Growth kinetics of nitride layers during microwave post-discharge nitriding

Abstract

An algorithm is presented to estimate the nitrogen diffusion coefficient during the growth of nitrided concomitant layers produced by microwave post-discharge nitriding. Diffusion coefficients in each phase are estimated by setting the inverse problem associated with growth of a compact nitrided layer γ′-Fe 4 N 1− x and the formation of an austenite layer. Both layers grow over a nitrogen diffusion zone in ferrite. The associated direct problem is a moving boundary one with conditions of Stefan type, where the diffusion coefficient in ferrite is assumed to be known. In this frame, the evolution of the layer width is studied from the initial states of the process. From the very beginning of the diffusion process, a nitrogen profile in “supersatured” ferrite is considered. The evolution of nitrogen profile concentration from supersatured ferrite to the formation of compact nitride layers is described. Nitrogen concentrations in each phase and diffusion zone are not considered to be bounded by their solubility limits. Evolution for large periods (quasi-steady periods), coincides with layer growth evolution considered in mass balance models.