Guidance Note: The design, specification and fabrication of structuralsteelwork that is to be galvanized. 1. Scope This document gives guidance on issues that should be considered when designing, specifying and fabricating structural steelwork that is to be hot dip galvanized. This guidance is particularly relevant to higher grade steels (such as grade S355 and above), and it includes advice on detailing, fabrication and the galvanizing process, to minimise any risks of steel cracking. It augments the general guidance already available from Galvanizers Association. 2. Introduction Cracking of steel happens from time to time, whatever the method of production, fabrication, corrosion protection and use. In galvanizing experience (dependent upon the manufacturing route of the material and its fabrication history) there are four ways in which steel being processed might crack: • Relief of very high residual stresses in the form of distortion cracking • Hydrogen cracking • Strain age embrittlement and • Liquid metal assisted cracking (LMAC) All of these mechanisms are rare, the first three being fairly well understood, and guidance from Galvanizers Association exists, allowing satisfactory control of these mechanisms. The interaction of the factors giving rise to LMAC are presently less well understood - they are complex. Its potential to affect work is limited to an extremely narrow sector of the galvanizing market, but it can apply to the structural steelwork sector. The paragraphs below describe, in fuller detail, these four potential mechanisms. 3. Potential mechanisms for steel cracking a) Distortion cracking If steel fabrications distort during galvanizing, this is usually due to release of in-built stresses as the steel is heated to the galvanizing temperature. Stresses may be inherent in the steel but can also be introduced by welding, cold forming, hole punching, and 1 holes that are present for other purposes may fulfil the requirements for venting and draining. 2 . General guidance on size for venting holes exists in the ‘Engineers & Architects Guide to Hot Dip Galvanizing’. venting and drainage will assist the galvanizer in minimizing the differential thermal stresses experienced by the fabrication when being dipped. In some cases it may be more economical to provide V or U shaped notches in the ends. therefore. due to variation in section sizes [and. • Holes for venting and draining should be diagonally opposite to one another at the high point and low point of the fabrication as it is suspended for galvanizing. a bath of molten zinc at about 450°C.these procedures provide ideal locations for venting and draining. again diagonally opposite one another. Thus any features that aid the access and drainage of molten zinc will improve the quality of the coating and reduce costs. differential temperatures] within the fabricated component. however. However. their application to structural steelwork is limited. Making adequate allowance in the design for filling. allowing the zinc to coat all surfaces and increase protection. holes should be provided. Efforts can be made at the design stage and elsewhere to minimize residual stresses. reducing the potential for un-vented voids. There may also be stresses associated with dipping. molten zinc must be able to flow freely to all surfaces of a fabrication. for example: • Avoid the use of thin plate with stiffeners • Arrange the weld seams symmetrically • Minimise the size of weld seams • Avoid large differences in structural cross-section that might increase differential thermal stresses during galvanizing • Consider the use of intermittent welds Intermittent welds have many advantages on galvanized structures. in other cases it may be necessary to provide extra holes for this purpose. For complete corrosion protection. With hollow sections or where there are internal compartments. • With hollow sections sealed at the ends. Best practice in “good design for galvanizing” requires: • Means for the access and drainage of molten zinc • Means for escape of gases from internal compartments (venting) It is important to bear in mind that the steelwork is immersed into. General principles for best practice in design for galvanizing and minimizing the potential for distortion cracking are: • Holes for venting and draining should be as large as possible.flattening. the galvanizing of the internal surfaces eliminates any danger of hidden corrosion during service. or to grind corners off rectangular hollow sections . With certain fabrications. as near as possible to the ends. and withdrawn from. reducing distortion. clients are recommended to discuss specific venting arrangements with the galvanizer who will be carrying out the work. particularly on hollow fabrications. off centre and as near as possible to the wall of the member to which the end plate is connected. tapered aluminium or plastic plugs are available and will prevent undesirable ingress of water Where there is an inherent tendency to distort.. Hydrogen embrittlement of steel occurs when atomic hydrogen diffuses into the steel lattice and affects the steel’s mechanical properties. provide useful informative annexes. Essentially however.g. as can the size and position of lifting holes or lugs. these annexes guide fabricators in choosing the most appropriate welding techniques to avoid hydrogen cracking [Annex C] whilst taking account of the welding procedures on the mechanical properties of heat affected zones [Annex D]. If required. Detailed design advice is available from Galvanizers Association or directly from the galvanizer. BS EN 1011 : Part 2 : 2001. In particular. 