The morphology of nitrided iron-chromium alloys; influence of chromium content and nitrogen supersaturation

Different iron-chromium alloys (4, 8, 13 and 20 wt.% Cr) were nitrided in NH3/H-2 gas mixtures at 580 degrees C. The nitrided microstructure was investigated by X-ray diffraction, light microscopy, hardness measurements and scanning electron microscopy. Composition depth-profiles of the nitrided zone were determined by electron probe microanalysis. Various microstructures develop, depending on the nitriding conditions and the alloy composition (chromium content). The initial development of coherent, sub-microscopical CrN nitrides leads to a state of hydrostatic stress allowing the uptake of excess nitrogen dissolved in the ferrite matrix. It is shown that the outcome of the subsequent discontinuous coarsening process, which leads to a lamellar microstructure, has a close relation to the nitrogen supersaturation. As a result, the occurrence of a distinct gradient in hardness across the nitrided zone can be understood.