Welding is one of the most critical processes in the manufacture of pressure vessels like the boiler of a thermal power plant. 

The temperature of the molten weld pool during the process is in the range of 2000 deg C. The heat increase is rapid and instantaneous. When this small strip of molten pool cools down the shrinkage results in thermal stresses that are locked up inside the metal. This also can change the macrostructure of the steel.

These residual stresses and macro structure changes, combined with operating stresses, can lead to catastrophic failure of the pressure vessels. 

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.