Effect of Microstructure on the Fatigue Crack Growth Behaviour in Low Carbon Steel

Rolling bearings are widely used in mechanical equipment. Under the drive of energy and/or resource saving there is a need to improve their performance and reliability. One cost effective way towards high performance and reliability is to optimize microstructure through heat treatment. In large scale bearings there may also be a variation in microstructure between the surface and interior regions. Hence the relationships between microstructure and mechanical properties need to be thoroughly understood. In the current study, fatigue life, fatigue crack growth threshold (ΔKth) and fracture toughness of two variant microstructures were investigated for a nickel-chromium-molybdenum steel 18NiCrMo14-6, typically used for large-size bearings. The two variant microstructures include martensite+bainite, and bainite+austenite. They were processed by austempering at 723 K followed by water quenching and then tempering at 453 K. Their fatigue and fracture properties were compared with three martensitic microstructures tempered at different temperatures to match the tensile strength of those of the two variant microstructures. It is observed that the ΔKth value of both dual-phase microstructures is higher than the martensitic counterparts. A noticeable reduction in fracture toughness is found in bainite+austenite microstructure compared with a martensitic structure with a similar strength. The fracture toughness of the martensite+bainite microstructure is slightly reduced. Compared again to the martensitic structure, the fatigue strength of the bainite+austenite microstructure is also reduced slightly whereas it is broadly similar for the martensite+bainite microstructure. In general the martensite+bainite microstructure offers superior all-round mechanical properties, especially the damage tolerance related properties. Such improvement benefits performance and reliability of the rolling bearings. The mechanism of microstructural influence on fracture mechanics related properties is studied using SEM metallography and fractography.