Short time nitriding and nitrocarburizing of martensitic stainless steel

Abstract

Plasma assisted thermochemical diffusion treatments have been extended to stainless steel and the usual process parameters are still in discussion regarding wear and corrosion performance as well as energy efficiency. In this work, a short time route for the thermochemical treatment for AISI 420 martensitic stainless was chosen, setting the duration to 2 h, changing other parameters such as temperature and N2 partial pressure. Temperatures of 420 and 440 °C were used, and two different atmospheres were applied, N2-H2 for nitriding, and N2-H2-CH4 for nitrocarburizing. The microhardness was measured, and microstructure was analyzed by XRD. Pin on disk tests were carried out according to G99 standard using alumina as counterpart, and abrasion wear tests were done following ASTM G65 standard. Corrosion behavior was evaluated using the salt spray fog test and electrochemical tests in NaCl solution. Modified layers between 5 and 15 μm thickness were obtained and surface hardness was always above 1000 HV. In XRD measurements martensite expanded by nitrogen/carbon and iron nitrides or carbonitrides were detected. The sample nitrocarburized at 440 °C showed better wear resistance under sliding conditions, however, the samples nitrided at 440 °C had the lowest mass loss in abrasive wear tests. Regarding the corrosion resistance, the sample nitrocarburized at 440 °C exhibited the best results in the salt spray fog test and in electrochemical tests. This behavior was related to the presence of the carbonitrides and the layer thickness.