Carbon steel with a carbon content of more than 0.35% like 45# steel or 40Cr/42CrMo needs to be hardened on its surface. It should be noted that after the high-frequency treatment of the gear, the hardness of the tooth surface and the tooth top will meet the requirements for use, but the tooth root part will not reach the quenching hardness. Therefore, the hardness is generally lower than the hardness of the carburizing and quenching treatment. And after the gear is high-frequency hardened, the accuracy will decrease. If it needs to be used as a high-precision gear, it is necessary to increase the subsequent process to grind the gear.