A numerical model of plasma nitriding of low alloy steels

A mathematical model has been developed to simulate the plasma nitriding process of low alloy steels, considering the simultaneous diffusion of nitrogen in ferrite, precipitation of fine scale alloy nitrides in the diffusion zone and development of γ′-Fe4N iron nitride layer on the surface. The present paper discusses various reactions occurring during the plasma nitriding process, mathematical formulations for various reactions to form the complete model of plasma nitriding, and a numerical solution to solve the model. Computation details and results for model Cr and Ti bearing steels and commercial steels En40B, En19 and Nitraps are also presented in the paper. It is shown that the model can predict the distribution of nitrogen in solution and in the form of alloy nitride precipitates, the iron nitride layer thickness developed during plasma nitriding, the incubation time for the formation of iron nitride layer and the nitride precipitation behaviour of various alloying elements in the steel. Application of the model to plasma nitriding of commercial low alloy steels demonstrated that the model describes the nitriding response of the steels to a quite satisfactory accuracy.