Carbon diffusion in cementite (Fe 3C) and Hägg carbide (Fe 5C 2)

Pure iron samples were carburised at 500 ∘C in CO–H 2–H 2O–H 2S gas mixtures to study the growth kinetics of metastable iron carbides. The carbon activity was varied in the range 7 ≤ a C ≤ 4580 . Small amounts of H 2S 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 a C < 150 parabolic growth of cementite (Fe 3C) was observed. At higher activities a C > 150 Hägg carbide (Fe 5C 2) formed on top of the cementite. From the dependence of the growth rate on carbon activity in the gas phase the self-diffusion coefficients D C ∗ of carbon at 500 ∘C were determined for both iron carbides Fe 3C and Fe 5C 2.