Influence of Material and Heat Treatment on the Formation of WECs on Test Rig FE8

Abstract

White etching cracks (WECs), or white structure flaking, is a failure phenomenon that has been reported as emerging in rolling bearing parts for more than the past two decades. This failure mode appears only if, in addition to the normal Hertzian load, a so-called additional load is applied. WECs can occur regardless of the bearing type and size and in both oil- and grease-lubricated bearings. By modifying the testing conditions of an FE8 type test rig, it is possible to alter the failure mechanism from classical wear failures to WEC formations. This allows for easy, fast, and reproducible testing of materials and heat treatment variants to check their WEC formation propensity. This paper explains the methodology of testing on a FE8 test rig, the subsequent failure analysis, and the testing results. The tested components are produced from a wide range of commonly used bearing materials that in these tests are through hardened variants of low and high alloyed steels, case carburized steel with high retained austenite content, and inductive hardened quenched and tempered steel. In addition to the spread in the base materials with different contents of carbon and alloying elements, the variations in materials and heat treatments offer the possibility of significantly varying the content of retained austenite and the residual stress distribution. The results show that all the tested low alloyed materials fail with the WEC mechanism, whereas a significant increase in lifetime can be observed in terms of inductive hardened quenched and tempered steel. Only the nitrogen alloyed stainless steel Cronidur 30 demonstrates an outstanding performance without WEC formation.

The influence of lubricant formulation on early thrust and radial bearing damage associated with white etching cracks

“…It is important to note that the representativity of such so-called ''WEC-critical'' oil formulation can be questioned compared to modern wind turbine oil formulations. Similarly, some authors 45 sub-surface cracks will initiate even in steels of highest cleanliness or toughness, as observed in structural fatigue.…”

“…It is important to note that the representativity of such so-called “WEC-critical” oil formulation can be questioned compared to modern wind turbine oil formulations. Similarly, some authors 45 conclude based on FE-8 81212 tests that the role of steel cleanliness has no effect with respect to WEC generation. Because the FE8 81212 tests with so-called WEC-critical oils might promote weakening of the bearing steel, early sub-surface cracks will initiate even in steels of highest cleanliness or toughness, as observed in structural fatigue.…”

Accelerated White Etch Cracking (WEC) FE8 type tests of different bearing steels using ceramic rollers

“…Several WEC resistant bearing materials based on surface treatments are currently available, including carburization, induction hardening and black oxide coating as well as bearings made from high-end expensive nitrogen rich steels. However, either these solutions do not offer 100 % guarantee against WEC or are significantly more expensive than current standard bearings [22][23][24].…”

“…This can include using hydrogen pre-charging of the specimens [26][27][28], using specific lubricants [29][30][31], introducing excessive slip [32][33][34] or external electrical currents [35,36]. One of the most common accelerated WEC tests is the FE8 type test rigs with axial cylindrical roller bearings, which have been extensively used for studying WEC [7,14,15,17,24,29,30,[37][38][39][40][41][42]. It relies on the inherent slip in 81212 type bearings, as well as using WEC inducing lubricants.…”

“…Therefore, tests must either include interruptions for changing the through hardened steel rolling elements or use ceramic rollers, which are unaffected by WEC as previously has been demonstrated in Ref. [24].…”