OXY-NITRIDING OF LOW-CARBON STEEL IN N-2-NH3-CO2 ATMOSPHERE

Low crabon steel (S15CK) specimens were oxy-nitrided in nitrogen-ammon ia-carbon dioxide mixed gas atmosphere. The microstructures of oxy-nit rided surface layers were examined by optical microscope, X-ray microa nalysis and X-ray diffraction. Corrosion-resistance and wear-resistanc e of oxy-nitrided layers were also studied. The gas composition greatl y affects the microstructure of oxy-nitrided surface layers, which is classified into white layer, gray layer with white layer and gray laye r. The white nitride layer is composed of an epsilon phase, Fe3N and g amma' phase. The main constituent of the white layer containing higher nitrogen is epsilon, and that with lower nitrogen is gamma'. The main constituent of the gray layer is found to be Fe3O4. The thickness of oxide layer increases as the premixed ammonia ratio decreases or the p remixed carbon dioxide ratio increases through activated oxidation. Wi th less than 45 vol% ammonia ratio and high carbon dioxide atmosphere, the oxy-nitrided layer is composed of only Fe3O4. The white nitride l ayer, epsilon, Fe3N and gamma' shows good corrosion resistance. The th ickness of nitrided layer decreases as the premixed carbon dioxide rat io increases and the thin gray Fe3O4 layer formed on the nitride layer . These layers show bad corrosion resistance. But the thick gray Fe3O4 layer shows good corrosion resistance. Wear-resistance of oxy-nitride d steels is improved about fourfold compared with untreated specimens. The nitride layer shows good wear resistance and it slightly decrease s as the thin gray Fe3O4 layer formed on the nitride layer.