A Study on Surface Properties and High Temperature Oxidation Behavior of Boro-nitrided FC-25 Gray Cast Iron

摘要

Surface properties and oxidation behavior were studied for FC-25 gray cast iron (GCI), ion nitrided GCI (N GCI), powder boronized GCI (B GCI) and powder boronized N GCI (B-N GCI). A boride (Fe_2B and FeB) layer with a tooth shape was formed by boronizing GCI at 650 deg C. The thickness and hardness of the layer were proportional to boronizing temperature and time. The hardness was decreased as the depth of the boronized layer increased. The activation energy for boride formation was about 74 kJ/mol. The thickness of the graphite layer formed by boronizing was increased by increasing temperature and time. A silicon layer insoluble to borides accumulated beneath the boride layer. The major phases were Fe_4N, Fe_3N and FeN in the nitride layer formed by ion nitriding. The thickness and hardness of the layer increased with increasing nitriding temperature and time under the Fe_4N decomposition temperature of 680 deg C. The hardness was maximum at the surface and decreased with increasing nitrided depth on B-N GCI at 700 deg C, above the Fe_4N decomposition temperature. Hardness of B-N GCI at the layer depth of 10 mu m was approx = 1000 Hv, which was lower than that of the B GCI at the same condition. Also the layer thickness was 50 mu m; 10 mu m deeper than that of B GCI. The reason was that the nitrogen gas given off by the decomposition made the layer porous and facilitated the diffusion of oxygen and other reactants. The samples were oxidated at 800 deg C for 48hrs under atmospheric air. Oxidation rate was in decreasing order of N GCI, GCI, B GCI and B-N GCI. Oxidation rates were greatly influenced by the diffusion of nitrogen gas formed by Fe_4N decomposition over the decomposition temperature of 680 deg C and the oxidation barrier of the less corrosive boride layer.