Abstract: In order to improve the uniformity of microstructures, and thus enhance the mechanical properties, especially the impact toughness, of 52100 bearing steels, modified quenching heat treatments with different austenitizing processes have been designed. The results show that 740 °C is the optimal holding temperature (approaching Ac₁ = 750 °C) to improve the impact toughness of the steel by 37% after quenching and low-temperature tempering. Microstructure observations reveal that 740 °C holding shortens the nucleation incubation period of austenitization phase transformation, and inhibits the overfast growth of austenite in some regions, which leads to simultaneous nucleation and growth of austenite during austenitizing, and homogeneous dissolution of carbides in different austenite grains. The changes in microstructure evolution improve the microstructure uniformity (carbide and grain size) after quenching. Uniformly distributed carbides and grain size effectively reduce stress concentration and retard crack propagation during impact loading, thus resulting in high impact toughness.

Key words: Austenitizing process, Carbides distribution, Impact toughness, 52100 steels