Evolution of microstructure and mechanical properties during tempering of M50 steel with Bainite/Martensite duplex structure

The evolution of microstructure and mechanical properties during tempering of M50 steel with bainite/martensite (B/M) duplex structure have been investigated. The microstructural observation shows that the redistribution of carbon atoms and precipitation of transition carbides in the B/M duplex specimens are slightly decreased during the initial stage of tempering, as compared with the fully martensitic specimens. After tempering of 550 °C, not only finer ferrite packets are obtained, but also the precipitation of nano-scale carbides is enhanced for the B/M duplex specimens. The mechanical characterization indicates that the hardness is decreased while the impact toughness is significantly improved when tempering temperature increases to 300 °C. When the tempering temperature increases to 550 °C, due to the decomposition of retained austenite (RA) and precipitation of alloy carbides, the hardness exhibits an obvious improvement while the toughness first decreases and then increases. The impact toughness of the B/M duplex specimens is higher than that of the fully martensitic specimens during the whole tempering process, although they have the same tendency varying with tempering temperature. Furthermore, the final wear resistance is significantly improved in the austempered specimens due to the enhanced precipitation during tempering.