The powder-pack nitriding process: Growth kinetics of nitride layers on pure iron

In this study, the growth kinetics of nitride layers that develop during the powder-pack nitriding process on the surface of ARMCO pure iron was estimated. The powder-pack nitriding of pure iron was performed according to the Pulnieren(H.E.F. Durferrit) method using a ?Pulnier? powder and an activator, at 798-848K with different exposure times (2-12h) for each temperature. In addition, for the entire set of nitriding conditions, three different activator/?Pulnier? powder ratios (0.20, 0.25, and 0.35) were used to evaluate the activation level during the growth of nitride layers. The kinetics of the nitride layers over the surface of ARMCO pure iron were estimated by two mathematical approaches, that consider the mass balance equations at the growth interphases. The resulting expressions for the effective diffusion coefficients in the nitride layers were evaluated as a function of nitriding temperatures and activator/?Pulnier? powder ratio. Finally, based on the experimental parameters ascribed to the powder-pack nitriding process, two expressions were proposed to estimate the nitride layer thicknesses at 798 and 823K after 9h of exposure for each temperature, to validate the diffusion models used in this work.