Browsing by Author "Schacherl, Ralf Erich"

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Growth kinetics and microstructure of gaseous nitrided iron chromium alloys
(2004) Schacherl, Ralf Erich; Mittemeijer, Eric Jan (Prof. Dr. Ir.)
Thin Fe-Cr foils with a thickness of about 150 to 200µm containing 4, 7, 13 and 20 wt.% chromium were nitrided in a gas mixture consisting of hydrogen and ammonia at temperatures of 450, 580 and 700 °C. The To investigate the nitrided samples light microscopy, scanning electron microscopy, hardness measurements, X-ray diffraction, electron probe microanalysis (EPMA), and Auger electron spectroscopy were applied. The samples with 4 and 7 wt.% Cr which were nitrided at 450 and 580 °C exhibit grains of relatively high hardness, containing finely dispersed submicroscopical chromium nitride precipitates, as well as grains of lower hardness containing discontinuously precipitated chromium nitride. The samples nitrided at 700 °C contain relatively coarse CrN particles. All samples were completely nitrided (i.e., throughout the whole sample cross-section), as confirmed by EPMA: a constant nitrogen content, independent of the occurring precipitation morphology, was observed. This nitrogen content is larger than expected on the basis of the Cr content and the solubility of N in Fe: excess N occurs. Local Cr and N enrichment was observed near pores which had developed mainly along grain boundaries. These pores are especially present in the samples which were nitrided at 700 °C. The evolution of the precipitation morphology, the presence of excess N and the local enrichment of Cr and N near pores were discussed. To investigate the growth kinetics of nitrided layers of Fe-Cr alloys, nitriding experiments were performed for Fe-Cr sheets (1mm thickness) with 4, 7, 13 and 20 wt.% Cr at a temperature of 580°C. The precipitation morphology of the nitrided samples was investigated with light optical and scanning electron microscopy. The elemental compositional variation was determined with electron probe microanalysis. To describe the evolution of the thickness of the nitrided layers a numerical model was developed 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. Fitting of the model to the experimental data demonstrated for the first time that mobile and immobile excess nitrogen is present in the nitrided layers and that the mobile excess nitrogen considerably influences the nitriding rate.