Effect of supercritical CO₂on steel ductility at 450°-650°C

The compatibility of ferritic-martensitic (FM) and conventional and advanced austenitic steels with supercritical CO2 (sCO2) is being explored at 450°-650°C to determine their maximum temperature capability. In addition to measuring reaction kinetics and reaction product thickness, bulk carbon content and post-exposure room temperature tensile properties were assessed by exposing both alloy coupons and 25 mm long dogbone tensile specimens. After 1-2 kh exposures in 300 bar research grade (RG) sCO2, ~9 and 12%Cr FM steels had similar behavior under these conditions. Consistent with the literature, higher Cr and Ni contents in alloy 316H provided lower reaction rates at 450° and 550°C, but limited benefit at 650°C with similar degradation of tensile properties and C ingress observed. An advanced austenitic Nb-modified 20Cr-25Ni alloy 709 provided the best compatibility even at 650°C with no C uptake detected after 1 kh and no significant loss in room temperature tensile properties after exposure. A clear correlation was observed under these conditions between the formation of a thin, protective Cr-rich oxide scale and the prevention of C ingress and tensile property degradation at 650°C.