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Guidance Note: The design, specification and fabrication of structural steelwork 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

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flattening. There may also be stresses associated with dipping, due to variation in section sizes [and, therefore, differential temperatures] within the fabricated component. Efforts can be made at the design stage and elsewhere to minimize residual stresses, 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, reducing distortion, reducing the potential for un-vented voids, allowing the zinc to coat all surfaces and increase protection. However, their application to structural steelwork is limited. Making adequate allowance in the design for filling, venting and drainage will assist the galvanizer in minimizing the differential thermal stresses experienced by the fabrication when being dipped. 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, and withdrawn from, a bath of molten zinc at about 450°C. Thus any features that aid the access and drainage of molten zinc will improve the quality of the coating and reduce costs. With certain fabrications, holes that are present for other purposes may fulfil the requirements for venting and draining; in other cases it may be necessary to provide extra holes for this purpose. For complete corrosion protection, molten zinc must be able to flow freely to all surfaces of a fabrication. With hollow sections or where there are internal compartments, the galvanizing of the internal surfaces eliminates any danger of hidden corrosion during service. 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. General guidance on size for venting holes exists in the ‘Engineers & Architects Guide to Hot Dip Galvanizing’, however, clients are recommended to discuss specific venting arrangements with the galvanizer who will be carrying out the work.

• 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.

• With hollow sections sealed at the ends, holes should be provided, again diagonally opposite one another, as near as possible to the ends. In some cases it may be more economical to provide V or U shaped notches in the ends, or to grind corners off rectangular hollow sections - these procedures provide ideal locations for venting and draining.

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• Where holes are provided in end plates or capping pieces, they should be placed diagonally opposite one another, off centre and as near as possible to the wall of the member to which the end plate is connected.

• Internal and external stiffeners, baffles, diaphragms, gussets etc., should have the corners cropped to aid the flow of molten zinc.

Holes that have been drilled for venting can be plugged, but this is mainly necessary for aesthetic reasons, because a galvanized coating covers all surfaces. If required, tapered aluminium or plastic plugs are available and will prevent undesirable ingress of water Where there is an inherent tendency to distort, e.g. in asymmetrically shaped fabrications, 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. The galvanizer should be consulted for advice at an early stage if this is being considered. The size and position of filling and drainage holes in fabricated vessels can have a major effect on distortion, as can the size and position of lifting holes or lugs, particularly on hollow fabrications. At the galvanizing temperature – usually around 450°C – the steel being processed will lose approximately 50% of its room temperature yield strength, regaining it on cooling after galvanizing. Much research has been carried out on the effects of galvanizing process on steel strength, revealing no measurable loss in strength due to the processing and indeed in almost all cases there was a slight increase in yield strength. Detailed design advice is available from Galvanizers Association or directly from the galvanizer. 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, the dominant feature contributing to hydrogen embrittlement is usually the welding process implemented as part of the fabrication of the structure, but this can be overcome by using the correct weld procedures and the correct choice, storage and use of the welding consumables. Latest standards for welding of steel articles, e.g. BS EN 1011 : Part 2 : 2001, provide useful informative annexes. In particular, 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], Hydrogen embrittlement of steel occurs when atomic hydrogen diffuses into the steel lattice and affects the steel’s mechanical properties. There are alternative ways of interpreting the actual mechanism for hydrogen embrittlement. Essentially however, it is considered that atomic hydrogen reacts in a number of ways in the steel, including recombination to form molecular or gaseous hydrogen that diffuses less easily than the atomic hydrogen, which is effectively trapped in the region of the dislocations. 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, 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. This condition is known as hydrogen embrittlement and is characterised by a loss in ductility of the steel.

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Prior to supply stage of the galvanized steelwork, atomic hydrogen may be formed as a result of the:

• Steel manufacturing process • Welding and fabrication process • Coating preparation process.

Once absorbed, atomic hydrogen diffuses to, and accumulates in, the regions of lattice defects, or dislocations, which are always present in the microstructure. 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. All structural steels may become embrittled to some extent. The extent of embrittlement depends upon the amount of strain, time at ageing temperature (e.g. galvanizing at 450°C), and steel composition – in particular nitrogen content. Elements that are known to tie up nitrogen in the form of nitrides are useful in limiting the effects of strain ageing. These elements include aluminium, vanadium, titanium niobium and boron. Cold working will include operations such as bending (to form tube or hollow section for instance), punching of holes and shearing. This type of cracking is not a concern in structural steelwork as the use of heavily cold worked, thin section material, or highly alloyed steels, is not usual. Where best practice fabrication guidelines have been followed, this type of cracking is unlikely to occur but in the few cases where this type of cracking has been evidenced, 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. 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. d) Liquid metal assisted cracking (LMAC)

This rare form of cracking will manifest itself only during the hot dip galvanizing process. 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. There are few explanations of exactly how and why LMAC occurs. Furthermore, during the past five years at least three laboratory research programmes in different countries, which were set up to investigate the phenomenon, failed to reproduce it under laboratory conditions. There are methods by which the risk of cracking can be minimised, thereby reducing the potential for an incidence of LMAC of steel during hot dip galvanizing. It is known that:

• More complex, highly restrained, fabrications are more prone to cracking • Cracking originates from surface stress concentrators such as discontinuities in

the surface of the steel, areas of greatest surface hardness and/or highest stress

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• Normally, 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, developed for instance through

rolling, finishing (at steel mill), welding, hardened surfaces, movement of the fabricated elements, pre-treatment and galvanizing.

