Electromagnetic induction heating, or induction heating for short, is used to heat conductor materials such asMetallic materialsA method of.It is mainly used for metal heat processing, heat treatment, welding and melting.
As the name implies, induction heating uses electromagnetic induction to generate current inside the heated material, and relies on the energy of these eddy currents to achieve the heating purpose.The basic components of induction heating system include induction coil, AC power supply and workpiece.The coil can be made into different shapes according to different heating objects.The coil is connected to the power supply, which provides alternating current for the coil. The alternating current flowing through the coil generates an alternating magnetic field passing through the workpiece, which makes the workpiece generate eddy current for heating.[1]
Induction heating is a quite new process. Its application is mainly due to its unique performance.When the rapidly changing current flows through the metal workpiece, the skin effect is generated, which makes the current concentrate on the surface of the workpiece, producing a highly selective heat source on the metal surface.Faraday discovered the advantage of skin effect and the noteworthy phenomenon of electromagnetic induction.He is also the founder of induction heating.Induction heating does not require an external heat source, but uses the heated workpiece itself as a heat source, and this method does not require the workpiece to contact the energy source, that is, the induction coil.Other features include the ability to select different heating depths based on frequency, accurate local heating based on coil coupling design, and high power density, or highpower density 。
The heat treatment process suitable for induction heating shall make full use of these characteristics, and complete equipment shall be designed according to the following steps.
First, the process requirements must be consistent with the basic characteristics of induction heating.This chapter will describe the electromagnetic effect, distribution of synthetic current and absorbed power in the workpiece.According to the heating effect and temperature effect produced by induced current, and the temperature distribution under different frequencies, different metals and workpiece shapes, users and designers can decide to discard them according to the requirements of technical conditions.
Second, the specific form of induction heating must be determined according to whether it meets the requirements of the technical conditions, and should also widely grasp the application and development situation, as well as the main application trend of induction heating.
Thirdly, after the suitability of induction heating and the best use method are determined, the inductor and power supply system can be designed.
Many problems in induction heating, similar to some basic perceptual knowledge in engineering, are generally derived from practical experience.It can also be said that without a correct understanding of the inductor shape, power frequency and thermal performance of the heated metal, it is impossible to designInduction heaterOr system.
The effect of induction heating is the same as that of flame quenching under the influence of invisible magnetic field.For example, byHigh frequency generatorThe higher frequency (above 200000 Hz) generated can generally generate intense, rapid and local heat sources, which is equivalent to the effect of small and concentrated high-temperature gas flame.On the contrary, the heating effect of medium frequency (1000 Hz and 10000 Hz) is relatively dispersed and slow, and the heat penetration is deeper, similar to the larger and open gas flame.[2]
Principle of induction heating
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principle
Induction surface hardeningIt is a quenching method that uses the principle of electromagnetic induction to generate high density induction current on the surface layer of the workpiece, rapidly heats it to the austenite state, and then rapidly cools it to obtain martensite structure,.When a certain frequency of alternating current passes through the induction coil, an alternating magnetic field with the same frequency of current change will be generated inside and outside the induction coil.When the metal workpiece is placed in the induction coil, under the action of the magnetic field, the induction current with the same frequency but opposite direction will be generated in the workpiece.Because the induced current forms a closed loop along the workpiece surface, it is usually called eddy current.This eddy current converts electric energy into heat energy, and rapidly heats the surface of the workpiece.The eddy current is mainly distributed on the surface of the workpiece, and there is almost no current passing through the interior of the workpiece. This phenomenon is called surface effect or skin effect.Induction heating is the use of skin effect, depending onCurrent heating effectThe surface of the workpiece is rapidly heated to the quenching temperature.The induction ring is made of copper tube, with cooling water inside.When the workpiece surface is heated to a certain temperature in the induction coil, it shall be cooled by spraying water immediately to make the surface layer obtain martensite structure.
Where: e - instantaneous potential, V; Φ - total magnetic flux of the area enclosed by the induced current circuit on the part, Wb, whose value increases with the increase of the current intensity in the inductor and the permeability of the part material, and is related to the gap between the part and the inductor.
Is the rate of change of magnetic flux, and its absolute value is equal to the induced potential.The higher the current frequency is, the greater the change rate of magnetic flux is, and the larger the induced potential P is correspondingly.The negative sign in the formula indicates the direction of induced potential and
The direction of change is opposite.
The direction of the eddy current induced in the part is opposite to the current direction in the inductor at each instant. The eddy current intensity depends on the induced electromotive force and the reactance of the eddy current circuit in the part, which can be expressed as:
Where, I - eddy current intensity, A; Z - self inductance reactance, Ω;R -- part resistance, Ω;X -- impedance, Ω.
