Growth kinetics of martensitic layers during high temperature gas nitriding of a ferritic – martensitic stainless steel

Abstract

A dual phase ferritic – martensitic AISI 410S stainless steel was nitrided in high purity N₂ atmosphere between 1373 and 1473 K. After this treatment a high nitrogen martensitic case, free of precipitates, was formed. The growth kinetics of these fully martensitic cases during high temperature gas nitriding is studied with the aim of predicting martensitic case depths as functions of nitriding times, temperatures and N₂ partial pressures. Thermocalc and Dictra software were used to calculate the equilibrium conditions and to solve the diffusion equations respectively. The results showed that the growth rate of the martensitic case is diffusion controlled and it can be calculated assuming local equilibrium. For the nitriding conditions used in this paper, the martensitic case depth is proportional to the square root of the nitriding time t^1/2 proportional to the nitriding temperature and roughly proportional to the logarithm of the N₂ partial pressure.