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Carbide-free bainitic steels for wear and rolling contact fatigue resistant applications



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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.





Ooi, Shgh Woei


carbide-free bainite, rolling contact fatigue, abrasion, impact, wear


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Ovako AB, SKF AB