| 摘要 |
PURPOSE:To impart a higher strength and toughness to a bushing, by providing a tempered part between high-frequency hardened parts. CONSTITUTION:In the first step, bushing blank is moved relatively to a product coil while rotating the blank with the axis thereof as a center, and an outside peripheral surface thereof is subjected to high-frequency hardening. In the second step, while rotating the bushing blank in the same manner as in the first step and cooling the outside peripheral surface thereof with water, the blank is moved relatively to the product coil, and an inside peripheral surface thereof is subjected to high-frequency hardening. As a result of the hardening of the inside peripheral surface, while the outside peripheral surface part retains the hardness as hardened, a central part of the wall thickness is softened by tempering under the effect of heating of the inside surface through heat conduction. In the third step, the bushing blank is subjected to low-temperature tempering, whereby the structure of the inside and outside peripheral surfaces is converted from martensite into tempered martensite, and the bushing 7 is completed. |
