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@PHDTHESIS{Mayweg:815142,
author = {Mayweg, David},
othercontributors = {Raabe, Dierk and Mayer, Joachim},
title = {{M}icrostructural characterization of white etching cracks
in 100{C}r6 bearing steel with emphasis on the role of
carbon},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2021-02467},
pages = {1 Online-Ressource : Illustrationen},
year = {2021},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2021},
abstract = {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 $at\%$ stabilize grain sizes down to less
than 10 nm. No evidence of carbide formation was found
inside WEAs. Carbon atoms partition between grain interior
and grain boundaries, both increasing as the total local
carbon content increases. The presented investigations
furthermore support the assumption that WEAs are always
present on both sides of a WEC, even if the appearance on
the two sides of cracks is strongly asymmetrical. This
asymmetrical distribution is construed as an indication that
cracks are a precondition for the formation of WEA.
Structure and chemistry at and around interfaces of WEAs and
bainitic matrix indicate that plasticity takes place there.
This finding indicates that transformation into WEA proceeds
from these interfaces into the base material. Structure and
chemistry at and around the WEA-matrix interfaces show that
decomposition occurs in a narrow region with a width of
several tens to a few hundred nm. This confined nature is
probably a consequence of cyclic loading. The smooth
morphology of WECs flanks as compared to ‘conventional’
faceted fatigue cracks and even WEC tips indicates that
crack surface rubbing is causing a smoothing of asperities.
However, crack surfaces are faceted on the nanoscale and
still exhibit puzzle-like fits, which is a strong indication
that WECs partially reweld during combined compressive and
shear loading and reopen during unloading. This mechanism is
thought to cause material transfer leading to lateral crack
displacement.},
cin = {523110 / 520000},
ddc = {620},
cid = {$I:(DE-82)523110_20140620$ / $I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2021-02467},
url = {https://publications.rwth-aachen.de/record/815142},
}
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