Effects of carbon and chromium on microstructure evolution and mechanical properties of friction stir weldment in medium-carbon steel

The carbon and chromium contents significantly affected the microstructure evolution in the stir zones of medium-carbon steels during the friction stir welding (FSW). The stir zone of the 0.2%C-4%Cr steel has a high strength over 1.5 GPa with a good elongation of 30.3%, while the stir zones of the 0.4%C-2%Cr and 0.4%C-4%Cr steels had ultrahigh tensile strengths over 2 GPa with low elongation. Auto-tempered martensite is the main reason for the excellent balance of strength and ductility in the stir zone of 0.2%C-4%Cr steel.