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Post Weld Heat Treatment (PWHT) is applied to relax the residual stresses within the weld and reduce the risk of brittle fracture by improving fracture toughness. However, with improved modern welding techniques is PWHT still necessary? 

The use of PWHT is a common requirement when using thick-walled components as part of the ASME boiler codes. PWHT is required for pipes exceeding 1 ¼” thickness or approximately 32mm.

The case study below investigates the requirement for PWHT, it looks at how Element’s energy team performed Engineering Critical Assessment (ECA) to avoid PWHT and investigates the client benefits to using this methodology. 

The challenge

Our client, in the gas transmission / Energy industry, approached us to investigate if using Post Weld Heat Treatment (PWHT) for one of their pipes with a 46mm wall thickness was required. 

This pipe had already been coated, both internally and externally, and the application of PWHT would badly damage these coatings. Consequently, if PWHT was avoided, these coatings could be preserved.

In addition, PWHT on such a large pipe was logistically difficult and time-consuming, and the client was working on a very tight schedule.

The solution 

To address this challenge, Element’s technical experts advised the client on the necessary input requirements to gain a full understanding of the component in service, and also guided the client on the testing required to perform the ECA. 

Element performed an ECA to establish if the maximum workmanship flaw, e.g. the largest permissible flaw in the structure, was acceptable in the absence of PWHT. The team investigated if PWHT added an overall benefit and concluded that even in the absence of PWHT the workmanship flaw was acceptable. The assessment showed that the PWHT was not providing any benefit to the integrity of the component. To verify this, Element performed project-specific fracture toughness and tensile testing, where the testing was conducted on non-PWHT material. 

Using the testing data combined with the design data specific to this component, the maximum workmanship flaws were assessed as per the guidance of BS 7910, which assesses the significance of flaws, using fracture mechanics approaches.

The result

Using ECA and Element’s integrated testing and analysis solutions helped avoid damage to the previously applied coatings and removed an unnecessary process that would take time and add significant cost to the project. As a result, the client could meet their schedule and save both time and money on their project.

The client was confident in the integrity of Element’s approach as industry-recognized codes and standards were followed to prove that their component was fully acceptable in the absence of PWHT.

The client has now performed additional testing, which demonstrates the actual material properties, such as fracture toughness and tensile properties, instead of relying on generic code compliance. The testing required for the ECA is more thorough testing than the testing required without the PWHT waiver and will increase the integrity of the component, reduce the cost of repairs, and can be used to extend the life of the component. 

To learn more about Post Weld Heat Treatment (PWHT) and Engineering Critical Assessment (ECA), or if you have any questions, contact our team today.