Abstract
The carburizing supercritical CO2 (sCO2) environment limits the use of lower cost steels in the lower temperature (450–650°C) portions of the sCO2 Brayton cycle because of concerns about internal carburization and embrittlement. Results on a ferritic–martensitic steel and conventional and advanced austenitic steels at 450–650°C in 30 MPa sCO2 with and without 1% O2 and 0.1% H2O additions have indicated that sCO2 environments will have lower maximum operating temperatures compared to steam plants. Pack Al and Cr coatings were evaluated at 650°C on T91 and 316H substrates and showed some benefit for up to 2000 h at 650°C, especially without impurities. However, characterization indicated Al2O3 was not formed and Cr-rich carbides formed in the Cr coatings. With the addition of impurities in the sCO2, the coatings were less protective at 650°C. Subsequent exposures at 600°C in sCO2 showed similar behavior. Postexposure evaluations included measuring the bulk C content and room temperature tensile properties. Improvements were indicated but the tensile results were complicated by the high temperature pack coating process affecting the substrate properties.
