Abstract
The reduced activation ferritc/martensitic 9Cr steel Eurofer 97 is a leading structuralmaterial candidate for next-generation commercial-scale tokamak experiments. During
service, plasma transients such as disruptions and edge localised modes may impose high
magnitude, short duration dynamic thermal loads on the tokamak's plasma-facing first wall.
Despite mitigation measures such as tungsten armouring and shattered pellet injection,
disruptions may briefy expose Eurofer 97 components of the first wall to temperatures of
up to 956 °C. Prior research on the thermal degradation behaviour of Eurofer 97 found
that its normalised and tempered ferritic/martensitic microstructure was significantly
altered by only 4 hours of isothermal ageing at 850 °C. A similar microstructural evolution
is hypothesised to arise from the cumulative effects of many plasma transients over the
lifetime of the reactor. While Eurofer 97's isothermal ageing behaviour is well understood,
there is a paucity of data regarding the microstructural effects of repeated exposure to
high-temperature transients. It is vital that the effects of this novel thermal degradation
mechanism on Eurofer 97 in both the bulk and welded condition are comprehensively
understood prior to the licensing of the DEMO tokamak.
This thesis explores the transient thermal ageing of Eurofer 97 via a multidisciplinary
approach. Finite element thermal analysis has been used to predict the maximum temperatures
Eurofer 97 will be exposed to during a mitigated plasma disruption in DEMO.
This modelling data was employed to design a laser transient heating experiment, which
utilised a novel thermoplasmonic functional surface to rapidly expose Eurofer 97 samples
to 700 °C and 850 °C thermal transients up to 1,500 times. This experimental campaign
was complimented by CALPHAD computational thermochemistry modelling of Eurofer
97's precipitate kinetics in the 600-850 °C range. Characterisation of transient-affected
Eurofer 97 was undertaken using a range of electron microanalysis techniques, including
scanning and transmission electron microscopy, electron backscatter diffraction, selective
area transmission electron diffraction, and energy dispersive X-ray spectroscopy. Quantitative
analyses of precipitate nucleation and growth was undertaken via SEM image
thresholding and segmentation, and microhardness indentation testing was also performed.
A single 700 °C transient lasting 51 seconds was found to promote signifcant coarsening of
Eurofer 97's tempered sub-grains. After 100 transients at 700 °C, sub-grain coarsening was
accompanied by Cr-rich M7C3 precipitation at grain boundaries. This meta-stable transition
phase appeared to dissolve back into the matrix after 500 transients, to be replaced by a
stable Cr-rich M23C6 phase after 1,000 transients at 700 °C. This dissolution-precipitation
mechanism has not been previously observed for the M7C3 > M23C6 transformation, and
may be unique to transient thermal ageing. The microstructural effects of 850°C transients
are also reported. Notably, after 1,000 transients at 850°C, Eurofer 97's grain sizes had
increased by an order of magnitude, and its hardness was found to have decreased by 32%.
The constitutive modelling and characterisation of Eurofer 97 first wall laser-keyhole
welds was also undertaken, combining finite element and precipitate kinetics modelling
with characterisation via electron and high-speed atomic force microscopy techniques.
The developed weld model agreed well with the results of microanalysis. Several large
void defects were discovered within the weld fusion zone, hyopthesised to arise from the
evaporation during welding of Ce-rich oxide inclusions present in the as-cast Eurofer 97.
| Date of Award | 7 May 2024 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Thomas Martin (Supervisor) & Ross S Springell (Supervisor) |
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