Calculation, measurement and evaluation of nitrogen diffusion in plasma nitriding process were carried out. Analytical models for ‘nitrogen concentration distribution’ and ‘compound layer growth’ were derived by applications of calculations on the diffusion laws. The compound region was considered as two separated layers including epsilon (?) and gamma prime (?') nitrides as well as a diffusion zone (?). The analytical models were constructed based on the binary Fe-N phase diagram below 592oC and up to 11wt% nitrogen. Considering relevant initial and boundary conditions, the Fick's second diffusion law was used for three separated ?, ?' and ? zones in a semi-infinite domain. Three nitrogen concentration equations in ?, ?' and ? phases were obtained versus nitriding temperature, time and diffusion distance. Using the Fick's first diffusion law and mass conservation rule, two equations were developed for predicting thickness of the ? and ?' layers versus nitriding temperature and time. Plasma nitriding was carried out on pure iron substrate at 550oC in an H2-N2 atmosphere for various nitriding periods. Optical microscopy and SEM for structural evaluation, microhardness for hardness profiling, profilometer for roughness measurements, XRD for phase analysis, EDS for semi-quantitative elemental analysis, GDOES for quantitative elemental depth profiling and SIMS for detection of nitrogen within the diffusion zone were employed to characterize nitrided samples. The calculated results were compared with the experimental data and a good agreement was achieved. The results showed that slope of the equations which represents growth rate of the compound layer, is different for ? and ?' nitrides at each nitriding temperature. In plasma nitriding process, opposite to gaseous nitriding, thickness of ? is much lower than that of ?' nitride. Using correction factors, the equations were modified for application in various nitriding conditions. It was found that the growth rate of both layers is high and increased at the primary stages of nitriding process, but gradually decreased and reached to the steady state condition. Increasing nitriding temperature leads to increasing the maximum nitrogen concentration at the ?'/? interface, Analytical calculation; Plasma nitriding; Nitrogen diffusion; Nitrogen depth profile; Compound layer thickne Diffusion zone; GDOES; SIMS.
