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TY  - THES
AU  - Mayweg, David
TI  - Microstructural characterization of white etching cracks in 100Cr6 bearing steel with emphasis on the role of carbon
PB  - Rheinisch-Westfälische Technische Hochschule Aachen
VL  - Dissertation
CY  - Aachen
M1  - RWTH-2021-02467
SP  - 1 Online-Ressource : Illustrationen
PY  - 2021
N1  - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
N1  - Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021
AB  - White etching cracks (WECs) are a characteristic premature failure phenomenon found in components that experience very high cycle rolling contact fatigue (> 109 cycles). The issue is most prevalent in bearings but in a similar form also affects other applications such as rails. The most notable WEC occurrences are related to failures in gearbox bearings of wind turbines. Most of the bearings affected are made from high carbon bearing steel 100Cr6 and similar grades used in a bainitic condition. WECs manifest in microstructural alterations that are called white etching areas (WEAs). They are nanocrystalline ferritic regions located directly adjacent to the cracks and extend several tens nm to several µm around WECs. Their white appearance in optical microscopy, which is caused by increased etching resistance compared to the unaltered material, is the defining feature of WECs. The current understanding is that cracks in large scale bearings initiate below the surface in several hundred µm depths at non-metallic inclusions, which are remnants of the steel manufacturing process. Once cracks have formed and propagate, the shear loading also leads to reciprocating sliding movements of the crack surfaces. This leads to severe plastic deformation, which causes decomposition of the initial microstructure resulting in nanocrystalline WEAs. What is currently lacking is a detailed understanding of the alterations at the nm scale and, most importantly, a mechanism-based explanation of the premature nature of WEC related failures. The present thesis aims at taking steps in this direction by contributing detailed microstructural analyses that aid in understanding the nature of WECs. To this end, a wind turbine gearbox bearing that failed due to WECs is investigated. Characterization techniques are used covering length scales from several mm to near-atomic distances. Analyses of the crystallographic structure are conducted using scanning and transmission electron microscopy techniques. Compositional analyses are performed employing spectroscopic methods such as X-ray spectroscopy and atom probe tomography. The results obtained show that, contrary to the expectation, the composition of WEAs does not equal the nominal alloy composition. In most cases, depletion in carbon is observed instead. Additionally, nanosized pure carbon deposits are found in WEAs with comparatively large grain sizes (several hundreds of nm). These results demonstrate that significant elemental redistribution occurs inside WEAs. The presence of pure carbon is in so far significant, as it renders the possibility of carbon being present at the crack surfaces. A consequence of this could be reduced friction that results in accelerated crack propagation. Furthermore, it was found that the varying grain sizes inside WEAs are corresponding inversely to the local carbon content. High carbon contents up to around ten at
LB  - PUB:(DE-HGF)11
DO  - DOI:10.18154/RWTH-2021-02467
UR  - https://publications.rwth-aachen.de/record/815142
ER  -