Abstract
A family of creep-resistant austenitic stainless steels based on alumina (Al2O3) scale formation for superior high-temperature oxidation resistance was recently identified. Results of long-term cyclic oxidation studies (100 h cycles and total exposure duration for up to 7500 h) from 650-800�aC in air and/or air with 10% water vapor for a series of 2.5, 3, and 4 wt.% Al AFA compositions, with varying levels of Nb and Ni additions, are presented. Water vapor was observed to enhance subscale Al consumption in the AFA alloys relative to dry air exposure, presumably via enhanced alumina scale cracking and reformation. Water vapor also increased the tendency for internal oxidation. Increased levels of Nb additions were found to significantly improve oxidation resistance, as were reactive element additions of Hf and Y. Computational thermodynamic calculations of the austenitic matrix phase composition and the volume fraction of MC, B2-NiAl, and Fe2Nb base Laves phase precipitates were used to guide interpretation of oxidation behavior alloy composition trends in terms of two-phase oxidation theory, reservoir effect, and the third-element effect of Cr. The implications of these findings for the upper-temperature service limit for this new class of alloys and the potential for AFA alloy modification for increased service temperatures are discussed.
