water Treatment. Plants (PWTP), R. E. Avery, S. Lamb, A. H. Tuthill, Nickel Development Institute, April 1998 Fabricating Stainless Steels for the Water Industry Nickel Development Institute Reference Book 11026, C. Powell, D. Jordan, October 2005 Microbiologically Influenced Corrosion – Case Studies in Australasia, ACCA 2007, Paper 21, L. H. Boulton Influence of Sulfate-Reducing Bacteria on Corrosion of 2205-Type Duplex Stainless Steel in Chloride Medium, ACCA 2007, Paper 72, P. J. Antony, R.K. Singh Raman, et. al. “Welding of Stainless Steels and other Joining Methods,” A Designers’ Handbook Series No. 9002, NiDI, Nickel Development Institute American Welding Society, “AWS Welding Handbook, Eighth Edition Volume 4, Materials and Applications Part 2”, 1998 ASM Handbook, Volume 6, “Welding, Brazing, and Soldering,” 1993 Stainless Steels, Properties, How to Weld Them, Where to Use Them, Kotecki, D. and Armao, F., 2003, The Lincoln Electric Company Welding of Austenitic Stainless Steels - a Guide to Best Practice, TWI Members Website Code of Practice for the Fabrication of Stainless Steel Plant & Equipment, 2001 Page 4 of 40 air and moisture acting on sensitized austenitic stainless steels. Stabilized grades with Ti (grade 321) or Nb (grade 347) are commonly used. to determine conformance to specified alloy composition. Sensitization is dependent on the composition (see sections 9 and 10. Cracking is due to sulfur acids forming from sulfide scale. Testing and Installation 4 Definitions and Abbreviations API ASSDA ASTM CRA CUI DSS Free Iron HAZ ISO MIC NACE NZSSDA PASCC American Petroleum Institute Australian Stainless Steel Development Association American Society for Testing & Materials Corrosion Resistant Alloy Corrosion under Insulation Duplex Stainless steel Surface contamination of SS with carbon or ferritic steels Heat Affected Zone International Standards Organization Microbiologically Influenced Corrosion National Association of Corrosion Engineers New Zealand Stainless Steel Development Association SCC which can occur rapidly under refinery shutdown or T&I conditions. including weld consumables and fabricated components. preventing the depletion in chromium. Positive Material Identification is a chemical analysis that ideally includes all alloying elements. Stabilization is an alloying method of preventing sensitization.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. Sulfide Stress Cracking Stainless Steel Page 5 of 40 Phase Balance Percentage of delta ferrite in duplex stainless steel PMI PWHT SCC SDSS Sensitization Stabilization SSC SS . It can be used to check all parts. Ti and Nb additions preferentially combine with the carbon. Postweld heat treated: Solution Anneal and rapid cool Stress Corrosion Cracking Superduplex Stainless Steel Formation of chromium carbides along grain boundaries which causes loss in corrosion resistance.2). ≤0. any of these environments may result in pitting attack. The stainless steels addressed in this best practice (BP) are primarily the 300-series austenitic stainless steels and the duplex stainless steels. these materials are not totally resistant to corrosion in other environmental conditions. they are sensitive to pitting and crevice corrosion in aerated water environments with chloride ions present at certain temperature conditions.08% C). the commissioning procedures should be optimized to minimize the corrosivity of the environment for the hydrotest period. Also. duplex and precipitation-hardenable (PH). adding less Ni and some Mo gives a duplex structure. Page 6 of 40 . On the basis of microstructure. Testing and Installation SSS 5 Super Austenitic Stainless Steel Introduction to Stainless Steels Stainless steels are selected for service in oil. austenitic. castings and other products. adding more Cr and > 8%Ni gives an austenitic microstructure. The simplified Alloy Tree (Figure 1) shows the relationship of these types of stainless. The fabricator should make every effort to optimize the quality of the weld and minimize the reduction of passive film quality. martensitic. Microbes introduced in the hydrotest water can promote MIC as another form of pitting. which is deleteriously affected in various ways by welding.5% Cr. In particular.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. five major families of stainless steels are recognized: ferritic. The protective thin chromium-rich passive film forms spontaneously in the presence of oxygen. Unfortunately. cryogenic and high temperature services. Stainless steel piping systems are typically hydrotested with water of varying quality from potable water through to raw water with or without various chemicals present. Stainless steel grades are specified for ambient. gas and downstream systems because of their high level of corrosion resistance in typical producing and refinery environments. This usually occurs at weak points such as heat tint oxides.5 – 14. building from a base case of 405 ferritic stainless steel (composition 11. that are associated with welding cycles. The purpose of this BP is to provide guidance on optimum practice at every step in order to give the very best confidence in the quality of fabrications. Adding C gives a martensitic structure. Depending upon the duration of exposure. Two approaches are therefore used for preventing pitting attack of stainless steels during hydrotesting and prior to service. The resistance to pitting is strongly affected by the stability of the protective passive film on the stainless steel. 6 Classes of Stainless Steel Stainless steels are broadly defined as steels that contain at least 11% Cr. they have a tensile yield strength of about 275 MPa (40 ksi) and are generally machined. Generally. These alloys are strongly magnetic and their heat-treated structure is body-centered tetragonal. Ferritic stainless steels are named because their body-centered-cubic (bcc) crystal structure is the same as iron at room temperature. even at cryogenic temperatures. Austenitic steels include 316 with Mo to increase pitting resistance. and then tempered for increased ductility and toughness. Like the ferritic alloys. formability. Testing and Installation finally.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. cold formed. have moderate corrosion resistance and are not susceptible to SCC. They possess excellent ductility. adding Cr. where creep and oxidation are the principal damage mechanisms (examples 304H and 310). Higher %C and heat resisting grades are specified for the highest temperatures. and their poor toughness limits their fabricability and the usable section size Martensitic stainless steels are similar to Fe-C alloys that are austenitized. welds and cold-worked austenitic materials can show appreciable magnetism. and cold worked in this condition. They are magnetic. they can be substantially hardened by cold work. like the high-temperature (900 to 1400°C) austenite form of iron. Austenitic stainless steels comprise the largest stainless family. Ni and Cu gives a precipitation hardening stainless steel. Page 7 of 40 . Their Cr content usually range from 11 to 18%. Their annealed yield strengths range from 275 to 350 MPa (40 to 50 ksi). and toughness. Austenitic alloys are nonmagnetic in the solution annealed condition. low C and weld stabilized grades (with Ti [321] or Cb [347]). In addition. Castings. in terms of number of alloys and usage. due to the presence of second phases such as delta ferrite or martensite. The austenite structure is face-centered-cubic (fcc). In the annealed condition. they cannot be hardened by heat treatment. ferritic stainless steels do not have particularly high strength. hardened by quenching. 03 -0. Higher Mn-containing alloys. Duplex stainless steels are Cr-Ni-Mo alloys that contain a balanced mixture of austenite and ferrite. Typically.08%C Duplex 2205 base: 22%Cr. and are therefore significantly magnetic. 0.03 -0. %Cr. 316. Typical applications include handling chlorinated seawater and for some heat exchanger tubing.0. compared with the austenitic stainless steels and improved toughness and ductility. 5%Ni.5 to 6. 6%Ni.03) Heat Resistant (310) & high C grades Austenitic Superaustenitic 304 base: 18PREN 8% Ni. 3%Mo. wear rings. such as 304. N Precipitation Hardening 15-5 PH 17-4 PH 13 Cr 13%Cr. Temperature limits typically range from -50 to +300ºC. 321 and 347 grades (see Table 1). Duplex stainless steels combine the optimum properties of austenitic and ferritic types.1 .03 -0. The higher Mn levels greatly improve resistance to wear including galling (a common problem with 300 series steels). which is approximately twice the strength level of either phase alone. 5%Ni. Applications have been limited by two factors: (1) weldability concerns due to formation of deleterious Page 8 of 40 .03 %C Martensitic 410 base: 12 %Cr. they contain 18 . 0. Testing and Installation Ferritic 405 base: 11 – 12% Cr < 0. They can have yield strengths ranging from 550 to 690 MPa (80 to 100 ksi) in the annealed condition. Duplex stainless steels have been utilized for seawater applications. compared with the ferritic stainless steels. with Mn exceeding 4%.08%C Superduplex PREN ⋝ 40 2507 & UNS S32760 2205 base: 22%Cr. 3%Mo. such as in Seawater Treatment Plants and some refinery heat exchanger tubing.26% Cr plus 4.5% Ni. ⋝ 40 0. are covered under 200 series stainless steel specifications.08%C UNS S31254 0.2%C Austenitic 304 base: 18 %Cr. Examples used in rotating equipment include UNS S20910 (Nitronic 50) and UNS S21800 (Nitronic 60) for shafts.