Crack Behavior around a Nonmetallic Inclusion under Hydrogen Precharged Rolling Contact Fatigue

Abstract

Wind power generation is a technology that contributes to the reduction of greenhouse gas emissions and that is expanding on a global scale. Recent reports have indicated that premature failure accompanied by white etching microstructural changes occurs in bearings used in the gearboxes of wind power turbines, thus necessitating a deep understanding of fatigue behavior and the implementation of countermeasures based on this understanding. This type of flaking is considered to be related to hydrogen behavior in steel that results in a unique rolling contact fatigue (RCF) process. As fatigue progresses, a needle-like microscopic fatigue structure, confirmed by nital etching, is likely to appear in the highly stressed region of the matrix of quenched and tempered high-carbon chromium-bearing steels. Flaking is thought to occur via the formation of microcracks, the connection of those cracks, and white microstructural changes around the cracks. However, the fatigue behavior around nonmetallic inclusions in the presence of hydrogen has not yet been elucidated. We conducted a unique experiment to clarify the role of nonmetallic inclusions. An RCF test was conducted after hydrogen precharging using specimens with artificial inclusions. In this experiment, spherical oxide particles of known chemical compositions and sizes were used to simulate nonmetallic inclusions, making it possible to easily observe crack behavior around inclusions under RCF. Here, we report new findings on fatigue crack behavior around inclusions under hydrogen precharged conditions and the relationship between steel cleanliness and bearing lifetime under these conditions.