What is the purpose of applying (PWHT) Post Weld Heat Treatment on the welding?

To reduce the over heating of metal

• To reducing the stress in the material  

According to DIN EN, heat treatment is a process or a combination of a plurality of processes for the treatment of a metallic workpiece, which is thereby heated and cooled again in a certain course in order to change material properties. In so doing, ambient agents may include changes in composition, e.g. The carbon or nitrogen content, or the crystal lattice

Is to reduce residual stresses as a method of harddness control or even to enhance material strength.

Is a method for reducing and redistributing the residual stresses in the material that have been introduced by welding