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. N Figure 1 – Stainless Steel Alloy Tree Austenitic stainless steels of the 300 series are the most commonly used. 8% Ni. Mo A486 CA6NM 316 (Mo-alloyed) Stabilized (Ti or Cb) or low C (≤ 0. etc. Their duplex structure results in improved SCC resistance. such as for pumps handling produced brine water. Page 9 of 40 . Nb or Cu.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. Testing and Installation intermetallic phases leading to loss of toughness and corrosion resistance. Where mechanical strength is important for design purposes. They are available in bar form for the production of heavy duty engineering components. Dual Certified and Stabilized Grades Types 304L or 316L are the standard grades of stainless steel used for welded applications. the slightly lower tensile and yield strengths of the "L" grades should be recognized. Dual certification means that it meets the 0. Steels with higher levels of chromium (Cr). Type 316L with 2 . such as Al. (2) low-to-moderate resistance to SSC. They generally form intermetallic compounds. These steels are sometimes referred to as superstainless steels (SSSs). semi-austenitic or austenitic.03% C is required for welded structures to minimize risk of sensitization. They combine the heat treatability of normal martensitic grades with the corrosion resistance of austenitics. The resistance to pitting is related to the composition using the empirical Pitting Resistance Equivalent Number (PREN) as follows: PREN = %Cr + 3. “L”. stainless steels with a PREN value above 40 are resistant to pitting corrosion in ambient temperature seawater. with high TDS and Cl levels. Standard Drawing AD-036821 for highly corrosive services. but in S17400 (17-4PH). Stabilized grades 321 and 347 are also specified for welded structures. Corrosion Resistance and Sensitization The resistance of stainless steels to pitting and crevice corrosion in aerated waters is strongly related to the chemical composition.3% Mo is more resistant to pitting and crevice corrosion and is preferred over Type 304L for more severe services. drive shafts and control valve parts. Typical uses of 17-4PH include valve stems. Ti. are used to achieve age hardening. The low C "L" grade which has a maximum of 0. It is increasingly common to encounter Dual Certified Type 304/304L and Type 316/316L stainless steel in warehouse stock. 7 Composition. Various alloying elements.03% C maximum requirement for the "L" grades and also meets the higher mechanical properties of the regular grades. molybdenum (Mo) and nitrogen (N) are more resistant. fine Cu precipitates are formed. PH stainless steels may be martensitic.3 x %Mo + 16 x %N Generally. especially with the superduplex grades. Superstainless cast and wrought grades are specified with PREN ≥ 40 in SAES-G-005. 45 43 – 49 Page 10 of 40 200 series Superaustenitic .15N 20Cr-18Ni-6Mo-N (254SMO) 21Cr-24Ni-6Mo-N (AL-6XN) Example Specification (ASTM/SAE) A240 A240 A240-410 A487 CA6NM A276 A240 S30400 A240 S31600 A240 S31700 A240 S32100 A240 S34700 A240 S31000 A240 XM-19 A240 S21800 A240-S31254 B688 UNS # / Grade S40500 S41008 S41000 J91540 S41500 S30400 S31600 S31700 S32100 S34700 S31000 S20910 S21800 S31254 N08367 PREN 10. including original heat treatment and welding. This means that even in the root region. ISO 15156-3 includes PREN ranges for stainless steels in the austenitic and duplex alloy classes.3 18 .26 29 -38 17. as a consequence of its lower PREN value.3N 17Cr-8. For this reason.8 – 17. Thermal history. SSS casting alloys must be in the solution annealed condition after weld repairs. microsegregation or surface condition. Field and laboratory investigations indicate the most likely cause of failure was insufficient (or no) addition of the recommended Inconel 625 filler material during welding of the root pass. with some dilution of the filler metal composition by the parent metal.20 17 . are major factors influencing ultimate performance.3 12.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication.08 12 Cr 13Cr-4Ni-0. It is important to ensure that the grades of stainless steels which are used in piping systems are not mixed up. Weld metal must have a pitting resistance equivalent to or better than that of the parent metal. The PREN does not factor alloy microstructure.19 24 .0 12.5Mo 18Cr-10Ni-Ti 18Cr-10Ni-Nb 25Cr-20Ni 22Cr-12.20.5Mo-0.5 – 14.5Ni-5Mn-2. A section of piping with lower resistance to pitting attack. may be more likely to initiate pitting when connected to higher alloyed stainless steel.20 23 .3 .28 28 .8 – 17.5Ni-8Mn-4Si-0. the pitting resistance will still be acceptable.5 11.5 – 14. as follows:   Corrosion damage initiated in the root pass of the UNS S31254 butt-welds.9 42 .5 – 12. Testing and Installation PREN data relates bulk chemistry to corrosion performance.5 11. but this is only a ranking guideline to compare different alloys. all of which strongly influence alloy performance in practice.5Mo 18Cr-8Ni-3.5Mo 13Cr-4Ni-0. superstainless steels such as UNS S31254 are fusion welded using overalloyed filler metal (Inconel 625).08 13 Cr-C≤0. Table 1 – Some Commonly Used Stainless Steel Grades Material Ferritic – 400 series Martensitic – 400 series Austenitic 300 series Nominal Composition 11 Cr-C≤0.5Mo 18Cr-9Ni 18Cr-8Ni-2. ALERT-93-011 (Saudi Aramco Technical Alert Number 11) provides additional information and advice concerning seawater service.33 17 . Consequently. 5Mo-N-W 25Cr-7Ni-3.38 A890 Gr 6A A240-S32750 A564 Type 630 40 . The (L grade) low carbon level limits the amounts of chromium carbides that form. N = a Ni-based alloy (including some higher grade stainless steels) and J = a casting specification.5Mo-1.5Mo-N UNS # / Grade N08020 (20Cb-3) N08904 (904L) S31803 S32205 J92205 S32760 J93380 S32750 S17400 PREN 26 . Use of low carbon or stabilized grades are methods of preventing sensitization. If a steel has been sensitized.Materials Selection and Effective Protection of Austenitic Stainless Steels and other Austenitic Alloys) provides practical advice on mitigation of PASCC in refinery environments. This Page 11 of 40 . welding must be performed with the continuous addition of filler. The degree of sensitization is dependent on the composition of the steel and the time it spends in the temperature range 370°C to 815°C (700°F to 1500°F).  Autogenous welds in UNS S31254 (without Inconel 625 filler) lack sufficient corrosion resistance to withstand seawater . In this system: S = a regular stainless steel. company standards and technical guidelines.5Cu 21Cr-25Ni-4. Testing and Installation Example Specification (ASTM/SAE) B464 B673 A182-F51 A890 Gr 4A Superduplex Precipitation Hardening 25Cr-7Ni-3.31 32 . Sour corrosion issues are different for refinery applications and NACE MR103 presents materials selection for CRAs for sour refinery environments.particularly chlorinated seawater.5Mo-N-W 25Cr-7Ni-4Mo-0.5 Note: The international UNS numbering system uses a letter prefix and numbers to designate specific alloy grades.46 38 . Sensitization gives rise to corrosion in the HAZ and is a prerequisite for polythionic acid stress corrosion cracking.3N 17Cr-4Ni-3Cu A276/A182-F55 Material Other grades Duplex Nominal Composition 20Cr-35Ni-2. SABP-A-001 (Polythionic Acid SCC Mitigation . 8 Materials Selection Guidelines and standards that cover selection and specifications of stainless steels include international standards. Sensitization is the formation of chromium carbides along grain boundaries that causes loss in corrosion resistance.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. thereby limiting chromium depletion at the grain boundaries.40 31 .5Mo-3. ISO 15156-3 presents materials selection for CRAs for sour chloride containing environments for upstream applications. Sensitization can occur in the HAZ adjacent to the weldment. corrosion resistance may be recovered only by the use of a full anneal (and suitably rapid quench) to dissolve the carbides.5Cu 22Cr-5Ni-3.46 39 .44 15 – 17. Therefore. Duplex stainless are attractive since they offer improved mechanical properties compared to austenitics and better corrosion resistance in saline environments. Foundries often make “minor” weld repairs without PWHT and this practice is generally satisfactory if L (lower carbon) grades are used. when internal stainless steel cladding is recommended. ISO 15156-3 specifies two different acceptable heat treatments for S17400. PWHT is omitted from ASTM A743 but is available as a Supplementary Requirement. UNS S41000 stainless steel (410 stainless steel) and other martensitic grades must be quenched and double tempered to a maximum allowable hardness level of 22 HRC. Wrought S17400 stainless steel is permitted for sour service but must be carefully processed to prevent SSC. lowering corrosion resistance by intergranular attack. as well as for a range of corrosive environments.