Microstructure evolution, fracture and hardening mechanisms of quenched and tempered steel for large sized bearing rings at elevated quenching temperatures

Based on 42CrMo steel, a steel with a higher C and Ni content is developed for use in large sized bearing rings. The impact energy and hardness of the quenched and tempered steel increase with the quenching temperature, but then decrease when the temperature is above 925 °C. When the temperature is below 925 °C, some larger M 23 C 6 -type carbides (with average diameters of 255.6 μm) exist in the quenched and tempered microstructure. The number of carbides is reduced as the quenching temperature increases. At the same time, the fracture modes change from microvoid coalescence and quasi-cleavage to microvoid coalescence. The number of round quasi-cleavage fractures, which are formed around the carbides, decrease as the number of carbides decrease. The existence of larger M 23 C 6 -type carbides leads to round quasi-cleavage fractures and decrease the impact energy. The precipitation strengthening of M 23 C 6 -type carbides increases the hardness at a quenching temperature of 925 °C.