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.

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.

Qualifying a Welder to AS 1796 Versus AS 3992 Several Weld Australia members have asked what the difference is between a welder qualifying to AS 1796 Certification of welders and welding supervisors, and a welder qualifying to a standard such as AS 3992 Pressure equipment—Welding and brazing qualification.

For a welder to qualify to a standard such as AS 3992, the welder is required to demonstrate their ability to qualify by welding with a specific welding process by following a specified weld procedure. If successful, the qualification is only valid whilst the welder continues to use the welding process within a defined period—typically six months. At the conclusion of the defined period, the welder will need to be requalified if they do not use the process within that time frame. The six month time period resets every time the welder uses the relevant welding process. This process is typically described as ‘welder qualification’ and it demonstrates that the welder is capable of following the defined welding procedure.

In comparison, AS 1796 is a qualification for life—it is not valid for a defined period. It tells an employer that—at a particular point in time—the welder was trained to weld using a specific welding process. Historically, in Australia, this process is referred to as welder certification because the welder has been granted a certificate showing their ability to weld using a predefined welding process. The welder is still required to separately qualify to weld specific weld procedures on the job as required by AS 3992.

NOTE: AS 1796 is currently under revision, and use of the term ‘welder certification’ is under review.