Effect of annealing and supercritical CO₂ exposure at 750°C on the tensile properties of stainless steel and Ni-based structural alloys

Subsize dogbone tensile specimens of alloys 304H, 310HCbN, and 740H were exposed to 30 MPa supercritical CO₂ (sCO₂) for up to 4000 h at 750°C. Similar exposures in laboratory air were used to assess the specific role of sCO₂ exposure on the alloy tensile properties. Type 304H specimens were only embrittled by exposure in sCO₂ due to the formation of a thick Fe-rich oxide and C ingress in the bulk of the alloy. For 310HCbN and 740H, no effect of the sCO₂ environment was observed and a decrease of the alloy's ductility was associated with previously observed microstructure evolutions, for example, grain boundary carbide formation. The 740H was tested in the solution annealed condition and self-aging took place during 750°C exposures in sCO₂ and air with the formation of fine gamma prime precipitates leading to significant tensile strength increase. It was found that both 740H and 310HCbN have adequate sCO₂ compatibility for high-temperature commercial applications.