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. there is an increased risk of either process-side or external chloride SCC. Page 12 of 40 . PWHT (solution anneal and rapid cool) should be considered for duplex stainless steel castings that have been weld repaired. which can also promote PASCC. Sensitization can occur in the HAZ of weld repaired areas of higher carbon austenitic grades especially. For sour services. The maximum hardness level is 33 HRC for both conditions. Solution treatment requirements are provided in ASTM A890. Duplex limitations include lower subzero impact toughness (-50ºC limit) and generally lower tolerance to sour environments. ASTM A744 requires that castings shall be PWHT (solution anneal and rapid cool) after all major weld repairs and after those minor weld repairs involving either of the following conditions: (1) welding on a wetted surface. PWHT is also required. ASTM A744 is preferred if non-L grade steels are specified or allowed. 2005) provides advice on duplex steels in refining. Above 60ºC. Solid austenitic stainless steels are used for cryogenic and other applications. or (2) welding that heats a wetted surface to or above 800°F [425°C]. since this material is strongly air-hardenable. Testing and Installation damage mechanism may also affect burner tips and other combustion equipment which are fired on sour hydrocarbons. API TR 938-C (Use of Duplex Stainless Steels in the Oil Refining Industry-First Edition. 9 Castings The chemical composition of cast stainless grades is adjusted to increase fluidity and minimize defects such as porosity and hot tears. Austenitic stainless steel grades offer a combination of good mechanical properties and material performance at both low (cryogenic) and high temperatures (heat resisting grades). but should normally be avoided for services above 60ºC. thus preventing oxidation whilst the weld metal and HAZ is hot. During manufacturing. the metal close to the weld is reheated to high temperatures. The oxide film which is formed at low temperature. Where welds are accessible. porous and cracked. Therefore. this oxide film is normally removed by pickling and passivating the surface in oxidizing acids. from the melting point at the fusion line to lower temperatures through the HAZ. is thin and dense.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. Once the weld is cooler. This „heat tint‟ oxide formed at the HAZ at lower temperatures is so thin that it forms interference colors. Characteristically. forming characteristic blue or rainbow colors. This oxide is far more easily broken down than the original passivated surface and there is a zone of metal under this oxide which is lower in chromium content relative to the bulk metal. excessive heat tint can be removed by pickling and passivating the surface using Page 13 of 40 . A straw-to-yellow heat tint (chart no. It presents a color guide (Figure 2) relating degree of discoloration to oxygen content in the backing shielding gas. This heat treatment gives a poor quality high temperature oxide film on the surface which is quite thick. Testing and Installation 10 Fabrication 10. AWS D18. 6 is normally removed by grinding. When welding is carried out. giving good protection to the stainless steel. pickling and passivating the surface in oxidizing acids. with a solution heat treated microstructure and a well-passivated surface oxide finish. Heat tint exceeding no. 3) is considered desirable but a blue tint up to chart no. the HAZ has to be protected from re-oxidising at high temperature during the welding process.6 is acceptable for standard applications. The as-delivered piping is therefore in an optimum condition. An unprotected HAZ which is exposed to air during welding will therefore show a high level of oxidation. the HAZ will re-oxidise slightly at lower temperatures but this oxide film is a more protective film than the high temperature oxide film. The heat tint oxide can be identified by a number corresponding to the oxygen level in the shielding gas. it would be black in color and in the worst condition may be visibly thick and porous. either in the passivating solution or just by exposure to air.1 Welding and Corrosion Resistance When stainless steel piping or components are manufactured they are normally subjected to a final high temperature solution heat treatment (typically at 1050ºC) followed by water quenching. This leaves the material in the solution annealed condition which is the optimum for corrosion resistance. This is done by shielding around the weld using an inert gas to exclude air.2:1999 addresses factors affecting weld discoloration inside a 316L austenitic stainless steel tube. Stainless steels should not be welded with the oxy-fuel Page 14 of 40 . The second part gives consumables and techniques that are specific to individual grades of stainless steel.1 Welding Processes The welding of stainless steels may be carried out using almost any welding process including all arc welding (Shielded Metal Arc. friction. Gas Tungsten Arc. resistance. Flux-cored Arc. It also provides guidance for inspection in fabrication facilities. If there is a conflict between this Best Practice and an approved welding procedure then the approved welding procedure shall govern. 10. This section is divided into two parts. which was attributed to the heat tint plus poor quality hydrotest and contributory MIC factors.2 Welding of Stainless Steels Introduction The purpose of this section is to give information on the correct welding of stainless steels.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. This best practice gives general information. Gas Metal Arc. Stainless steel lines have failed prematurely in service at heavily heat-tinted HAZ locations. 10. Surfaces should be fully rinsed with clean water to ensure removal of any residual acids on the surface. It is intended for internal Saudi Aramco operations and maintenance applications. Testing and Installation standard products. The first part gives information and techniques that are pertinent to the general classifications of stainless steels. All conflicts shall be brought to the attention of CSD.2. Figure 2 – Weld Discoloration and Heat Tint Numbers Scanning electron microscopy examination of heavily heat tinted surfaces has revealed a craze-cracked oxide surface that is enriched in Cr (Figure 3). The craze-cracked surface topography may promote bacterial colonization. laser and electron-beam welding techniques. Submerged Arc and Plasma Arc). titania covered electrode (also called Acid– Rutile).0 mm] and smaller may be used in all positions of welding. SMAW may be used for the fill passes on butt welds in pipes larger than 2" diameter and all socket welds. These electrodes are usable with DCEP (electrode positive) only. Electrodes with the -26 designation are recommended for welding only in the flat and horizontal fillet positions. These electrodes tend to weld easier out of position but are not as smooth. They give superior impact properties at cryogenic temperatures. Electrode sizes 5⁄32 in. For example E316L electrode is for type 316L stainless steel. This is a lime (also called basic) coated electrode. Testing and Installation welding processes. The GTAW process can also be used for socket welds. SMAW electrodes for stainless steels come with four usability classifications. This electrode produces a smooth. [4. This is a lime .silica . Electrode sizes 5⁄32 in. This is a lime . Within Saudi Aramco we only use two welding processes: Shielded Metal Arc (SMAW) and Gas Tungsten Arc (GTAW). This designation is for those electrodes that are designed for flat and horizontal fillet welding and that have limited out of position characteristics. SMAW Welding The SMAW welding electrodes for stainless steels are given in AWS A5. the number of the SMAW welding electrode is equivalent to the type of stainless steel.0 mm] and smaller may be used in all positions of welding. Page 15 of 40 . -17.4 specification. -16. The welding consumables for the commonly found stainless steels are given in Table 2. The GTAW process is always used for root passes on butt welds and for butt welds on pipes smaller than 2" diameter. Fabricators and contractors may use a wider range of welding processes depending on their needs. For example.