Beneficial effects of nitrogen atomization on an austenitic stainless steel

Fully dense nitrogenated austenitic stainless steels were produced by gas atomization and HIP consolidation. The base alloy, 304L, contained about 0.15 wt pct nitrogen when melted under a nitrogen atmosphere, and a modified version of 304L with 23 wt pct Cr contained 0.21 wt pct nitrogen. A series of experiments using various combinations of N₂ and Ar as the melt chamber backfill gas and atomizing gas demonstrated that the nitrogen content of the powder was largely controlled by the backfill gas and that the fraction of hollow particles was determined by the atomization gas. The hollow powder particles, which are common in inert-gas atomized materials, were virtually eliminated in the nitrogen atomized powders. Additional atomizing experiments using copper and a nickel-base superalloy indicate that low gas solubility in the metal leads to gas entrapment. Hardness and compression behavior (yield strength and flow stress) are substantially improved with the addition of nitrogen. The results of this study suggest that the properties of nitrogenated stainless steels fabricated in this manner are comparable to other high nitrogen austenitic alloys. G.M. JANOWSKI, formerly NRC/NIST Postdoctoral Research Associate, National Institute of Standards and Technology.