The carburization behavior of alloy 800HT in high temperature supercritical-CO₂

The contribution of high temperature supercritical-CO₂ (S-CO₂) exposure (650 °C for 1000 h) on the carburization behavior of Alloy 800HT was studied. For comparison, Alloy 800HT specimens were also exposed to air and vacuum environments. The S-CO₂ exposed specimens showed carbon-accumulated layer at the oxide-matrix interface. All specimens showed subsurface carbide precipitation but it was more pronounced for the S-CO₂ exposed specimen, suggesting inward carbon diffusion from the carbon-accumulated layer at oxide-matrix interface. The pronounced subsurface carbide precipitation for the S-CO₂ exposed specimens resulted in much increased hardening and reduction in ductility. Thus, we clearly identified that S-CO₂ exposure caused the formation of the interfacial carbon-accumulated layer and more subsurface carbide precipitates, and resulting in additional hardening and reduction in ductility for Alloy 800HT.