Isothermal aging treatment and its effect on the mechanical properties of Fe-Cr-Mn high-nitrogen austenitic steel

Abstract The high-temperature solution nitriding process is a suitable treatment for producing high-nitrogen steels by the diffusion of nitrogen from the surface to the center of steels under high nitrogen pressure at high temperatures. On the other hand, long-term solution nitriding at high temperatures can cause the formation of coarse-grained austenite. This study focused on the grain size, strength, and elongation properties of Fe-Cr-Mn high-nitrogen austenitic steels by isothermal aging. For this purpose, we produced high-nitrogen steels by three-step phase transformations: (i) high-temperature solution nitriding, (ii) isothermal aging, and (iii) reaustenitization. After solution nitriding, high-nitrogen austenitic steel was treated with isothermal aging to induce austenite decomposition. Supersaturated austenite (ϒ) transformed to less nitrogen-saturated austenite (ϒ′) and discontinuous cellular precipitation (DCP) during the isothermal aging treatment. Subsequently, the decomposed structure was reversed into austenite through reaustenitization. The results suggested that austenite grain refinement and increasing hardness were achieved by nucleating reversed austenite grains after reaustenitization treatment. On the other hand, a noticeable increase in elongation appeared after reaustenitization treatment for 30 min.