OS1409 750℃水蒸気環境中における617合金のき裂発生経路に及ぼす微細組織変化の影響

Abstract

Advanced ultra supercritical (A-USC) steam power generation, in which high-pressure steam is raised to beyond 700℃, are being studied internationally in order to utilize high efficiency power generation. As candidate alloys for structural components in high temperature section, Ni-based superalloys, such as alloy 617, have been considered from high temperature creep strength, material workability, welding property. Since these materials are exposed to high temperature steam, it is desirable to investigate the effect of the environment on the degradation. Up to now, there are not many investigations about the evaluation of micro crack initiation and its growth behavior of Ni-based superalloys in high temperature steam environment. In this current investigation, in order to evaluate the influence of local oxidation and microstructural change on micro crack initiation and its growth behavior of alloy 617, constant stress tension tests were performed in a steam and a inert gas environment at 750℃. In both environments, fine grain structures were confirmed after the test. Such grain is thought to be formed as dynamic recrystallization under tensile loading at the high temperature. Oxidation along the recrystallized grains was not so significant and aluminum enriched oxides in addition to the chromium oxide were found. It is exhibited that almost all of the cracks initiated from the ordinary grain boundaries.