Characterisation of non-stoichiometric cementite (Fe3C)

In order to measure the composition range of cementite, a sample has been prepared by carburisation at 700 degrees C at a high carbon activity a(C) = 100. Such strongly carburising conditions normally result in a quick onset of the high-temperature corrosion process metal dusting, which leads to cementite decomposition at the surface. The effect of sulphur to retard this corrosion process was used by adding a small amount of 1 ppm H2S to the gas mixture. The measurements of the carbon contents of the cementite particles formed at the sample surface were performed by using electron-probe microanalysis (EPMA). It was found that cementite (Fe3C) which was formed at a very high carbon activity a(C) >> a(C) (Fe3C/alpha-Fe) reveals a measurable carbon gradient. The homogeneity range of cementite at 700 degrees C in the investigated sample was determined to be of at least 0.6 at.% carbon. Note: this study has been performed at Max-Planck-Institut fur Eisenforschung already two decades ago, but it was not yet published internationally. However, the respective experimental results were already described in [1].