Volume 3, Issue 5 (May 2006)
Study on Flaking Process in Bearings by White Etching Area Generation
White etching area (WEA) is widely known as a microstructural change caused by rolling contact fatigue of a bearing. It has been reported that early flaking accompanied by the WEA occurred in such bearings as those for automotive alternators, and effective measures have been demanded. The WEA type microstructural change has so far been studied in detail, particularly for butterfly, which was metallographically characterized as follows: First, the WEA coexists with microcracks, which was initiated by nonmetallic inclusions and extended at the angle of 45 deg to the raceway. Second, according to the TEM study, the WEA consists of ultrafine grain as small as approximately 10 nm in diameter. The authors want to establish a counter measure against the early flaking from a material side. Its realization requires the clarification of flaking processes, especially the cause-effect relationship between microcrack and WEA, so that an appropriate measure is taken in the prevention of microcrack or WEA itself. In this study, high carbon chromium bearing steel (JIS SUJ2) specimens containing voids with a few µm diameter were prepared through powder metallurgy, and were subjected to rolling contact fatigue tests by thrust-type testers. Many 45 deg microcracks and WEA initiated by the voids were successfully reproduced just below the raceway. It was found by observation that a microcrack forms first and then WEA generation follows. In addition, the stress analysis by computer simulations found out that WEA generates only in the hydrostatic compressive region localized by the presence of the microcrack. Therefore, WEA in rolling contact fatigue is a secondary phenomenon preceded by a microcrack. It was concluded that an effective counter measure against the early flaking with WEA is to improve the resistance to microcrack initiation during rolling contact fatigue.