Tempering properties of a new ultrahigh strength martensitic steel

A new ultrahigh strength martensitic steel with low-alloy composition system was designed, and the tempering properties at different tempering temperatures were studied by tensile tests and impact tests at room temperature and the microstructural characterization. The results show that the martensite block size was the finest (450 nm) at a tempering temperature of 220 °C, leading to the optimum yield strength (∼1323 MPa). Besides, the tensile strength of the tempered samples decreased as the tempering temperature increased from 200 °C to 250 °C, which was attributed to the disappeared dislocation and the increasing C content in retained austenite. In addition, an extended plateau occurred in the work hardening rate curve of 220T sample during deformation, and the transformation induced plasticity effect of retained austenite lasted longer when the tempering temperature was 220 °C. Moreover, the tempered samples were subjected to ductile fracture after impact tests. The impact toughness after tempering at 220 °C was significantly influenced by the spheroidization of carbides and the presence of the largest dimple size.