Carburisation of ferritic Fe–Cr alloys by low carbon activity gases

Model Fe–Cr alloys were exposed to Ar–CO2–H2O gas mixtures at 650 and 800 °C. At equilibrium, these atmospheres are oxidising to the alloys, but decarburising (aC ≈ 10−15 to 10−13). In addition to developing external oxide scales, however, the alloys also carburised. Carbon supersaturation at the scale/alloy interface relative to the gas reflects local equilibrium: a low oxygen potential corresponds to a high pCO/pCO2pCO/pCO2 ratio, and hence to a high carbon activity. Interfacial carbon activities calculated on the basis of scale–alloy equilibrium are shown to be in good agreement with measured carburisation rates and precipitate volume fractions, providing support for the validity of the thermodynamic model.

► Model Fe–Cr alloys are exposed to Ar–CO2 and Ar–CO2–H2O at 650 and 800 °C.
► Alloy microstructures are extensively modified by carburisation.
► Chromium carbides precipitate at both temperatures, and martensite forms at 800 °C.
► Elevated carbon activities reflect local equilibrium at the scale/alloy interface.