The Development of Zhengzhou Guoyun Induction Heating Technology and Equipment
Induction heating originates from the electromagnetic induction phenomenon discovered by Faraday, which means that alternating current generates induced current in a conductor, thereby causing the conductor to heat up. In 1890, Swedish technicians invented the first induction melting furnace - the slotted core furnace, and in 1916, Americans invented the closed-slot core furnace, marking the gradual practical application stage of induction heating technology. Induction heating equipment
The rapid development of power electronic devices and technology in the 20th century greatly promoted the development of induction heating technology. Induction heating equipment
In 1957, the United States developed the thyristor, a milestone in power electronic devices, marking the beginning of modern power electronics technology and triggering a revolution in induction heating technology. In 1966, Switzerland and West Germany were the first to use thyristors to develop induction heating devices, thus starting the rapid development of induction heating technology. Induction heating equipment
After the 1980s, power electronic devices experienced another rapid development with the emergence of GTOs, MOSFETs, IGBTs, MCTs, and SITs. Induction heating devices gradually abandoned thyristors and began to adopt these new components. Currently, IGBTs and MOSFETs are more commonly used, with IGBTs used for higher power applications and MOSFETs for higher frequency applications. It is reported that abroad, IGBTs can be used to achieve induction heating device powers exceeding 1000kW and frequencies exceeding 50kHz. MOSFETs are more suitable for high-frequency applications, typically used in medium and small power situations of a few kilowatts, with frequencies reaching above 500kHz or even several megahertz. However, abroad, there are also large-power induction heating devices using MOSFETs, such as a 2000kW/400kHz device developed in the United States. Induction heating equipment
The real application of induction heat treatment technology in China started in 1956, introduced from the Soviet Union, mainly applied in the automotive industry. With the manufacturing of power supply equipment in the 20th century, induction hardening process equipment also developed accordingly. Now, the domestic induction hardening process equipment manufacturing industry is expanding, with diverse product varieties. Previously imported equipment is gradually being replaced by domestic products, saving foreign exchange while promoting the development of related domestic enterprises. Currently, the main service objects of the induction heating manufacturing industry are automotive manufacturers, but in the future, the modern metallurgical industry will have a significant demand for induction heating. Induction heating equipment
I. Characteristics of Induction Heating
Induction heating technology has features such as rapidness, cleanliness, energy-saving, easy automation and online production, high production efficiency, internal heat source, non-contact heating method, high power density, highly flexible selection in heating surface and depth, operation in various carrier gases (air, protective gas, vacuum), extremely low loss, no physical pollution generation, compliance with environmental protection and sustainable development policies, making it one of the green and environmentally friendly heating processes. Together with controlled atmosphere heat treatment and vacuum heat treatment technologies with minimal oxidation, they have become the mainstream development direction of heat treatment technology.
Its main applications include:
(1) Metallurgy: Smelting of non-ferrous metals, heat treatment of metal materials, through-heating for forging, extrusion, rolling and other profile production, weld seams in welded pipe production.
(2) Machinery Manufacturing: Heating for quenching of various mechanical parts, as well as tempering, annealing, and normalizing after quenching; through-heating before pressure processing.
(3) Light Industry: Sealing of cans and other packaging, such as the famous Tetra Pak sealing packaging.
(4) Electronics: Heating for vacuum degassing of electron tubes.
(5) Special Applications: Such as plasma, overlay welding, etc.
Taking FAW as an example, in the production of medium-sized vehicles, light vehicles, and sedans, nearly 200 types of parts require induction heating quenching treatment. From the shape and size of induction heating quenched parts, they can be described as having a wide variety of patterns and sizes. With the continuous development of induction quenching technology, the proportion of induction quenched parts has increased to around 50% of all heat-treated parts. Relevant data shows that in China's automotive industry, the application of induction heat treatment is entering the ranks of world advanced levels.
II. New Induction Heating Processes
Induction heating processes are the main manifestation of induction heating technology levels and the basis for technological development. Advanced induction heating process technology can effectively leverage the characteristics of induction heating, achieving efficient and energy-saving local heat treatment.
(1) Longitudinal Induction Hardening Quenching: Already used in the automobile and tractor industries. The longitudinal heating of half-shafts is a single quenching process. In Germany and the United States, there are dedicated machines for single quenching of half-shafts, completing heating, correction, and quenching on one machine, improving productivity. The floor space for equipment with the same output for single quenching and continuous quenching is 40m² and 115m², respectively.
(2) Fillet Quenching of Crankshaft Journals: After fillet quenching of crankshaft journals, fatigue strength doubles compared to normalizing. Crankshafts for Cummins and NH engines produced in China already use this process.
