Improving laser welded joint performance of medium-Mn steel through C and Mn partition by intercritical annealing process

In this paper, the fracture failure mechanism of laser welded (LW) joints of medium-Mn steel is studied. And the mechanical properties of the joints were greatly improved by designated intercritical annealing (IA) process. Experimental results showed that the elongation of the as-welded joint of medium-Mn steel is very low, and the fracture shows typical brittle fracture characteristics. Electron probe micro-analyzer (EPMA) test showed that there were many Mn segregation bands in the joint, and electron backscatter diffraction (EBSD) surface scanning test further demonstrated that many Mn23C6 carbides were produced at the martensite grain boundary (GB) of the fusion zone (FZ), which was the main reason for the occurrence of brittle fracture in the FZ. The IA process improves the segregation of Mn element at the GB of the joint and reduces the intergranular brittleness. At the same time, more austenite was distributed by C and Mn elements, and more Fe3C phases were found near austenite in the EBSD diagram, which provided nucleation centers for the formation of austenite. The product of strength and elongation (PSE) of the IA treated joint was dramatically increased by 2693.65 %.