Evaluation of Dark Etching Regions for Standard Bearing Steel under Accelerated Rolling Contact Fatigue

Abstract

Subsurface microstructural alterations are formed in the later stages of rolling contact fatigue (RCF) under high contact pressure. The subsurface changes observed as a dark contrast under optical microscopy are classified as Dark Etching Regions (DERs). Despite the fact that DERs have been presented for several decades, the understanding of its development and growth is yet to comprehend. Current research employed a modified high-speed microprocessor rotary tribometer to conduct systematic RCF study under accelerated testing conditions with variable temperatures and contact pressures. Comprehensive RCF data has been acquired, analysed and is reported for the very first time with ball-on-ball point contact loading conditions. The subsurface microscopic investigations have shown the ongoing progression and development of DER extent and are reported to be associated with the accumulation of plasticity during RCF. The comparison of the DER with the responsible stress components have revealed that DER formation is more closely related to the von Mises stresses when superposed with residual stresses. The experimentally observed area fraction of dark etching zones has been evaluated in terms of DER% and compared with the dislocation assisted carbon diffusion model for DER formation. The overprediction of the numerical model in comparison with the presented results in current research manifests its limitations which can be improved with the incorporation of cyclic plasticity governed by evolved von Mises stresses. The evaluated DER results are further presented as a 3D DER% maps incorporating the combined effects of contact stress, temperature and rolling cycles simultaneously which provides a comprehensive understanding of RCF from a microstructural point of view and thus can be used as guidelines for DER formation models.

A 3D Finite Element Model of Rolling Contact Fatigue for Evolved Material Response and Residual Stress Estimation

“…The spread of dotted lines represents the development of minimum and maximum depth of microstructural alterations from a few million cycles to tens of million cycles. It can be seen that the wider spread of von Mises maxima with progressive RCF cycles corroborates well with formation of DERs which is comparable from a recent study [36]. With the subsurface plasticity accumulation, lowering of von Mises stress profile indicates lowering of yield stress which coincides with the depth range of WEBs formation.…”

“…40 °C, as thermal diffusivity of Carbon can significantly affect the formation of WEBs as discussed in previous studies [36,41]. Nevertheless, the evolved Mises criteria can define the evolution of microstructural alterations in well agreement with the experimental data [36] and can be used to develop formation models for DERs.…”

“…dark etching regions (DERs) and white etching bands (WEBs), are directly associated with the accumulation of plasticity which in turn related to the applied alternating stress range; the evolution of such Mises stress profile also helps to retard the development of visible microstructural alterations. Recent studies [34,36] have shown that microstructural alterations are governed by the maximum distortion energy yield criteria during multiaxial RCF loadings where subsurface plasticity is governed by equivalent von Mises stresses. Inclusion of deep zone residual stresses with von Mises stress profile can better explain the formation mechanism of such microstructural alterations.…”

“…12 Simulated and numerical stress profiles acquired for (a) 4 GPa contact stress and (b) 6 GPa contact stress. Dotted lines represent microstructural alteration depth range under contact track, *obtained from 4-ball RCF testing [36], whereas the spread of these lines shows the continuous growth with progressive stress cycles and 6 GPa contact stress are obtained from [36] and plotted in Fig. 12.…”

“…In this configuration, three lower balls are kept in a cup which acts as outer race of the bearing. The upper ball is mounted on a shaft with the help of a collet and rotates on top of lower three balls which forms a contact track similar to the inner raceway of a bearing [35,36], following Fig. 1.…”