VACUUM INDUCTION MELTING

The development of vacuum induction melting technology and the application of induction smelting technology in different occasions. According to the structure of different vacuum induction furnaces, their advantages and disadvantages are compared. Looking forward to the future development of vacuum induction furnace, the development trend of vacuum induction furnace is expounded. The development and progress of vacuum induction furnace are mainly reflected in the gradual improvement of the overall structure of the equipment, the increasingly obvious trend of modularization and the more intelligent control system.

1. VACUUM INDUCTION MELTING TECHNOLOGY

1.1. Principle

Induction heating technology usually refers to the vacuum conditions, through the principle of electromagnetic induction to better inductive magnetic materials to obtain induction current, to achieve the purpose of heating a technology. Current passes through the electromagnetic coil around the metal at a certain frequency, and the changing current generates the inductive magnetic field, and makes the metal produce the inductive current, and generates a large amount of heat, which is used to heat the material. When the heat is relatively low, it can be used in vacuum induction heat treatment and other processes. When the heat is high, the heat generated is enough to melt the metal, used to prepare metal or alloy materials.

Fig. 1 electromagnetic induction principle

1.2, application

1.2.1, VACUUM INDUCTION MELTING

Vacuum induction melting technology is currently the highest heating efficiency of metal materials, the fastest, low energy-saving and environmentally friendly induction heating technology. The technology is mainly realized in induction melting furnace and other equipment, and its application scope is very extensive. When the electric current flows through the induction coil, the induction electromotive force is generated and the eddy current is generated inside the metal charge. When the electric current is more calorific than the heat dissipation rate of the metal charge, the metal melts from the solid state to the liquid state as the heat accumulates to a certain extent. To the purpose of smelting metal. In this process, because the whole process takes place in a vacuum environment, it is conducive to the removal of gas impurities in the metal, and the obtained metal alloy material is more pure. At the same time, through the control of vacuum environment and induction heating, the melting temperature can be adjusted and the alloy metal can be supplemented in time to achieve the purpose of refining. In the melting process, because of the characteristics of induction melting technology, liquid metal materials in the crucible due to the interaction of electromagnetic force, can automatically realize stirring, make the composition more uniform, which is also a major advantage of induction melting technology.

Fig. 2 agitation of induction melting

Compared with traditional smelting, vacuum induction smelting has many advantages, such as energy saving, environmental protection, good working environment and low labor intensity. Using induction melting technology, the final casting of the alloy material with less impurities, adding a more appropriate proportion of alloys, can be more in line with the technical requirements of the material properties.

Vacuum induction melting technology has been widely used at present. From a few kilograms induction furnace for experimental research to a large induction furnace with a capacity of tens of tons for actual production, it has great application because of its simple operation process, easy control of melting temperature and rapid melting process, and uniform composition of metals smelt. The prospect has been developing rapidly in recent years.

1.2.2 AND VACUUM INDUCTION SINTERING

Vacuum sintering refers to the sintering of metal, alloy or metal compound powders into metal products and billets at a temperature below the melting point in a vacuum (10-10-3 Pa). In vacuum sintering, there is no reaction between metal and gas, and there is no effect of adsorbing gas. It not only has good densification effect, but also can purify and reduce the sintering temperature. The sintering temperature and the sintering ratio at room temperature can be reduced by 100 ~150 C. It can save energy, improve the life of sintering furnace and obtain high quality products.

Some materials need to be heated by atom migration to achieve the binding between particles, and induction sintering technology in this process has played a heating role. The advantages of vacuum induction sintering are that it can help to reduce harmful substances (such as water vapor, oxygen, nitrogen and other impurities) in the atmosphere and avoid a series of reactions such as decarbonization, nitriding, carburizing, reduction and oxidation. In the process, the amount of gas in the pores is reduced, the chemical reaction of gas molecules is reduced, and the oxide film on the surface of the material is removed before the liquid phase appears, so that the material bonds more tightly and the wear resistance and strength are improved when the material melts and combines with each other. In addition, vacuum induction sintering has certain effect on reducing product cost.

Because of the relatively low gas content in vacuum environment, the convection and conduction of heat can be neglected. Heat is mainly transmitted from heating module to material surface by radiation. It is also very important to select suitable heating module according to the specific sintering temperature and physical and chemical characteristics of material. Compared with vacuum resistance heating, induction sintering uses medium frequency power supply heating, to a certain extent, avoids the high temperature insulation problem of vacuum furnace with resistance internal heating mode.

At present, induction sintering technology is mainly used in steel and metallurgy. In addition, induction sintering enhances the bonding of solid particles on special ceramics, which is conducive to grain growth, compression of voids, thereby increasing density and forming a dense polycrystalline sinter. Induction sintering technology is also being applied more widely in the research of new materials.

1.2.3, VACUUM INDUCTION HEAT TREATMENT

 At present, more induction heat treatment technologies should be focused on induction hardening technology. The workpiece is placed in the inductor (coil), when the inductor is connected with a certain frequency of alternating current, alternating magnetic field is generated around. The electromagnetic induction of alternating magnetic field causes a closed eddy in the workpiece. Because of the skin effect, i. e. the distribution of induced current on the workpiece section is very uneven, the current density on the surface of the workpiece is very high, and gradually decreases inward.
 The electric energy of the high density current on the surface of the workpiece is transformed into heat energy, which makes the temperature of the surface rise, that is, the surface heating is realized. The higher the current frequency, the greater the current density difference between the workpiece surface and the interior, the thinner the heating layer is. When the heating layer temperature exceeds the critical point temperature of steel, it can be cooled rapidly, so that the surface hardening can be realized. According to the principle of induction heating, by adjusting the frequency of induction coil passing through the current, the penetration depth of the current can be properly changed, and the depth can be adjusted, which is also a great advantage of induction heat treatment. But induction hardening technology is not suitable for complex mechanical parts because of its poor adaptability. Although the surface of the workpiece after quenching has larger compressive internal stress, the ability to resist fatigue and fracture is higher. But it is only suitable for pipelined production of simple jobs.
 Development and trend of vacuum induction melting technology

  

Fig. 3 skin effect
 At present, induction hardening technology is mainly used for surface hardening of crankshaft camshaft and other parts in automotive industry. Although these parts are simple in structure, but the working environment is bad, they have certain wear resistance, bending resistance and fatigue resistance requirements for the performance of parts. Through induction hardening, their wear resistance and fatigue resistance are improved. Labor ability is also the most reasonable way to meet the performance requirements. It is widely used in parts processing of automobile parts.