The electric furnace is a heating furnace that converts the electric energy in the furnace into heat to heat the workpiece. Compared with the fuel furnace, the advantages of the electric furnace are: the furnace atmosphere is easy to control, and even the vacuum can be pumped; the material is heated quickly, the heating temperature is high, and the temperature is easy to control. .
advantage
The electric furnace is a heating furnace that converts the electric energy in the furnace into heat to heat the workpiece. Compared with the fuel furnace, the advantages of the electric furnace are: the furnace atmosphere is easy to control, and even the vacuum can be pumped; the material is heated quickly, the heating temperature is high, and the temperature is easy to control. The production process is easier to mechanize and automate; the labor and sanitation conditions are good; the thermal efficiency is high; the product quality is good; and the environmental protection is a good product for the increasingly serious environmental problems. The electric furnace in the metallurgical industry is mainly used for the melting, heating and heat treatment of steel, iron alloys, non-ferrous metals and the like. At the end of the 19th century, industrial-scale electric furnaces appeared. Since the 1950s, due to the increase in demand for advanced metallurgical products and the decline in electricity costs with the development of the power industry, the proportion of electric furnaces in metallurgical furnace equipment has increased year by year. Electric furnaces can be classified into electric resistance furnaces, induction furnaces, electric arc furnaces, plasma furnaces, electron beam furnaces, and the like.
Type introduction
Resistance furnace
An electric furnace in which Joule heat generated by current passing through a conductor is a heat source. According to the electric heating method, the electric resistance furnace is divided into direct heating and indirect heating. In the direct heating resistance furnace, the current directly passes through the material, and since the electric heating power is concentrated on the material itself, the material is heated quickly, and is suitable for a process requiring rapid heating, such as heating of a forged billet. This electric resistance furnace can heat the material to a very high temperature, such as a carbon material graphitization electric furnace, which can heat the material to over 2500 °C. The direct heating resistance furnace can be used as a vacuum resistance heating furnace or a protective gas resistance heating furnace. In powder metallurgy, it is commonly used for sintering tungsten, tantalum, niobium and other products. When using this furnace to heat, you should pay attention to:
1. In order to make the material heat evenly, it is required that the conductive cross section and electrical conductivity of each part of the material are consistent;
2. Because the resistance of the material itself is quite small, in order to achieve the required electric heating power, the working current is quite large, so the contact between the power transmitting electrode and the material is better, so as to avoid the arc burning material, and the resistance of the power transmitting busbar is small to reduce the circuit loss. Most of the resistance furnaces are indirect heating resistance furnaces, which are equipped with resistors specifically designed to achieve electro-thermal transitions, called electric heaters, which transfer heat energy to the materials in the furnace
The electric furnace shell is made of steel plate, and the furnace is lined with refractory material such as ceramic fiber, and the material is placed inside.
The most commonly used electric heaters are iron chromium aluminum electric heating body, nickel chromium electric heating body, silicon carbide rod and molybdenum disilicide rod, silicon carbon rod and zirconium diboride ceramic composite heating element. The atmosphere in the furnace may be a normal atmosphere, a protective atmosphere or a vacuum, as needed. The general power supply voltage is 220 volts or 380 volts, and an intermediate transformer with adjustable voltage can be configured if necessary. Small furnace (<10 kW) single-phase power supply, large furnace three-phase power supply. For materials with a single variety and large batch volume, continuous furnace heating should be used. Resistance furnaces with furnace temperatures below 700 □, most of which are equipped with blowers to enhance heat transfer in the furnace to ensure uniform heating. The electric resistance furnace for melting fusible metals (lead, lead-bismuth alloy, aluminum and magnesium, alloys, etc.) can be made into a crucible furnace; or a reverberatory furnace having a molten pool, and an electric heating body is installed on the top of the furnace. The electroslag furnace is an electric resistance furnace that realizes electrothermal transformation by slag (see electroslag remelting).
Induction furnace
Induction Furnace Introduction An electric furnace that uses materials to induce electric heating effects to heat or melt materials. The basic component of the induction furnace is an induction coil wound with a copper tube. An alternating voltage is applied to both ends of the induction coil to generate an alternating electromagnetic field. The conductive material is placed in the induction coil. The electromagnetic induction generates eddy currents in the material, and the electric energy is converted into heat energy by the electric resistance to heat the material; therefore, it can also be considered Induction heating is a direct heating type of electric resistance.
The characteristic of induction heating is that the electric heating power (current distribution) transformed in the material to be heated is very uneven, the surface is the largest, and the center is the smallest, which is called the skin effect. In order to improve the electrothermal efficiency of induction heating, the power supply frequency should be suitable. The small-scale melting furnace or the surface heating of the material adopts high-frequency electricity, and the large-scale melting furnace or medium-frequency or power-frequency electricity is used for deep material heating. The induction coil is a load with a relatively large inductance, and its power factor is generally low. In order to improve the power factor, the induction coil is generally connected in parallel with an intermediate frequency or high frequency capacitor, called a resonant capacitor. The gap between the induction coil and the material should be small. The induction coil should be made of square copper tube. The water inside the tube should be cooled. The inter-turn gap of the induction coil should be as small as possible and the insulation should be good. Induction heating device, mainly used for heating and casting of steel, copper, aluminum and zinc, with fast heating, less burning loss, high mechanization and automation, suitable for deployment on automatic lines.
