Microstructure and Mechanical Properties of Cold-Rolled Martensitic Steel Welded Joint for Carriage Structure by Cold Metal Transition Welding

Cold metal transition lap welding was performed on 1 000 MPa cold-rolled SCR1000 martensitic steel. The phase composition, microstructure, hardness, tensile shear properties and fatigue properties of welded joint were studied. The results show that the weld and the heat affected zone were mainly composed of Fe, Fe₂O₃ and Fe₃O₄ phases. The weld structure was acicular ferrite. The structure of coarse grain zone in heat affected zone was coarse martensite, the fine grain zone structure was fine martensite, the incomplete phase transformation zone structure was mainly martensite and small size ferrite, and the tempering zone structure was tempered martensite and granular carbide. The hardness of coarse grain zone and fine grain zone in heat affected zone was higher than that of the base metal, which was the hardening zone. The hardness of incomplete phase transformation zone and tempering zone in heat affected zone and weld was lower than that of the base metal, which was the softening zone. The tensile shear strength was 707 MPa, and the fracture absorption energy was 51.223 J. The fracture location was the softening zone near the base metal, the tensile shear fracture characteristic was ductile fracture. The fatigue strength was 54 MPa, and the fracture position was weld toe. The fatigue fracture surface showed fatigue striation features and cleavage formed by brittle fracture.