N<SUB>2</SUB>-NH<SUB>3</SUB>-CO<SUB>2</SUB>ガス雰囲気における低炭素鋼の酸窒化

Abstract

Low crabon steel (S15CK) specimens were oxy-nitrided in nitrogen-ammonia-carbon dioxide mixed gas atmosphere. The microstructures of oxy-nitrided surface layers were examined by optical microscope, X-ray microanalysis and X-ray diffraction. Corrosion-resistance and wear-resistance of oxy-nitrided layers were also studied. The gas composition greatly affects the microstructure of oxy-nitrided surface layers, which is classified into white layer, gray layer with white layer and gray layer. The white nitride layer is composed of an ε phase, Fe3N and γ′ phase. The main constituent of the white layer containing higher nitrogen is ε, and that with lower nitrogen is γ′. 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 premixed carbon dioxide ratio increases through activated oxidation. With less than 45 vol% ammonia ratio and high carbon dioxide atmosphere, the oxy-nitrided layer is composed of only Fe3O4. The white nitride layer, ε, Fe3N and γ′ shows good corrosion resistance. The thickness of nitrided layer decreases as the premixed carbon dioxide ratio 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-nitrided steels is improved about fourfold compared with untreated specimens. The nitride layer shows good wear resistance and it slightly decreases as the thin gray Fe3O4 layer formed on the nitride layer.