Electrolyte heating surface quenching is to apply high-voltage (150~300V) direct current to the electrolyte, which conducts electricity due to ionization, and the negative electrode releases hydrogen and the positive electrode releases oxygen. Hydrogen forms a gas film around the negative electrode, which has great resistance. When the current passes through, it will generate a lot of heat to heat the cathode. When quenching, the workpiece immersed in the electrolyte is connected to the negative electrode, and the liquid tank is connected to the positive electrode. Turn on the power supply and heat the unmanned part of the workpiece (the quenching temperature can be reached within 5~ 10s). After power failure, it can be cooled in electrolyte, or it can be taken out and put into a separate quenching tank for cooling. There are many electrolytes suitable for surface quenching, among which 5%~ 18% W (sodium carbonate) aqueous solution is the most widely used, and the temperature should not exceed 60℃, otherwise the hydrogen film will be unstable, which will affect the heating effect. The corners and sharp parts of the workpiece are easy to overheat, so refractory materials should be used for insulation. The end face of the workpiece is usually insulated to avoid overheating. Electrolyte surface quenching is most suitable for rod-shaped workpiece, rim or plate-shaped workpiece.

The purpose of surface quenching is to obtain a surface with high hardness and wear resistance, while the core still maintains its original good toughness, which is often used in machine tool spindles, gears, engine crankshafts and so on. Surface quenching is a local quenching method, which hardens the surface layer of steel to a certain depth, while the core part remains unquenched. During surface quenching, the surface of steel parts is quickly heated to the quenching temperature, and cooled immediately before the heat reaches the center of the workpiece to realize local quenching. Induction heating surface quenching is to introduce alternating current with a certain frequency (including high frequency, intermediate frequency and power frequency) into the induction coil, so that an alternating magnetic field with the same frequency is generated around the induction coil, and the workpiece placed in the magnetic field will generate an induction current with the same frequency and opposite direction to the induction coil. This current is called eddy current. Due to the skin effect, eddy current is mainly concentrated on the surface of the workpiece. The resistance heat generated by eddy current quickly heats the surface of the workpiece to quenching temperature, and then sprays water on the workpiece to harden the surface of the workpiece.

Its heating speed is extremely fast, usually only a few seconds.

The current frequency range in practical application is from 50Hz to 500KHz. Among them, 50Hz to 10KHz is the intermediate frequency, mainly silicon controlled rectifier (thyristor), which is used for deep hardening, such as rollers; 10KHz to 50KHz, which is super audio, mainly IGBT, and has the widest application range; Above 50KHz, it is called high frequency, which is applied to the processing of fine parts, mainly electron tubes and useful MOS tubes.

At present, the development momentum of IGBT in China is relatively strong, especially the second generation IGBT induction power supply and the third generation IGBT induction power supply. The frequency range covers intermediate frequency, super audio frequency and high frequency, up to 200KHz. IGBT is famous for its stable performance.