Plasma nitriding of stainless steels at low temperatures

To avoid the drop in corrosion resistance of stainless steels in conventional nitriding (precipitation of CrN), low-temperature techniques like ion implantation, plasma immersion ion implantation (PIII, PI3) and low-temperature plasma nitriding were developed. In this investigation, four stainless-steel grades (ferritic: X6Cr17, austenitic-ferritic: X2CrNiMoN22.5.3, austenitic: X8CrNiTi18.10 and X5CrNi18.10) were plasma-nitrided between 250 and 500 degrees C. Nitrogen-enriched layers with a high nitrogen content were produced, leading to a significant increase in surface hardness. X-ray diffraction indicated that CrN did not precipitate if treatment temperatures did not exceed 400 degrees C. 'Expanded austenite' formed in the austenitic and duplex steels and E-nitride (Fe2N1-x) in the ferritic steel. The optically visible structure of the nitrided cases is comparable with that of the PIII layers, with higher charging densities being possible in the plasma nitriding. Also, in comparison to conventional ion implantation, large charges and parts with complicated shapes can be treated. (C) 1999 Elsevier Science S.A. All rights reserved.