Effect of Specimen Thickness on the Degradation of Mechanical Properties of Ferritic-Martensitic P91 Steel by Direct-fired Supercritical CO₂ Power Cycle Environment

In this study, degradation of the mechanical properties of 9Cr ferritic-martensitic P91 steel by supercritical CO₂ (sCO₂) power cycle environment was investigated. P91 tensile specimens of 2.54 mm and 0.5 mm thicknesses were exposed to a simulated direct-fired sCO₂ power cycle environment at 650 °C for 1000 hours. The exposed specimens were tensile stressed at room temperature and the resulting deformation behaviors were studied. While the thicker P91 had ductile failure with decreased ductility, the failure of the thinner P91 was completely brittle and at a significantly lower tensile stress. These results indicate that the mechanical properties of the thinner P91 were severely degraded by the exposure. Microstructural analyses reveal that the thinner P91 became carbon saturated. This resulted in breakaway oxidation, where fast growing iron-rich oxide layers consumed a significant portion of the specimen thickness. The thicker P91 also carburized during the exposure, however it did not become carbon saturated and growth of iron-rich oxide layers consumed a much smaller portion of the thickness. Hardness tests performed on specimen cross-sections showed increased hardness for both thicknesses. However, the increase was considerable for the thinner P91, where a thick oxide layer and extensive carbide formation in the alloy caused its brittle failure.