Tempering influence on microstructural evolution and mechanical properties in a core of CSS-42L bearing steel

The microstructures in the core of CSS-42L bearing steel, including the hierarchical packet-block-lath martensite, residual austenite (RA) and carbide were tailored by employing different heat treatment with the emphasis on tempering and then carefully characterized. The microstructure, strength and toughness were investigated in detail as a function of tempering temperature. The results reveal that the tempering at 490 °C and 520 °C primarily induce fine secondary M6C precipitates which are responsible for the precipitation strengthening. When tempered at 550 °C, the strength further promoted significantly, which could be contributed to the dominating coherent M2C precipitation by RA decomposition. Toughness of this steel is mainly determined by the number density and mechanical stability of austenite grains retained since the propagating cracks can be trapped by them, leading to improved toughness. At 520 °C, both the impact toughness and the fracture toughness reach their peak values, which are attributed to the more RA possessing mechanical stability in the microstructure.