An Update from Weld Australia’s Hotline
Weld Australia offers a ‘Hotline’ service to all Corporate Members. The purpose of the Hotline is not to provide a solution, but to advise the enquirer on practical next steps. For further advice, Weld Australia’s highly experienced welding consultants can speak to you over the phone or visit your site in person. If you have a Hotline query, complete our online contact form and we will respond as soon as possible: www.weldaustralia.com.au/hotline. Post Weld Heat Treatment in Structural Steel Fabrications Over recent months, Weld Australia’s Hotline has received a number of enquiries regarding the need for post weld heat treatment (PWHT) on structural steel fabrications. Whilst many application standards specify specific requirements regarding PWHT, AS/NZS 1554.1 Structural steel welding—Part 1: Welding of steel structures, is mostly silent on its use. In structural steel welding, PWHT is usually specified by the client where the fabricated item is to be machined, and accurate tolerances are applicable to the final machined structure. PWHT serves to stress relieve the structure prior to machining. This means that, as a structure is machined, there is minimal movement of the components being machined as material is removed during the machining process. In comparison, structures that have not undergone PWHT, contain locked up stresses that are released during machining. Release of these stresses can cause unintended or unwanted movement during
This Hotline update covers a specific query encountered during the last few months. Whilst accuracy in welding is critical, it is impossible to report in detail the full circumstances of the query. As such, Weld Australia recommends that further technical advice is sought in relation to specific, individual circumstances.
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the machining process. The one situation in which AS/NZS 1554.1 indicates that PWHT may be beneficial can be found in Clause B4.3.3 of Appendix B of the Standard. Specifically, if a structure has been subjected to a significant degree of outer bend fibre strain during processing, its design service temperature (i.e. resistance to brittle fracture) can be restored through the application of PWHT. Reference is made to AS 4458 Pressure equipment Manufacture for the performance of PWHT. Two questions then arise: does the weld procedure need to be qualified with the test plate tested in the PWHT condition, and is the application of PWHT an essential variable? Dealing with the second question first, many Standards regard the application of PWHT as an essential variable. However, AS/NZS 1554.1 is silent on this. This means that to answer the first question, engineering judgement must be applied. In doing so, reference to the official commentary on the Standard should be made: Weld Australia’s Technical Note 11. Clause A4.11, which discusses essential variables, indicates that where PWHT is specified, if the welding consumables are classified in the PWHT condition, and provided that the steel manufacturer can confirm that the steel can be stress relieved, it could be argued that the joint does not need to be qualified in the PWHT condition. If the consumable is not classified for use in the PWHT condition, or, it cannot be confirmed that the steel can be subject to the required PWHT without loss of properties, then the procedure should be treated as non-prequalified and fully qualified in the PWHT condition.
