Welding residual stresses have important consequences on the performance of engineering components. However, residual stress often remains the single largest unknown in industrial damage situations. They are difficult to measure or theoretically estimate and are often significant when compared with the in-service stresses on which they superimpose. Residual stresses in weld joints and repairs are sometimes reduced by heat treatment or by mechanical stress relieving. The main aim of Post Weld Heat Treatment (PWHT) is to reduce or completely relax the residual stresses induced by the welding process. The main purpose of this experiment is to obtain detailed knowledge of residual stresses relaxation during in-situ PWHT of welded component. Additionally, the results before and after PWHT will be compared with the current safety assessment procedures, BS 7910 and R6.