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1-1 characteristics of induction heating
Before the induction heating was born, the flame furnace was the only choice for forging industrial heating. In fact, if it is purely economical, the flame furnace is superior to any electric heating including induction heating equipment. This is because, from the occurrence of electricity to the induction heating of the billet, a number of energy conversion processes have taken place. And each conversion is accompanied by loss. Taking thermal power generation as an example, the conversion of thermal energy obtained from fuel combustion into mechanical energy and electrical energy will consume a portion of energy; from the power plant to the induction heating device, part of the electrical energy is converted into a rise and fall transformer, high and low voltage transmission lines. The heat is consumed; the remaining electrical energy is converted into magnetic energy in the induction heating device, and the magnetic energy can be converted into heat to heat the blank and bring about new losses.
Figure 1-1 Energy distribution diagram of induction heating of steel billet *This article was originally published in Mechanical Workers (Hot Processing) No. 1 (No. 544)
However, in the forging industry, the heating of the billet uses induction heating with respect to the furnace heating to have the following two characteristics:
Feature 1: Induction heating has a unit power of up to 500-1000 kW/m2 (due to the high magnetic flux density and a certain amount of effective heating layer depth established in the billet) and short diathermy time (due to heat from the billet itself) .
The benefits of this feature for the heating of forged billets are:
1 The heating speed is fast, and even if the surface of the billet is heated in air, the oxidation and decarburization are very small. The forged parts have a reasonable streamline distribution, which allows the billet to achieve higher fatigue performance. Heating in a flame furnace, the amount of scale is 3%-4%, and can be controlled by induction heating at 0.05%-0.5%; induction heating has no fuel product in the heating zone, so that a relatively clean blank can be obtained, the pair of blanks High quality manufacturing is a very beneficial factor.
2 Short heating time facilitates local heating. The temperature of the heating zone can be established quickly, and the temperature transition zone is relatively narrow, which not only reduces the loss of electric energy, but also facilitates the forging forming of some locally heated workpieces.
3 Energy saving and material saving.
Feature 2: Induction heating is reproducible, that is, the power required for induction heating for a given billet size, material, initial forging temperature, and tempo is substantially constant. Therefore, the beat can be used to determine the precise heating temperature, the process repeatability is good, and the product quality is stable.
The benefits of this feature for the heating of forged billets are:
1 Due to the compact equipment and adjustable power, the induction heating equipment can be combined with the forging equipment to form a production line for simultaneous production.
2 Induction heating method can effectively prevent the heating temperature of the billet from being insufficient or over-burned under certain power and tempo conditions, thus fundamentally guaranteeing the quality of the forging.
1-2 Basic principles of induction heating
Induction heating is based on two fundamental physical phenomena: Faraday electromagnetic induction, Joule effect.
When the alternating current I1 is passed through the coil, an alternating magnetic field is established in the space around the coil. An induced electromotive force E is generated in the metal blank in the alternating magnetic field.
1 power supply 2 eddy 3 metal blank 4 induction coil Figure 1-2 induction heating schematic...
1-3 Electromagnetic effect of induction heating
(1) The alternating current in the skin effect coil conductor and the eddy current in the metal blank are unevenly distributed in the current density of the cross section, and the maximum current density appears on the surface layer of the cross section, and the centripetal portion is exponentially functioned. Attenuation, this phenomenon is called the skin effect.
Figure 1-3 Causes of current density along the surface layer skin effect: When an alternating voltage U is applied across the metal blank, an alternating electric field is established in the metal blank. Due to electromagnetic induction, the alternating magnetic field formed by the current in the billet produces an induced potential e in the opposite direction. Since the core of the billet penetrates more magnetic flux than the surface, the induced electromotive force e1 of the core is greater than the induced electromotive force e2 of the surface, that is, U-e1<U-e2, so the surface current density i2>the core current density i1.
(4) The end effect is divided into the end effect of the blank and the end effect of the induction coil.
The skin effect describes the magnetic field distribution on the cross section of the metal blank, while the end effect shows the magnetic field distribution at the ends of the blank and the induction coil. It will affect the power distribution along the axial direction of the blank and the distribution of the billet heating temperature.
For non-magnetic blanks in a longitudinal magnetic field, the end effect of the blank increases the power absorbed at the ends of the blank. For magnetic blanks, the end effect increases the power absorbed by the blank, depending on the radius of the blank, material properties, frequency and Magnetic field strength.
Induction heating is a combination of the above four effects. The function of the inductor coil system is the ring effect, and the billet system exhibits the skin effect. The proximity effect and the end effect are between the two.
The following is a brief introduction: the current frequency, power, heating time of induction heating and the design and calculation examples of the equivalent pitch and variable pitch sensor.
 Tang Jingming. Application of induction heating technology and equipment design experience [M]. Beijing: Mechanical Industry Press, 1975.
 (Russia) AE Slohotsky, CE Reskin. Sensors for induction heating of machine parts (design and manufacture)
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(Russia) Ð¡.Ð•. Reskin (Ð¡.Ð•.Ð Ñ‹ÑÐºÐ¸Ð½), Xiong Dazhang translation. Calculation and design of induction heaters [M]. Beijing: Mechanical Industry Press, 1957.
 (English) John Davis (J. Davies), (English) Peter Simpson (P. Simpson), Zhang Shufang, et al. Translational induction heating manual [M]. Beijing: National Defense Industry Press, 1985 .
 (Russia) Lozinski, Ðœ.Ð“., Wang Dongsheng, Yang Jibao translation. Industrial application of induction heating [M]. Shanghai: Shanghai Science and Technology Press, 1962.
 (Russia) (Riskin (Ð Ñ‹ÑÐºÐ¸Ð½, Ð¡.Ð•.), Huang Fuwan translation. Application of penetration induction heating in industry [M]. Beijing: National Defence Industry Press, 1982.
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