Non-steady state carburisation of martensitic 9–12%Cr steels in CO2 rich gases at 550 °C

• Exposure of 9–12%Cr steels in Ar–50%CO2 at 550 °C results in internal carburisation.
• A new model is proposed to describe non steady-state carburisation kinetics.
• Differences in carburisation kinetics can be correlated with scale morphologies.

Two martensitic steels were reacted with Ar–50%CO2 at 550 °C for exposure times up to 150 h. Microstructural analyses and glow discharge optical emission spectroscopy revealed that internal carburisation of both steels beneath external oxide scales occurred, in spite of the very low equilibrium carbon activity of the atmosphere. The carbon concentration at the metal–scale interface increased throughout the reaction and the carbon uptake varied approximately linear with time. A model is proposed to describe the non steady-state carburisation kinetics whereby the usual diffusion equation describing carbon movement into the alloy is modified to account for loss of carbon as precipitated carbides.

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