Microstructure change of bearing steel in rolling contact fatigue
Material degradation is the main cause of rolling bearing failure. The best way to predict bearing failure is to have a deep understanding of material degradation mechanisms. This not only helps to select the best steel and heat treatment process for the specific application to achieve the desired performance of the bearing, but also enables more accurate prediction of the remaining life of the bearing.
The rolling contact of the bearing raceway produces a circulating stress that extends from the surface to the subsurface area. If the cyclic stress exceeds the local strength limit of the material, the microstructure will be damaged or degraded, and the raceway surface may eventually peel off due to rolling contact fatigue.
Under conditions of poor lubrication (viscosity ratio κ< 1), metal to metal contact on rough surfaces will produce high surface stress. Even surface traction caused by a small amount of sliding of the contact surface results in a higher surface stress. This stress can lead to fatigue damage to the surface, which usually manifests as micro-spalling or pitting.
Under good lubrication conditions (κ> 2) For running rotating bearings, the maximum shear stress exists at a certain depth below the raceway surface. Cyclic shear stress resulting from rolling contact can cause fatigue damage and eventually lead to sub-surface origin spalling of raceways (Figure 1). Under rolling contact fatigue conditions, two types of material degradation can occur. If the maximum contact stress exceeds a certain limit (called the elastic stability limit), progressive plastic flow occurs in the subsurface region of the material, causing progressive microstructure changes that eventually lead to raceway spalling.
However, when the contact stress is below the elastic stability limit, local damage (depending on local stress conditions) may still develop from material defects such as non-metallic inclusions due to stress concentration effects. This type of damage has what is known as a butterfly damage feature, in which a single or multiple crack initiation occurs from an inclusion or pore. The propagation of cracks is accompanied by the development of white etched areas. Under certain conditions, the expansion of secondary surface cracks may also lead to raceway surface spalling.
Local fatigue damage caused by bearing steel material defects has been discussed in previous articles [Reference 1]. These types of fatigue damage are closely related to large and medium-sized roller bearings. Based on a recent review paper [Reference 2] published by the authors, this paper aims to summarize the studies published in the literature on the microstructure change of bearing steel, i.e. large-scale material degradation under rolling contact fatigue. The latter type of material degradation is closely related to small and medium-sized bearings operating at higher contact pressures.
