Mechanisms of microstructural damage during rolling contact fatigue of bearing steels

摘要

Bearings are employed in a number of applications under extremely demanding conditions. During long operation times, the material undergoes rolling contact fatigue where microstructural damage manifests as dark-etching regions and white-etching areas, which display different properties from the surrounding region. The aim of this study is to identify the mechanisms for such damage and to suggest models that can explain the influence of the initial microstructure and test conditions.In order to appraise the stress state in rolling contacts, two testing techniques wereemployed and it was examined if the testing methods could reproduce the same damageas in bearing operation. During ball-on-rod fatigue testing, microcracks were generatedadjacent to inclusions and some were decorated with white-etching areas. Repetitive pushtests showed a similar extent of subsurface hardening compared to the ball-on-rod tests,and allowed the strain per stress cycle to be measured.The microstructural alterations in a white-etching area were studied both on a macroscaleand on an atomic-scale. The degree of stress concentration near a microcrack was calculated employing a nite element method. The microstructure, as well as the segregation behaviour of alloying elements in the white-etching area, were investigated byemploying transmission electron microscopy and atom probe tomography. A nanocrystalline structure with scattered carbide particles was observed in the white-etching area. Carbon and silicon segregation was highly pronounced in some boundaries of dislocation cell structures.Models were suggested to account for the microstructural alterations during rollingcontact fatigue. Carbide coarsening in dark-etching regions was modelled by consideringhow carbon di usion is assisted by dislocation glide. The predicted hardness evolutionwas consistent with experimental observation. The kinetics of carbide dissolution inwhite-etching areas was calculated by taking two processes into account: deformation accumulation and carbon diffusion. These models suggest that the microstructural changes during bearing operation can be controlled by tailoring the initial microstructure and managing the test conditions.