Abstract
The high efficiency pulverized coal fired power generation so called an Advanced Ultra SuperCritical plant, in which the main steam condition of 30MPa of pressure and 700℃ in temperature are considered, has been aiming to establish in order to increase its efficiency of the power generation. In such condition, it is desirable to utilize the Ni-base alloys as structural material due to the severe environmental condition. The effect of steam on degradation behavior of Inconel 718 under bending stress in high temperature environments was investigated. Stress accelerated grain boundary oxidation is considered to be a prior mechanism in the environmental effect. The four-point bending tests at 750℃ in air and steam environments were performed. It was found that local delamination of oxide layer in the specimens exposed in a steam environment. Surface cracks were found after four-point bending test in steam and were thought to be formed subsequent to the local separation of surface oxide layer. Cross-sectional analysis using SEM-EDX showed that Nb oxide and Ti oxide were apparent adjacent to the crack. These results exhibit that the cracks were attributed to break of these oxides under bending stress.
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