PWHT is required where there is a risk of environmentally-assisted cracking. PWHT of C-Mn steels is typically carried out at approximately 600°C, for one hour per 25mm of thickness.
In most cases, stress relieving is done to regain dimensional tolerance and to reduce distortion. Distortion occurs due to the rapid and uneven heating and cooling of the weld metal and the surrounding base metal. As the molten weld metal cools it contracts and does so at a different rate than the base metal around it.
Post Weld Heat Treatment (PWHT), or stress relief as it is sometimes known, is a method for reducing and redistributing the residual stresses in the material that have been introduced by welding.