Corrosion and Mechanical Performance of Grade 92 Ferritic-Martensitic Steel After Exposure to Supercritical Carbon Dioxide

Grade 92 ferritic-martensitic steel is a candidate alloy for medium temperature (< 550 degrees C) components for the supercritical carbon dioxide (s-CO2) Brayton cycle. 1000 hours exposures were performed on base and welded material in s-CO2 at temperatures of 450 degrees C or 550 degrees C and compared to samples aged in Ar at 550 degrees C. Both s-CO2 exposures resulted in a duplex oxide growth and carburization, with 450 degrees C exhibiting carburization in a power law diffusion profile up to a depth of 200-250 mu m, while 550 degrees C showed a linear profile up to a depth of 100 mu m. The different profiles indicate much slower precipitation and coarsening of carbides at the lower temperature, allowing carbon to diffuse deeper into the material. However, 450 degrees C produced improved mechanical properties while 550 degrees C produced deteriorated properties. This was due to the higher density of carbon near the metal-oxide interface which leads to significant carbide coarsening and, subsequently, crack initiation and early failure. Additional exposure at 450 degrees C is predicted to increase deposited carbon, but further study would be needed to understand if and when carburization will produce a negative mechanical effect.