Formation of white etching cracks at manganese sulfide (MnS) inclusions in bearing steel due to hammering impact loading

Abstract

Wind turbine gearbox bearings (WTGBs) are failing prematurely, leading to increased operational costs of wind energy. Bearing failure by white structure flaking (WSF) and axial cracking may both be caused by the propagation of white etching cracks (WECs) and have been observed to cause premature failures; however, their damage mechanism is currently not well understood. Crack initiation has been found to occur at subsurface material defects in bearing steel, which may develop into WECs. One hypothesis for WEC formation at these defects, such as non‐metallic inclusions, is that repetitive impact loading of a rolling element on a bearing raceway, due to torque reversals and transient loading during operation, leads to high numbers of stress‐concentrating load cycles at defects that exceed the material yield strength. In this study, a number of tests were carried out using a reciprocating hammer‐type impact rig. Tests were designed to induce subsurface yielding at stress concentrating manganese sulfide (MnS) inclusions. The effects of increasing surface contact stress and number of impact cycles, with and without surface traction, were investigated. Damage adjacent to MnS inclusions, similar to that observed in a failed WTGB raceway, was recreated on bearing steel test specimens. It has been found that increasing the subsurface equivalent stresses and the number of impact cycles both led to increased damage levels. Damage was observed at subsurface equivalent stresses of above 2.48 GPa after at least 50,000 impact cycles. WECs were recreated during tests that applied surface traction for 1,000,000 impacts. Copyright © 2016 John Wiley & Sons, Ltd.

Threshold Maps for Inclusion-Initiated Micro-Cracks and White Etching Areas in Bearing Steel: The Role of Impact Loading and Surface Sliding

“…In general, the main forms of damage were observed at MnS inclusions, which were similar to those found in both failed WTG bearings [ 45 , 46 , 49 ] as shown in Fig. 1 and tested specimens [ 51 ].…”

“…In which case, it is likely that WEAs may have occurred at lower sliding/contact pressure thresholds if the number of RCF load cycles was higher. Evidence to support this hypothesis is presented in a study investigating the effect of impact loading on inclusion-initiated damage [ 51 ], since WEAs at inclusions were only found in the long-term impact tests with high load cycles.…”

“…The impact test rig described in Ref. [ 51 ] was used to seed damage in the test specimens at two opposite locations on the circumference of each disc representing bearing raceways. As illustrated in Fig.…”

“…It has been hypothesized that impact loading is a key factor affecting subsurface damage initiation at inclusions [ 51 ]. Figure 2 describes a hypothesis, showing that if a high-impact load event happens prior to a series of lesser loading events, then the damage is more serious than that if these events occur in the reverse order, due to the initiation of fatigue cracking at inclusions (this will be analysed in detail in Sect.…”

Defining the role of ‘zero wear volume’ in percussive impact

“…In order to ensure that the impact would repeatedly occur in the same contact region, thus reflecting the actual contact mechanism of components, and to enable a detailed study of that region under different impact parameters, a modified version of the reciprocating hammer type impact wear apparatus initially designed and then used previously by the authors [6][7][8] was used for this work.…”