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Title: The role of secondary phases in the long term degradation of stainless steels at high temperatures
Author: Warren, Alexander David
ISNI:       0000 0004 5915 2973
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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During prolonged service at raised temperatures the microstructure and phases present in stainless steels are known to evolve. In this thesis these phase evolution processes are studied in detail, with particular focus on the influence of secondary phases formed during prolonged ageing (approximately 87,000 hours) of an AISI Type 316H austenitic stainless steel in the temperature range 490 °C to 550 0c. These are the conditions of the UK's fleet of AGR nuclear reactors, and thus knowledge about the ageing behaviour of steels is critical for continued safe operation. The project provides an in-depth investigation of these secondary phase precipitates providing direct observation at micro and nano scale lengths using trad itional and novel materials analysis techniques. Pitched against direct measurement, the project tests the validity of thermodynamic computer modelling (JMatPro) to predict steel phase transformation during thermal ageing. The model shows good correlation with the volume fractions of phases recorded and the differences in predicted and observed formation rates have been assessed and discussed. Whilst understanding the type and distribution of secondary phases is important, a further requirement is to better understand their influence on the physiomechanical properties of the steel. A previous study noted the relationship between creep cavities and an unknown secondary phase precipitate in ex-service AISI Type 316H steel. In this thesis, the secondary phase is identified as two distinct Bee phases: ferrite and chi-phase. These phases are found to form during ageing in chromium and molybdenum rich defect zones generated during the initial casting process. The relationship between these phases and creep cavitation is rigorously assessed, and a mechanism for the enhancement of creep cavitation due to the presence of these phases is proposed. As such, this thesis has increased the general understanding of the role of secondary phases in creep cavitation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available