Preparation of nanocrystalline high-nitrogen stainless steel powders by mechanical alloying and their hot compaction

Abstract

Mechanical alloying (MA) was performed, with Fe-N alloy powders as nitrogen source, on stainless steels of Fe-Cr-Ni-N (Cr-Ni type) and Fe-Cr-Mn-Mo-N (Cr-Mn type) systems in an Ar atmosphere using a planetary ball mill. In the MA processing, high nitrogen nanocrystalline stainless steel powders with 0.45-0.90 mass%N were readily manufactured after about 500-700 ks of processing. Nitrogen highly enhanced austenitizing of the powder products. The onset temperature (Md) of strain-induced martensite formation in nitrogen-free 19Cr-11Ni stainless steel MA powders was estimated to be about 100 K or more higher than those obtained under conventional plastic deformation processes. Compaction of the MA powders at 1173 K using a spark-plasma sintering (SPS) process still retained their nanostructure. When 5 vol% of dispersion particle agents AlN or NbN were added to the MA powders, grain growth in the compacts was greatly suppressed during SPS processing. On SPS processing, nitrogen in the Cr-Mn type system was completely retained but about 10-15 mass% of nitrogen in the Cr-Ni type powder samples was observed to escape, depending on the composition. However, when Mn, Nh or Ta was added to the samples or Cr content was increased, the nitrogen loss was greatly decreased. Furthermore, by simultaneous addition of these elements, such nitrogen loss was almost completely prevented. The nitrogen retention depending upon the chemical composition during SPS processing and the marked raising of Md temperature due to the MA processing are thermodynamically explained.