Hard austenitic stainless steel screw and a method for manufacturing the same

摘要

This invention allows the surface of an austenitic stainless steel screw surface to be formed into a hard nitrided layer so as to harden and a part such as a screw head which is in contact with outside air is removed its own ultra hard surface layer in the hard nitrided layer by scouring or the like to be rust preventive. Even if the ultra hard surface layer is thus removed, an inner hard layer in the hard nitrided layer is present beneath the surface layer to be able to maintain a hard state of the screw surface. In the method for manufacturing the austenitic stainless steel screw according to the invention, upon forming said hard nitrided layer on the screw surface by nitriding, the austenitic stainless steel screw surface is cleaned with a fluorine- or fluoride-containing gas prior to nitriding. Thereby remained foreign matter, oxidized layer and the like on the screw surface are removed and at the same time the screw surface is activated and so N atoms easily penetrate and diffuse when nitriding to form a uniform nitrided layer.