Plasma nitriding behavior of Fe-18 mass%Cr alloy

Microstructure and growth kinetics of the nitrided zone in an Fe-18 mass%Cr alloy plasma-nitrided at temperatures in a range between 743 K and 943 K were investigated. Morphology of the nitrides changes drastically depending on the nitriding temperatures. During nitriding at 943 K, Cr2N and CrN are formed through continuous precipitation in original ferrite grains, and there is no change in an orientation of ferrite grains. At 843 K, on the other hand, elongated columnar-shaped ferrite grains containing regularly spaced lamellar CrN precipitates are formed in the nitrided zone. Most of the columnar ferrite grains hold 19 coincidence site lattice orientation relationships with respect to the original ferrite grain. In spite of such a difference in the morphology of the Cr nitrides, a square of the nitrided zone thickness are nearly proportional to nitriding periods and an activation energy for the growth of the nitriding zone is almost constant in all the temperature range studied.