Study on the effect of post weld heat treatment parameters on the relaxation of welding residual stresses in electron beam welded P91 steel plates

Residual stresses are created by localised heating effects that occur during the welding process. Post weld heat treatment (PWHT) is the most convenient method for stress relief of welds. But PWHT cannot completely eliminate the residual stresses. So, it is essential to determine the influence of PWHT parameters like holding temperature and time on the stress relaxation for optimising the process. The selected material is modified 9Cr-1Mo (Grade 91) steel in the form of plates welded together using a high intensity electron beam. To facilitate the study, a numerical thermo-elastic-plastic model has been developed to simulate the welding of the plates. As P91 steels undergo phase transformations, the corresponding volumetric change and transformation plasticity are taken into consideration during the analysis and welding residual stresses are predicted. PWHT is implemented using Norton creep law and the residual stresses after relaxation are determined. The developed model and the predictions are validated using neutron diffraction measurements on as welded and post weld heat treated plates. A good agreement has been achieved between the measurements and predictions. The validated model has been used to study the effect of variation of heat treatment parameters like holding temperature and time on the relaxation of welding stresses.