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Metallurgical risk factors in grade 91 steel
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Jabar, Sharhid (2022) Metallurgical risk factors in grade 91 steel. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3909352
Abstract
The quantification of key microstructural parameters as a function of creep conditions is commonplace in the assessment of Grade 91 and other creep strength enhanced ferritic (CSEF) power-plant steels. This thesis analyses a broad range of Grade 91 steels which include, ex-service, developmental and more recently produced steels; each distinct in terms of creep performance and composition. Characterisation was performed using a combined series of techniques that provided through-length scale chemical analysis and site-specific quantification.
Initial steps involved quantitatively assessing the chemical homogeneity of the steels using micro-X-ray fluorescence (μ-XRF) and scanning electron microscope (SEM) based chemical mapping. All steels displayed micro-segregation although it was greatest in the ex-service steels. In these steels the micro-hardness was found to vary from 190 to 225 HV primarily due to compositional heterogeneity caused by micro-segregation. Focussed ion beam (FIB-SEM) imaging and thin foil lift-outs analysed using scanning transmission electron microscopy (STEM) chemical mapping showed area coverages of M23C6 carbides and V-rich carbo-nitrides were greater in positively segregated regions. Site-specific quantification using SEM-back scattered electron (BSE) imaging of the Laves phase showed a difference of up to a factor of two in the number density of particles and area coverage due to micro-segregation. For ex-service steels this was consistent with thermo-dynamic predictions. A procedure for quantification of inclusions was developed which considered optimal voltage parameters investigated through studies and simulations, micro-segregation effects and fine (down to 0.25 μm) inclusions. These are all important deviations from conventional automated SEM procedures.
This work shows that micro-segregation can have significant effects on micro-hardness and characteristics of various second phase particles. Since the performance of a material is likely to be controlled by local microstructural extremes rather than average properties, these findings have important implications for the characterisation and failure assessment of Grade 91 and other CSEF steels.
Item Type: | Thesis (PhD) | ||||
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Subjects: | T Technology > TA Engineering (General). Civil engineering (General) T Technology > TN Mining engineering. Metallurgy |
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Library of Congress Subject Headings (LCSH): | Steel alloys -- Microstructure, Steel alloys -- Creep, Steel, Heat resistant, Steel -- Metallurgy, Segregation (Metallurgy), Power-plants -- Materials | ||||
Official Date: | March 2022 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Warwick Manufacturing Group | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | West, Geoffrey D. ; Strangwood, Martin | ||||
Sponsors: | Electric Power Research Institute | ||||
Format of File: | |||||
Extent: | 273 pages : illustrations, charts | ||||
Language: | eng |
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