Abstract
Substantial microstructural changes, such as Dark Etching Region (DER), White Etching Bands (WEBs) and White Etching Cracks (WECs), can occur in typical bearing steels such as AISI 52100 and 4320 due to Rolling Contact Fatigue (RCF). Although it has been reported that DER and WEBs typically appear in bearings over extended rolling cycles (>100×106) as a result of material deterioration under high-stress RCF while WECs are found to occur in much earlier of bearing’s life (<20% the calculated L10 life), differences and relationship between DER/WEBs and WECs are not fully investigated or understood. This study investigated the microstructural alterations in WECs in an AISI 52100 through-hardened martensitic bearing using a combination of characterisation methods and the results are directly compared with those observed in DER and WEBs due to high-stress RCF (Šmeļova et al., 2017). The results show that, although the features maybe initiated by different causes, there are many similarities between the microstructural alterations in WECs and DER/WEBs, such as the carbide disintegration, and carbon and chromium movement that plays a role in the formation and development of microstructural alterations in all three features.
Highlights
In recent years, ambitious worldwide renewable energy targets have resulted in significant growth of the wind energy sector’s market share
This paper presents the results from analogous detailed analyses on the microstructure alterations in White Etching Cracks (WECs) using a combination of modern microstructure characterisation techniques similar to those used for the Dark Etching Region (DER) and White Etching Bands (WEBs) in [1], including Scanning Electron Microscope (SEM), Electron Backscatter Diffraction (EBSD), Energy-dispersive X-ray Spectroscopy (EDX), Transmission Electron Microscopy (TEM), and nanoindentation
The microstructural variations observed in the WEC using a range of advanced techniques are firstly presented which are compared with those found in DER and WEBs [1]
Summary
Ambitious worldwide renewable energy targets have resulted in significant growth of the wind energy sector’s market share. The subsurface initiation hypothesis suggests that WECs initiate from material defects or stress raisers (e.g. inclusions, voids, large carbides, etc.) in the maximum shear stress region under the bearing contacting surface network and propagate towards the bearing surface leading to axial cracking or WSF failures. The term ‘white etching’ comes from the white appearance of the microstructure after being etched, typically with Nital or Picral, and observed under Light Optical Microscopy (LOM) It is still unclear whether microcracks or WEAs form first in a WEC or whether it is a cooperative growth of the two. It was suggested that elongated grains within WEA may have the tendency to be etched more with Nital due to its chemical composition, for example different carbon content, and had been interpreted as microcracks. A comparison between the WECs and DER/WEBs is given in the discussion section to provide an insight into the relations between these microstructural alterations in the AISI 52100 through-hardened martensitic bearing steel
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