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. E316L-16. This produces a smoother weld bead appearance than the -15 but is more difficult to weld out-ofposition and a smaller diameter should be selected. [4. -26. concave weld bead. -17 and -26. These are indicated by the two numbers following the alloy designation -15.titania (also called basic–rutile) covered electrode. It is generally is not as weldable out-of-position as the -16. Generally. The significance of these usability classifications are as follows: -15. -16. however. this is not a general rule and the electrode supplier should be contacted for advice on a case by case basis. This may lead to problems with hot cracking. Failure to do this can result in hydrogen cracking on the non-austenitic side of the joint. Moisture in the flux can lead to defects such as wormholes and porosity. Neutral fluxes give sound weld deposits and permit some oxidation and loss of alloy to the flux. Ferrite control in submerged arc welds is therefore very important. more than any other arc welding process. silicon pickup from the slag can be quite significant. but this may be problematic due to the difficulty of control over dilution in the weld. Dissimilar metal welds in hydrocarbon service must be made with a nickel based consumable. SAW Welding The high heat input from SAW leads to larger weld beads and slow cooling rates. Hot cracking is caused by the segregation of minor elements to the liquid phase during freezing. An arc is established between a tungsten electrode and the work piece. One of the elements that can cause this phenomenon is silicon and. however. Careful control must be maintained over the arc during welding as small variations in penetration can greatly affect the dilution levels. Base metal dilution during SAW can vary between 10 and 75%. Since the filler material does not form part of the arc. storage and conditioning Although hydrogen cracking is not a major problem in austenitic steels. It is very important to keep electrodes for dissimilar metal welds in a low hydrogen condition. electrodes should not be exposed to humid environments. no AWS specification exists to cover these fluxes. due to the slow cooling rates from SAW. The filler material is added separately into the molten weld puddle. Fluxes can be susceptible to pickup of moisture and should be baked as per the manufacturer's instructions prior to use. Basic fluxes contain additions of alloying elements that are imparted to the weld when molten. Flux for SAW is usually designed to be either basic or neutral with respect to the steel. there will be extremely low loss of alloying elements during welding. giving Page 16 of 40 . Testing and Installation Electrode care. Dissimilar metal welds are not permitted in sour service exposure. It may be possible to recover moist electrodes by baking.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. Electrode wire is readily available from a number of different suppliers. GTAW Welding GTAW (Gas Tungsten Arc Welding also referred to as TIG) is similar to Gas Metal Arc Welding (GMAW) in that the arc is protected by an inert gas. All grades of weldable stainless steel may be welded using GTAW welding. GTA welding has extremely wide applicability and can be used to weld in all positions. Austenitic Stainless Steel Properties Affecting Welding The coefficient of thermal expansion for the austenitic stainless steels is almost 40% greater than that of carbon steel.e. but is most suited to thin sections. Contact between the electrode and the weld pool is to be avoided to avoid tungsten contamination. 4 for 75 . no gas). and G for unspecified).g.. Y denotes the applicable positions for welding (1 for all position and 0 for flat or horizontal only) and Z the shielding gas (1 for CO2. However.22. Helium shielding provides deeper penetration.. FCAW Welding Electrodes for FCAW are designated according to AWS specification A5. This designation takes the form EXXXTY-Z wherein XXX denotes the chemical specification of the steel (e. bismuth additions should not be employed for components which will operate at high temperatures or require a postweld heat treatment. this facilitates ease of flux removal after welding. (i. This is due to the link between bismuth and reheat cracking susceptibility. Electroslag Welding Electroslag is frequently used for overlay welding in fabrication shops.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. Argon is typically used for this purpose. 308). This also makes the weld puddle more fluid and difficult to control. Only inert gases are used during GTA welding. Back-purging to protect the root of the weld is important when GTA welding stainless steels to control oxidation of the weld area. It may be used to weld any thickness of material. It has very low dilution and can be used for single or multi-pass overlays. Page 17 of 40 . Saudi Aramco does not approve the use of self-shielded (No shielding gas) FCAW welding for stainless steel. Testing and Installation a high quality weld. The thermal and electrical conductivity of austenitic stainless steel is lower than that of carbon steel. which is useful for thicker sections. Less welding heat is required to make a weld because the heat is not conducted away from a joint as rapidly as in carbon steel. Argon is primarily used for both shielding the arc and back-purging. 3 for self-shielding. This means that distortion is generally more of a problem with austenitic stainless steels and must be considered during welding.80% Ar in CO2 . It is not unusual for bismuth to be added to the flux of flux cored arc welds. This gas protection is known as a shielding gas and a purge gas.5) Pickling pastes or baths should be used at such strength and duration so as to remove fully the oxide and chromium depleted layer.05% (500 ppm). The chromium oxide has a very high melting point (2435°C). be de-mineralized water. It will interfere with the fusion of the welding process and cause an unacceptable weld profile. Areas that have been exposed to high temperatures but did not have. This chromium oxide can adversely affect the corrosion resistance of the weld and heat affected zone. along with the base material being depleted of chromium immediately under the tint. Rinsing solutions should. Testing and Installation Purging and Shielding Gas A chromium oxide layer forms when stainless steel is exposed to air and normally acts as a thin passive layer. The corrosion resistance of the component can only be restored by removing the oxide and chromium depleted layer. The SMA welding process does not require a shielding gas because it generates its own protective shielding from the flux during welding. Argon is normally used as the purging gas. Safety Caution Argon is heavier than air. Never enter a pipe if it has a purge running. This layer becomes thicker at higher temperatures. Argon is normally used as the shielding gas on the torch for the GTA welding process. This is much higher than the stainless steel base or weld material which is nominally 1450°C. For groove welds in pipe the back side of the weld must be protected by a purging gas. preferably. but not stain or unduly corrode the surface. A three-step process of grinding. Argon/Helium and helium gas can also be used but are not common. Purge dams must Page 18 of 40 . vessels or other areas where argon can accumulate. Though it is not toxic it can cause death by asphyxiation. gas shielding will form a 'heat tint‟. or had insufficient.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. This heat-tinted area will have comparatively poor corrosion resistance when compared with standard plate due to the heat tint oxide giving little protection to the base metal underneath. Always be aware of this when purging in tanks. For these reasons the weld and Heat Affected Zone (HAZ) must be protected from the air by an inert gas. pickling and rinsing is best. (section 10. For the purge to be effective it must reduce the oxygen content on the back-side of the weld to less than 0. The flow rate shown for this table is 50 CFH (22. wood.. then the inside of the first portion of root welded must be examined through the root gap to make sure that there is no excessive oxidation. If there is excessive purge flow it Page 19 of 40 . Testing and Installation be installed to contain the purge gas.e. It will not be possible to achieve a good purge if the purge dam is porous or leaks. This table is only applicable if there is no leakage of gas through the purge dam. balloons.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. See the sketch below.5 L/H). The purging volume should be kept as small as possible. It is critical that the purge dam is tight and can retain the purging gas. Water soluble paper must be installed with water soluble tape and water soluble glue. Water soluble dams should be the last choice for purging dam material.) There are commercially available purging devices which are recommended because they seal well and reduce the purging volume. cardboard. If this table is used. plastic. There are many types of purging dams (i. and water soluble paper. Once the purge has been established the flow rate should be reduced to 10CFH or just enough to maintain the purge level. Figure 3 – General Purge Dam Configuration Water soluble paper is porous and several layers of it must be glued together to make it gas tight. If a purge oxygen monitor is not available then the following table can be used as a guideline for minimum purging time. Preheat. The governing code. To limit sensitization and embrittlement. Use the above values for 300 mm for any shorter length. For this reason preheat is not used for these materials. Purge Times for Stainless Steel Pipe Diameter 2 and less 4 6 8 10 12 16 18 24 Purging time Minute 0.5 1 2 3 4 6 10 12 22 Minimum purging time based on pipe size and 6 volume changes of gas The above table assumes use of argon gas at a flow rate of 50 CFH (22. Interpass Temperature. Super-Austenitic. Duplex and Precipitation Hardening Stainless Austenitic.