Fundamental Understanding of Macro to Nano-scale Structures and Transformations in High Carbon Steel: Formation of Novel Ultrahard Surface using Waste Materials through an Innovative Hybrid Layering Approach

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Copyright: Hossain, Rumana
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Abstract
Although high carbon martensitic steels with dual phase, i.e. retained austenite and martensite, are well known for their industrial utility in high abrasion and extreme operating environments, due to their hardness and strength, the compressive stability of their retained austenite, phase transformation behaviour under different load and the implications for the steels’ performance and potential uses, is not well understood. These aspects need to be understood in depth for creating a set of information which can be used for designing new application for this steel or for improving the performance of the steel. The phase transformation mechanism was identified, from the macro to the nano level which shows that, at the early stage of plastic deformation ε-martensite formation dominates, while higher compression loads trigger α’-martensite formation. Different strain rates transform austenite into martensite at different volume, simultaneously activate multiple micromechanisms, i.e., dislocation defects, nanotwining, etc. that enhanced the phase stability and refined the microstructure which led to an increase in the hardness. Increasing Cr %, altered the morphology and stability of the phases and the overall structure. Also, post-tempering heat treatment facilitates redistribution of carbon, decreased the hardness of martensite and overall hardness but increased the stability of austenite significantly. This research also identifies the hybrid structure of the white layer in high carbon steel and demonstrates the combination of phase transformations, strain hardening, and grain refinement led to a hybrid microstructure. This comprehensive study could enable the understanding of the precise control of the microstructures of high carbon martensitic steels, and hence their properties. Microstructural engineering through a controlled high compact deformation has been used to produce nano-grain martensitic structure (~40nm) which has ceramic-like hardness with metal-like toughness. An innovative method of transforming steel surface into multi layered ceramic-diffusion-metal structure using the waste source. Through a controlled high-temperature reaction, the outer layer of a steel surface was produced as an ultra-hard ceramic surface and the inner layer is produced as a metal matrix enriched with carbides. The result reveals that by turning the normal metal surface into a complex ceramic-diffusion-metal structure, extremely high hardness can be achieved.
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Author(s)
Hossain, Rumana
Supervisor(s)
Sahajwalla, Veena
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Publication Year
2019
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Thesis
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PhD Doctorate
UNSW Faculty
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