Study on Flaking Process in Bearings by White Etching Area Generation

Abstract

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.

Critical Assessment 13: Elimination of white etching matter in bearing steels

“…in making tougher steel. 30 Such a hypothesis is able to explain all the observations listed in the preceding section. Hydrogen enhances the formation of white etching matter because it reduces the fracture toughness of the steel and hence promotes cracking.…”

Tribological advancements for reliable wind turbine performance

“…A catalytic interaction with Fe is the proposed mechanism that dissociates hydrogen atoms from water and/or organic molecules; however, this type of surface chemistry is only known to occur on transition metal surfaces in ultrahigh vacuum. Several studies by Hyde (1996), Ochi et al (1999) and Hiraoka et al (2006) have observed that high cycle loading applied to highly stressed regions around nonmetallic inclusions like Al 2 O 3 in bearing steels are known to produce 'butterfly' cracks, and white-etch areas were found to occur on one side of the crack.…”

“…In their study of AISI 52 100 bearing steel that had endured 3.9 GPa cyclic stresses, Hiraoka et al (2006) proposed that microcracks form first, which then generate white-etch areas. More recently, Grabulov et al (2007) performed rolling contact fatigue testing at 2.6 GPa for 130 million revolutions on AISI 52 100 articles, then examined the microstructure in the wear tracks using transmission electron microscopy on specimens removed by focused ion beam milling.…”

Recent developments in bearing steels

“…7 It is likely that the white matter is a symptom of microscopic cracks which form first, but then develop into severely deformed regions due to the beating of their parted surfaces, caused by the cyclic stresses associated with rolling contact fatigue. [8][9][10][11][12] The presence of hydrogen makes the problem worse by reducing the fracture toughness of the steel, which for common bearing steels is poor even without hydrogen.…”