Modelling Simultaneous Reactions of Niobium Precipitates in Ferritic Stainless Steels

Abstract

Niobium is used as an important alloying element in the design of heat-resistant ferritic stainless steels for automotive exhaust systems.When in solid solution, the niobium improves both the high temperature strength and the resistance to thermal fatigue. However, it also forms several kinds of precipitates during service at elevated temperatures, making it important to understand the kinetics of its precipitation and coarsening. In the present work, the kinetics of simultaneous reactions of niobium precipitates in ferrite have been modelled, taking into account the multicomponent nature of the diffusion process whilst at the same time allowing for capillarity effects. The model can predict not only volume fraction changes but also particle size. The experimental results on Fe₂Nb Laves phase and Fe₃Nb₃C carbide in a 19Cr-0.8Nb steel have been obtained and shown to be comparable to be the calculations with new simultaneous model.