Studying the strengthening mechanisms and mechanical properties of dissimilar laser-welded butt joints of medium-Mn stainless steel and automotive high-strength carbon steel

In this study, a post-weld heat treatment (PWHT) at a high temperature of 700 °C for 10 min was proposed to significantly improve the mechanical performance of dissimilar laser-welded butt joints between medium-manganese stainless steel (MMn-SS) and high-strength carbon steels (HS-CS) as base metals (BMs). Electron backscatter diffraction and transmission electron microscopy were employed to characterise the microstructures and study the deformation mechanisms in the paired metals of the dissimilar joints during PWHT and tensile straining. Micro-indentation hardness tests were performed to analyse the mechanical responses in the weld zones after PWHT. The results revealed that PWHT at 700 °C significantly affected the microstructure and mechanical strength of the BMs and their weldments. A fully tempered martensitic microstructure with recrystallised grain regions was formed in the fusion zones (FZs) of the weldments, which were processed at low and high specific point energies (SPEs) of 19, 25, and 30 J. BM HS-CS was significantly strengthened owing to the interphase precipitation hardening of vanadium carbide precipitates during the proposed PWHT. The strength-ductility balance of BM MMn-SS was improved by combining the martensitic transformation and mechanical twinning mechanisms. Although, the tensile strength of BM HS-CS increased from 500 to 630 MPa and that of the metal in the FZ decreased from 1100 to 800 MPa owing to PWHT. Hence, HS-CS is a weak metal for dissimilar joints, and joint failure was promoted in it.