Post Weld Heat Treatment (PWHT), also referred to as stress relieving, is an application of uniform heat applied after welding to a structure, or portion thereof, at a sufficient temperature – below the transformation temperature range and with the intention of retaining the material’s strength/quality and ductility.
The primary benefits of post weld heat treatment are recognized as relaxation of residual stress, tempering, and hydrogen removal. If performed incorrectly, or neglected altogether, it can lead to reduced tensile strength, creep strength, notch toughness of the component, as well as the increased susceptibility to corrosion. An important aspect of post weld heat treatment is to prevent brittle trans-granular cracking that can potentially result in catastrophic failures, such as the collapse of a pressure vessel or major piping component.
For that reason, post weld heat treating applications must be executed within strict uniform temperature ranges and follow code specifications which are dependent on the material’s composition, thickness and prior thermal or mechanical process applications.