Effects of tempering temperature on morphology of metastable austenite and mechanical properties of Mn-Ni steel

Effects of tempering temperature on morphology of metastable austenite and mechanical properties of Mn-Ni steel were investigated by X-ray Diffraction (XRD), Electron Back Scattering Diffraction (EBSD) and Transmission Electron Microscope (TEM). The results show that the volume fraction of metastable austenite at room temperature increased with the increase of tempering temperature. When tempering temperature was 600 and 625 ℃, metastable austenite mainly precipitated in the shape of thin film along the lath boundaries of tempered martensite, paralleling to the surrounding martensitic laths. This thin film metastable austenite had a dense distribution, with the thickness ranging from 60 to 100 nm, and had a good stability; when the tempering temperature was 650 ℃, thin film metastable austenite decreased dramatically and large block metastable austenite precipitated unevenly on the intersections of grain boundaries of tempered martensite. Analysis indicates that block metastable austenite is beneficial to the ductility of test steel, but it goes against to improving low temperature toughness; while thin film metastable austenite can improve low temperature toughness significantly.