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What is induction heating?

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Author : FOCO induction
Update time : 2021-06-22 11:34:16
Electromagnetic induction heating, referred to as induction heating, is a method that uses the principle of electromagnetic induction to heat conductive materials such as metal materials. It is mainly used for metal heat treatment, hardening, brazing and melting.

Principle of induction heating

Induction heating uses electromagnetic induction to generate electric current inside the heated conductive material, which will make the electrons inside the metal very active, collide with each other, and rub against each other to generate heat energy, achieving the effect of quickly heating the metal itself. The main principles involved in the application of induction heating include induction heating, Faraday's law, skin effect, and heat conduction principles. The basic composition of induction heating system includes induction coil, AC power supply and cooling system. According to different heating parts, the coil can be made into different shapes. The coil is connected to the power supply, and the power supply provides alternating current to the coil. The alternating current flowing through the coil generates an alternating magnetic field through the part, which causes the part to generate eddy currents to heat the part itself.
The essence of induction heating is that an alternating magnetic field can produce an alternating electric field, an alternating electric field can also cause an alternating magnetic field.

When an alternating current is passed through one or several turns of the coil, an alternating magnetic field is generated around the coil, and the conductor (metal part) placed in the alternating magnetic field will generate an electromotive force E(V), that is
In the formula
f——current frequency (Hz)
Φ——the amplitude of the magnetic flux
n——the number of turns of the loop
When heating a metal part, the circuit is the part itself, and n=1 at this time.
The conductor generates eddy current under the action of electromotive force, and its value can be expressed by the following formula:
I = E/Z
In the formula
I—— Eddy current (A)
E-electromotive force (V)
Z-the impedance of the metal conductor.
This eddy current will heat the metal itself, causing the metal's temperature to rise. According to the Joule's Law, the heat Q(J) generated by the eddy current can be calculated by the following formula:
In the formula
I—— Eddy current (A)
R——Resistance of metal parts (Ω)
t——heating time (s)
When the heated metal part is a ferromagnetic material, the alternating magnetic field will cause the ferromagnetic material to be repeatedly magnetized, the direction of the magnetic field will continue to turn, and friction will be generated, which consumes energy, which is called hysteresis loss. The hysteresis loss also makes the metal part heat, but the heat in this part is much smaller than the heat of the eddy current.

Benefits of induction heating

Heating methods for conductors (metal materials) include coal heating, oil heating, gas heating, resistance furnace heating, electric oven heating, etc. Why do we use induction heating? Because compared with traditional heating methods, induction heating has many advantages.
High productivity. The speed of induction heating is very fast, and heat is directly generated inside the metal parts, and the part can be heated to above 1000°C in 1 second.
Energy saving. Induction heating has high thermal efficiency and can save energy by more than 30% compared with traditional heating methods. In addition, induction heating directly heats the inside of the metal, reducing heat loss.
Automatic control. Induction heating equipment can be equipped with a PLC control system to realize automatic control, reduce labor costs, and improve production efficiency in large-scale production lines.
Environmental protection. The induction heating equipment will not produce any harmful gas when it is working, and the heat radiation is also very small.



Induction heating can be used to heat metal materials, and can be used for surface hardening, annealing, brazing, smelting, shrink fitting, and preheating. In addition to metal materials, induction heating can also be used to heat semiconductor materials, plastic film heating and so on.
Induction furnace
Induction furnace, also called induction melting furnace, is mainly used to melt metal materials, such as steel, iron, aluminum, copper, lead, tin, gold, silver and some alloys. It mainly uses the principle of electromagnetic induction to heat the metal in the induction coil made of copper tube to melt the metal. The melting capacity of the induction melting furnace ranges from a few kilograms to hundreds of tons. During the production process, the heat radiation is small, the noise is relatively small, and no harmful gases are generated. It is a very environmentally friendly and clean production method. According to the special needs of certain metal melting, inert gas protection can be used or a vacuum induction melting furnace can be used.
Induction hardening and tempering
In the application of industrial heat treatment, induction heating is most widely used for induction hardening of metal parts, especially steel hardening. Using induction heating, the part can reach 1000°C within a few seconds, and the part can be locally heated as needed to achieve local hardening. After hardening, induction heating can also be used to achieve the purpose of tempering. The use of induction heating equipment for quenching and tempering of the part can realize the automation of the entire heat treatment process and improve production efficiency.
Induction brazing
In induction brazing, the brazing part of the part is placed in an alternating magnetic field, and the heating of this part of the base material is realized by the resistance heat of the induced current generated in the alternating magnetic field. Usually the induction coil is made of pure copper tube, and the tube is cooled by water during operation. There should be a gap between the induction coil and the part to avoid short circuit, but in order to improve the heating efficiency, the gap between the induction coil and the part should be minimized, so the induction coil is generally coated with insulating material. Induction brazing is widely used for brazing steel, copper and copper alloys, stainless steel, high-temperature alloys and other parts with symmetrical shapes. It is especially suitable for brazing of pipe fittings, pipes and flanges, shafts and sleeves, turning tools, cutter heads and saw blades. For the brazing of aluminum alloys, induction brazing can also be used.
Induction heating can also be used to weld plastic materials or relatively thin metal materials. The more common ones are the welding of plastic film and the welding of steel pipe joints. Welding of steel pipes is generally a method of welding steel strips into steel pipes. UHF induction welding machines are commonly used, and the frequency can reach more than 400kHz.
Shrink fitting
When the industrial parts are assembled, the mating parts can be expanded by heating to realize the combination of the two parts. Induction heating is mainly used for shrink fit of bearings, motor rotors, etc.
Induction preheating
Induction heating is often used to preheat the part in advance to facilitate the next processing procedure. For example, before the welding process, the weld seam needs to be preheated. Its purpose is to reduce the cooling rate of the welded joint, avoid the formation of hardened structure and reduce the welding stress and deformation, it is an effective method to prevent welding cracks.


Frequency of induction heating machine

The higher the frequency of the induction heating machine, the shallower the heating depth and the lower the permeability. High frequency is suitable for surface heating of metal parts, such as quenching. The appropriate frequency can be used according to the requirements of the depth of the hardened layer.
Medium frequency heating has a large penetration depth, and is often used for heating large parts or for smelting. For example, the heating forging of billets, the melting of various metals, and the greater depth of the hardened layer can also be used for medium frequency heating.


Induction coil

The induction coil is made of copper and is generally an open or closed circular coil. According to special needs, it can also be a special shape to ensure the consistency of the heating depth and heating quality of the part. The gap between the induction coil and the part should not be too large to ensure the best heating efficiency. The copper pipe needs to be cooled with water to avoid the temperature of the copper pipe being too high.