| 摘要 |
PROBLEM TO BE SOLVED: To form a hardened layer only on both side surfaces of a work deeper in a center portion than at end portions close to an upper surface and a bottom surface by achieving the induction hardening on only the side surfaces of the work. SOLUTION: A long work W having a groove portion WM formed along the longitudinal direction is heated by a heating coil 200 having a heating conductor part 210 along the longitudinal direction. The heating conductor part 210 is opposite to right and left side surfaces WL, WR of the long work W with a specified interval therebetween, opposing to the groove portion WM. A first connection conductor part 220 and a second connection conductor part 230 of the heating coil 200 are arranged away from the long work W by at least a specified distance, and since the induced current runs in the longitudinal direction of the long work W, only both sides WL, WR are heated. After the heating, a refrigerant is sprayed to form a hardened layer WA on both surfaces WL, WR. The hardened layer is not formed on an upper surface WU or a bottom surface WD, but formed deeper in a center portion than at end portions close to the upper surface WU and the bottom surface WD, and a through-hole can be easily opened between the upper surface WU and the bottom surface WD.
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