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

International audienceModel 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/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