Because the frequency of induction power supply is too high, the power is too high, the quenching fixture is biased, the top and bottom centers of quenching machine are not concentric, and the gap between inductor and gear is uneven, which will cause induction hardening cracks. When the single tooth of the gear with large modulus is heated, the adjacent tooth top and tooth surface are affected by the heat because the adjacent teeth are not shielded. When the heated single tooth is cooled, the heated tooth top part and tooth surface are also cooled, resulting in quenching cracks.