Effect of heat treatment process on microstructure and mechanical properties of M390/304 CMT welded joints

In order to improve the mechanical properties of M390 high carbon martensitic stainless steel and 304 austenitic stainless steel welded joints, especially the hardness of welded joints to meet the requirements of advanced knives production, different heat treatment processes of welded joints were performed for M390 high carbon martensitic stainless steel and 304 austenitic stainless steel joints obtained by cold metal transfer welding. Tensile, Vickers microhardness tests and scanning electron microscopy (SEM) were used to characterize the mechanical properties and microstructure evolution of welded joints with different heat treatment processes, the carbide distributions of M390 base metal, M390 fine-grained region and M390 coarse-grained region in welded joints under different heat treatment processes were calculated, the fracture mechanism of welder joints under different heat treatment processes was studied. The research results show that the water quenching at 1 150 ℃ heat treatment process not only meets the hardness requirements of advanced knives, but also has good mechanical properties, and can be used as the best heat treatment process for M390/304 welded joints, the tensile strength and elongation of the corresponding welded joints reach 502 MPa and 20.8%, the tensile strength and elongation of the corresponding welded joints reach 98% and 95% of as-welded joint. The average size of carbides in M390 base metal, fine-grained region and coarse-grained region of water quenching at 1 150 ℃ heat treatment process is the smallest, and the carbide morphology is uniformly distributed in small blocks. With the increase of quenching temperature, both the tensile strength and elongation showed a trend of first decreasing and then increasing. With the decrease of cooling rate, both the tensile strength and elongation showed a decreasing trend. The welded joints under different heat treatment processes are fractured at the interface between M390 coarse-grained region and weld metal, where the hardness varies greatly, and there is a phenomenon of uneven stress distribution.