The effect of microstructure on the oxidation and carburisation of 9Cr-1Mo steel exposed to CO2

[Display omitted] •Substrate carbide distribution is dictated by the substrate microstructure.•Formation of carbides leads to elemental segregation, dependant on microstructure.•Martensitic 9Cr-1Mo shows greater carburisation relative to ferritic 9Cr-1Mo steel.•Preferential oxidation of Cr leads to the formation of the internal oxidation zone.•Shadows of the prior microstructure are present in the oxide due to Cr segregation. To understand the effect of microstructure on the oxidation characteristics of 9Cr-1Mo steel, experimental material with two different starting microstructures (ferritic and martensitic) were exposed to a CO2 rich atmosphere at 600 °C and 640 °C for up to ∼7000 h. The microstructure influences the size and distribution of carbides forming within the 9Cr-1Mo steel exposed to a CO2 rich atmosphere. These differences lead to elemental segregation during carburisation and oxidation. This elemental segregation influences the oxidation characteristics of the 9Cr-1Mo steel and the morphology of the internal oxidation zone, and subsequently the spinel structure shows ghosts of the prior substrate microstructure.