Topy Kogyo Kabushiki Kaisha

Kimio Goto

Masahiro Nakajima

Hiroyuki Takeno

Isao Yoshida

A method for heat-treating a hollow cylindrical workpiece includes quench-hardening including a first step and a second step. In the first step, during continuously conveying the workpiece in a horizontal direction without a space between adjacent workpieces, an entirety of the workpiece is subjected to induction-heating conducted from an outside surface of the workpiece only, and a temperature of the workpiece is made uniform before the workpiece reaches a cooling portion spaced from a heating portion. Then, cooling the workpiece is initiated before the temperature of the workpiece falls to Ar,temperature and an entire cross-section of a wall of the workpiece is quench-hardened. In the second step, during continuously conveying the workpiece in the horizontal direction, induction-heating is conducted only to an outside portion of the workpiece, the induction-heating being conducted from the outside surface only. Immediately after that, the workpiece is cooled from the outside surface thereby re-quench-hardening the outside portion, so that both the outside portion and the inside portion of the workpiece have a hardness greater than an effective hardness, and a core portion of a wall of the workpiece has a hardness less than the effective hardness.

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Chigasaki, Japan

Kimio Goto