Abstract: Heat treatment processes to improve the cryogenic toughness of Cr-Ni-Co-Mo maraging stainless steel were investigated, including preheating at 600 ℃+low temperature solution treatment at 750 ℃, and direct solution treating once or twice at 750 ℃ without preheating respectively, all after solution treatment at 1000 ℃. The volume fractions of martensite, retained austenite and reversed austenite phases were determined respectively, and the tensile strength at room temperature and -196 ℃, the notched tensile strength and impact toughness at -196 ℃ were analyzed. The results show that compared with the conventional heat treatment, more retained austenite exists in the steel after adding low temperature solution treatment at 750 ℃. Moreover, more reversed austenite is formed after subsequently aging at 500 ℃, which can significantly improve the cryogenic toughness. Further comparative analysis shows that the process of direct solution treatment at 750 ℃ is relatively simple, which can lead to the highest tensile strength at both room temperature and -196 ℃, the highest notch tensile strength at -196 ℃, and lower notch sensitivity at -196 ℃. Therefore, the direct solution treatment at 750 ℃ has advantage over other treatments.

Key words: maraging stainless steel, cryogenic toughness, low temperature solution treatment, retained austenite, reversed austenite