Microstructure Development of Oxide Scale during Steam Oxidation of the Fe–20Cr–30Ni–2Nb (at%) Austenitic Steel at 1073 K

Abstract

Novel austenitic steel of which composition is Fe–20Cr–30Ni–2Nb (at%) is one of the promising material for further advanced steam power plants, which will be operated at 973 K or higher temperature such as 1073 K. This steel is strengthened by intermetallic compounds and has creep rupture strength 80 MPa at 973 K for 10⁵ h. Further investigations are needed to improve both creep rupture strength and steam oxidation resistance of the steel for application at 1073 K. This paper focuses on steam oxidation resistance of the steel at 1073 K. Steam oxidation has been conducted to clarify microstructure development of the oxide scale and to evaluate long-term steam oxidation resistance based on the kinetic data. Microstructure observation reveals that Cr₂O₃ layer is easy to form at the scale/steel interface compared with the commercial austenitic steels, because the transition from internal to external oxidation has finished within about 20 ks. Surface oxygen potential measurement by using oxygen concentration cell is also useful technique to detect Cr₂O₃ formation in the scale. Based on the kinetics data, this steel has a good steam oxidation resistance compared with the commercial austenitic steel such as type 316 steel and is applicable to further advanced steam power plants.