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A comparison of two high spatial resolution imaging techniques for determining carbide precipitate type and size in ferritic 9Cr-1Mo steel.

The microstructure of the oxide scale and metal bulk in ferritic 9Cr-1Mo steel was observed in order to explore the oxidation and carburisation mechanisms upon exposure to a CO2 gas environment at high temperature and high pressure. An experimental 9Cr-1Mo steel sample that had been oxidised at 580ºC for more than 165000 hrs in the coolant gas consisting primarily of CO2 gas was analysed. In order to elucidate the oxidation characterisation, scanning Raman spectrometry was used to analyse the oxide close to the metal/oxide interface. Carbon was found to be deposited in the spinel layer. The microstructure and the distribution of elemental chemical composition were examined and analysed using optical and high-resolution scanning electron microscopy combined with energy dispersive X-ray analysis. Raman signals from the oxide scale were analysed in order to explore the possibility of deriving strain information close to the oxide-metal interface. The results are discussed with relation to understanding the mechanisms of oxidation and carburisation which aim to underpin extension of the service life of components fabricated from this steel.