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. They can be heated to remove moisture prior to welding if required. Heat Input and Post Weld Heat Treatment (PWHT) The following section provides general information on the preheat. Austenitic. the maximum interpass temperature is restricted for these materials. Listed times are for each 300 mm of pipe length to be purged (multiply by actual length). For types 304L and 316L Page 20 of 40 . heat input and PWHT for stainless steel materials. duplex and precipitation hardening stainless steels are susceptible to sensitization and secondary phase embrittlement when exposed to temperatures in the range of 450°C to 850°C. interpass temperature.5 L/H). Saudi Aramco Standards and approved welding procedure must be followed. super-austenitic. Testing and Installation can buildup pressure on the inside of the pipe and cause the root to be defective or unweldable. The Interpass temperature is restricted to 315°C. The Interpass temperature is restricted to 315°C. For the duplex materials there is a heat input range that is maintained in order to obtain the “duplex” microstructure in the weld. This treatment involves heating the material to a temperature of approximately 1000°C to 1200°C. This range will be indicated on the welding procedure. This can reduce the corrosion resistance and lead to cracking.4. Contamination As discussed further in section 10. PWHT is not normally required for these materials unless the thickness exceeds 38 mm. Post weld heat treatment (PWHT) is not normally recommended for these materials because it can cause sensitization. 1. Filler material must always be added or the area should be ground. it is important to avoid contamination of the stainless steel with particles of carbon steel or elements which can adversely affect the corrosion resistance or cause Page 21 of 40 . the “L” grades and types 321 and 347 will not be sensitized by a normal PWHT. Martensitic Stainless Steels These materials will readily form martensite when air cooled. If heat treatment must be performed it is often specified as a Solution Anneal heat treatment. However. 2. The heat input for these materials must also be kept as low as possible.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. This should never be done. For all other stainless steels in this group the interpass temperature shall not exceed 100°C. After welding they should be slow cooled to 120°C (martensitic transformation temperature) and then tempered at 750°C. The preheat temperature range is from 250°C to 450°C. This relieves the stresses from forming and welding and is above the temperature at which sensitization occurs. Testing and Installation the interpass temperature shall not exceed 177°C. The material is then quickly cooled through the sensitization range. The heat input is not a serious concern for these materials. Sometimes welders will want to weld without filler consumable to smooth a rough weld profile. The heat input is not a serious concern for these materials. Ferritic Stainless Steels Preheat must be used for the ferritic stainless steels. GTA Welding without the addition of filler material is expressly prohibited for this material. This is because these materials can be hardened due to the fast cooling from the welding temperature. 250°C preheat should be used. e. lead and other metals may be picked up from fabrication tools and can lead to cracking. It also is generally regarded as detrimental to toughness in cryogenic service. For certain alloys it is not essential to have ferrite in the weld deposit but normally 3 – 7FN ferrite will prevent cracking. Other contamination can be from grease. Brushes and grinding disks that have been used on carbon steel must never be used on stainless steel. Ferrite may have a detrimental effect on corrosion resistance in some environments. all slag and surface oxide must be removed between passes. These elements are strongly linked to hot cracking due to the formation of low melting point alloys. Ferrite can be measured on a relative scale by means of various magnetic instruments. One of the greatest sources of carbon steel contamination is through grinding dust from the carbon steel. at temperatures exceeding 650°C).” Brushes and grinding wheels used on stainless steels should either be new or only previously used on similar stainless steels.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. Testing and Installation cracking. All grinding discs must be halogen. Cracking is more of a problem when the welds are restrained or the joints are large. sulfur and iron free as indicated by a statement “for stainless steel. If a multipass weld is being produced. it is essential to ensure that all of the components are clean. Before any welding operation. Ferrite In Weld Deposits One of the problems associated with welding of austenitic stainless steels is that of solidification cracking. Ferrite increases the weld strength level. Ferrite percentage can also be determined by Page 22 of 40 . zinc. contaminated lifting equipment and contaminated tools. Stainless steel must be fabricated in a separate shop from carbon steel. These can cause significant pickup of carbon and other elements which may affect the corrosion resistance. Stainless steel wire brushes must always be used. In high-temperature service delta ferrite can transform into the brittle sigma phase. This is a product of the solidification and transformation sequence experienced at elevated temperature. Copper. tin. This problem is often referred to as 'hot cracking' since it occurs before the weldment has cooled. The presence of ferrite in the weld reduces the susceptibility of the metal to hot cracking. This ferrite is referred to as “Delta ferrite” and can be observed in the as-deposited microstructure of austenitic stainless steels welds.. paint and dirt. This can be embedded into the stainless steel by walking on the stainless steel with contaminated shoes. Materials with a ferrite number greater than 10 will be susceptible to the formation of excessive quantities of sigma phase and should not be used for elevated temperature service (i. The WRC diagram should be used for ferrite predictions and gives a ferrite number. Testing and Installation metallographic examination. is currently performed with a magnetic measuring technique and is more commonly referred to in terms of percentage. It is based on the cooling rate and alloy interaction. Two rule-of-thumb conversions from FN to percentage are: 1. For higher percentages (duplex) 0.twi.uk/j32k/protected/toolkits/Ferrite/IntroInstructions. DeLong. For low percentages of ferrite (less than 10) the FN is the same as the percentage. SAES-W-014 gives ferrite requirements for overlays.co. TWI has weld simulation software online that can predict the percentage ferrite http://www.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. Ferrite can be expressed as a “percent ferrite” or a ferrite number (FN). Page 23 of 40 . These diagrams use nickel and chrome equivalence empirical relationships. These diagrams are known as the Schaeffler. however. 2. and the Welding Research Council (WRC).html. SAES-W-016 gives ferrite measuring requirements for welding procedure qualification and production and special requirements for duplex materials. The amount of ferrite in austenitic stainless steel welds can also be predicted from the chemical composition of the weld deposit using one of several constitution diagrams.70(FN) = % ferrite Measurement of ferrite content in DSS. The relationship between the percent ferrite and ferrite number is complex. Less frequently are GTAW and GMAW and FCAW welding processes. Dilution of the weld pool with molten base material may be a problem during cladding since the underlying materials may contain elements detrimental to the performance of the stainless steel (e. SAW is ideally suited to the task since relatively large areas may be deposited with high deposition rates. dilution could be minimized but this may be difficult since it is very sensitive to arc length and voltage. This layered component can provide corrosion resistance and/or wear resistance without the expense of producing it entirely from a higher alloy material. There is a variety of different equipment available. Alternatively. Testing and Installation Figure 4 – WRC Ferrite Number Weld Overlay Cladding Weld overlay cladding is the process of depositing a layer (or layers) of austenitic stainless steel (or other corrosion or wear resistant material) onto the surface of a base material (such as mild steel).. employing either strip or between one and six wires. Electroslag welding is often frequently used. Submerged arc welding is by far the most common method of cladding with austenitic stainless steel. In these instances. multiple layers can be deposited so that dilution with the parent metal is reduced in each progressive pass. carbon).Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. Multiple layer overlays are preferred and single layer overlays require special approval. Joining of Clad Materials There are two techniques for joining of carbon steel which is clad with Page 24 of 40 .g. 84). The solution is swabbed on the part. and left to stand for 6 minutes minimum. 