Nano-sized austenite and Cu precipitates formed by using intercritical tempering plus tempering and their effect on the mechanical property in a low carbon Cu bearing 7 Ni steel

High toughness, especially cryogenic toughness, is usually hard to be obtained in precipitation strengthening steels. In this report, intercritical tempering plus tempering were applied on a low carbon Cu-contained 7Ni steel. Results reveal that high strength (819MPa, 25 degrees C) combined with high cryogenic toughness (138 J, 196 degrees C) can be realized simultaneously by two steps tempering. The microstructural evolution during intercritical tempering and tempering was investigated systematically by scanning electron microscopy (SEM), transmission Kikuchi diffraction (TKD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was found that both Ni-rich regions and Ni-depleted regions exist after first step intercritical tempering and no stable austenite was found, whereas nano-sized austenite (with average thickness of 20 nm) was produced on the Ni-rich regions after following second tempering process. Thermo-dynamic calculation and DICTRA simulation was applied to systematically study formation of austenite and partition of Ni during two steps tempering process. Simulation results coincides well with experiments. Meanwhile, it is suggested that high cryogenic toughness is ascribed to the stable austenite because of its nanometer thickness and high Ni content. Precipitation of Cu-rich phases at second tempering process maintained high strength during final tempering.