Keywords –Diffusible hydrogen, dynamic loading, lubricant degradation, retained austenite, white etching areas
Despite more than two decades of research in white etching areas (WEAs), WEAs formation still persisting in wind turbine gearbox bearings, resulting in a serious reduction in desired life of the bearings (L10 life) by up to 90 percent. There are various driving factors that contribute to the WEAs formation; among which dissolved hydrogen is considered one of the root causes. Diffusible hydrogen reduces the critical resolved shear stress for dislocation motion to initiate. This phenomenon leads to accelerating the motion and accumulation of dislocations around stress raisers, resulting in the localisation of strain. The influence of hydrogen on such microstructure degradation can be minimized by introducing hydrogen traps in the microstructure, which reduces its mobility inside the matrix. Stabilising face centred cubic (FCC) retained austenite (RA) phase within martensite/tempered martensite matrix is known to act as irreversible trapping sites due to the high solubility and low diffusivity of hydrogen in FCC structures. Further, the tendency to form WEAs in bearing steel depends on the stability of carbides against severe plastic deformation and resistance to hydrogen induced degradation. In this research work, the role of size and volume fraction of carbides, and retained austenite against WEAs formation tendency in bearing steel is studied. Bearing ball samples with different carbide sizes and retained austenite volume fraction are tested against dynamic loading in the presence of two different lubricants using dynamic load pin-on-disc (PoD) test rig. The results from the study show that reducing the size of carbides improved their stability against plastic deformation leading to the stagnation of WEAs formation. Further, stabilizing RA in the microstructure proved to be beneficial in reducing hydrogen permeation and degradation of microstructure in a high hydrogen atmosphere.