P/M PROCESSING ROUTES FOR HIGH NITROGEN MARTENSITIC STAINLESS STEELS

摘要

High nitrogen martensitic stainless steels were obtained through four different P/M processing routes namely: (1) die-compaction + simultaneous nitriding/sintering + hot isostatic pressing (2) nitriding of the uncompressed powder +hot isostatic pressing, (3) nitriding of the uncompressed powder + hot pressing, and (4) mechanical alloying (Fe+Cr+chromium nitride)+sintering +hot isostatic pressing. Nitrogen addition and hot pressing treatments were made at temperatures between 1273 and 1473 K under N2 pressure, while hot isostatic pressing was carried put at 1423 K under 150 MPa argon pressure. Stainless steel with relative densities of 85 percent to 99.5 percent and nitrogen contents ranging from 0.47 to 2.9 wt percent N was obtained. The microstructure of heat-treated specimens with less than 0,73 wt percent N was composed by martensite plus retained austenite, while increasing amounts of precipitated nitrides were observed in the alloys with higher nitrogen contents. The steels were ranked according to their corrosion and corrosion-erosion properties, which were measured through slurry wear and polarization tests. The negative effect of porosity was greater on corrosion-erosion resistance (measured in slurry wear tests) than in electrochemical corrosion resistance in acid solution (measured in polarization tests). On the other band, increasing the nitrogen content of the specimens beyond the nitrogen solubility limit strongly reduced their localized corrosion resistance, although practically no effect was observed in corrosion-erosion tests.