| 摘要 |
PURPOSE: To provide a screw coupling for an oil well which is constituted in such a manner that even through repetition of fastening and unfastening, seizure to an oil well pipe hardly occurs. CONSTITUTION: In a screw coupling for an oil well steel pipe formed of a high chrome steel containing 10% or more Cr in a weight ratio and having a screw part and a metal seal part, surface hardness of the screw part and the metal seal part of either a male and a female is increased than that of the other by at least 10Hv (Vickers hardness) or more and surface average coarseness of the side where the hardness is increased is set to 0.25-1.00μm.
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