Plasma Processing of Iron Alloys-(I) Plasma Nitriding of Fe-Cr Alloys

Abstract

Plasma nitriding behavior has been studied for Fe-Cr alloys nitrided by plasma in a mixted gas of N₂ and Hz at temperatures from 450 to 550°C in order to discuss an application of plasma nitriding process for providing a functional gradient material. Two nitrided layers are formed:an external layer of γ '-Fe₄N and an internal layer where small CrN particles disperse in α-Fe matrix. The growth of the internal nitrided layer is controlled by diffusion of nitrogen in the matrix metal. The internal nitrided layer exhibits higher hardness due to dispersion of small CrN particles. Hardness in nitrided Fe-3%Cr alloy gradually decreases as the distance from the specimen surface increases. The change in hardness is ascribed to that in volume fraction of CrN particles.