3 . it is considered that atomic hydrogen reacts in a number of ways in the steel. In the presence of a tensile stress – either residual or applied – and depending on the number of dislocations and the amount of available atomic hydrogen. revealing no measurable loss in strength due to the processing and indeed in almost all cases there was a slight increase in yield strength. Holes that have been drilled for venting can be plugged. the dominant feature contributing to hydrogen embrittlement is usually the welding process implemented as part of the fabrication of the structure. This condition is known as hydrogen embrittlement and is characterised by a loss in ductility of the steel. Internal and external stiffeners. should have the corners cropped to aid the flow of molten zinc. because a galvanized coating covers all surfaces. The galvanizer should be consulted for advice at an early stage if this is being considered. At the galvanizing temperature – usually around 450°C – the steel being processed will lose approximately 50% of its room temperature yield strength. but this can be overcome by using the correct weld procedures and the correct choice. the effect can be minimised or possibly eliminated by restricting the fabrication to such a size and design that it can be rapidly immersed in a single dip. in asymmetrically shaped fabrications. including recombination to form molecular or gaseous hydrogen that diffuses less easily than the atomic hydrogen. storage and use of the welding consumables. e.• • Where holes are provided in end plates or capping pieces. Much research has been carried out on the effects of galvanizing process on steel strength. e. but this is mainly necessary for aesthetic reasons. which is effectively trapped in the region of the dislocations. b) Hydrogen embrittlement Galvanizers use inhibited acid to reduce any generation of hydrogen during the chemical cleaning stages of the pre-treatment for galvanizing however. an internal gas pressure can be induced locally that may become so high that plastic deformation of the microstructure or even initial cracks may occur. gussets etc.g. There are alternative ways of interpreting the actual mechanism for hydrogen embrittlement. diaphragms. Latest standards for welding of steel articles. baffles. The size and position of filling and drainage holes in fabricated vessels can have a major effect on distortion. they should be placed diagonally opposite one another. regaining it on cooling after galvanizing. the regions of lattice defects.Prior to supply stage of the galvanized steelwork. It will only occur while the article is in the galvanizing bath. The steel is not in an embrittled state after removal from the galvanizing bath. Furthermore. This type of cracking is not a concern in structural steelwork as the use of heavily cold worked. is not usual. fabrications are more prone to cracking • Cracking originates from surface stress concentrators such as discontinuities in the surface of the steel. These elements include aluminium. during the past five years at least three laboratory research programmes in different countries. Once absorbed. Cold working will include operations such as bending (to form tube or hollow section for instance). areas of greatest surface hardness and/or highest stress 4 .g. which were set up to investigate the phenomenon. which are always present in the microstructure. punching of holes and shearing. atomic hydrogen diffuses to. The extent of embrittlement depends upon the amount of strain. thin section material. galvanizing at 450°C). Steel fabrications that have been subject to heavy cold work may be susceptible to strain age embrittlement and should be stress relieved prior to galvanizing. or dislocations. failed to reproduce it under laboratory conditions. thereby reducing the potential for an incidence of LMAC of steel during hot dip galvanizing. It is known that: • More complex. titanium niobium and boron. atomic hydrogen may be formed as a result of the: • Steel manufacturing process • Welding and fabrication process • Coating preparation process. or highly alloyed steels. highly restrained. time at ageing temperature (e. Elements that are known to tie up nitrogen in the form of nitrides are useful in limiting the effects of strain ageing. this type of cracking is unlikely to occur but in the few cases where