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A thermogravimetric technique has been employed to measure the kinetics of the carburization of high purity FeNiCr alloys containing 25 wt% Cr with 5, 20 and 35 wt% N; additions at 1000°C in methane/hydrogen gas mixtures of unit carbon activity. The carburization has been found to follow parabolic kinetics and the parabolic rate constants have been calculated from the experimental data. Estimation of the phase relationships in the FeNiCrC system has permitted the development of a mathematical model of the carburization process. This model is based upon internal carburization with rate control by carbon diffusion through a subscale of M₇C₃. The model predicts parabolic rate constants in good agreement with the measured values. The model has then been adapted to allow for a surface reaction limitation with a linear increase in surface soluble carbon with time. Surface reaction coefficients have …