Effect of Solute Content on Plasma Nitriding Behavior of Fe-Cr Alloys.

Fe-Cr alloys with 3.0 to 18.6wt%Cr were plasma-nitrided at 650°C for 16h in a mixed gas of N2:H2=1:3. A nitriding layer, which is so called "internal nitriding layer", was formed parallel to the specimen surface, but the formation of a surface nitriding layer of γ'-Fe4N was not observed. The Cr nitride formed in the nitriding layer was determined by XRD experiments to be CrN, not Cr2N. The hardness Hv of the nitriding layer increased as the Cr content increased. The hardness increase is due to the dispersion of fine CrN particles in α-Fe matrix. The thickness of the nitriding layer decreased linearly as the Cr content increased.