Pre and Post Weld Heat Treatment (PWHT) is the most widely used form of stress relieving on completion of fabrications of welded structures. The principle is that as the temperature is raised, the yield stress and the elastic modulus of the material fall. Pre-and Post-Weld Heat Treatment done before and after welding/machining to improve the chemical mechanical properties of weldment/machined surfaces. In concept, PWHT covers many different potential treatments. A point is reached when the yield stress no longer supports the residual stresses and some localized plastic deformation occurs. Pre and Post Weld Heat Treatment in the context described here refer to the process of reheating a weld to below the lower transformation temperature at a controlled rate, holding for a specific time and cooling at a controlledrate.