Post Weld Heat Treatment (PWHT) reduces the residual stresses formed during welding. It also restores the macrostructure of the steel. Mandatory in high-pressure applications, constructors have to strictly follow PWHT requirements to avoid component failures. These residual stresses and macrostructure changes, combined with operating stresses, can lead to catastrophic failure of the pressure vessel.
PWHT eliminates these effects by heating, soaking, and cooling the weld area in a controlled manner to temperatures below the first transformation point, giving the macro structure sufficient time to readjust to its original state and removing the residual stress.