Tempering-Induced Microstructural Evolution and Precipitation-Controlled Strengthening in Cronidur 30 Bearing Steel

The effects of tempering temperature on the microstructure and mechanical properties of Cronidur 30 bearing steel were investigated by XRD, SEM, TEM, EBSD, and mechanical testing. The microstructure evolves systematically with increasing tempering temperature. At 400 °C, the matrix is dominated by lath martensite with fragmented boundaries. At 450 °C, the martensitic laths are refined, the geometrically necessary dislocation (GND) density reaches its maximum, and carbonitrides are most finely and uniformly distributed. When the tempering temperature increases to 500 °C, cementite enrichment along lath boundaries becomes evident. Further increasing the temperature to 550–600 °C promotes the formation of tempered sorbite and coarsening of carbonitrides. At 450 °C, the average surface carbonitride size decreases to approximately 0.39 μm, while the GND density reaches 20.80 × 1014 m−2. Under this condition, the hardness and tensile strength reach peak values of 60.3 HRC and 2226.5 MPa, respectively. The hardening response at 450 °C is attributed to the combined effects of fine carbonitride strengthening, high dislocation density, and grain-boundary strengthening.