Creep behavior of austenitic steels in CO2 and the effect of specimen thickness

Steels have a proven track record of safe operation in steam power plants for decades. Interest in developing supercritical CO₂ power cycles as a more efficient and sustainable alternative to steam cycles has driven a need to understand steel performance in these new environments. In particular, the potential of the high temperature CO₂ environment to influence the creep behavior of the steel must be determined. The prior research between the 1960s and 1980s on this topic has raised the possibility that carburization during CO₂ exposure may strongly affect the creep behavior with conflicting conclusions. Where higher temperatures may benefit from carburization strengthening, and lower temperatures may be embrittled. Of particular concern is thin-sectioned components such as compact heat exchangers, where even small rates of carburization can become problematic over long operating lifetimes. To shed light on this issue, this research investigates the creep behavior of austenitic stainless steel 347H and 309H (a higher Cr alternative) at 650°C. Specimens of 0.5, 1.0, and 2.0 mm thickness were tested to further assess the effect of steel thickness. Both steels show a reduction in creep life in CO₂ relative to air and analysis is ongoing to determine the reason for degraded creep properties.