Mechanical and Corrosion Response of 316SS in Supercritical CO2

The supercritical carbon dioxide (s-CO2) Brayton cycle is currently being explored as a replacement for the steam Rankine cycle due to its potential for higher efficiency and lower cycle cost. 316 stainless steel is a candidate alloy for use in s-CO2 up to roughly 600 degrees C, but the mechanical effects of prolonged exposure of base and welded material in s-CO2 have not been analyzed. The potential for carburization makes this an important concern for the implementation of 316 and similar austenitic stainless steels in the s-CO2 environment. In this study, welded and base material of two types of 316-316L and 316H-were exposed in either s-CO2 or argon at 550 degrees C or 750 degrees C for 1000 h. 550 degrees C s-CO2 exposure yielded a thin (< 1 mu m) Cr oxide with occasional nodules of duplex Fe oxide and Fe-Cr spinel that were up to 5 microns thick. However, tensile results from s-CO-2 exposure matched those of 550 degrees C thermal aging in Ar, indicating that no mechanically detrimental carburization occurred in either 316 variant after 1000 h exposure. Conversely, 750 degrees C s-CO2 exposure produced roughly 10 x the oxide thickness, with a more substantial Fe oxide (3-5 mu m) on the majority of the surface and nodules of up to 40 mu m thick. In comparison to aged samples, tensile testing of 750 degrees C CO2-exposed samples revealed ductility loss attributed to carburization. Projections of 316L performance in s-CO2 indicate that mechanically detrimental carburization-equal to that shown here for 750 degrees C, 1000 h-will likely be present after 7-14 years of service at 550 degrees C.