Plasma nitriding of stainless steels at low temperatures

To avoid the drop in corrosion resistance of stainless steels in convention al nitriding (precipitation of CrN), low-temperature techniques like ion im plantation, plasma immersion ion implantation (PIII, PI3) and low-temperatu re plasma nitriding were developed. In this investigation, four stainless-s teel grades (ferritic: X6Cr17, austenitic-ferritic: X2CrNiMoN22.5.3, austen itic: X8CrNiTi18.10 and X5CrNi18.10) were plasma-nitrided between 250 and 5 00 degrees C. Nitrogen-enriched layers with a high nitrogen content were pr oduced, leading to a significant increase in surface hardness. X-ray diffra ction 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 PII I layers, with higher charging densities being possible in the plasma nitri ding. Also, in comparison to conventional ion implantation, large charges a nd parts with complicated shapes can be treated. (C) 1999 Elsevier Science S.A. All rights reserved.