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

Pure iron samples were carburised at 500 ring operatorC 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 image of carbon at 500 ring operatorC were determined for both iron carbides Fe3C and Fe5C2.