Nitrogen bearing martensitic stainless steels: Microstructure and properties

Three experimental nitrogen bearing martensitic stainless steels (nitrogen content ranging from 1 600 to 1 900 ppm) were produced in an air induction furnace and the N was added into the melt as Fe-Cr-N master alloy or gas nitrogen. These alloys with (C + N) content equal to 0.32 wt% were compared with a commercial AISI 420 steel. The alloys were homogenized, forged, quenched in air (alloys I and II) or in oil (alloys III and AISI 420) from temperatures between 1 073 to 1 423 K. The austenitizing temperature of 1 273 K was chosen and the specimens were tempered in the range of 373 to 973 K for 1 h. SEM of the 773 K tempered nitrogen steels did not show any visible precipitates. The AISI 420 alloy, however, exhibited a high density of chromium carbide precipitates when heat treated in the same manner. TEM observation of the alloy III did not show clearly fine and well distributed precipitates. The nitrogen bearing steels showed better corrosion resistance in the 773 K tempered condition than the as quenched AISI 420 as a consequence of lower precipitate size, stoichiometry and distribution of precipitates.