Abstract: Effects of QT and QLT treatments on mechanical properties of the 9Ni steel were studied, and the toughening mechanism was discussed. The results show that QT and QLT treatments have a significant impact on the mechanical properties of 9Ni steel. Compared with QT treated specimen, the tensile strength and yield strength of the specimen treated by QLT are decreased, but the elongation is increased slightly, the low temperature impact property is increased significantly by nearly 40 J. QT treated specimen has typical ductile fracture characteristics. There is a coarse martensite structure near the impact fracture. The segregation zone is mainly lath martensite, and there is a trend of multilateralization between martensitic lath. There is a certain amount of bright areas in the structure, which are mainly reversed austenite formed during tempering, and can significantly improve the low temperature toughness of the 9Ni steel. However, the lath martensite of the QLT treated specimen is finer and more uniform, there are more bright areas between the martensitic lath, and the specimen after low-temperature impact test has more dimples with smaller size and uniform distribution. After low temperature impact test, the volume fraction of reversed austenite in QT and QLT treated specimens is 0.88% and 6.60%, respectively, therefore the low temperature toughness of the QLT treated specimen is better, while the reversed austenite absorbs the solid solution strengthening elements in the substrate, resulting in the reduction of yield strength.

Key words: 9Ni steel, quenching and tempering treatment, quenching, intercritical quenching and tempering, microstructure, low temperature toughness