The kinetics of the nitriding of Fe–7Cr alloys;the role of the nitriding potential

Abstract

The nitriding behaviour of the Fe–7Cr alloy was studied at 580°C in a gas mixture of ammonia and hydrogen. The nitriding potential was varied from 0.03 to 0.818 atm^−1/2. Microstructural analysis of the nitrided specimens was performed by applying light microscopy, hardness measurements, X-ray diffraction, and electron probe microanalysis. The nitrided zone is composed of both regions with finely dispersed small chromium nitride (CrN) precipitates (continuous precipitates) in ferrite (α–Fe) grains and, mainly near the surface, regions where the precipitates have discontinuously coarsened leading to a lamellar CrN/α–Fe morphology. The nitrogen content within the nitrided zone is larger than expected on the basis of the chromium content and the solubility of nitrogen in (stress free) ferrite: excess nitrogen occurs. The hardness maximum in the nitrided zone and the nitriding depth increases with increasing nitriding potential as long as no iron nitride layer develops at the surface of the specimens. To describe the evolution of the nitrogen concentration depth profile of the nitrided layers, a numerical model was applied that has as important (fit) parameters: the surface nitrogen content, the solubility product of chromium and nitrogen dissolved in the ferrite matrix, and a parameter defining the composition of the precipitated chromium nitride. The nitriding depth depends roughly linearly on the square root of the nitriding potential. Analysis of the concentration depth profile data demonstrated that the amount of excess nitrogen considerably influences the nitriding kinetics.