The purpose of gear quenching is to make undercooled austenite undergo martensite or bainite transformation to obtain martensite or bainite structure, and then temper at different temperatures to greatly improve the strength, hardness, wear resistance, fatigue strength and toughness of steel to meet the different use requirements of various mechanical parts and tools. When a workpiece is placed in an inductor (coil), an alternating magnetic field will be generated around the inductor when a certain frequency of alternating current is applied. The electromagnetic induction of alternating magnetic field produces a closed induced current-eddy current in the workpiece. The induced current is unevenly distributed on the cross section of the workpiece, and the current density on the surface of the workpiece is very high, which gradually decreases inward. This phenomenon is called skin effect. The electric energy of high-density current on the surface of the workpiece is converted into heat energy, which makes the surface temperature rise, that is, surface heating is realized. The higher the current frequency, the greater the current density difference between the surface layer and the inside of the workpiece, and the thinner the heating layer. When the temperature of heating layer exceeds the critical temperature of steel, surface quenching can be realized. A) Tooth surface hardness: 48~56HRC (important gear 50 ~ 56 HRC

B) Effective hardened depth of tooth surface (0. 15~0.35)mn, mm;

Generally, the deviation of effective hardened layer depth of tooth surface is not specified, and the important gear is15% of the effective hardened layer depth;

C) tooth bottom hardness: > 40HRC (important gear > 45hrc);

Depth of hardened layer of tooth bottom: (0. 10~0.30)mn, mm;

Generally, the depth deviation of the hardened layer at the bottom of the tooth is not specified;

D) Hardening and tempering hardness of the core;

E) There should be no crack on the tooth surface;

F) Important gear materials shall be tested for mechanical properties.