Effects of PWHT on the Residual Stress and Microstructure of Bisalloy 80 Steel Welds

Quenched and tempered (Q & T) steels have numerous applications, particularly in the defence industry with welding as the main fabrication route. Since welding imparts stresses due to thermal gradients development during welding, plus the fact that the Q & T fabricated structures are expected to function in a complex loading environment, it is critically important to relax the welding stresses before exposing the parts to service conditions. The present study reports on the generated residual stresses when Bisalloy 80 is welded by pulsed gas metal arc welding (GMAW-P) and verifies the effects of post-weld heat treatment (PWHT) on the microstructural changes, removal or reduction of residual stresses and the resulting mechanical properties of the welded Q & T steel joints. Neutron diffraction was utilized to measure the residual stresses in the as-welded and after PWHT of the Bisalloy 80 steel weldments. High levels of tensile residual stresses reaching to the yield strength of the weld metal were present (642 ± 24 MPa) in the as-welded joints but were substantially reduced after PWHT (145 MPa ± 21 MPa, which is ~23% of the yield strength of the weld metal). PWHT led to microstructural changes in different regions of the parent and weld metals, including the formation of coarsened polygonal ferrite grains and bainitic ferrite laths. This finding is in line with hardness measurements, where hardness reductions were evident in the heat-affected zone (HAZ) and the weld metal (WM) of the heat-treated specimens.