Modelling and characterisation of stress-induced carbide precipitation in bearing steels under rolling contact fatigue

The nucleation and growth of lenticular carbides (LCs) in bearing steels occur near to deformed ferrite bands after exposure to prolonged rolling contact fatigue (RCF). Since the first observations in 1947, a large number of attempts have been made to explain the formation mechanisms of such stress-induced microstructural alterations, but a reliable model was still not available. In this research, a novel theory is proposed to describe the carbon redistribution process during LC formation. The theory suggests a dislocation assisted LC growth mechanism on the basis of the classic Cottrell atmosphere formation theory. The mechanism considers (1) JLC=Jd, the carbon flux equilibrium between LC thickening (JLC) and dislocation-assisted carbon migration (Jd), and (2) M0=MLC+Mb, the carbon mass conservation of the system, where M0 denotes the total amount of carbon within the system, MLC denotes the amount of carbon within a LC, and Mb denotes the amount of carbon left within the ferrite band, respectively. The solution to these two equations, which addresses the problem that has been puzzling researchers for several decades, makes good predictions on LC thickening rate under various testing conditions. The stress-induced carbide precipitation was examined using high resolution characterisation techniques such as scanning and transmission electron microscopy, obtaining significant evidence to support the postulated theory. The successful description of LC growth implies a potential extension of the theory to other types of stress induced microstructural changes in bearing steels where carbon redistribution occurs. The model presented here provides a more comprehensive understanding of RCF from a microstructural point of view, and thus can enhance the accuracy of traditional bearing life prediction approaches.