Volume 2, Issue 10 (November 2005)
Effects of Neutron Irradiation on Precipitation in Reactor Pressure Vessel Steels
Several MnMoNi pressure vessel steels with varying Ni, Cu, and Mo contents have been investigated in the irradiated and unirradiated condition. Examination using analytical transmission electron microscopy was performed in order to understand more fully the nature of both the inter- and intra-granular precipitation in these alloys. In particular, attention has been paid to the carbide population, which exists in the material in addition to the copper precipitates, which have received detailed study elsewhere. The most dramatic revelations concern the discovery that fine, intra-granular (10 nm diameter) Fe3C particles are present in these steels, in addition to the normally expected copper-rich precipitates of a similar size. Neutron irradiation increases the volume fractions of inter- and intra-granular precipitation of all phases (Fe3C, Cu-rich, and Mo2C). Neutron irradiation has the least effect in increasing volume fraction in high Ni; high Cu alloys (WV). The effects of neutron irradiation on precipitation rates are greatest in low Ni; high Cu alloys (SG (SH)). Inter-granular precipitation is most prevalent in the high Ni; low Cu material (WF). It is significant that this material contains the least intra-granular fine-scale precipitation. This is probably because the Cu precipitation is minimal. Mo and C have an important influence on the amount of Mo2C phase formed, with large amounts being observed in high Mo-C, irradiated material (SH) (0.48 % Mo). These results are discussed in terms of various models for inter-and intra-granular precipitate nucleation and growth.