Fabrication of Fine-grained High Nitrogen Austenitic Steels through Mechanical Alloying Treatment.

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Abstract
Mechanical alloying (MA) treatment was applied for the fabrication of fine-grained high nitrogen stainless steels. Chromium nitride (Cr2N) powder was mixed with Fe-Cr binary alloy powder to control its mean chemical composition to be Fe-23mass%Cr-1mass%N which is enough to stabilize austenitic structure at room temperature. The powder mixture was mechanically alloyed up to 360 ks in an argon gas atmosphere (MA powder). The MA powder was packed in a stainless steel tube in a vacuum and consolidated by warm rolling at 1 073 K. The consolidated materials were finally heated to various temperatures (1 173-1 473 K) for austenitizing and then quenched without holding at the temperatures. Although the materials heat-treated below 1 323 K had bcc (martensitic) matrix, those heat-treated above 1 373 K had stable austenitic structure with a small amount of Cr2N. The grain size of matrix was maintained to be fine due to dispersed oxide particles within matrix in all steels. For example, the materials heat-treated at 1 473 K had fine austenitic structure in which the grain size was 2.2 µm and the solute nitrogen concentration was 0.86 mass%. The steel had very high yield strength of 1.1 GPa and moderate elongation of 30%. Such a high strength of the steel was explained by the combined strengthening mechanism of nitrogen solid solution strengthening and grain refining strengthening.

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