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Title: Characterisation of the microstructural evolution in welded austenitic stainless steels during accelerated ageing for use in the nuclear power plant industry
Author: Green, Graham
ISNI:       0000 0004 7971 0573
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2015
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This thesis studies the microstructural evolution of welded austenitic stainless steels that are commonly used in power plant applications. This work has been funded by EDF Energy and will be used to aid with the plant life extension work for the Advanced Gas Cooled Reactor nuclear power plants currently operating in the UK. Extending the life of power plants puts demands on the steel pipe work, as the steels could be exposed to high temperatures for many years past their originally planned decommissioning date. This could cause changes in microstructure which may result in worsening of mechanical properties The plant life extension will help to meet the rising demand for energy in the UK, but the effect on materials must be understood in order to achieve this. Various grades of austenitic stainless steels were artificially aged at temperatures of 650°C and 750°C for up to 15,000 hours and then analysed using a variety of techniques. This analysis showed differences in the microstructural evolution between the alloy grades, as well as between the parent and weld metals of the same alloying grade. The 321 and 347 grade austenitic stainless steels primarily formed the brittle intermetallic sigma phase during thermal ageing. Whereas the 316 grade stainless steel showed a more complex precipitation sequence due to differences in the chemical composition. The size of the particles was measured using ion beam imaging as it provided excellent contrast between the matrix and the intermetallic particles. The particles formed in all of the alloys rapidly grew during the first 1,000 hours after which the growth rate decreased. The Larson Miller Parameter was used to relate the equivalent ageing times between ageing at different temperatures. It was found that to make the Larson Miller Parameter fit for the 316H parent metal a constant of 10 for the area fraction and 15 for the particle size was required. Whereas, the 316L, 321 and 347 alloys fit better with a constant of 5.81 due to having a different precipitation sequence to the 316H alloy. A low temperature ferrite phase was found to form in the 321 and 347 grade austenitic stainless steels in the unaged condition of both the parent and weld metals. In the parent metal the ferrite phase was characterised using Electron Back Scatter Diffraction, while in the weld metals Energy Dispersive Xray Spectroscopy was also used to aid in the differentiation between the alpha and delta ferrite. The presence of alpha ferrite was unusual for these steels as they were expected to be primarily austenitic. As such, various techniques were used to investigate the behaviour of this phase, this included dilatometry, Differential Scanning Calorimetry, high temperature X-ray diffraction and various heat treatments. It was found that ferrite dissolved at between 800°C and 900°C but re-formed again on cooling at around 200°C. Analysing the chemical composition of these steels showed that the nitrogen content was particularly low, as nitrogen is a strong austenite stabiliser it was thought to be the cause of the austenite instability. It was also found that, during thermal ageing, the area fraction of ferrite increased. This was postulated to be caused by the diffusion of nitrogen into the sigma phase which reduces the stability of the austenite, which can then transform to ferrite during cooling.
Supervisor: Not available Sponsor: EDF Energy
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Materials Engineering not elsewhere classified ; Stainless steel ; Ferrite ; Sigma phase ; Precipitation ; Austenite