Evaluation of nitrogen diffusion in thermo-mechanically nanostructured and plasma nitrided stainless steel

•We investigate the nitrogen diffusivity in nanostructured austenitic stainless steel.•Nanostructured grains reduce the nitrogen diffusivity in austenitic stainless steel.•Nanostructured grains improve the mechanical properties of nitride layer.•Chromium atoms and crystal defects affect nitrogen diffusivity in ASSs.

The effect of grain size on diffusion depth of nitrogen in AISI 321 stainless steel during plasma nitriding was investigated. The repetitive cold rolling and subsequent annealing were conducted to achieve nano/ultrafine grains in AISI 321 stainless steel. The grain size range of 130 nm up to 45 μm was obtained under these conditions. Plasma nitriding was performed at temperatures of 400, 450 and 500 °C for duration of 5 h. Microstructural evolutions were conducted by OM, SEM and TEM. The microstructure and composition of the nitrided layer were characterized by SEM and GDOES. Mechanical properties of the S phase were evaluated by micro-hardness testing. Results indicated that nitrided layer of the nanostructured steel have uniform appearance with no CrN precipitates, while CrN precipitations were formed in nitrided layer of the micro-grain one. The corrosion resistance of the nitride layer was improved in nanostructured condition because of uniform appearance of the nitride layer. Furthermore, the hardness of the S phase improved by decreasing substrate grain size. Increasing austenite grain size from 130 nm up to 45 μm, caused to increase surface nitrogen concentration in nitride layer. Decreasing austenite grain size led to decreases the S phase thickness, while the nitrogen diffusion mechanism is the same.