Effect of microstructure on the fatigue crack growth behaviour in low carbon steel

In order to optimize heat treatments of large-scale rolling bearings, the relationships between microstructure and mechanical properties need to be thoroughly understood. In the current study, the effect of microstructure on fatigue life, fatigue crack growth threshold (ΔK_th) and fracture toughness were investigated for 18NiCrMo14-6, typically used for large-size bearings. The martensite microstructure was processed by conventional quenching and tempering, and the two-phase microstructures, martensite+bainite, and bainite+austenite were processed by austempering. Fatigue crack propagation properties of two-phase microstructures were compared with three martensitic microstructures tempered at different temperatures to match the tensile properties of those of the two-phase microstructures. Fatigue crack growth tests were carried out and the ΔK_th values of both dual-phase microstructures are higher than that of the martensitic counterparts. In contrast, it was found by uniaxial tensile fatigue tests using cylindrical specimens, that the fatigue strength at 10⁷ cycles of the two-phase microstructure is lower than that of the martensite single-phase microstructure. It is also postulated from fracture surface observations that the variations in ΔK_th seen in different microstructures result from plasticity-induced crack closure and roughness-induced crack closure.