625合金の750°C超臨界水環境中における局所酸化形態分析

抄録

Ni-base superalloys have been considered as the candidate materials of the high temperature components of advanced ultra supercritical (A-USC) power plant. Those materials are known to form variety types of the precipitates after those exposing to the high temperature environments. The localized oxidation influenced by those precipitates, particularly those of carbides, are sometimes concerned as an acceleration factor to the material degradation. In this study, in order to confirm the long-term reliability of A-USC power plant, oxidation tests in supercritical water (SCW) which simulated the main steam condition of A-USC were carried out using alloy 625. The cross-sectional analyses were performed to examine the elemental distribution in and beneath the oxide film. A laves phase contained Ni, Mo, Nb, and Mo rich carbides were observed along the grain boundaries. The oxidation of the laves phase was not recognized though those were present adjacent to the oxide film. The voids formed beneath the oxide film were observed and those surroundings were enriched in carbon, therefore it was considered that particular carbides influenced to the formation. A large number of pits were confirmed, and those were initiated on the Nb rich carbides. The Nb and Cr rich oxide was continuously formed at the pit and alloy matrix interface, which layer was speculated to play as the protective one. According to the observations, it is considered that further growth of those pits beyond the size of the existed Nb rich carbide was not pronounced.