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Post Weld Heat Treatment (PWHT)

A number of different types of post-weld heat treatments are used for different reasons and for different materials.

a)  Post-weld heat treatment is most generally used for stress relief.  The purpose of stress relieving is to remove any internal or residual stresses that may be present from the welding operation.  Stress relief after welding may be necessary in order to reduce the risk of brittle fracture, to avoid subsequent distortion on machining, or to eradicate the risk of stress corrosion. 

b)  For some alloy steels, a thermal tempering treatment may be necessary to obtain a suitable metallurgical structure.  This treatment is generally performed after the weld has cooled, but under certain circumstances, it may be necessary to perform this treatment before it has cooled to prevent cracking.

c)  Extremely coarse weld structures in steel, such as those obtained with the electro-slag welding process, may require normalizing after welding.  This treatment will refine the coarse grain structure, reduce stresses after welding, and remove any hard zones in the heat-affected zone.

d)  The precipitation hardening alloys, such as the heat treatable aluminum alloys, are sometimes required to undergo post-weld heat treatment to regain their original properties.  In some cases, only an aging treatment is used, although a full solution heat treat and artificial aging treatment will provide better recovery of properties after welding.

When the welding operations involve preheating and/or post-weld heat treatment, it is important that the welding inspector understand these requirements in order to ensure that they are being conducted correctly and in terms of the relevant welding procedure specifications and/or code requirements.