The value of Z is small, so the value of I is large.
The heating heat of parts is:
Where Q -- heat energy, J; t -- heating time, s.
yesFerromagnetic material(such as steel), the heat effect produced by eddy current heating can rapidly increase the temperature of parts.Steel parts areHard magnetic materialIt has a large remanence. In an alternating magnetic field, the direction of the magnetic pole of the part changes with the direction of the magnetic field of the inductor.Under the action of the alternating magnetic field, the magnetic molecules will generate intense friction and heat due to the rapid change of the direction of the magnetic field, which also plays a role in heating the parts, which is the hysteresis heat effect.This part of the heat is much smaller than the heat effect of vortex heating.The hysteresis heat effect of steel parts only exists below the magnetic transition point A2 (768 ℃). Above A2, the steel parts lose magnetism. Therefore, for steel partstwoBelow point, the heating speed ratio is AtwoIt is fast when the point is above.[3]
Frequency selection
Selection of induction heating frequency: select the frequency according to the requirements of heat treatment and heating depth. The higher the frequency, the shallower the heating depth.
High frequency (above 10KHZ) heating depth is 0.5-2.5mm, which is generally used for heating small and medium-sized parts, such asSmall module gearAnd small and medium-sizedShaft partsEtc.
Medium frequency (1~10KHZ) heating depth is 2-10mm, which is generally used for heating shafts with large diameter and gears with large and medium modulus.
Power frequency (50HZ) heatingHardened layer depth10-20mm, generally used for heat transfer of large size parts, and the surface of large diameter parts (diameter above 300mm, such as rolls, etc.)quench。
Empirical formula
Induction hardeningThe depth of surface hardening layer depends on the thickness of heating, and the thickness of heating depends on the frequency of alternating current. Generally, if the frequency is high, the heating depth is shallow, so is the depth of hardening layer.The relationship between frequency f and heating depth δ is as follows:
Where: f isfrequency, in Hz;δ is the heating depth, in mm.
Specific application
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Induction surface hardeningIt has the advantages of good surface quality, small brittleness, difficult oxidation and decarbonization of quenched surface, small deformation, etc., so the induction heating equipment is used toSurface heat treatmentHas been widely used.
Induction heating equipment
Induction heating equipment is a kind of equipment that generates induction current of specific frequency for induction heating and surface quenching.
Surface hardening
Place the workpiece in the inductor wound by hollow copper tube, apply medium frequency or high frequency AC current, form induction current of the same frequency on the workpiece surface, rapidly heat the surface of the part (800~1000 degrees can be heated within a few seconds, and the center is still close to the room temperature), and then immediately spray water to cool (or oil immersion quenching), so as to harden the surface layer of the workpiece.
1. The heating speed is extremely fast, which can expand the transformation temperature range of body A and shorten the transformation time.
2. After quenching, the surface of workpiece can be very fineCryptocrystalline martensiteAnd slightly higher hardness (2~3HRC).Low brittleness and high fatigue strength.
3. The workpieces treated by this process are not easy to be oxidized and decarburized, and some workpieces can be directly assembled for use after treatment.
4. The hardened layer is deep, easy to control and operate, easy to realize mechanization and automation.
technical study
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modernInduction heating power supplyIt is developing towards high power and high frequency.This pair of modernPower electronic devicesIt is a considerable challenge.The traditional method is to use series parallel connection of devices, but it is difficult to share current and voltage between devices. Especially when there are many series parallel connections of devices, it is necessary to ensure accurate synchronous signals to avoid circulating current between devices damaging power electronic devices.But in many cases, this is difficult to guarantee accurately.Especially when there are many series and parallel devices and the power level is very high, its excellent characteristics can effectively reduce the circulating current between parallel inverters. Through parameter design, the power distribution of each bridge can be balanced, and the loss of devices can be reduced, thus effectively solving some problems in parallel inverters, which is conducive to the parallel connection of multiple bridges of induction heating power supply, and improving the output power and reliability.
Circulating current analysis of induction heating parallel module
The biggest advantage of LLC resonant load is that it is conducive to the parallel connection of multiple machines in induction heating. It does not need to add any components between inverters. Even if the signal delay angle of each bridge is large, it can ensure the system to work steadily, suppress the circulating current between each bridge, and adjust the output power of each inverter.
Future characteristics of induction heating equipment
With induction heat treatment production lineAutomatic controlIt is necessary to strengthen the development of complete sets of heating process devices for the improvement of the degree and high reliability requirements of power supply.At the same time, the induction heating system is developing towards intelligent control, with computer intelligent interface, remote control and automatic fault diagnosis, miniaturization, suitable for field operation, high efficiency and energy savingInduction heating power supplySystem is becoming the future development goal.
