Thermal-diffusion finite element analysis of nitriding process for arc plasma surface hardening of steel

Abstract

Two-dimensional finite-element modelling of kinetics of nitriding during arc plasma surface hardening is considered in the paper. The nitrogen distribution in the metal surface layer is found by means of the joint calculation of the non-steady state heat conduction equation and the diffusion equation. The distribution of plasma jet heat flux on the metal surface is represented by an exponential expression obtained from the experimental data. The boundary conditions for the diffusion equation result from the calculation of the kinetic Boltzmann equation which describes the non-equilibrium heat-mass transfer processes in gaseous phase within Knudsen layer near metal surface and connects the transfer processes between gaseous and solid phases. The calculations were carried out for steel with low nitrogen content for the regimes of the plasma surface hardening. The influence of the heat flux density and speed of plasma jet moving along the metal surface on the kinetics of nitrogen flux into the metal and nitrogen distribution in the steel surface layer is investigated for different values of the gas pressure. The results are compared with the data obtained from one-dimensional modelling.