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Title: Nano-structuring of micro-alloyed steels via nano-precipitate formation
Author: Clark, Samuel
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2019
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This Thesis presents work undertaken in order to develop the fundamental understanding of nano interphase precipitation in steels. Small angle neutron scattering (SANS) was employed to characterize the precipitates and their size distributions in an Fe-0.047C-0.2V-1.6Mn (in wt.%) alloy isothermally transformed at 700 °C up to 600 min. Analysis suggested the major axis diameter increased from 18 nm after 3 min to 35 nm after 600 min. The precipitate volume fraction correspondingly increased from 0.009 to 0.022 over the same period and number density fell from 2 × 1021 to 5 × 1020 m−3. In-situ Electron Dispersive Backscatter Diffraction (EBSD) and High Temperature Laser Confocal Microscopy (HT-CSLM) were applied to elucidate the influence of γ → α transformation upon the extent of interphase precipitation. Analysis of the γ/α orientation relationship during cooling at 2 and 10 ℃s−1 suggests the proportion of ferrite likely to hold interphase precipitation varies little with cooling rate. At cooling rates in excess of 20 Ks−1, interphase precipitation is increasingly suppressed due to the Widmanstätten γ → α transformation. An analytical model is presented which suggests the inter-sheet spacing of interphase precipitates is controlled by a complex interplay between the interfacial energy and interfacial segregation. It is shown that the general trend of refining inter-sheet spacing with growing ferrite half-thickness can be well predicted. The multi-phase field method, is used to investigate the pairing of interphase precipitates in micro-alloyed steels. Where only the interfacial energy is considered it is shown that this energy can lead to a neck between two neighbouring precipitates. When both strain and interfacial energies act on the system, the bridge between the particles becomes stabilized. A procedure for the 'layman' to optimise interphase precipitation in any given alloy is proposed. For the Fe-0.047C-0.2V-0.18Si-1.6Mn (in wt.%) alloy considered in this study specific recommendations of processing parameters are given.
Supervisor: Not available Sponsor: Tata Iron and Steel Company
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TN Mining engineering. Metallurgy