1 ml sulfuric acid H2SO4 (specific gravity 1. The “stripped back” area shall be checked with a copper sulfate solution to verify that all of the stainless steel has been removed. Figure 5 – Single-sided Cladding 2. (Reference ASTM A967) The carbon steel shall be completely welded and NDE inspected prior to welding the cladding portion. Single-sided When welding clad materials from one side (for example a small diameter pipe joint) the entire weld will be made with a consumable that is compatible with the dissimilar metal welding of the base material and the CRA.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. The stainless steel must be “stripped back” by grinding or machining to no closer than 10mm from the edge of the carbon steel weld. the part should be carefully rinsed and dried. then free iron is on the surface of part. If copper deposit is observed. Testing and Installation stainless steel: Single-sided and double-sided: 1. Page 25 of 40 . Double-sided The entire weld is made from both sides (See Saudi Aramco Drawing AB-036367). 250 ml of distilled water. Copper sulfate solution preparation: 4 gm copper sulfate pentahydrate CuSO4.5H20 (use reagent grade). Figure 7 – Duplex Microstructure. Never weld stainless steel electrode into nickel base material. Duplex stainless steels (DSS) contain 35 to 65% ferrite and the remainder is austenite. good resistance to chloride stress corrosion cracking and relatively good notch toughness. magnification approximately 200X Page 26 of 40 . good corrosion resistance. Testing and Installation Figure 6 – Double-sided Cladding Caution: Never weld carbon steel or low-alloy steel into high alloy. The percentage of ferrite is also called the phase balance.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. These materials are characterized by high strength. stainless steel or nickel base material. Welding of Duplex and Superduplex Duplex stainless steels (DSS) are broadly divided into standard and superduplex stainless steels. Duplex stainless steels that are currently produced tend to have slightly more austenite that ferrite. The table below gives an approximate comparison between the chemistry of the Duplex and the Superduplex. The heat input is based on the following formula. Testing and Installation Table 3 – Typical Chemistry of Duplex and Superduplex Stainless Steels Cr (%) Ni (%) N (%) Mo (%) Fe Standard Duplex Chemistry Superduplex Chemistry 22 25 5 7 0. whereas welding at higher heat input levels promotes lower ferrite levels. Page 27 of 40 .5 Balance Balance Besides the phase balance.5 KJ/mm but the heat input restrictions on the welding procedure must be followed.2 3.25 3.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. The maximum interpass temperature for duplex stainless steels is 100°C per the Saudi Aramco specification SAES-W-016. Both the upper and lower ends of the heat input range must be established on the welding procedures in order to achieve the proper phase balance. the range on heat input for duplex stainless steel is 0. there is a second major concern with duplex stainless steels. These phases can be eliminated and the original properties restored if the material receives a solution anneal heat treatment at 1040°C and is rapidly cooled through the critical temperature range. or when welding heavy sections under restraint. These phases can significantly reduce the toughness and corrosion resistance of these materials. HeatInput  amps * volts * 60 TravelSpeed Preheating is not recommended with duplex stainless steels except to dry the surface or when the temperature is below 5°C [40°F]. Intermetallic phases (Sigma and chi) form at the temperature range of 540 to 950°C. The characteristics of welding duplex stainless steel are very similar to welding 300 series stainless steels. The low interpass temperature reduces the amount of sigma formation in the HAZ. The same welding processes. Therefore. it is critical that sufficient nitrogen be present in these alloys. Welding at lower heat inputs promotes higher ferrite levels.12 0. There is an increased emphasis on heat input control. Some of the differences are noted below. joint details and techniques are generally followed. The addition of nitrogen significantly delays formation of these phases.5 to 2. Generally. air or steam) or external attachments. there is no concern regarding obtaining a balance of austenite and ferrite. Consideration must be given to the possibility of galvanic corrosion in this type of joint. Nickel alloy filler metals have also been used to weld both the standard and super DSSs. It is very important to keep electrodes for dissimilar metal welds in a low hydrogen condition. Since the nickel alloy welds are fully austenitic.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. such as E/ERNiCrMo-3. It is also advisable to consult the alloy producer of proprietary alloys for their filler metal recommendations and filler metal availability. type 309) may only be used for dissimilar metal welds in non-hydrocarbon. This is frequently stainless steel welded to carbon steel or a low alloy steel. Dissimilar metal welds are not permitted in sour service exposure. One such nickel alloy filler metal for which data is available is AWS A5. For this reason all welding electrodes must be handled to insure that they are low hydrogen. This material meets the strength. Suggested filler metals are shown in Table 2. Testing and Installation Autogenous welding (welding without filler material) should not be performed on duplex stainless steels. All surfaces to be welded must be free from contaminants that could cause hydrogen pickup. Austenitic stainless steel (i.14 ERNiCrMo-3. Failure to do this can result in hydrogen cracking on the non-austenitic side of the joint. corrosion resistance and impact resistance of the base materials. Because of the ferrite present in the duplex stainless steels there is a possibility of delayed hydrogen cracking. DSS welds tend to be higher in ferrite than the base metal being welded. Dissimilar Metal Joints Dissimilar metal weld joints are welds between stainless steel and another type of alloy. water.e.e.. Sensitization Sensitization occurs in the HAZ adjacent to the weldment and may be minimized by using either low carbon or stabilized grades of stainless Page 28 of 40 . Dissimilar metal welds between DSS and austenitic stainless steels such as Type 304 or 316 or for welding to carbon steel should be made with ENiCrMo-3 or ERNiCrMo-3. There are near matching composition proprietary filler metals for many of the standard and super DSSs. Dissimilar metal welds in hydrocarbon service must be made with a nickel base consumable. non-sour service (i. If a steel has been sensitized. Smaller microfissures are often invisible to the Page 29 of 40 . independent of any precipitation. The classic sigma-phase is nominally FeCr composition. meaning that thick plate can be successfully multi-pass welded. corrosion resistance may be recovered only by the use of a full anneal (and suitably rapid quench) to dissolve the carbides. This may cause alloy embrittlement during long term use. The presence of such phase has proven to be highly sensitive to alloy processing parameters such as the cooling rate after a final heat treatment. High molybdenum high strength stainless steels can contain the Chi phase (Fe36Cr12Mo10). may occur where micro-segregation and coring within dendrites is particularly severe. If these precipitates form a continuous network there can be a corresponding reduction in ductility. This is the reason that high molybdenum grades must be welded with nickel base fillers. The precipitation of any secondary. Testing and Installation steel. This can often be the case if a matching filler. which normally occurs at grain boundaries. Sensitization gives rise to corrosion in the HAZ and is a prerequisite for polythionic acid stress corrosion cracking. Recent alloy developments have included the addition of significant amounts of nitrogen to high alloy stainless steels. Microsegregation Pitting of the weldment.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. depletes the chromium content leading to intergranular corrosion. Crevice Corrosion Crevice corrosion is another frequent cause of failure in weldments. autogenous weld or high molybdenum stainless steel (4-6% Mo) is used or if there are large surface-lying dendrites. but it can have a more complex. chromium-rich phase will deplete the local area of chromium. Possibly the most frequent cause of crevice corrosion are solidification cracks and microfissures. this acts to retard the nucleation of both and phases. variable composition. The presence of this phase. Low carbon grades or grades with Ti or Nb stabilization can prevent or reduce sensitization. typically in the region of 900°C. Both and phases can be easily formed by the decomposition of ferrite. Embrittling Phases Another phenomenon similar to sensitization is the precipitation of Chi and Sigma intermetallic phases on the grain boundaries in the HAZ. at 540-950°C for phase and 650-950°C for phase. toughness and corrosion resistance. Some degree of corrosion resistance may be recovered by a stabilization anneal. pores.2. welding should only be carried out using proper fume extraction. and weld start/stop cracks and craters. 