Beneficial effects of nitrogen
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I.
INTRODUCTION
ADDITIONS of nitrogen have been shown to enhance the strength, corrosion resistance, and hardness of austenitic stainless steels without sacrificing lowtemperature ductility and toughness (e.g., References 1 and 2). An additional advantage of nitrogenated stainless steels is that the precipitation of nitrides along grain boundaries occurs at a slower rate than does the formation of carbides, ljl Therefore, nitrogen-containing stainless steels are less prone to sensitization and more suitable for high-temperature exposure. These qualities make nitrogenated stainless steels candidate materials for applications where moderately high strength, good corrosion resistance, good oxidation resistance, and high toughness are required of nonmagnetic alloys. One of the limiting factors in the application of nitrogenated stainless steels is the difficulty of incorporating a controllable, homogeneous concentration of nitrogen into the metal. Methods have included melting and casting under high pressure gaseous nitrogen13m or adding nitrogenous materials to the slag during electroslag remelting, lsl A processing route which has not been previously explored is entraining nitrogen during gas atomization followed by forming and/or consolidating the resulting powder by methods such as hot isostatic pressing (HIPping), conventional sintering, or injection molding. By using this fabrication approach, the nearnet-shape capabilities of powder metallurgy and the microstructural refinement of rapid solidification can be combined with the improved properties of nitrogenated austenitic stainless steels. The present article reports on initial results of G.M. JANOWSKI, formerly NRC/NIST Postdoctoral Research Associate, National Institute of Standards and Technology, is Assistant Professor, Department of Materials Science and Engineering, The University of Alabama at Birmingham. Birmingham, AL 35294-4461. F.S. BIANCANIELLO and S.D. RIDDER, Metallurgists, are with the Metallurgy Division, MSEL, National Institute of Standards and Technology, Gaithersburg, MD 20899. Manuscript submitted January 10, 1992. METALLURGICAL TRANSACTIONS A
fabricating a high nitrogen austenitic stainless steel by consolidating powders which were melted under and/or atomized with nitrogen gas. It will be shown that nitrogen is dissolved in the molten metal and retained in the powders during atomization using the NIST supersonic inert gas metal atomizer (SIGMA). In addition, the roles of backfill and atomizing gas in hollow particle formation were examined. Hardness, yield strength, and microstructural data will be presented to demonstrate the viability of this process for manufacturing nitrogenated stainless steels. II.
EXPERIMENTAL PROCEDURE
The 304L rods that were used for melt stock were supplied by Carpenter Technology Corporation (Reading, PA); the composition as given by the supplier is contained in Table I. A modified alloy with 23 wt pct Cr was prepared by adding high purity Cr to the 304L melt stock. This alloy was produced in order to enhan
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