Carburization/Oxidation Behavior of Alloy Haynes-214 in Methane–Hydrogen Gas Mixtures

Abstract

The excellent resistance of Alloy Haynes-214 to carburization at elevated temperatures is attributed to the formation of a protective surface layer of Al2O3, resulting from the reaction of trace oxygen impurity in the gas phase with the Al-enriched surface. Lower carburization resistance below 1000 °C is manifested by increased carbon pick-up and degradation of mechanical properties. Exposures in CH4/H2 gas mixtures containing trace levels of oxygen below 1000 °C reduce the supply of outwardly diffusing aluminum from the bulk of the alloy to the metal surface, thus enhancing inward-oxygen diffusion and the subsequent formation of fine lamellar dispersion of internal Al2O3 within the sub-surface zone. In the meantime, inward-carbon diffusion forms internal carbides, thus leading to increased weight gain, relative loss of protection, and possible degradation of mechanical properties. Effective mitigation of carburization is limited only to exposure at extreme temperatures (1100 °C) which enhances the outward diffusion of aluminum towards the gas/metal interface as well as the nucleation of Al2O3 at the surface, thus causing an early transition from internal to external oxidation, which promotes the formation of an external protective layer of Al2O3 which effectively blocks carbon ingress in the alloy.