Carbides precipitation and their evolution of Cr15Co10Mo5-alloyed heat-resistant bearing steel after tempering at different temperatures

The carbides precipitation and their evolution at elevated tempering temperature in Cr15Co10Mo5-alloyed heat-resistant bearing steel were investigated by means of the transmission electron microscope. The results show that there is no carbide precipitated from the martensitic matrix when the sample was tempered at 480 °C. However, when the sample was tempered at 540 °C, a large number of stable spherical M₆C carbides precipitated in the test steel. Nevertheless, there are three types of carbides precipitated from the matrix including M₆C, M₂C and M₂₃C₆ carbides when the tempering temperature reached 600 °C. The mechanical properties also present a correlation with the evolution of carbides during tempering at different temperatures. This indicates that the strengthening mechanism of the steel is mainly attributed to the precipitation of carbides and their evolution, including the morphologies and types of carbides at different temperatures. In addition, the austenitic layers with a thickness of about 30 nm have been obtained between the martensite laths after tempering at 600 °C. The austenitic layers will produce transformation-induced plasticity effects to improve the toughness of the steel.