(3) Low Hardenability Steel Gear Quenching: In the 1970s, China conducted research on 55DT, 60DT, and 70DT steels and achieved preliminary results, but later discontinued due to unstable hardenability of the steel. In 1992, K.3ЩЕПЕЛЯКОВСКЦЦ, the founder of low-hardenability steel in Russia, came to China to lecture and investigated the conditions for smelting low-hardenability steel at a certain steel plant, finding that the plant was fully capable of producing low-hardenability steel. YB 20091981 "Low Hardenability Titanium-containing High-Quality Carbon Structural Steel" controls alloy elements differently from Russia ((Ru)105474, 58(55П П) steel element content requires Mn, Cr, Ni, Cu sum <0.5% (mass fraction), while YB20098155Ti steel requires Cr, Ni, Cu sum <0.5% (mass fraction), which may be the key.
Russian low-hardenability steel and controlled hardenability steel have been widely used in automotive and tractor rear axle gears, excavator gears, transmission cross shafts, rolling bearings for train carriages, automotive leaf springs, and railway spiral springs, achieving significant economic benefits.
(4) Induction Resistance Quenching: It is well-known that brazing tool welding equipment uses induction resistance quenching for the gear part of steering racks. There are more than three imported machines domestically in production. A machine in the UK applies this process to gear production, discovering that the gear barely deforms after quenching and can then enter the assembly process.
(5) Fixed Heating Quenching of Crankshaft Journals: The new equipment is called Gr ankproTM, replacing the figure-eight semi-ring rotating heating inductor with two semi-ring fixed heating inductors. This set of equipment can perform quenching and tempering on crank necks, offering advantages over old processes such as energy savings, smaller footprint, less workpiece deformation, and longer inductor life.
III. Induction Heating Power Supply and Technology
In terms of power supplies, thyristor medium frequency has replaced motor-type generators. In the early 1990s, domestic thyristor power supply factories sprouted up like mushrooms after rain, spreading everywhere. After competition based on survival of the fittest, the number of production plants has now stabilized. Currently, thyristor power supplies are developing towards IGBT transistor power supplies, while electron tube high-frequency will evolve into MOSFET transistor power supplies. Handheld transistor super-audio and high-frequency power supplies face fierce market competition, and their future lies in who can maintain quality and technical level.
Domestic medium frequency power supplies currently all use parallel resonant type inverters. Therefore, while researching and developing larger capacity parallel resonant medium frequency power supplies, developing structurally simple, easily frequently startable series resonant medium frequency power supplies remains an issue to be resolved in the domestic medium frequency induction heating device field. Why do workpieces need to be quenched?
Quenching processes are widely applied in modern mechanical manufacturing industries. Important parts in machinery, especially steel parts used in automobiles, airplanes, and rockets, almost all undergo quenching treatment. To meet various parts' vastly different technical requirements, various quenching processes have been developed. For instance, according to the treated area, there are overall, partial, and surface quenching; according to whether phase transformation during heating is complete, there are complete quenching and incomplete quenching (for hypoeutectoid steel, this method is also called subcritical quenching); according to the content of phase transformation during cooling, there are分级quenching, isothermal quenching, and under-speed quenching, etc., especially in smelting and casting applications, series resonant power supplies easily achieve constant power output across all operating conditions (which helps reduce electric energy per ton consumption) and multi-load power distribution control, making them more worthy of promotion and application.
In the super-audio (10～100kHz) range, due to the limitations of thyristor switch characteristics and other parameters, developing power supplies within this frequency band presents significant technical challenges. Although in the 1980s Zhejiang University used thyristor frequency multiplication circuits to develop 50kW/50kHz super-audio power supplies and time division circuits to develop 30kHz thyristor super-audio power supplies, the double resonance circuit coupling caused the load to present non-linear characteristics, making the matching debugging of time-varying heating load parameters and resonance circuit parameters quite complex. Additionally, the control and main circuit structure of the time division circuit are complex, with low utilization of inverter tubes, so they did not gain widespread application.
From the late 1970s to the early 1980s, people combined modern semiconductor micro-integration processing technology with power semiconductor technology, successively developing a large number of fully controlled power electronic semiconductor devices (GTR, MOSFET, SIT, SITH, and MCT, etc.), laying a solid foundation for the development of fully solid-state super-audio and high-frequency power supplies.
In the high-frequency (above 100kHz) range, internationally, we are currently transitioning from traditional electron tube power supplies to transistorized fully solid-state power supplies. Some Japanese companies use SITs, achieving power supply levels of 1000kW, 200kHz, and 400kW, 400kHz by the late 1980s.
