Induction Heat Treatment on Metal Materials

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Surface heat treatment process in which the workpiece is locally heated by induction current. This heat treatment process is often used for surface quenching, and can also be used for local annealing or tempering, and sometimes it is also used for overall quenching and tempering.

In the early 1930s, the United States and the Soviet Union successively began to apply induction heating methods for surface hardening of parts. With the development of industry, induction heating and heat treatment technology has been continuously improved, and the scope of application has been continuously expanded.

Basic principle: Put the workpiece into the inductor (coil), and when an alternating current of a certain frequency is passed through the inductor, an alternating magnetic field will be generated around it. The electromagnetic induction of the alternating magnetic field produces a closed induced current-eddy current in the workpiece. The distribution of the induced current on the cross-section of the workpiece is very uneven, and the current density on the surface of the workpiece is very high and gradually decreases inward. This phenomenon is called the skin effect. The electric energy of the high-density current on the surface of the workpiece is converted into heat energy, so that the temperature of the surface is increased, that is, surface heating is realized. The higher the current frequency, the greater the current density difference between the surface and the interior of the workpiece, and the thinner the heating layer. After the temperature of the heating layer exceeds the critical point temperature of the steel, it is rapidly cooled to achieve surface quenching.

Classification: According to the frequency of the alternating current, induction heating treatment can be divided into 5 categories: ultra-high frequency, ultra-audio frequency, intermediate frequency, and power frequency.
1. The current frequency used in UHF induction heating heat treatment is as high as 27 MHz, and the heating layer is extremely thin, only about D.15 mm, which can be used for surface quenching of thin-layered workpieces such as circular saws.
2. The current frequency used in high-frequency induction heating heat treatment is usually 200-300 kHz, and the depth of the heating layer is 0.5-2 mm. It can be used for surface quenching of gears, cylinder liners, cams, shafts and other parts.
3. The frequency of the electric current used in the super-audio induction heating heat treatment is generally 20~30 kHz. The small-modulus gear is heated by super-audio induction current. The heating layer is roughly distributed along the tooth profile, and the performance is better after simmering.
4. The current frequency used in the intermediate frequency induction heating heat treatment is generally 25~10 kHz, and the depth of the heating layer is 2~8 mm. It is mostly used for surface quenching of workpieces such as large modulus gears, shafts with larger diameters, and cold rolls.
5. The current frequency used in the power frequency induction heating heat treatment is 50-60 Hz, and the depth of the heating layer is 10-15 mm, which can be used for surface quenching of large workpieces.

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