Evaluation of Alumina-Forming Austenitic Foil for Advanced Recuperators

A corrosion- and creep-resistant austenitic stainless steel has been developed for advanced recuperator applications. By optimizing the Al and Cr contents, the alloy is fully austenitic for creep strength while allowing the formation of a chemically stable external alumina scale at temperatures up to $900°C$⁠. An alumina scale eliminates long-term problems with the formation of volatile Cr oxy-hydroxides in the presence of water vapor in exhaust gas. As a first step in producing foil for primary surface recuperators, three commercially cast heats have been rolled to $∼100μm$ thick foil in the laboratory to evaluate performance in creep and oxidation testing. Results from initial creep testing are presented at $675°C$ and $750°C$⁠, showing excellent creep strength compared with other candidate foil materials. Laboratory exposures in humid air at $650–800°C$ have shown acceptable oxidation resistance. A similar oxidation behavior was observed for sheet specimens of these alloys exposed in a modified 65 kW microturbine for 2871 h. One composition that showed superior creep and oxidation resistance has been selected for the preparation of a commercial batch of foil.