Study On Process Of N₂-H₂ Mixed Plasma Source Nitriding AISI 304 Austenitic Stainless Steel

The AISI 304 austenite stainless was modified by the plasma source nitriding technique under low voltages with N2-H2 mixture, and a good effect of nitriding was achieved. The modified samples were characterized by optical metallurgical microscope, electron probe micro-analyzer, X-ray diffractometer, surface profile measurement, micro-hardness tester and electrochemical workstation. In this paper, the effects of process parameters, including N/H ratio, nitriding pressure, sample bias and position, on microstructure, nitrogen concentration-depth distribution, phase structure, surface roughness, micro-hardness and corrosion resistance of nitrided layer were studied systematically. A changing law between process parameters and nitriding effect was given qualitatively.At nitriding pressure of 300 Pa, floating potential, a distance of 100 mm between screen and sample, nitriding time of 6 h and N/H ratios of 3:1-1:5, all the nitrided layers were consisted of mono-phase yN phase. Compared with other N/H ratios, the nitriding effect with N/H ratio of 1:3 was the best, and the thickness, peak value of nitrogen concentration, surface roughness and micro-hardness of nitrided layer was 9.8 μm,16.5 wt.%,0.124 μm and 10.4 GPa, respectively. When the N/H ratio was larger than 1:3, the higher N/H ratio the worse nitriding effect. When the N/H ratio was smaller than 1:3, the lower N/H ratio the worse nitriding effect.For N/H ratios of 1:3, floating potential, a distance of 100 mm between screen and sample, nitriding time of 6 h and nitriding pressure of 220-500 Pa, all the nitrided layers were consisted of mono-phase γN phase. Compared with other nitriding pressures, the nitriding effect with pressure of 300 Pa was the best.For nitriding pressure of 300 Pa, N/H ratios of 1:3, a distance of 100 mm between screen and sample, nitriding time of 6 h and negative bias voltages of 200-400 V, all the nitrided layers were consisted of mono-phase γN phase. Compared with other negative bias voltages, the nitriding effect with negative bias voltage of -200 V was the best, and the thickness and micro-hardness of nitrided layer were 10.9 μm and 11.3 GPa respectively. With applying the negative bias voltage, the samples were clean-activated, and the thickness, micro-hardness and corrosion resistance of nitride layer were all improved.For nitriding pressure of 300 Pa, N/H ratios of 1:3, negative bias voltages of -200 V, nitriding time of 6 h and a distance of 50-250 mm between screen and sample, all the nitrided layers were consisted of mono-phase γN phase. Compared with other distances between screen and sample, the nitriding effect with a distance of 100 mm was the best. When the distance between screen and sample was 50 mm, the thickness of nitrided layer was increased but with an underdense structure, which led to a lower micro-hardness and a worse corrosion resistance.