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Austenitic stainless steel SUS 316L was nitrided by active screen plasma nitriding (ASPN) using screens with various open areas to investigate the effect of the screen's open area ratio on the nitriding response. The sample was placed on the sample stage in a floating potential and isolated from the cathodic screen and anode. The screen, which was SUS 316L expanded metal mesh with 38%, 48%, or 63% open area ratio, was mounted on the cathodic stage around the sample stage. Nitriding was performed in a nitrogen-hydrogen atmosphere with 25% N2 + 75% H2 for 18 ks at 693 K under 600 Pa by the ASPN process. After nitriding, the nitrided microstructure was examined using a scanning electron microscope and an electron probe microanalyzer. The phase structures on the nitrided surface were determined by X-ray diffraction. In addition, the surface hardness and cross section of the nitrided samples were measured by the use of a Vickers microhardness tester. The thickness of the nitrided layer of the S-phase decreased with increasing open area ratio of the screen.KEY WORDS: surface engineering; active screen plasma nitriding; cathodic cage; S-phase; stainless steel; open area ratio.

Abstract. Plasma nitriding is a surface modification process with a low environmental impact. Active screen plasma nitriding (ASPN) is one of the new plasma nitriding technologies, and can eliminate problems related to conventional direct current plasma nitriding (DCPN). In this study, ferritic stainless steel SUS430 samples were treated by ASPN to increase their wear resistance without decreasing their corrosion resistance. ASPN was performed in a nitrogenhydrogen atmosphere with 25%N 2 + 75%H 2 for 18 ks at 623 K, 673 K, 723 K, 773 K, and 823 K under 600 Pa using an SUS304 screen. When the sample was treated at 673 K by ASPN, the pitting corrosion resistance and wear resistance of its surface were improved because of the formation of the S α phase and a deposited layer containing Ni on the sample surface.

An austenitic stainless steel AISI 304 was active screen plasma nitrided using a 304 steel screen to investigate the effect of the gas pressure on the nitriding response. The sample was treated for 18 ks at 723 K in 25 % N 2 ϩ 75 % H 2 . The gas pressure was changed to 100, 600, and 1200 Pa. The distance between the screen and the sample was also changed to 10, 30, and 50 mm. The nitrided samples were characterized by observing their appearance and surface roughness by optical microscopy, X-ray diffraction, and microhardness testing. After nitriding, polygonal particles with a normal distribution were observed at the center and edges of all the nitrided sample surfaces. The particles on the sample surface became finer with an increase in the gas pressure. The nitrided layer with a greater and homogeneous thickness was obtained at a low gas pressure of 100 Pa.