PURPOSE:To uniformly harden internal and external surfaces of a hollow piston pin and to improve fatigue strength particularly at the bore surface by vertically holding a hollow piston pin of the transformation point or above after carburizing treatment, spraying quenching media on the internal and external surfaces of the piston pin, and allowing the quenching media to flow down along the surface of the piston pin. CONSTITUTION:A hollow piston pin 1 subjected to carburizing treatment and heated at a temp. of the transformation point or above is held vertically via a pedestal 2 and a presser 4 of a fog quenching device 3. Then, quenching media is sprayed on the internal and external surfaces of the above hollow piston pin 1 through respective jets 5a, 6a of an external cylinder 5 and an internal cylinder 6. The quenching media after the conclusion of cooling is allowed to flow down along the internal and external surfaces without stagnation, by which both the internal surface and the external surface of the hollow piston pin 1 can be sufficiently subjected to quenching media spraying and quenched. By this method, uniform hardening intensity can be provided to both the bore surface and the outside cylindrical surface, and further, uniform compressive residual stress is generated at the bore surface and sufficient fatigue strength can be provided to the hollow piston pin 1.
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