Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.785484
Title: Characterisation of the effects of long term isothermal ageing on stabilised stainless steels and their autogenous welds in the context of advanced gas cooled reactors
Author: Jackson, Charles P.
ISNI:       0000 0004 7970 9943
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2017
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Abstract:
Stainless steels have been used for decades in nuclear power stations, particularly in pipe and tube applications. For these applications, welding is often required to join sections of components or pipes/tubes due to their large sizes and lengths. These components may be required to withstand the harsh operating conditions inside nuclear power stations (high temperatures and pressures) for durations of more than 25 years. During long term ageing at high temperatures, the microstructures of materials are known to evolve and change as a function of time at temperature. These changes must be characterised and understood to ensure the safe and economic operation of nuclear power stations. This thesis investigates five different stainless steels and their response to long term isothermal ageing in the context of advanced gas-cooled reactors. The five different materials include three base materials: A wrought type-321 stainless steel and two cast Nb-stabilised stainless steels. The other two materials are two welds made from welding each cast steel to the wrought type-321. The materials were aged at both 650°C and 750°C for up to 15,000 hours in an inert atmosphere. All five materials were characterised using optical microscopy, scanning electron microscopy, focussed ion beam imaging and transmission electron microscopy. A displacive form of ferrite was observed in the wrought type-321 material after ageing for 1000 hours at 750°C, which was found to increase in volume fraction during long term thermal ageing. Sigma phase was also detected during ageing at both temperatures along with secondary intergranular Ti(C,N) particles in the wrought material. An extensive sigma phase transformation took place in both of the cast materials, with a large proportion of delta ferrite transforming to sigma phase gradually during ageing. This sigma phase transformation was accelerated in both cast materials at 750°C compared to at 650°C. The sigma phase regions displayed a tendency to crack after longer ageing times, which is a significant concern with regard to the mechanical properties of these alloys during long term thermal ageing. Both welds exhibited a large scale martensitic transformation during long term ageing, which grew gradually inwards to the centre of the weld bead from the base materials. The martensitic regions were localised, with several small regions being present in the same parent austenite grain. The martensitic regions maintained a consistent orientation relationship with the parent austenite grains, along with being chemically indifferent to the parent austenite. The martensitic regions were found to increase in size at longer ageing times at both temperatures. The welds both also exhibited sigma phase precipitation after 1000 hours at 650°C and 500 hours at 750°C with particles found to increase in size at later ageing times with the general size of the sigma phase particles consistently being larger at 750°C than at 650°C when compared at the same ageing time.
Supervisor: Not available Sponsor: EPSRC ; EDF Energy
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.785484  DOI: Not available
Keywords: Materials Engineering not elsewhere classified ; Stainless steel ; Steel ; Weld ; AGR ; Nuclear ; Nuclear energy
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