this type of cracking has been evidenced. There are methods by which the risk of cracking can be minimised. and steel composition – in particular nitrogen content. c) Strain age embrittlement Strain age embrittlement of steel is caused by highly cold working the material followed by ageing at temperatures less than 600°C or by warm-working steels at temperatures less than 600°C. There are few explanations of exactly how and why LMAC occurs. All structural steels may become embrittled to some extent. Where best practice fabrication guidelines have been followed. the cracks are usually confined to the corners or edges of the heavily cold worked material extending along the edge from the open end of the fabrication. and accumulates in. d) Liquid metal assisted cracking (LMAC) This rare form of cracking will manifest itself only during the hot dip galvanizing process. vanadium. LMAC will not occur. to critical or sensitive areas of the fabrication (notified to the galvanizer) that might be subject to higher levels of post-galvanizing inspection. Any tendency to LMAC is removed once the article is removed from the galvanizing bath.g. finishing (at steel mill). or steel from an unknown supplier • Where choice of steel stock exists. The guidelines set out below are aimed at controlling one or more of the factors that can act as contributors to this mechanism. articles affected by this mechanism can be repaired satisfactorily. pre-treatment and galvanizing. molten zinc during the galvanizing operation Unless all four factors are present. Reference should also be made to the National Structural Steelwork Specification (NSSS) handbook in this regard (available from BCSA whose contact details are at the end of this guidance document). hardened surfaces. Where choice of steels is possible and/or flexibility of design allows: • Use steel which conforms to the required specification and is not downgraded (from a higher grade) material. Usually. only steels of Grade S355 and higher would be considered susceptible From this it can be deduced that four factors must come together in order for LMAC to occur: • The presence of a susceptible material substrate • The presence of a surface crack initiation or stress concentration site • High levels of residual stress in the fabrication. Specification & Design • Follow the design guidance contained within BS EN ISO 14713 : 1999 & Galvanizers Association’s publication ‘Engineers & Architects’ Guide to Hot Dip Galvanizing’ • Maximise provision for good venting and drainage • Make provision for handling of the article during the galvanizing process • Consideration should be given in the agreed quality plan. the guidelines relate to removing surface discontinuities (stress concentration or crack initiation sites). reducing the presence of hardened surfaces and minimising differential thermal expansion (and therefore any stresses associated with this) during the galvanizing process. In particular. developed for instance through rolling.• Normally. welding. Galvanized structures which have been designed. • Use open ended sections in preference to closed sections 5 . fabricated and galvanized to the best practice set out below will have an extremely low susceptibility to this type of cracking. • The presence of a molten metal e. movement of the fabricated elements. reducing stress in the fabricated articles. the material with the lowest carbon equivalents should be used in the fabrication in order to improve weldability and reduce the potential for the development of hardened steel surfaces after fabrication. g. supply standard. for example: o Ensuring that the steel conforms to the required supply specification o Observing best practice in the welding procedures and testing • Adopt sound detailing practices. commensurate with achieving the required thickness and quality of the coating and balancing off requirements for minimization of potential for distortion. 6 . for example: o Consideration should be given to dressing of critical welds o 'Stress raisers' should be avoided. • The steelwork should be galvanized in accordance with BS EN ISO 1461:1999 or any specific customer requirements • Galvanizing procedures should be in accordance with Galvanizers Association’s publication ‘General Galvanizing Practice’ • Steelwork should be pickled in inhibited acid for the minimum time required to clean the steelwork • Thermal stresses should be minimised during the galvanizing process. differential thermal stresses during galvanizing should be minimised by taking these into account when considering individual component and fabricated component form. the material suppliers should be informed that protective treatment is to be achieved by hot-dip galvanizing • The galvanizer should be provided with information as to the steel used within the fabrication for example. jagged edges and sharp notches should be avoided • Hardened surfaces should be avoided wherever possible • Where possible. undercutting. e. it is prudent that the