Characterisation and Development of Nanostructured, Ultrahigh Strength, and Ductile Bainitic Steels
University of Cambridge
Department of Materials Science and Metallurgy
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Sherif, M. (2006). Characterisation and Development of Nanostructured, Ultrahigh Strength, and Ductile Bainitic Steels (Doctoral thesis).
The purpose of the present work was to characterise and further develop a novel nanostructured type of bainitic steel. Three chemical compositions were considered with different concentrations of Al and Co. The addition of Al and Co is believed to be necessary to produce the desired nanostructure at very low temperatures within a reasonable transformation time. An overview of the mechanical performance of fully bainitic steels vs other steel systems is presented in Chapter 1. An introduction to metallurgical concepts regarding the design and performance of bainite steels is presented in Chapters 1 and 2. Chapter 2 focuses on the design concepts by which the steel chemical composition was optimised, primarily on the basis of cost and the avoidance of carbide precipitation. Chapter 3 deals with the evolution of the microstructure during uniaxial tension, studied using X-ray diffraction. The effect of tempering deformed and undeformed structures, and heating to high temperatures, have also been investigated. In this context, data on bainite-containing steels in the literature are found to be rather limited. Chapter 4 is a comprehensive assessment of the mechanical behaviour of the steels subjected to a variety of processing routes. It is demonstrated that it is possible to outperform current commercially available steels. The microstructural behaviour of strain-aged and as-transformed steels during uniaxial tension studied using in situ neutron diffraction is described in Chapter 5. The evolution of texture with plastic deformation was confirmed as previously observed using conventional X-ray analysis. Evidence regarding the presence of two populations of carbon-depleted and carbon-rich austenite and their response to strain, grain rotation, anisotropy, stress partitioning between phases and the lack of work-hardening to overcome the onset of necking are presented.
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