Influence of Deep Cryogenic Treatment on Microstructure, Mechanical Properties and Dimensional Changes of Vacuum Heat-treated High-speed Steel

摘要

On the basis of the results of extensive tests performed on the ESR high-speed steel AISI M2, it has been confirmed that the microstructure can be modified substantially by vacuum heat treatment in conjunction with deep cryogenic treatment, and that the ratio between the hardness and the fracture toughness K_(Ic) of this steel can be optimised. From the results, we can conclude that the fracture toughness K_(Ic) of the investigated steel, calculated using equation (1), can be used to optimise the parameters for vacuum heat treatment alone or in conjunction with deep cryogenic treatment. We have proved that, for given vacuum heat treatment conditions, the volume fraction of retained austenite and the hardness have a significant effect on the fracture toughness K_(Ic) of the steel. The important factor increasing the steel1s fracture toughness is an increased volume fraction of retained austenite that is stable after tempering in the steel initially austenitised at 1230 deg C. The highest hardness and the lowest fracture toughness are obtained in the case of vacuum heat treatment conditions under which virtually all the retained austenite in the matrix is transformed to martensite (K and N). The deep cryogenic treatment of metallographic specimens and modified "Navy C" rings, performed after vacuum quenching, prolongs the austenite-to-martensite transformation and, therefore, greater dimensional stability (external and internal diameter A and B) results after any subsequent tempering. However, the shape distortion (separation at opening D), due to the transformational stresses in conjunction with the thermal stresses, might increase significantly. The rod-like carbide precipitation in the investigated AISI M2 high-speed steel occurs for given vacuum heat-treatment conditions only during tempering following deep cryogenic treatment. The size of these particles, precipitated from the martensite, depends on the tempering temperature. They are coarser after tempering at 500 deg C and finer after tempering at 540 deg C.