Fabrication of high nitrogen austenitic stainless steels by powder metallurgy

A few kinds of high nitrogen powders (nitride (Cr₂N), atomized 23mass%Cr steel with 4 mass% N (AT-4N), mechanically milled 23mass%Cr steel with 4mass%N (MM-4N)) were mixed with nitrogen-free powders (atomized 16mass%Cr steel, atomized 23mass%Cr steel (AT), mechanically milled 23mass%Cr steel (MM)) to fabricate the high nitrogen stainless steel of the mean chemical composition; Fe-23mass%Cr-1mass%N. The powder mixtures were packed in stainless steel tubes in vacuum and then consolidated by hot rolling for densification. The consolidated materials were annealed at 1473K for 1.8ks for the purpose of homogenization. In the powder mixtures, in which the Cr₂N or the AT-4N powder was added as the nitrogen supplier, full densification was not performed. In the case that the MM-4N powder was added as the nitrogen supplier to the nitrogen-free MM powder, the powder mixture was consolidated to the material with full density. The structure is of austenite with the grain size of about 5 μm. The austenitic steel has much higher strength and elongation than the sintered material with the same composition, because of its fine grained structure and its full density.