Local Dynamics of Chemical Kinetics at Different Phases of Nitriding Process

The local dynamics of chemical kinetics at different phases of the nitriding process have been studied. The calculations are performed under the conditions where the temperature and composition data are provided experimentally from an in-service furnace. Results are presented in temporal variations of gas concentrations and the nitrogen coverage on the surface. It is shown that if it is available in the furnace, the adsorption of the N₂ gas can seemingly start at temperatures as low as 200 °C. However, at such low temperatures, as the diffusion into the material is very unlikely, this results in the surface poisoning. It becomes clear that, contrary to common knowledge, the nitriding heat treatment with ammonia as a nitrogen-providing medium is possible at temperatures like 400 °C. Under these conditions, however, the presence of excess amounts of product gas N₂ in the furnace atmosphere suppresses the forward kinetics in the nitriding process. It seems that the best operating point in the nitriding heat treatment is achieved with a mixture of 6% N₂. When the major nitriding species NH₃ is substituted by N₂ and the N₂ fraction increases above 30%, the rate of the forward reaction decreases drastically, so that there is no point to continue the furnace operation any further. Hence, during the initial heating phase, the N₂ gas must be purged from the furnace to keep its fraction less than 30% before the furnace reaches the temperature where the reaction starts.