Microstructure in a plasma-nitrided Fe-18 mass% Cr alloy

Microstructures in a nitrided zone of an Fe-18 mass% Cr binary alloy plasma-nitrided at temperatures between 743 and 943 K were investigated by means of electron microscopy. The morphology of the nitrided zone changed drastically depending on the nitriding temperatures. On nitriding at 943 K, disk- and rod-shaped CrN and rod-shaped Cr₂N were observed in the original ferrite grains. Examination of orientation relationships between the ferrite matrix and the rod-shaped Cr nitrides suggested that a transition from Cr₂N to CrN occurs during nitriding at 943 K. On nitriding at 843 K, elongated columnar-shaped ferrite grains containing regularly spaced lamellar CrN precipitates developed from the specimen surface and grew inwards. Most of the columnar ferrite grains held Σ9 coincidence site lattice relationships with respect to the original ferrite grains. The columnar ferrite tends to form at a lower nitriding temperature or a higher Cr content.