Role of plastic deformation on the efficiency of a nitriding treatment: modelling of the hardness-depth profile

Abstract

To increase the hardness of ion-nitrided stainless steels, a plastic deformation is performed previously to the nitriding treatment in order to generate dislocations used afterwards as diffusion paths for atoms. In this work, influence of different plastic strain rates on the surface hardness improvement is studied. To obtain hardness values very close to the outer surface is not possible by classical hardness-depth profile, a simple model based on indentations performed perpendicularly to the surface is proposed. The predicted hardness-depth profile in different situations of work-hardening clearly demonstrates the efficiency of the prior plastic deformation on the global hardness improvement of the nitrided stainless steel. As a result, the hardened layer thickness increases from 20 µm to 35 µm when the plastic deformation rate increases from 0% to 80% of the maximum one. This is directly linked to the augmentation of the dislocation density by the work-hardening process.