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Carbide-free bainitic steels for wear and rolling contact fatigue resistant applications 10.17863/CAM.44019 Gola, Adam Marek 0000000285807836 Ooi, Shgh Woei Carbide-free bainitic (CFB) steels are one of the novel solutions for applications requiring high strength with increased toughness. In this work, knowledge about the transformation mechanism and its significance for mechanical properties are summarised. The role of carbon, which partitions after displacive growth of bainitic ferrite, is stressed as the main strengthcontributing factor. Alloy design together with the thermomechanical treatment methods are discussed with the aim of achieving optimal mechanical properties for wear resistance and rolling contact fatigue. The idea of using carbide free bainitic steels for these kinds of applications is not new; however, a straightforward relationship between microstructural features and the ultimate performance has yet to be found. It is believed that the retained austenite fraction, size and morphology are closely related to its mechanical stability, which is crucial in affecting its transformation to martensite. To investigate the evolution of the microstructure, multiple tests were conducted on a variety of CFB steels differing in carbon content to replicate the working conditions in real-life applications. Two-step austempering and sub-zero cooling post-treatment were utilized as ways of reducing excess of blocky retained austenite. Microstructural alterations were observed with use of optical and electron microscopy methods, as well as quantified by X-ray diffraction, including a layer-by-layer method developed specially for this work. The research proves the beneficial role of retained austenite for wear resistance, as CFB specimens outperformed martensitic benchmarks of the same initial hardness - measured mass loss was up to 20-25% lower for bainitic samples. Microstructural defects, such as voids and microcracks, generated in CFB by rolling contact fatigue were identified. The presence of coalesced bainite in high carbon specimens was observed – a feat that has not been reported in the literature before. University of Cambridge 2019-10-26 Thesis Doctoral Doctor of Philosophy (PhD) en Ovako AB, SKF AB https://www.repository.cam.ac.uk/handle/1810/296977 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/9f49388d-4793-41dd-9b79-9bd585489a47/download 475a4429b029efb0b7bfd87a10c08036 https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/8aff97ae-9b1f-4adb-96e5-1b614d42437b/download 87eda9de84448d1f82354d60eee3eb5f https://www.rioxx.net/licenses/all-rights-reserved/ carbide-free bainite rolling contact fatigue abrasion impact wear