Analysis of localized oxides and oxide films formed on alloy 625 and 617 in supercritical water and superheated steam environments at 750°C

Abstract

Oxidation tests using coupon specimens of alloy 625 and alloy 617, and four-point bending tests for a duration of 100 hrs were carried out in supercritical water (SCW) environment at a pressure of 24 MPa and in superheated steam (SHS) environments at 750°C. The noticeable difference between the oxidation in two environments appeared in the oxidation behavior of primary precipitates like Nb rich MC carbide for alloy 625. In the SCW environment, a large number of pitting was observed. Those were thought to be made by electrochemical reactions, while in SHS most of the carbides were oxidized as Nb oxides with some extent of volume expansion. In the case of alloy 617, the pitting formation was also observed in both environments. It seemed to have a dependence on the oxidation behavior of the primary carbides similar to that of alloy 625. The effect of the applied stress was not apparent under current experimental conditions because permanent deformation was observed in all the specimens and stress relaxation subsequently occurred during the test. In addition, no cracks were observed on both alloys after four-point bending tests. In the outer layer of the oxide film formed in the SCW, the Cr content exhibit to have smaller in both alloys, and the oxide thicknesses were 0.53 to 0.88 times thinner than those in the SHS.

Subcritical and supercritical Rankine steam cycles, under elevated temperatures up to 900°C and absolute pressures up to 400 bara

“…8 If the water is heated while it is also compressed (pressurized) to a high pressure above the critical pressure for water such that its temperature exceeds another water-specific constant called the critical temperature of water, which is 374.0°C 9 ; this water is described as ''supercritical water'' or ''supercritical fluid.'' 10 What is unique about this form of water is that it does not exhibit a liquid-tovapor or a vapor-to-liquid phase change at a constant temperature. Instead, it represents an intermediate condition between a liquid and a vapor.…”

“…When steam has just been generated by boiling liquid water which is converted into a gaseous phase; once the last droplet of liquid is vaporized by the effect of heat, the resulting steam is specifically called “saturated vapor” or “dry steam.” 6 If this freshly-formed hot water vapor is heated further while it is in the gaseous phase, it is called “superheated steam” provided that its pressure does not exceed a special value called the critical pressure of water, which is a physical constant equal to 220.6 bara, 7 and this is about 217.7 times the atmospheric pressure. 8 If the water is heated while it is also compressed (pressurized) to a high pressure above the critical pressure for water such that its temperature exceeds another water-specific constant called the critical temperature of water, which is 374.0°C 9 ; this water is described as “supercritical water” or “supercritical fluid.” 10 What is unique about this form of water is that it does not exhibit a liquid-to-vapor or a vapor-to-liquid phase change at a constant temperature. Instead, it represents an intermediate condition between a liquid and a vapor.…”

Precipitation Behavior of Ni-Based Superalloy Alloy 625 for A-USC Power Plants

“…The Ni-based superalloy Alloy 625 has been adopted as a candidate material for boiler piping in A-USC thermal power plants 9) because of its excellent high-temperature strength 10) and superior oxidation resistance. 11) The hightemperature strength of Alloy 625 is generally considered to be attributed to solid-solution strengthening by transition metals, such as Cr, Mo, and Nb, in addition to precipitation strengthening by six kinds of precipitates. 12) When Alloy 625 is subjected to aging treatment at high temperatures, three kinds of intermetallic compounds can precipitate within the γ matrix phase, namely, γ ′′-Ni 3 (Nb, Al, Ti) with a body-centered tetragonal DO 22 structure, δ-Ni 3 (Nb, Mo) with an orthorhombic DO a structure, and Ni 2 (Cr, Mo) with a Pt 2 Mo-type structure, in addition to the three types of carbides (i.e., MC, M 6 C, and M 23 C 6 ).…”