To provide a coil for simultaneously induction-hardening a wheel part and a tooth part of a drive plate; a holder therefor; and a quenching method therefor.

The method for induction-hardening the drive plate 1 having a tooth form 4 formed in a circumferential rim portion 3 of the wheel part 2 comprises the steps of: preparing a semicircle-shaped coil which is formed of one pair of two conductors each having approximately a quarter arc shape, and of which the circuit is formed of 4 sector-shaped conductors 11 to 14 having four different diameters respectively; arranging the coil concentrically with the drive plate to be heated; simultaneously heating the outer surface of the tooth form 4 with the first conductor 11 and the plane of the wheel part 2 with the second to fourth conductors 12 to 14; press-hardening the front and back of the wheel part by a cold plate provided with radial channels through which water flows radially from the vicinity of the center, and simultaneously cooling the tooth form with a cooling means; and inserting a shield plate 34 into a draining hole 7 of the drive plate having the draining hole in the wheel part, and heating the drive plate.

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