10. for example.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. solidification cracks may be avoided by proper control of weldment delta ferrite content. Failure to do this may result in crevices being formed between the component and the backing ring. Particular care should be taken to minimize exposure to weld fumes. poorly adherent weld spatter may also create crevices. Any welding defect that causes a crevice or confined feature can also lead to crevice corrosion. Chromium and nickel are both known carcinogens and typically have maximum exposure limits as specified in the MSSG. if essential. Sensitization may be as minor as mild irritation. Austenitic stainless steels also generally contain nickel and this may lead to sensitization of the skin. or as major as gross swelling. should be fully consumed during welding. Testing and Installation naked eye and therefore microfissure crevice corrosion is often mistaken for simple weldment pitting. therefore.2 Fume and Welding Safety Normal safety considerations should be taken when welding austenitic stainless steels. Bad joint design and/or poor welding practice can also cause crevice corrosion. Microfissuring and crevice corrosion in high molybdenum alloys (4-6%) is best avoided by the use of nickel base fillers. As previously stated. Preheat and PWHT Austenitic Stainless Steels Parent Material Type 304 and 304L 304H 316 and 316L 317L 310 321 330 Filler Material SMAW Electrode E304L E304H E316L E317L E310 E347 E330 GTAW Bare Wire ER304L ER304H ER316L ER317L ER310L ER347 ER330 Preheat 20° and dry 20° and dry 20° and dry 20° and dry 20° and dry 20° and dry 20° and dry PWHT none none none none none none none ENiCrFe-3 and ERNiCr-3 can also be used Page 30 of 40 Comments . It should be noted that MIG and TIG welding can also give rise to ozone. Moreover. Table 2 – SMAW And GTAW Consumable Selection. Fumes from the welding of stainless steels may also contain significant quantities of nickel and hexavalent and trivalent chromium. this also necessitates suitable ventilation. non-removable backing rings are to be avoided or. Such defects include flux layers or inclusions. Chromium is toxic by inhalation. SAW Bare Wire ER410 ER430 Preheat 250°C 250°C PWHT required required Comments Ferritic Stainless Steels Parent Material Type 405 410S Filler Material SMAW Electrode E309 E309 GTAW Bare Wire ER309 ER309 Preheat 250°C 250°C PWHT Not normally required Not normally required Comments Superaustenitic. Testing and Installation Austenitic Stainless Steels Parent Material Type 347 Filler Material SMAW Electrode E347 GTAW Bare Wire ER347 Preheat 20° and dry PWHT none Comments ENiCrFe-3 and ERNiCr-3 can also be used Martensitic Stainless Steels Filler Material Parent Material Type 410 430 SMAW Electrode E410 E430 MIG.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. TIG. Precipitation Hardening Parent Material Type 904L 254SMO Alloy 20 17-4 PH Filler Material SMAW Electrode E385 ENiCrMo-3 ENiCrMo-3 ENiCrMo-3 GTAW Bare Wire ER385 ERNiCrMo-3 ERNiCrMo-3 ERNiCrMo-3 Preheat 20° and dry 20° and dry 20° and dry 20° and dry PWHT none none none none Comments E/ER NiCrMo-3 may be used Duplex and Super Duplex Parent Material Type S31803 S32205 J92205 S32550 S32750 S32760 CD-4MCu (cast) Filler Material SMAW Electrode E2209 GTAW Bare Wire ER2209 Preheat 20° and dry PWHT none Comments E2595 ER2594 20° and dry none E/ERNiCrMo-3 may be used Page 31 of 40 . Plates and sheets should be stored vertically in racks and not be dragged out of the racks or over one another.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. chains and wire ropes shall be kept from coming into contact with stainless steel equipment”. x200. dry and well ventilated. NiDI publication 11003 highlights that: “experience has shown that (surface) cleanliness and weld quality are far more critical to successful performance than Cl-ion concentration”. Absorbent wrappings such as interleaving paper. Figure 8 – Microcracked Surface at Heat Tint Oxide Films on 316L Stainless Steel. provided that they have not previously been used on non-stainless steels. 10. NZSSDA (Code of Practice for the Fabrication of Stainless Steel Plant & Equipment. Outdoor storage of stainless steels adjacent to carbon steels should be avoided (Figure 9). 2001) stresses that the fabricator must ensure that the fabrication is clearly identified and protected from damage and contamination.3 Storage Storage areas must be clean. Cleaners which can be used include stainless steel wool and stainless steel wire brushes. Plastic-coated or SS weld overlayed parts should be used for handling SS. Testing and Installation 10. The shop should have a separate area where only stainless steel is fabricated. This will prevent the cross-contamination of the stainless steel surface with iron particles. Some suggested methods for the removal of surface contamination and defects are presented in Table 1. Racks should be protected to prevent iron contamination. cardboard and timber should be kept dry to prevent surface staining. Page 32 of 40 . “Mild steel lifting forks. hooks.4 Shop Fabrication Figure 9 – Unsuitable Materials Separation and Outdoor Storage The number one problem with unsuccessful fabrication of stainless steel is surface contamination. Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. Passivation and Iron Removal Chemical passivation is normally only used to remove “free iron”. The parts are picked by immersion in a bath and thoroughly rinsed and blown air dried afterwards. high temperature oxides and other surface contamination arising from processing and handling. Figure 10 – Poor Shop Material Segregation 10. Page 33 of 40 .5 Figure 11 – Iron Contamination Caused by Operator Grinding Carbon Steel Nearby Pickling. Figure 11 shows an example rust spotting caused by iron contamination. clean surface which can then be passivated  Air passivate or be chemically passivated Pickling is normally performed during manufacturing processes. where carbon and stainless steels are being processed together. Pickling treatments provided in ASTM A380 include the following:  Typical austenitic stainless steel pickle solution: 10% HNO3 + 2% HF at 50ºC  Removes oxide film plus 25 – 40 μm of surface  Effectively removes welding heat tint  Removes embedded iron particles  Can improve corrosion resistance of ground. Testing and Installation Figure 10 shows an unacceptable example of poor material segregation. wire brushed and blasted surfaces by removing surface contamination and exposed impurities in the metal. such as sulphides  Exposes a new. such as following solution heat treatment of welded stainless pipe. ceramic (glass) beads. Testing and Installation Pickling can also be performed after shop or site welding operations.2.5. garnet or walnut shells. Pickling with nitric-hydrofluoric acid removes free iron and a thin surface layer of metal that may contain surface defects. Acceptable methods include the use of medium to fine-grit abrasives such as clean flapper wheels.. the surface should show no evidence of rust stains or other corrosion products. Alternative Page 34 of 40 . is also normally used during manufacturing after pickling treatments. etc. After completion of this test. SAES-W-014 recommends that only stainless steel brushes. The “Rust Bloom” water wetting and drying procedure described in ASTM A380 para 7. The metal surface is then passive and in the most corrosion resistant state. flexible disks or blasting with clean abrasives such as glass beads. must be coated with suitable materials such as plastic. The wet-dry cycles should be such that the sample remains dry for a total of 8 h in a 24-h test period. Free iron and heat tint can also be removed by a hand-held electropolishing probe.. 10.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. rubber or weld overlayed with SS. The procedure calls for wetting the surface with distilled or deionized water or fresh water followed by drying. potable tap water. A solution of nitric acid and potassium ferricyanide is sprayed onto the surface and free iron contamination is disclosed by the development of a dark blue color within 30 s. ASTM A967 Practice E (Potassium Ferricyanide–Nitric Acid Test) provides further details of this type of test. Free Iron Contamination is detected using a ferroxyl test. using nitric acid treatments. using proprietary pickle pastes or swabbing followed by thorough rinsing in clean water. forklift truck forks and handling tools. the free iron can be removed mechanically. Storage racks. Formation of rust stains may be accelerated by periodically wetting the surface with preferably distilled or deionized water or clean. If a nitric-hydrofluoric acid pickle is not practical. The solution should be removed after a few minutes with a damp cloth or water spray.1 is an effective test to check for the removal of free iron.6 Field Fabrication Shop fabrication is normally better controlled and potentially much more serious field fabrication errors could be overlooked. Chemical passivation. Some contractors apparently lack knowledge and/or experience of appropriate handling techniques for stainless and other CRAs. or stainless steel grit shall be used to mechanically clean the weld overlay surfaces. etc. iron-free grit. which is recommended for application only on 200 and 300 series stainless steels. fresh. Page 35 of 40 . not covering the storage facilities so the stainless steel does not rest on carbon steel and other items. Table 4 – Suggested Removal Methods for Various Surface Defects and Contamination The following photographs (Figures 12 to 15) show improper handling of SS and are from an actual jobsite. Carbon steel strapping must not be allowed to come into contact with the stainless steel surfaces. Conveyor tables should be designed and operated to avoid damage and contamination. using carbon steel lifting brackets to place shell plates. Contractors seemed totally unaware of appropriate techniques for good fabrication practice for stainless steels.