Carbon diffusion in cementite (Fe3C) and Hägg carbide (Fe5C2)

Pure iron samples were carburised at 500 ∘C in CO-H2-H2O-H2S gas mixtures to study the growth kinetics of metastable iron carbides. The carbon activity was varied in the range 7≤aC≤4580. Small amounts of H2S were added to the atmosphere to prevent graphite deposition and metal dusting from occurring. The gas composition was optimised to obtain parabolic carbide layer growth over a sufficiently long time period. At activities aC<150 parabolic growth of cementite (Fe3C) was observed. At higher activities aC>150 Hägg carbide (Fe5C2) formed on top of the cementite. From the dependence of the growth rate on carbon activity in the gas phase the self-diffusion coefficients DC∗ of carbon at 500 ∘C were determined for both iron carbides Fe3C and Fe5C2.