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

Pure iron samples were carburised at 500 degrees C in CO-H-2-H2O-H2S 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 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 a(C) < 150 parabolic growth of cementite (Fe3C) was observed. At higher activities a(C) > 150 Hagg 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 D-C* of carbon at 500 degrees C were determined for both iron carbides Fe3C and Fe5C2. (c) 2006 Elsevier Ltd. All rights reserved.