Welding residual stress prediction with varying cooling rate in phase transformation

•The accuracy of welding residual stress prediction can be improved by considering cooling rate in phase transformation.•The proposed method characterizes austenite decomposition and formation of retained austenite under varying cooling rates.•A higher cooling rate promotes the formation of double peaks in welding residual stress profile.•Martensitic transformation volumetric strain, unlike the austenitic counterpart, has profound impact on the residual stress. This work investigates the role of cooling rate on solid-state phase transformation (SSPT) during laser welding, as well as its effect on generating residual stress in single pass weldments of P91 steel. Diverse cooling rates are usually encountered in welding, exerting a substantial influence on both microstructural development and residual stress distribution within the weld bead. However, the relation between cooling rate and welding residual stress has not been thoroughly elucidated in the context of phase transformation. To depict the cooling rate-dependent volume fraction evolution during phase transformation, the transformation kinetics is reproduced herein by Kolmogorov-Johnson-Mehl-Avrami (KJMA) rate equation. In this study, the KJMA model is formulated into a thermal-metallurgical-mechanical numerical procedure. Both the solid-state phase transformation and thermal elastic–plastic response are computed in the updated temperature field. The results indicate that predicted residual stress, when considering SSPT and cooling rate, agree well with the available neutron diffraction experiment. Using the KJMA model can reasonably account for the influence of cooling rate and improve the accuracy in numerical prediction of welding residual stress. It has been found that lower cooling rate inhibits austenite decomposition. This promotes the formation of retained austenite. While the austenitic phase transformation is well represented, the residual stress distribution along the weld line exhibits a distinctive double-peak profile due to high cooling rate. The austenitic transformation volumetric strain has been discovered to have much less influence on the final residual stress than the martensitic transformation volumetric strain.