Thermodynamic Consideration on Steam Oxidation Resistance of Austenitic Stainless Steels Forming Intermetallic Compound

Steam oxidation resistance of Fe­20Cr­30Ni (at%) austenitic stainless steels with Nb, Mo, Ta, and W was investigated at 700°C. The Cr2O3 layer uniformly formed at the boundary between a spinel-type oxide scale and a metal substrate of alloys containing Nb, Mo, or W inhibits the growth of the oxide scale to improve steam oxidation resistance. During the heating at a high temperature, intermetallic compounds consisting of Fe and the added elements precipitate in the alloy and alter the chemical composition of the solute elements. The Cr activity gradient becomes large between the alloy substrate and the Cr depletion zone beneath the oxide scale since the amounts of precipitation differ between them, leading to enhancement of the Cr flux outward. It is indicated that an increase in the Cr flux accelerates the formation of the Cr2O3 layer. [doi:10.2320/matertrans.M2018261]