Role of cementite on tensile properties in auto-tempered 0.15C–5Mn martensitic steel

The effects of austenitizing temperature and cooling rate on microstructure and subsequent tensile properties were investigated in 0.15C–5Mn martensitic steels. Tensile strength and elongation increased with decreasing austenitizing temperature (1000 °C to 800 °C) in both air-cooled and water-quenched steels. Improvement of tensile properties originated from the promoted transformation-induced plasticity (TRIP) effect, as lower austenitizing temperature limited Mn diffusion to make cementite preferred site for Mn partitioning, thereby increasing the fraction of retained austenite after heat treatment. Cooling rate also affected the mechanical properties, as air-cooled samples showed lower strength and higher elongation compared to water-quenched samples. During the air-cooling, supersaturated carbon in martensite is redistributed to form additional carbides by auto-tempering effect. The formation of carbides softened the martensite to decrease strength while increased tensile elongation. Fraction of retained austenite influenced by the austenitizing temperature and formation of carbide affected by the cooling rate acted as competing mechanisms affecting the tensile properties of the martensitic steels, indicating that heat treatment should be controlled carefully to obtain the desirable mechanical properties.