Modeling nitriding enhancement resulting from the NanoPeening treatment of a Pure Iron

Nanocrystalline materials are well known to have enhanced diffusion properties, especially due to their high grain boundary density which are fast diffusion channels in the material. In this paper, a highly pure iron sample is treated by Nano Peening® to obtain a nanostructured layer with nanometric grains in the top surface. Then, the grain size distribution is measured by EBSD and optical micrography to fit with an analytical law. It is shown that an exponential law is a good estimator for the grain size distribution after Nano Peening®. Then, using this grain size distribution, the diffusivity distribution is calculated with an analytical model we recently proposed. A 1D finite element simulation of nitrogen diffusion is then performed with this distribution and the concentration profiles are compared with those extracted from a simulation with a constant grain size distribution. Simulations show that Nano Peening® significantly enhances diffusion, dividing the nitriding duration by 2 or by obtaining a 120% gain in the nitriding depth.