Microstructural degradation of bearing steels
Microstructural degradation of bearing steels（轴承钢显微组织的失效过程）Abstract
The aim of the work presented in this thesis is to clarify one of the most fundamental aspects
of fatigue damage in bearings steels through critical experiments, in particular whether
damage in the form of cracks precedes hard “white-etching matter" formation, which is carbon
supersaturated nanoscaled ferrite. Heat treatments have been designed to create four
different crack types and distributions: scarce martensite plate cracks, fine grain boundary
cracks, abundant martensite plate cracks, and surface cracks. Subsequent rolling contact
fatigue experiments showed that the amount of hard white-etching matter is higher in precracked
samples compared to those without prior damage and that its formation mechanism
is the frictional contact of disconnected surfaces within the bulk that elevate the temperature
and localise deformation. These key experiments indicate that hard white-etching matter
is the consequence, not the cause, of damage. Therefore, one way to avoid white-etching
matter is by increasing the toughness of the material. The macroscopically homogenous
distribution of microcracks proved also to be a useful rolling contact fatigue life enhancer
due to damage deflection via crack branching and a powerful trap for diffusible hydrogen.
Successful trapping was corroborated by the inability of hydrogen to cause crack propagation
via embrittlement or accelerate white-etching matter generation during rolling contact
fatigue. By also studying the behaviour of a nanostructured bainitic steel under rolling
contact fatigue, it was found that its degradation mechanism is ductile void formation at
bainitic ferrite/stress-induced martensite interfaces, followed by growth and coalescence
into larger voids that lead to fracture along the direction of the softer phase as opposed
to the conventional damage mechanism in 52100 steel of crack initiation at inclusions and
propagation. Given the relevance of phase quantification in nanobainite and the possible
surface artefacts introduced by preparation, alternative methods to X-ray diffraction such as
magnetic measurements were also investigated. The lack of hard white-etching matter obtained
in the carbide-free nanostructured bainite led to conclude that an alternative route to
mitigate hard white-etching matter could be by eliminating pre-eutectoid carbides from the
microstructure, therefore restricting their dissolution and ultimate carbon supersaturation of
the mechanically deformed and homogenised nanoferrite.
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