A MECHANICAL SPECTROSCOPY STUDY OF NEUTRON-IRRADIATION AND THERMAL AGING OF REACTOR PRESSURE-VESSEL STEELS

Neutron irradiation and thermal ageing are known to cause embrittlemen t of pressure vessel steels, but the underlying microstructural proces ses 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 to rsion pendulum. Our studies have revealed a marked effect on the dampi ng due to neutron irradiation, which, according to previous work, is r elated to interactions between dislocations and neutron induced defect s. We also found that the magnitude of this effect depends on the stee l type (JRQ, Doel,...). Thermal ageing shows similar effects of harden ing. A difference in the damping can also be observed between base and weld metal, giving an indication of the importance of the chemical co mposition. At a critical amplitude the behaviour of the damping with v ibration amplitude changes considerably. This can be related to the on set of microplasticity. Our measurements have shown that the critical amplitude is higher for thermally aged steel than for unirradiated spe cimens, indicating an increase in microyield due to thermal ageing. Ho wever, such an effect has not yet been established beyond doubt for ir radiated steel specimens. A comparison of these results with tensile t est results will be presented.