A Mechanical Spectroscopy Study of Neutron Irradiation and Thermal Ageing of Reactor Pressure Vessel Steels

Abstract
Neutron irradiation and thermal ageing are known to cause embrittlement of pressure vessel steels, but the underlying microstructural processes are not well known. In order to acquire a better understanding of embrittlement, we investigate pressure vessel steels in unirradiated, thermally aged and neutron irradiated conditions, using an inverted torsion pendulum. Our studies have revealed a marked effect on the damping due to neutron irradiation, which, according to previous work, is related to interactions between dislocations and neutron induced defects. We also found that the magnitude of this effect depends on the steel type (JRQ, Doel, ...). Thermal ageing shows similar effects of hardening. A difference in the damping can also be observed between base and weld metal, giving an indication of the importance of the chemical composition. At a critical amplitude the behaviour of the damping with vibration amplitude changes considerably. This can be related to the onset of microplasticity. Our measurements have shown that the critical amplitude is higher for thermally aged steel than for unirradiated specimens, indicating an increase in microyield due to thermal ageing. However, such an effect has not yet been established beyond doubt for irradiated steel specimens. A comparison of these results with tensile test results will be presented.