Description
From the automotive industry to next-generation turbines, bearings are critical components in a wide range of machinery for varied applications. Their capacity to resist wear and fatigue during service is a critical factor in improving maintenance intervals and reduces the risk of premature or catastrophic failure. The work presented in this thesis primarily aims to elucidate the relationship between processing parameters of steels for bearings and mechanical and thermal performance. Particularly but not exclusively, the role of retained austenite in the microstructures of these steels is examined, in which there is a long-standing tenet that carbon content in austenite acts to stabilise it from phase transformation. The existence of retained austenite within a microstructure has complex implications on a bearing's performance characteristics, including dimensional stability, wear resistance in contaminated environments, and work hardening behaviour. A wide range of processing parameters were applied to SAE 52100 steel that were designed to promote carbon diffusion into austenite retained during quenching. Firstly, the industry standard quenching and tempering process was examined via dilatometry with in-situ synchrotron X-ray radiation in order to elucidate the phase transformation kinetics during processing, and provide fundamental insight into microscopic behaviour of martensite formation and tempering and generally serve as a reference point for the thesis. This was then compared to phase transformations during bainitic austempering, with particular emphasis on lattice strains in parent austenite and bainitic ferrite, and carbon segregation. Significant carbon loss in bainitic ferrite was observed due to the precipitation of nano-scale carbides and carbon partitioning into austenite. The thermal and mechanical performance of these microstructures was then tested again with in-situ synchrotron X-ray diffraction. It was observed that solid solution strengthening dominated the mechanical stability of austenite, but carbon content of austenite and strain relaxation of the bainitic ferrite matrix experienced during austempering increased the thermal stability of austenite. A higher carbon content in retained austenite did indeed increase its mechanical stability across a parameter space of like heat treatment types, but a tempered martensite matrix outperformed bainite matrices in all cases.| Period | 1 Aug 2023 |
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| Examinee | Daniel Foster |
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| Degree of Recognition | International |