Oxidation and Cracking Behavior of Nickel Base Superalloys Under Bending Stress in Advanced Steam Condition Beyond 700°C

Abstract

The high efficient pulverized coal fired power generation employing advanced ultra supercritical condition (A-USC) with the main steam condition beyond 700°C in temperature has been aiming to increase its efficiency of the power generation and reduce CO2 emission. The steam condition applied to the units depends on the materials’ properties like mechanical properties, creep strength and fatigue limit in the temperature. Ni-base superalloys had been considered to be a candidate for the rotating structural materials used in high temperature section of A-USC. The oxidation in a high temperature steam environment is recognized to exhibit differently as compared with its behavior in the gas environments. In addition, cracking behavior of those alloys in these conditions is not well known. In this current investigation, four-point bending tests using Inconel 718, 617 and 625 at 750°C in air and steam environments were performed. Surface cracks were found in Inconel 718 after four-point bending test in steam and were thought to be formed subsequent to the local separation of surface oxide layer. Cross-sectional elemental analysis showed that Nb oxide and Ti oxide were apparent adjacent to the crack. In addition, applied stress enhanced the overall oxidation rate. Stress accelerated grain boundary oxidation (SAGBO) mechanism is thought to be a possible cracking mode for the conditions.