Phase transformation in white etching area in rolling contact fatigue

Yun-Shuai Su,Li-Biao Wan,Si-Yuan Lu,Shu-Xin Li,G Hénaff

doi:10.1051/matecconf/201816511004

Abstract

Rolling contact fatigue (RCF) involves microstructural change in the subsurface of contact. The changed microstructure is generally termed as white etching area (WEA) as it appears white under optical microscope when etching in nital solution. WEA has been acknowledged as one of the primary failure modes in RCF since it causes severe local inhomogeneity of microstructure. It was reported that WEA consists of nano ferrites as martensite grains and carbides are significantly refined in the WEA. Some carbides are dissolved. In some cases, an amorphous-like structure was occasionally observed in the WEA, indicating that phase transformation may possibly occur. The WEAs were studied by using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Electron back scattered diffraction (EBSD). The result showed that WEA is dominated with an amorphous phase with martensite, austenite and carbides embedded interior. A distinct interface between the matrix and the WEA was present. In addition to grain refinement down to nanometers, phase transformation including amorphization and austenitization happened in WEAs. The content of austenite was increased from 2% in the matrix to 20% in the WEA. The analysis showed that phase transformation is controlled by plastic deformation mechanism.

Highlights

The rolling bearings are always failed under rolling contact fatigue with much shorter lives than that designed, especially in wind turbine gearbox bearings
white etching area (WEA) was formed at the contact pressure of 0.8 GPa with the SRR of 10%, but was absent at 1.0 GPa and 15%
The same phenomenon occurred at 1.2 GPa and 10% and 1.4 GPa and 15%, where WEA was observed under the former condition but not under the latter

Summary

Introduction

The rolling bearings are always failed under rolling contact fatigue with much shorter lives than that designed, especially in wind turbine gearbox bearings. When inspecting cross-sections of failed bearings or samples after RCF using optical microscope, white etching matters such as white etching bands [1,2], white etching areas (WEAs) [3,4,5,6,7] and white etching layers (WELs) [8] are observed in different depths below the contact surface. These white microstructural changes are different from base materials in terms of compositions and hardness. We have provided the evidence for an amorphous phase exist in the WEA [3,4]

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Conclusion