A Comparative Study on the Microstructure and Mechanical Properties of Laser Welded Joints among Ferritic SUS443 and Austenitic SUS304 Stainless Steel

In this study, 0.4-mm-thickness ferritic SUS443 and austenitic SUS304 stainless steel were laser welded by combining 304/304, 443/443 and 304/443. Predictions of phase evolution were made using CALPHAD, with microstructures observed via metallography. Grain size in the heat-affected zone (HAZ) was measured, and initial weld quality observations were made. The mechanical properties of joints were evaluated through hardness testing, tensile experiments, and fracture analysis. The joints almost solidified in a columnar dendritic mode. Different combinations resulted in distinct microstructures: 304/304 joint consists of δ-ferrite and γ-austenite, 443/443 joint shows α-ferrite and precipitates of carbide or nitride, and 304/443 joint is composed of α-ferrite, γ-austenite, and α'-martensite, leading to the highest hardness. The fracture behavior of 304/443 and 443/443 is similar, with fractures occurring at the base metal of SUS443 in both cases. This indicates that the strength of the joint is greater than that of the base metal. The enhanced strength of the 304/443 joint is attributed to the formation of α'-martensite, whereas the strength improvement of the 443/443 joint is due to the tiny second-phase precipitation and slight grain growth. The fractures exhibit massive plastic deformation, featuring a distinct crack source, radial zone, and shear zone. Non-isometric ductile dimples clustered together. In laser welding of 304/304, fracture occurs at the joint, and tiny ductile dimples show poor plasticity. Although the joint has lower plasticity, it has high strength, with deformability provided mainly by the base metal SUS304.