<p>Rolling
contact fatigue (RCF) is a common cause of failure in tribological
machine
components such as rolling-element bearings (REBs). Steels selected for RCF applications are
subject to various material processes in order to produce martensitic
microstructures. An effect of such
material processing is the retention of the austenitic phase within the steel
microstructure. Retained austenite (RA)
transformation in martensitic steels subjected to RCF is a well-established
phenomenon. In this investigation, a
novel approach is developed to predict martensitic transformations of RA in steels
subjected to RCF. A criteria for phase
transformations is developed by comparing the required thermodynamic driving
force for transformations to the energy dissipation in the microstructure. The method combines principles from phase
transformations in solids with a damage mechanics framework to calculate energy
availability for transformations. The
modeling is then extended to incorporate material alterations as a result of RA
transforming within the material. A continuum
damage mechanics (CDM) FEM simulation is used to capture material
deterioration, phase transformations, and the formation of internal stresses as
a result of RCF. Crystal lattice
orientation is included to modify energy requirements for RA transformation. Damage laws are modified to consider residual
stresses and different components of the stress state as the drivers of energy dissipation. The resulting model is capable of capturing
microstructural evolution during RCF.</p>
<p>The development and stability of
internal stresses caused by RA transformation in bearing steel material was
experimentally investigated. Specimens
of 8620 case carburized steel were subjected to torsional fatigue at specific
stress levels for a prescribed number of cycles. X-ray diffraction techniques were used to
measure residual stress and RA volume fraction as a function of depth in the
material. A model is set forth to
predict compressive residual stress in the material as a function of RA
transformation and material relaxation.
Modeling results are corroborated with experimental data. In addition, varying levels of retained austenite (RA) were
achieved through varying undercooling severity in uniformly treated case
carburized 8620 steel. Specimens were
characterized via XRD and EBSD techniques to determine RA volume fraction and
material characteristics prior to rolling contact fatigue (RCF). Higher RA volume fractions did not lead to
improvement in RCF lives. XRD
measurements after RCF testing indicated that little RA decomposition had
occurred during RCF. The previously
established RCF simulations were modified to investigate the effects of RA
stability on RCF. The results obtained
from the CDM FEM captured similar behavior observed in the experimental
results. Utilizing the developed model,
a parametric study was undertaken to examine the effects of RA quantity, RA
stability, and applied pressure on RCF performance. The study demonstrates that the energy
requirements to transform the RA phase is critical to RCF performance.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/15079056 |
| Date | 30 July 2021 |
| Creators | Dallin S Morris (11185158) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Investigation_of_Microstructural_Effects_in_Rolling_Contact_Fatigue/15079056 |
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