Theoretical modelling of iron nitriding coupled with a nanocrystallisation treatment. Application to numerical predictions for ferritic stainless steels

► Nitriding on ferritic steels is modelled by a simultaneous multiscale/multiphysics approach. ► A duplex nanocrystallisation/nitriding treatment is investigated. ► Numerical results show a significant influence of nanocrystallisation on penetration depth. This paper analyses a recently developed duplex process combining nitriding with nanocrystallisation. A model is proposed to show how nitrogen diffusion mechanisms are modified within ferritic steels due to the nanostructure near the top surface. This model is based on micro-mechanical and micro-physical approaches, and also on the thermodynamics of irreversible processes. It takes into account size effects influencing the nitrogen diffusion, including mechanical stresses at the different length scales. Several models are investigated and numerical applications are performed. The results are compared to literature in order to demonstrate the generality of the present methodology.