Effects of chromium content on the nitrided layer of binary Fe-Cr alloys

Binary alloys were gas-nitrided at a temperature of 520 °C for 70 h and with a N₂/NH₃ ratio of 15 to 20%. The thickness of the nitrided layers was evaluated by Vickers hardness testing. The morphology and composition of the nitrided layers were investigated by light and scanning electron microscopy (SEM). Phase depth distributions were calculated by comparison of peak intensities method, and the elemental analyses were made by EDS. Increasing chromium content increases the solubility of nitrogen in both the compound and the diffusion layers, and then their hardness. It also increases CrN amount and the surface porosities, it reduces the nitrided depth but it does not affect the thickness of the compound layer. Continuous precipitation of CrN is noted until 5%Cr. For higher chromium content, discontinuous precipitation occurs hampering the diffusion of nitrogen and coarsening the precipitates. The compromise between nitrogen diffusion and CrN growth is carried out for 3%Cr.