625合金の750°C過熱蒸気環境中酸化挙動に及ぼす圧力の影響

抄録

Ni-base superalloys have been regarded as candidate materials for the components in high temperature and high pressure section of the advanced ultra supercritical power plant. These components are supposed to be utilized for a few decades, with exposed to supercritical water beyond 700°C. However the knowledge of oxidation behavior in supercritical water at such high temperature is not enough until now. Therefore, with aiming to evaluate the dependence of oxidation behavior of Ni-base superalloy on steam pressure, oxidation tests were performed using coupon specimens of alloy 625 for the duration of 100 hr at 750°C in superheated steam (0.1 MPa), subcritical water (10 MPa) and supercritical water (24 MPa) environments. A lot of pittings were observed on the coupons in subcritical water and supercritical water. Those were thought to be caused by not the dissolution of Nb rich MC carbide oxides but the interaction between the oxidation of the carbides and its transport to the water. The results obtained from the weight gain analysis and the elemental depth profiles by Auger electron spectroscopy were compared. It was expected that the higher pressure promoted higher oxidation rate in both the matrix and the Nb rich MC carbides, and releasing of those oxides.