Diffusion of nitrogen and thickness of nitrided layer in ultrafine and micron grain size AISI 304L was evaluated in the process of plasma nitriding. Improving the surface hardness of austenitic stainless steels can be obtained by plasma nitriding. Operating temperature is limitation of this process for austenitic stainless steels. Precipitation of chromium nitride (CrN) is formed above 480 °C which leads to decreasing of corrosion resistance. AISI 304L austenitic stainless steel samples in the grain size of micrometer to nanometer were processed by process of severe cold roll and reversion annealing of strain induced martensite. Then plasma nitriding was performed with the same conditions, 450 °C for 5h, and the gas composition was nitrogen and hydrogen with a mass ratio of 1 to 3. The evaluation of grain size and observation of microstructure was performed by scanning electron microscopy (SEM) and optical microscopy (OM). Surface morphology and distribution of precipitates in structure of plasma nitrided samples was evaluated by field emission scanning electron microscopy (FESEM). Spectroscopy energy distribution (EDS) was used to obtain the profile of concentration of nitrogen. Identification phases before and after plasma nitriding was performed by X-Ray diffractometer (XRD). It was shown that increasing the thickness of nitrided layer from 4.8 to 10.6 micrometer was attributed to decreasing of grain size from 11 micrometers to 135 nanometers. Concentration profiles of nitrogen were consistent in different samples. Decreasing of grain size led to increasing the penetration depth of nitrogen. The concentration profiles of nitrogen and the hardness profiles were consistent. Grain refinement enhanced the formation of CrN precipitates by X-ray diffraction patterns. Results of the FESEM showed that the volume percent of CrN in ultrafine samples was increased in comparison to micrometer grain size samples. In addition to the effect of grain size on the percentage of precipitates, the type of phases and percent of them had significantly effect on the formation of precipitations in the AISI 304L austenitic stainless steel. It was observed that retained martensite after reversion annealing caused the formation of iron nitride (-'? Fe 4 N) in ultrafine samples. Keywords : Austenitic stainless steel AISI 304L, plasma nitriding, cold rolling and reversion
