Influence of surface nano/ultrafine structure formed via pre-deep rolling process on the plasma nitriding characteristics of the AISI 316L stainless steel

t Influence of deep rolling prior to plasma nitriding on microstructure and hardness of the AISI 316L stainless steel was investigated in this paper. Deep rolling using ‘ball-point’ tool was conducted on the 316L stainless steel bar at multiple passes. Then, plasma nitriding was performed on the as-received and deep-rolled kinds at 450 C temperature for 5 h. Structural characterisation was done using optical microscope, field emission scanning electron microscope, feritscope, X-ray diffractometer, and glow discharge optical emission spectroscope as well as hardness measurement by a Vickers micro-hardness tester at 0.1 kgf. An ultrafine structure and a nitrogen-rich layer were, respectively, formed on the rolled and nitrided surfaces. Surface hardness was increased from 210 up to 450, 670 and 1050 HV0.1 after the rolling, nitriding, and rolling– nitriding processes, respectively. Thickness of the nitrided layer was increased from 12 to 20 lm and diffusion depth of nitrogen from 12 to 25 lm via conducting the deep rolling before the nitriding process. The rolling–nitriding process was resulted in rising of nitrogen concentration by a factor of about 3 at near-surface regions.