galvanizer ensures that appropriate visual inspection for suitability of the steelwork for galvanizing is carried out prior to the galvanizing operation (by galvanizers’ personnel) and where the results of this review give cause for concern. size and connections • At material ordering stage. nevertheless.Fabrication • Ensure that the provisions of the National Structural Steelwork Specification relating to materials and fabrication are met (unless varied by other contractual requirements). This will embrace. profiles should be cut to maximum reasonable radii. grade of material and mechanical properties/chemistry Galvanizing • Assuming all the above has been carried out. This may be achieved by using one or more techniques below: o Processing the work through a heated pre-flux solution o Heating in a drying oven o Holding the article over the galvanizing bath prior to dipping • The article should be dipped with a slow immersion rate which is. the details noted and are then referred back to the customer. or to agreed criteria set out in the quality plan.g. Remediation If cracking occurs. The level of inspection should be related to the criticality of the structure. These might include for example. • Identification of the element • Indication of the specific areas inspected.uk • British Constructional Steelwork Association (BCSA) – Tel: (0207) 839 8566 • Corus – Tel: (01709) 820 166 7 . with appropriate post-weld testing.org. The agreed inspection regime will include details of which records should be retained for future reference. When galvanized coating repairs are required. Reference should be made to the specification for the job. EN ISO 1461 : 1999. work should be inspected for the presence of cracks. it can normally be repaired by gouging and re-welding to an approved procedure. and should be familiar with the fabrication details and possible crack initiation sites. may include. E-mail: ga@hdg. magnetic particle inspection [MPI] or eddy-current testing). The inspection.g. 6. which should take place as soon after galvanizing as practicable.g. e.4. The detailed inspection regime on the post-galvanized work at the galvanizer should be agreed between galvanizer and client. Stripping and re-galvanising after crack repair is not essential. e. The contacts below can provide useful guidance and supporting information. they should be made in accordance with the relevant standard. e. • Visual inspection (up to 100%) • Non-destructive testing on areas identified by visual inspection or areas identified for NDT on the drawings (NDT methods might include for instance. • Galvanizers Association (GA) – Tel: (0121) 355 8838. welds • Date of inspection • Name of the inspector and their company • Outcome of the inspection. for example. The inspectors should be experienced in fabrication inspection. Inspection After completion. satisfactory or not • Recommendations for any further actions required 5. Contacts and signposts for further information Information and guidance on control of cracking of high strength steel in fabrications is best found from those with experience of the phenomenon. Specification for hot dipped galvanized coatings BS EN 1011: Part 1:1998. Coatings on metal fasteners . Publications include: • • • • • The Engineers and Architects Guide to Hot Dip Galvanizing The Engineers and Architects Guide to Hot Dip Galvanized Fasteners Directory of General Galvanizers ‘GalvAction 21’ information sheets on a whole variety of areas including specification and design for galvanizing. Technical delivery requirements. Technical delivery conditions.org. General guidance for arc welding BS EN 1011: Part 2: 2001.org. Protection against corrosion of iron and steel in structures Zinc and aluminium coatings .steelconstruction. Galvanizers Association Wrens Court 56 Victoria Road Sutton Coldfield West Midlands B72 1SY Website: www. Hot finished structural hollow sections of non-alloy and fine grain structural steels.galvanizing.uk British Constructional Steelwork Association • The National Structural Steelwork Specifications (NSSS) Contact details: The British Constructional Steelwork Association 4 Whitehall Court Westminster LONDON SW1A 2ES Website: www. Galvanizers Association Detailed information on a wide range of subjects relevant to galvanizing is available from Galvanizers Association. Arc welding of ferritic steels BS EN 10025: 1993.Specifications and test methods BS EN ISO 14713: 1999. Relevant standards • • • • • • • • BS EN ISO 1461: 1999. Publications & contacts for further information a. Hot dip galvanized coatings on fabricated iron and steel articles .uk 8 . renovation of damaged galvanized coatings and interpretation of relevant standards for galvanizing Case histories on performance of galvanizing Contact details: b. BS EN 10210-1: 1994. Hot rolled products of non-alloy structural steels. General delivery conditions. Hot rolled products in weldable fine grain structural steels.7. 8.Guidelines BS 7371: Part 6 : 1998. BS EN 10113-1: 1993.