The main feature of induction heating is the direct heating of workpieces. Its advantages include good working conditions for operators, fast heating speed, easy temperature control, no metal impurities mixed in during the heating process, and minimal metal loss, so it is widely used. The main disadvantage of induction heating is that the equipment is complex. At present, induction heating is mainly used for quenching, through heating, melting, brazing, and sintering in the machinery manufacturing and metallurgical industries.
(1) Quenching
Heating a workpiece to a certain temperature and then cooling it down rapidly to change the metallographic structure of the workpiece, thereby increasing the hardness and wear resistance of the workpiece, is called quenching. However, after quenching, the workpiece becomes brittle and its fatigue resistance becomes worse. To solve this contradiction, surface quenching appears, that is, only the surface of the metal is heated (the interior is not heated), and then cooled down rapidly. In this way, the surface of the metal is quenched, increasing the hardness and wear resistance of the workpiece, while the interior is not quenched and still maintains a certain strength. Induction heating is the most ideal heating method for surface quenching. Due to the skin effect and proximity effect, only a layer with a penetration depth of △ near the outer surface of the workpiece is directly heated, and the interior generates very little heat. If the heating time is controlled and the heating power is increased, so that the heat of the directly heated layer has no time to be transferred to the interior of the workpiece, this can meet the requirements of surface quenching.
The depth of the quenching layer is an important process requirement, which has a great impact on the quality of the product. The depth of the quenching layer mainly depends on the frequency of the heating power supply. The higher the frequency, the smaller the penetration depth, and the shallower the directly heated layer and the quenching depth. The frequency of the quenching heating power supply is divided into two categories: medium frequency and high frequency.
(2) Through Heating
Heating a workpiece so that the interior and surface temperatures are roughly equal is called through heating. Through heating is mainly used for heating before forging, hot stamping, extrusion, hot rolling, and pipe bending, as well as for annealing of strips, tubes, and wires. The main advantages of induction heating for through heating are improving working conditions for operators, high heating efficiency, fast speed, reduced metal loss, easy temperature control, ensuring heating quality, and being easy to form automatic production lines, so its application is becoming more and more widespread.
(3) Melting
The induction coil of a medium-frequency induction melting furnace is made of copper tubes, which are cooled by water inside. When a large medium-frequency alternating current passes through the coil, the furnace charge in the furnace hearth can be heated and melted, and the liquid metal can be heated to the required temperature.
The advantages of the induction melting furnace are good working conditions for operators, high efficiency, minimal loss of alloying elements in the melt, and easy composition control, so the product quality is high, especially suitable for melting various high-alloy steels. Its disadvantage is that the equipment is complex and expensive. In the induction melting furnace, the surface temperature of the melt is low, and there is electromagnetic stirring, which is not conducive to the melting process requiring high-temperature slag making.
(4) Brazing
Brazing is to heat the brazing material to its melting temperature to connect two or more parts together. Common tin soldering and copper soldering are all brazing. The induction heating power supply is a new brazing power supply. Its characteristics are good working conditions for operators, high productivity, safe operation, and no need for pickling and removing oxide scale. The frequency of brazing power supplies in China is generally 8kHz and 20kHz.
(5) Vacuum Sintering
Sintering is used in powder metallurgy and is often used to manufacture products such as cemented carbide cutting tools. Induction heating is a relatively ideal heating method for vacuum sintering, which can improve product quality. The frequency of the power supply for sintering can be 2.5kHz or 8kHz.
