There are various ways of classifying the numerous types of rolling bearing damage. One option is to make a rough distinction between wear and fatigue damage. The causes of wear damage range from incorrect installation and incorrect design to lubrication problems. In the event of damage, we can use materialography to make all of these microstructural errors visible to you. Since all microstructural changes have different causes, you can use the information obtained from metallography to prevent them in the future and extend the service life of your bearings, gears or other machine parts!
Conventional microstructural changes due to fatigue include the formation of dark etching regions (DER), white etching bands (WEB) and butterflies (left image). These changes all occur in the region of maximum shear stress (Hertzian stress).
DERs and WEBs are named after their appearance after etching and are significantly softer than the surrounding material. Butterflies form on impurities in the steel, with the "wings" represented by a white etching area (WEA) and are significantly harder than the matrix.
Another non-conventional change in the microstructure due to fatigue is white etching cracks (WEC) (right image), sometimes also called white structured flakings (WSF). WECs are a common cause of premature failure of bearings in wind turbines. WECs are very thin cracks that grow radially or tangentially in the rolling bearing rings and on which white etching areas (WEA) form (Figure 04). The causes of the formation of WEC have not yet been sufficiently clarified; the hydrogen hypothesis is the most likely. According to this theory, the penetration of hydrogen into the material locally reduces the yield point and the aforementioned structural changes occur. The diffusion of atomic hydrogen is probably promoted by a combination of material defects, high loads and chemical lubricant problems. The WEAs on the WECs can be characterized as nanocrystalline ferritic structures supersaturated with carbon, which have a higher hardness than the surrounding matrix.
