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Title: Effect of a pressurised CO2 environment on deformation and failure behaviour of Type 316H stainless steel under steady and cyclic creep conditions
Author: Palko, Sandor
ISNI:       0000 0004 9354 2772
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2020
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A significant number of stainless steel components within the boilers of the UK AGR plants are subjected to oxidation, carburisation and other changes in the underlying microstructure of the material during operation. This results from exposure to the pressurised CO2-based primary circuit coolant at temperatures from about 500 to 650oC. It is believed that there is a synergistic relationship between the pressurised CO2 coolant environment and creep-fatigue initiation and cracking. Devising and implementing an evaluation methodology to account for oxidation and carburisation to enable conservative lifetime assessments is essential to manage plant lifetime. Therefore, the development of a new and fundamental understanding of environmentally assisted degradation and failure mechanisms is required. It has been postulated that the mechanism underlying the initiation of cracks is carburisation associated with the presence of a duplex oxide layer. In this study, the material-environment interaction for Type 316H stainless steel under simulated AGR conditions has been investigated to increase the understanding of the combined effects of stress, strain and surface preparation, for example, on oxidation and cracking behaviour. Experimental data are presented which show that the presence of an ultrafine grained layer induced by surface grinding promotes the formation of a thin and protective oxide scale. Conversely, the removal of the surface deformed layer by chemical polishing using an oxide dispersion polishing solution (OPS), leads to the formation of a much ticker duplex oxide that protrudes along the grain boundaries. Furthermore, an increased surface hardness due to carburisation has been observed for the polished surface only, suggesting that carburisation occurs at an early stage on a chemically polished (OPS) surface. In fact, the elevated hardness was observed only at regions associated with creep deformation. It is found that when the substrate is plastically deformed and under the effect of active stress, the thin oxide on the work hardened (abraded) surface can be disrupted, resulting in similar oxidation behaviour to a chemically polished surface but with a better resistance to carbon ingress.
Supervisor: Scenini, Fabio ; Ainsworth, Robert Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Creep ; 316H Stainless Steel ; Oxidation ; Carburisation ; Hardening