In Europe and America, due to the high conduction losses (SIT operates in the non-saturated region) and other defects of SITs, MOSFETs are primarily used as high-frequency power devices. With the modularization and large-capacitance development of MOSFET power devices, the capacity of MOSFET high-frequency induction heating power supplies has rapidly developed. Spain's current-type induction heating power supply manufacturing level using MOSFETs reaches 600kW, 400kHz, Germany's current-type MOSFET induction heating power supply level developed in 1989 reaches 480kW, 50～200kHz, and Belgium's InductoEiphiac company's current-type MOSFET induction heating power supply level can reach 1000kW, 15～600kHz. Zhejiang University developed a 20kW, 300kHz MOSFET high-frequency power supply in the 1990s, which has been successfully applied to surface heat treatment of small tools and thermal stress testing of aircraft turbine blades.
Currently, induction heating power supplies mainly use thyristors in the medium-frequency band, IGBTs in the super-audio band, and MOSFETs in the high-frequency band due to the high conduction losses and other defects of SITs. Although induction heating power supplies use resonant inverters, which are beneficial for power devices to achieve soft switching, induction heating power supplies usually have large power, imposing many special requirements on power devices, passive devices, cables, wiring, grounding, and shielding. Therefore, achieving high-frequency induction heating power supplies still involves many application-based fundamental technologies that need further exploration, especially with the advent of new high-frequency large-power devices (such as MCT, IGBT, and SIT power devices), which will further promote the development of high-frequency induction heating power supplies.
Considering the large-capacity development of induction heating power supplies from a circuit perspective, large-capacity technologies can be divided into two categories: one is the series and parallel connection of devices; the other is the series and parallel connection of multiple bridges or multiple power supplies. In the series and parallel connection methods of devices, it is necessary to carefully handle the voltage balancing issue of series-connected devices and the current balancing issue of parallel-connected devices. Due to the discreteness of device manufacturing processes and parameters, the number of devices connected in series or parallel is limited, and the more devices are connected in series or parallel, the poorer the reliability of the device becomes. The series and parallel connection technology of multiple power supplies is an effective means to further increase capacity based on the series and parallel connection technology of devices. With reliable power supply series and parallel connection technology, large-capacity devices can be simply obtained through series and parallel operation modes when the single-machine capacity is appropriate, with each single machine being just a unit (or module) of the device.
The inverter of the induction heating power supply mainly includes parallel inverters and series inverters. The output of a series inverter can be equivalently regarded as a low-impedance voltage source. When two voltage sources are connected in parallel, differences or fluctuations in amplitude, phase, and frequency between them can lead to significant circulating currents, resulting in serious current imbalance in the inverter devices. Therefore, it is difficult to expand the capacity of series inverters by paralleling. On the other hand, for parallel inverters, the DC large inductor at the input end of the inverter can act as a current buffer link between the parallel inverters, allowing the AG/DG or DG/DG links at the input end to have enough time to correct the deviation of the DC current, achieving multi-unit parallel expansion. The adoption of parallel inverter structures in transistorized super-audio and high-frequency currents, and the ease of modularization and large-capacitance development of parallel inverters, are among the main reasons.
The load objects of induction heating power supplies vary widely, and the power supply inverter and load form an organic whole. Generally, matching transformers connect the power supply and the load inductor. Matching transformers for high-frequency and super-audio power supplies are being optimized and improved from magnetic materials to winding structures. Simultaneously, from a circuit topology perspective, three passive elements can replace two passive elements to eliminate transformers, achieving efficient and low-cost matching.
Induction heating power supplies, including thyristor, transistor, and electron tube types, are all produced domestically. Thyristor power supplies have been produced and applied for many years. Currently, IGBT power supplies are more widely adopted by users due to their more advantages. MOSFET power supplies have high electrical efficiency and low voltage but are relatively expensive and are gradually replacing electron tube high-frequency power supplies. Handheld small high-frequency power supplies are inexpensive and convenient, widely used domestically, and even entering foreign markets.
Ultra-high-frequency power supplies (27.12MHz) were previously dependent on imports, but now at least two domestic enterprises have started production, solving the needs of special processes such as blades and saw blades.
With the increasing degree of automation control in induction heat treatment production lines and the requirement for high reliability of power supplies, it is necessary to strengthen the development of complete sets of heating process equipment. At the same time, induction heating systems are moving towards intelligent control, and induction heating power supply systems with computer intelligent interfaces, remote control, and automatic fault diagnosis functions are becoming the next-generation development goals.