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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.

No abstract

Flaking failure in rolling contact fatigue (RCF) of hardened bearing steel under well controlled lubrication is known to originate primarily from non-metallic inclusions. Among several different types of defects inevitably present in steels, the influence of sulphides on the RCF performance of steel is an issue that still raises many questions and controversies. In the present study, our objective is to investigate this matter by observing cracks initiating from sulphides after RCF testing. To accelerate bearing failure, high Hertzian stresses of 5·3 GPa were used. It was found that cracks initiated from the elongated tips of sulphides and propagated to a direction parallel to the direction of load movement. This rule was true regardless of the relationship between the direction of load movement and the elongation direction of sulphides. We concluded that sulphides could be a dominant factor in RCF life when the harmful oxide effect was excluded.

The improvement in rolling contact fatigue life is a key subject to increase the reliability of bearings. It has been well known that the fatigue properties are significantly affected by nonmetallic inclusions in steel. The rolling contact fatigue life may be divided into two types, according to the type of reliability requirements. One is “L10 life,” which represents general bearing performance. The other is “accidental short life,” where a bearing prematurely fails in its service period and its calculated life is hardly met. Since the probabilities of these failures are significantly different, the distribution densities of the corresponding life-limiting inclusions are also expected to be widely different. When nonmetallic inclusions are evaluated as an index of fatigue life, therefore, testing volume should be appropriately determined according to the concerned type of life. From this point of view, it is effective for the reliability assessment of bearing steel to combine the evaluations of large microscopic inclusions by statistics of extreme value and macroscopic inclusions in larger volume by a high-frequency ultrasonic test. It was found that the L10 life determined by thrust-type rolling-contact fatigue tests was well correlated with maximum micro-inclusion diameter predicted by the statistics of extreme value, where both oxide and sulfide played as life-limiting inclusions. Fifteen MHz ultrasonic testing was employed to evaluate the macroscopic inclusions. The combination of these two procedures was able to demonstrate the difference in reliability clearly, even when other cleanliness indices could not characterize the concerned materials.