| 摘要 |
PURPOSE:To obtain a slender and thick steel pipe being suitable for a high pressure fuel jet pipe, etc., which has no intermetallic compound whose componet is N, at all, on at least the surface of the inside circumferential wall of the steel pipe, and also is provided with a hardened layer accompanied with a diffusion layer on which nitrogen has been solid-melted directly. CONSTITUTION:To a slender and thick steel pipe 1, a nitriding preventive treatment is performed in advance on the surface of its outside circumferential wall by a plating film of Ni, etc., or without performing this treatment, a tough-dried treatment is performed in a state energized in the axial core direction in a molten bath of NaCN at 570-580 deg.C. As a result, to say nothing of an inside dircumferential wall surface 3 of the steel pipe 1, a diffusion layer 4 or 4, 4' on which nitrogen has been solid-melted are generated on an outside circumferntial wall surface 2, too, as nesessary. Subsequently, these diffusion layers are heat-treated for a desired time in an N2 gas or atmospheric gas furnace of about 600-750 deg.C. As a result, a fragile metallic compound consisting of Fe3N or Fe4N, which has been formed simultaneously on the layer 4 or 4, 4' is decomposed and removed, and it is possible to obtain a slender and thick steel pipe on which a hardened layer 5 or 5, 5' having hardness of about Hv=200-260.
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