| Popis: |
White Etching Cracks (WECs) in bearings are found to cause unexpected failures and can result in economic losses in many applications, such as wind turbine gearbox and automotive components. Although the problem of WECs is established as important and many researchers have studied the phenomenon, the effect of different drivers, whether the initiation point is subsurface or surface and the order of formation of cracks and white etching areas (WEA) are still the subject of debate. A better understanding of these particular issues will help to inform the reliability of roller bearings by improving their design through further mechanistic insight. A systematic literature review was carried out first to assess previous relevant research related to WECS, such as the detailed microstructural features observed, proposed drivers of the phenomenon and initiation mechanisms in proposed bearing steel. In this project, the initiation mechanisms of WECs have therefore been further studied via experimental observation of a series of AISI 52100 steel bearing samples tested on an FAG-FE8 rig at Schaeffler under a contact pressure of 1.35 GPa over a range of test durations, where WECs at different stages were created. All bearing tests used the “WEC critical lubricant”, a common lubricant to create WECs. Microstructure characterisation techniques, including optical microscopy, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), were then used to characterize the different stages of WEC formation, with an especial focus on the initiation stages. The bearings with the longest test duration that have surface damage were examined first to help to determine the area where WECs were likely to initiate. These areas in tested samples with shorter testing duration time were then observed in detail to find the earliest stages of WECs. By comparing the different microstructure characteristics of WECs at these different stages and comparing with prior knowledge in the literature, a growth mechanism of WEA related to a transitional area containing crack-like structures and an initiation mechanism of WECs related to the aligned/clustered carbide aggregation have been hypothesised. |
