Effect of austenitizing temperature on isothermal quenching microstructure and mechanical properties of 52100 bearing steel

In general, isothermal quenching heat treatment in salt bath can improve the toughness but sacrifice the strength and hardness of 52100 bearing steel. Many previous studies focusing on the isothermal quenching process have been carried out to balance the strength and toughness. And the optimal mechanical properties are limited to approximately 60–61 HRC hardness and 100 J impact toughness. In this study, in order to further improve the mechanical properties, the influences of austenitizing temperature on microstructural evolution and strengthening-toughening mechanism in isothermal quenched bainite in 52100 bearing steel have been systematically investigated. The increase of austenitizing temperature decreases the volume fraction of undissolved carbides, and increases the carbon concentration in austenite matrix. These changes not only strongly influence the kinetics of bainite phase transformation and bainite microstructure, but also cause the conflict between the coarsening of prior austenite grains, as well as bainite sheaves, and the refining of substructure of lower bainite. Austenitizing at 870 °C can balance this conflict well and obtain a good combination of hardness and toughness. It is found that the carbon content and dislocation density in bainite ferrite reach the peak value at 870 °C austenitized specimens, which effectively strengthens the bearing steels. At the same time, the 870 °C austenitized specimens retain appropriate content of undissolved carbides, which avoids stress concentration and inhibits crack propagation, and finally improve the impact toughness. Therefore, the optimal austenitizing temperature (870 °C) achieves the improvement of mechanical properties with the hardness and toughness of 59.7 HRC and 273 J, respectively.