• The presence of a molten metal e.g. molten zinc during the galvanizing operation Unless all four factors are present, LMAC will not occur. Any tendency to LMAC is removed once the article is removed from the galvanizing bath. Usually, articles affected by this mechanism can be repaired satisfactorily. The guidelines set out below are aimed at controlling one or more of the factors that can act as contributors to this mechanism. In particular, the guidelines relate to removing surface discontinuities (stress concentration or crack initiation sites), reducing stress in the fabricated articles, reducing the presence of hardened surfaces and minimising differential thermal expansion (and therefore any stresses associated with this) during the galvanizing process. 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). Galvanized structures which have been designed, fabricated and galvanized to the best practice set out below will have an extremely low susceptibility to this type of cracking. 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, to critical or sensitive

areas of the fabrication (notified to the galvanizer) that might be subject to higher levels of post-galvanizing inspection.

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, or steel from an unknown supplier

• Where choice of steel stock exists, 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.

• Use open ended sections in preference to closed sections

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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, 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; for example: o Consideration should be given to dressing of critical welds o 'Stress raisers' should be avoided; e.g. profiles should be cut to maximum

reasonable radii, undercutting, jagged edges and sharp notches should be avoided

• Hardened surfaces should be avoided wherever possible • Where possible, differential thermal stresses during galvanizing should be

minimised by taking these into account when considering individual component and fabricated component form, size and connections

• At material ordering stage, 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; supply standard, grade of material and mechanical properties/chemistry

Galvanizing

• Assuming all the above has been carried out, 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, the details noted and are then referred back to the customer.

• 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. 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, nevertheless, commensurate with achieving the required thickness and quality of the coating and balancing off requirements for minimization of potential for distortion.

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4. Inspection After completion, work should be inspected for the presence of cracks. The detailed inspection regime on the post-galvanized work at the galvanizer should be agreed between galvanizer and client. The level of inspection should be related to the criticality of the structure. The inspection, which should take place as soon after galvanizing as practicable, may include, for example;

• 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, magnetic particle inspection [MPI] or eddy-current testing).

The inspectors should be experienced in fabrication inspection, and should be familiar with the fabrication details and possible crack initiation sites. The agreed inspection regime will include details of which records should be retained for future reference. These might include for example;

• Identification of the element • Indication of the specific areas inspected, e.g. welds • Date of inspection • Name of the inspector and their company • Outcome of the inspection, e.g. satisfactory or not • Recommendations for any further actions required

5. Remediation If cracking occurs, it can normally be repaired by gouging and re-welding to an approved procedure, with appropriate post-weld testing. Reference should be made to the specification for the job. Stripping and re-galvanising after crack repair is not essential. When galvanized coating repairs are required, they should be made in accordance with the relevant standard, e.g. EN ISO 1461 : 1999, or to agreed criteria set out in the quality plan. 6. 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. The contacts below can provide useful guidance and supporting information.

• Galvanizers Association (GA) – Tel: (0121) 355 8838, E-mail: ga@hdg.org.uk • British Constructional Steelwork Association (BCSA) – Tel: (0207) 839 8566 • Corus – Tel: (01709) 820 166

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7. Relevant standards

• BS EN ISO 1461: 1999, Hot dip galvanized coatings on fabricated iron and steel articles - Specifications and test methods

• BS EN ISO 14713: 1999, Protection against corrosion of iron and steel in structures - Zinc and aluminium coatings - Guidelines

• BS 7371: Part 6 : 1998, Coatings on metal fasteners - Specification for hot dipped galvanized coatings

• BS EN 1011: Part 1:1998, General guidance for arc welding • BS EN 1011: Part 2: 2001, Arc welding of ferritic steels • BS EN 10025: 1993, Hot rolled products of non-alloy structural steels. Technical

delivery conditions. • BS EN 10113-1: 1993, Hot rolled products in weldable fine grain structural steels.

General delivery conditions. • BS EN 10210-1: 1994, Hot finished structural hollow sections of non-alloy and fine

grain structural steels. Technical delivery requirements. 8. Publications & contacts for further information a. Galvanizers Association

Detailed information on a wide range of subjects relevant to galvanizing is available from Galvanizers Association. 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, renovation of damaged galvanized coatings and interpretation of relevant standards for galvanizing

• Case histories on performance of galvanizing

Contact details: Galvanizers Association Website: www.galvanizing.org.uk Wrens Court 56 Victoria Road Sutton Coldfield West Midlands B72 1SY

b. 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.steelconstruction.org.uk