Materials Engineering, The Open University, Walton Hall, Milton Keynes, MK7 6AA.
Structural materials used in nuclear power
plantsexperience temperatures in excess of 450°-500°.At thesetemperatures, creep behaviour is an
important factor affecting
material behaviour. Creep is the slow deformation of materials under a constant stress and is
significant at high temperatures.
Nuclear power plants are subject
to shut downs and start-ups
(both hot and cold) during their lifetime. During these procedures, stress and or temperature change and sometimes have an effect on the overall response of the material. Assessment procedures used in power plants to determine the maintenance routines should be robust to take these effects into consideration.
A Rao, J Bouchard, M
Fitzpatrick and M Rist
tests were done on austenitic steels which simulated loading conditions during power plant shut downs and start-ups. Tests involved application of load
and temperature for a certain
duration and removing the load
and /or temperature for a fixed time.
A phenomenon called anelasticity
was observed and influenced
the material behaviour greatly.
Fig. 1 Test Conditions
650°C 180MPa. Sample was loaded for 48 hours and unloaded for 48 hours.
Fig.3 Test involving both load and temperature removal
Fig.8Precipitate Densities during the Initial
Stages of the Unload.
This work was carried out as part of the TSEC programme KNOO and as
such we are grateful
to the EPSRC for funding under grant EP/C549465/1
load-on durations and strains showed significant
deviations compared to creep-rupture data. An increase of 2-3 times was seen in the rupture life but
a reduction of more than
50% was seen in the failure strain
Fig.5 On-load curves
for tests where the specimens were
loaded for 48 hours
studies were done on samples stopped at various stages of the unload. Dislocation/particle densities
and distributions were
Fig.6 Interaction ofdislocations
Fig.7 Pinning of
dislocations by precipitates
and Future Work
operation should be taken into consideration when creep assessment procedures are
is observed in all the load/temperature removal tests. The mechanism responsible is due to the interaction of dislocations with
precipitates. The role of long-range internal stresses is being determined.