Effect of Tempering and Deep Cryogenic Treatment on Microstructure and Tribological Properties of M50 Steel

The quenched M50 steel was subjected to triple tempering, tempering + deep cryogenic treatment + tempering, and deep cryogenic treatment + double tempering separately; the tempering process was at 550 ℃ for 2 h and the deep cryogenic treatment was at −196 ℃ for 1 h. The effects of tempering and deep cryogenic treatment on the microstructure and tribological properties of the test steel were studied. The results show that after quenching and different tempering and deep cryogenic treatments, fine secondary carbides were precipitated in the steel, the retained austenite content decreased, the martensite content increased, the hardness of test steel was improved and both the friction coefficient and wear rate of test steel decreased. With the introduction of deep cryogenic treatment, the content of retained austenite was lower, the secondary carbides became finer and more numerous, and the martensite structure was further refined. Therefore, the hardness and the wear resistance of test steel was improved. The test steel subjected to deep cryogenic treatment + double tempering exhibited the lowest retained austenite content, the highest hardness, and the best wear resistance. After introducing deep cryogenic treatment, the main wear mechanism of the test steel was adhesive wear. The test steel treated with deep cryogenic treatment + double tempering showed the flattest wear surface, and showed slight oxidative wear feature.