Oxidation Behavior and Mechanism of Inconel 740H Alloy for Advanced Ultra-supercritical Power Plants Between 1050 and 1170 °C

Abstract

The oxidation behavior and mechanism of as-cast Inconel 740H superalloy, newly developed for advanced ultra-supercritical power plants superheater and reheater pipes, were investigated in the temperature range of 1050–1170 °C in air. Isothermal oxidation testing, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were used in this study. The results showed that the oxidation of Inconel 740H was a diffusion-controlled process and controlled by the outward diffusion of metallic elements and inward penetration of oxygen. Owing to the complicated chemical composition of the alloy, different oxides tended to form in the oxidation process. Cross-sectional characterization investigation indicated that the oxide-scale layer consisted of three parts when the temperature was high or the oxidation time was long: an internal oxidation zone composed of Al2O3 and TiO2, a continuous and dense middle layer of Cr2O3 containing precipitates of TiO2 and an outmost layer of NiCr2O4 spinel. In the formation process of NiCr2O4, NiO was found in the early transient stage. With further oxidation, TiO2 finally could not be detected in the internal oxidation zone as a result of outwards diffusion of Ti and its limited concentration in the matrix.