CARBURIZATION OF FE-NI-CR STEELS IN CH4-H-2 MIXTURES AT 850-1000-DEGREES-C

Three heat resistant FeNiCr steels have been carburized in mixtures of CH4 + H-2 in the temperature interval 850-1000 degrees C. The carbon activity in the gas phase was larger than one. Under these conditions, the carburization process involved carbon deposition on the outer sur face of the alloy, dissolution and diffusion of carbon into the alloy as well as carbide formation ((Cr, Fe)(7)C-3 + (Cr, Fe)(23)C-6). The r ate determining step was diffusion of carbon into the alloy. Two model s, which both were based on Fick's 2. law, have been applied on the re sults. Wagner's model for internal oxidation, which give an analytical solution to Fick's 2. law, did not fit the obtained results. With the model based on a finite difference technique. which gives a numerical solution to Fick's 2. law, it was possible to obtain a good fit. Howe ver, the kinetical and thermodynamical data used in the calculation de viate from those found in literature.