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. wooden bearers should be inserted between the carbon steel strapping and the stainless steel surfaces. Examples included: contractors off loading stainless steel pipe with carbon steel hooks. If used. using wire brushes and grinding wheels that were first used on carbon steel. Packaging materials and methods used must help prevent surface damage. Testing and Installation lifting equipment materials to those used in carbon steel fabrications shops are also recommended. Where hardened ferrous tools (such as roll. they must be completely cleaned (solvent/steam cleaned). Fabric or rope slings should be used rather than steel chains. presses and angle rolls) must be used. using carbon steel wedges to support stainless steel during tank fabrication. Heavy paper sheets are sometimes used to prevent direct contact between tool and stainless steel. Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. The key actions are summarized:     Hydrotesting should be carried out with clean treated water Do not use untreated raw water. check that there are no areas of ponding (stagnant water) – if necessary. wipe / mop to dry Figure 12 – Carbon Steel Wedges on Stainless Steel Tank Bottom Figure 13 – Carbon Steel Fitting Plates Used to Fit Stainless Steel Shell Plates Page 36 of 40 . or be biocide treated. or contaminated recycled water Drain water from inside plant promptly after hydrotesting After draining hydrotest water. Hydrotest water should also be verified as low chloride and low SRB bacteria count. Testing and Installation 11 Hydrostatic Testing SAES-A-007 paragraph 6 limits the chloride content of hydrotest water to 50 ppm to minimize the risk of chloride pitting or SCC during startup. seawater. For example. to verify chloride level on-site (Figure 16). use Kitagawa or Draeger tubes or similar field techniques. Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. The morphology of MIC failed components is usually consistent with localized corrosion and pitting at or adjacent to austenitic stainless welds (Figures 17. MIC can be avoided as follows:  Ideally. 18). as these favor bacterial colonization Use only treated water for hydrotesting and for cooling water: microorganisms cannot live in treated (chlorinated) water Introduce a biocide such as chlorine into the water system Page 37 of 40    . including duplex stainless grades. Testing and Installation Figure 14 – Carbon Steel “Key Nut” on Stainless Tank Figure 15 – Carbon steel Fixturing (Wedges and Key Shims) on Stainless Tank Figure 16 – Kitagawa Chloride Test Tube # 201SB 12 Microbiologically Influenced Corrosion (MIC) MIC is a fairly common problem with stainless steels. weld spatter should definitely be removed or controlled (Table 4). Failures can occur rapidly in microbially contaminated waters containing relatively low chlorides (<100 ppm). Remove heat tints. Though this may be impractical in many engineering applications. ensure all internal welds are polished smooth – bacteria can form colonies on rough weld surfaces. According to the ASSDA. These failures can occur too rapidly (in terms of weeks to months) to be attributed to conventional chloride crevice corrosion. filled with stagnant water Note frequently there is confusion between the “chloride” that causes stress corrosion cracking and the “chlorine” found in the biocides used to treat water. These forms of “chlorine” are sometimes referred to as “free chlorine. MgCl2. Regarding draining and drying. pipes and vessels. Insulation is normally encased in aluminum sheet wrapping. CUI damage can potentially occur as SCC. showing preferential corrosion of welds by MIC. Causes of CUI-related failures can Page 38 of 40 . or circulate water for about one hour daily if the water cannot be drained. or other salts. For SSs. Figure 18 – Microsection through a SS tank weld showing undercut pitting due to MIC in weld filler metal. pitting and crevice corrosion. The “chlorine” that is present in water from some of the biocides is HOCl and OCl-.” They do not break down or change into the harmful “chloride” ion that causes stress corrosion cracking. for example. stainless steels operating at low and high temperatures ranges are insulated and normally the SS is coated. should not be overlooked and must be drained fully.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. such as tanks. 316L plate and filler metal were damaged in 6 weeks due to MIC in raw water Remedial measures include: drain promptly and completely after hydrostatic testing.1 Coating Under Insulation To mitigate CUI. “Chloride” is the ion of Chlorine (Cl-) that is present in water from dissolving NaCl. Figure 17 – Stainless steel water tank after 8 months. Testing and Installation  Do not leave stainless equipment. valves. where heat tints were not removed. 13 Coating Stainless Steel 13. protective coating shall be used. chloride contamination of insulation and damaged or non-weatherproof casings.Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. Page 39 of 40 . External coating selection shall be reviewed and approved by the CSD Coatings Team. to ensure an optimum performance and reliable installation using stainless steels. 14 Summary The flowchart (Figure 19) summarizes five principal process steps for the specification. Select halide free coating materials which need minimum surface preparation either by solvent cleaning or by abrasive blasting using aluminum oxide abrasive without causing damage to the passivation layer. It is also very important to specify and install low leachable chloride insulation materials for SSs (conforming to ASTM C871). fabrication. Testing and Installation include moisture entrapment under insulation. stainless steel pipes are susceptible chloride stress corrosion cracking due to exposure to soil chloride contamination and soil stress. NACE RP0198 (The Control of Corrosion Under Thermal Insulation and Fireproofing Materials . fabrication. In addition. hyrotesting and protection of stainless steels.2 Coating above Grade Piping Exposed to Atmospheric Conditions Exposure to chloride contamination due to washing down with saline or raw water and proximity to seawater breeze or splash can cause significant damage to plant piping and equipment made of stainless steel. The controls should be used as a checklist to document the use of correct materials and proper storage. The controls associated with each step should be carefully followed and documented where required.3 Coating Buried Pipes All buried stainless steel pipes shall be coated to prevent adverse interference with the cathodic protection system. and hence. hydrotesting and insulation and external coating (where appropriate). 13. To prevent chloride induced stress corrosion cracking caused. halide free external coating shall be applied on stainless steel pipes after appropriate surface preparation with solvent cleaning and/or abrasive blasting using aluminum oxide abrasive.A Systems Approach) provides practical guidelines for managing CUI. 13. Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication. fabrication. Testing and Installation Stainless Steel Selection and Specification  Consider service. Page 40 of 40 . tools and equipment  Welding procedure control – PMI of consumables & fabricated parts  Verify hydrotest water chloride (≤ 50 ppm) and low SRBs (SAES-A-007)  Completely drain and dry water after test  Don’t leave trapped stagnant water Fabrication and Assembly – Shop and Field Hydrotest Procedure and Documentation Protection Insulation and External Coating Process Step  Verify low leachable chloride insulation  External coating holiday check  Water tightness of sheathing Control Figure 19 – Flowchart summarizing principal process steps and associated controls in the specification. hydrotesting and protection of stainless steels 25 August 2008 Revision Summary New Saudi Aramco Best Practice. PREN  Review heat treatment of wrought and cast parts  Perform PMI Storage and Segregation  Store SSs separately in dry conditions  Avoid free iron contamination  Use non-chlorinated solvents and chloride-free marking inks  Control or remove welding heat tints  Check for and remove any iron contamination  Check appropriate handling.