Modelling carburisation in 9Cr-1Mo ferritic steel tube substrates in experimental CO2 atmospheres

[Display omitted] •Both geometry and chemistry play a vital role in breakaway oxidation initiation.•Carbide precipitation impedes carbon diffusion leading to saturation and breakaway.•X-ray microanalysis combined with thermodynamic modelling identifies carbide phases.•2-D coupled thermodynamic-kinetic model predicts the microstructural evolution.•Modelling explains many features observed experimentally and elucidates breakaway. By combining modelling and experimental work, new insights have been gained into the combined oxidation/carburisation of 9Cr-1Mo steels in CO2 rich environments. The breakaway-oxidation process is preceded by the rejection of carbon into the scale due to the poor solubility of carbon in the metal. Experimental TEM work reveals the formation of a carbide rich layer near the substrate surface which forms a further barrier against carbon ingress. The eventual oxidation of this layer could contribute to breakaway oxidation. A 2D finite-difference based diffusion model developed in combination with Thermo-Calc software underlines the role of specimen geometry on breakaway initiation.