Effect of annealing and supercritical CO2 exposure at 750 degrees 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 (sCO(2)) for up to 4000 h at 750 degrees C. Similar exposures in laboratory air were used to assess the specific role of sCO(2) exposure on the alloy tensile properties. Type 304H specimens were only embrittled by exposure in sCO(2) 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 sCO(2) 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 degrees C exposures in sCO(2) 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 sCO(2) compatibility for high-temperature commercial applications.