Investigation on microhardness and fatigue life in spot welding of quenching and partitioning 1180 steel

Quenching and partitioning (Q&P) steels are among the most critical third-generation steels for realizing vehicle weight reduction and improving safety performance. It is critical for the structure durability to have an excellent fatigue life of the welds. In this study, Q&P1180 steel was laser spot welded (LSW) in order to evaluate the microhardness, mechanical properties, and fatigue behavior of its joints, which were compared with resistance spot welding (RSW). In both types of joining methods, the welded zone is composed of lath martensite with high microhardness value. Due to the martensite tempering, the sub-critical heat affected zone of Q&P RSW welds was soften, resulting in a decrease of 52 HV of microhardness than the LSW joint. The presence of stress concentration in the boundary influenced tensile strength and all-welded samples failed in the boundary. Furthermore, the fatigue limit of the RSW joint (1.139kN) was about 91.5%t of the LSW joint (1.245kN). Fatigue specimens of the LSW joint failed between the fusion zone and heat-affected zone because of their higher sensitivity to stress concentration than the RSW joint, resulting in double eyebrow failure and pull-out failure. Fatigue crack originated from the specimen surface and propagated through fatigue striations together with secondary cracks. In addition, it was found that the strength of the base metal has inconsequentially affected the fatigue life of the welds when these welds were produced by the same welding method and have similar geometries.