Induction furnace series heating furnace features:
(1) Fast heating speed, high production efficiency, less oxidative decarburization, saving material and forging die cost.
Since the principle of medium frequency induction heating is electromagnetic induction, the heat is generated in the workpiece itself. The average worker can use the medium frequency electric furnace to carry out the continuous work of the forging task ten minutes after going to work. The professional workers do not need to burn the furnace and seal the furnace in advance. . There is no need to worry about the waste of the heated billet of the coal stove caused by power outage or equipment failure. Since the heating method has a fast heating rate, the oxidation is extremely small, and at least 20-50 kg of steel raw materials are saved per ton of forgings compared with the coal burning furnace, and the material utilization rate is up to 95%. Since the heating method is uniform in heating and the temperature difference of the core surface is extremely small, the life of the forging die is greatly increased in forging, and the surface roughness of the forging is also less than 50 um.
(2) Excellent working environment, improve workers' working environment and company image, no pollution, low energy consumption.
Compared with the coal stove, the induction heating furnace will no longer be bake and smoked by the coal stove under the scorching sun, and can meet the requirements of various environmental protection departments, and at the same time establish the external image of the company and the future development trend of the forging industry. . Induction heating is the most energy-efficient heating method in electric heating furnace. The power consumption of ton forgings heated from room temperature to 1100 °C is less than 360 degrees.
(3) The heating is uniform, the temperature difference of the core meter is very small, and the temperature control precision is high.
Induction heating furnace has the advantages of small size, light weight, high efficiency, excellent thermal processing quality and favorable environment. It is rapidly eliminating coal-fired furnaces, gas furnaces, oil-fired furnaces and ordinary resistance furnaces. It is a new generation of metal heating equipment.
The induction melting furnaces used in the industry include a crucible furnace (coreless induction furnace) and a grit furnace (core induction furnace). Made of refractory or steel, the capacity ranges from a few kilograms to tens of tons. Its smelting characteristics are that the melt in the crucible is subjected to electric power, forcing the liquid level of the molten pool to bulge, and the melt flows from the center of the liquid surface to the periphery to cause circulating flow. This phenomenon is called electrokinetic effect and can make the melt composition uniform. The disadvantage is that the slag is biased to the periphery and the coverage is poor. Compared with the melting furnace, the furnace is flexible in operation and has a high melting temperature, but the power factor is low and the power consumption is high. The inductor of the furnace is composed of an iron core, an induction coil and a lining of a molten groove, and the molten groove is one or two strip-shaped annular grooves filled with a melt which is in communication with the molten pool. In principle, the grit furnace can be regarded as a core transformer with only one winding and short circuit in the secondary. The induced current flows in the melt of the melt to achieve an electrothermal transition.
In the production, after the melting of each furnace metal, the molten pool can not be emptied, otherwise it is easy to dry, and it is necessary to keep a part of the melt as the melt of the next furnace. The temperature of the molten ditch is higher than that of the molten pool and is subject to the erosion of the melt flow, so the lining of the molten ditch is easily damaged. For the convenience of maintenance, the inductor of the modern furnace is made into an assembly that is easy to replace. The capacity of the furnace is from a few hundred kilograms to more than one hundred tons. The grit furnace is used for power frequency. Since the iron core made of silicon steel sheet is used as a magnetic path, the electrical efficiency and power factor are high. The grit furnace is mainly used for the melting of cast iron, copper, zinc, brass, etc., and can also be used as a fusion melter for storing and heating the melt.
Electric arc furnace
An electric furnace that smelts metals and other materials using the arc heat effect (Fig. 3 electric arc furnace type). According to the heating method, there are three types: 1 indirect heating electric arc furnace. The arc is generated between the two electrodes, does not contact the material, and heats the material by thermal radiation. This kind of furnace is noisy, low efficiency, and is gradually being eliminated. 2 Directly heat the electric arc furnace. The arc is generated between the electrode and the material to directly heat the material; the steel-making three-phase electric arc furnace is the most commonly used direct heating electric arc furnace (see electric arc furnace steelmaking). 3 submerged arc electric furnace, also known as reduction electric furnace or mine electric furnace. One end of the electrode is buried in the material layer, an arc is formed in the material layer and the material is heated by the resistance of the material layer itself; it is commonly used for smelting iron alloy (see iron alloy electric furnace)
Vacuum electric arc furnace
It is an electric furnace in which a molten metal is directly heated by an electric arc in a vacuumed furnace body. The gas in the furnace is thin, and the arc is mainly caused by the vapor of the molten metal. In order to stabilize the arc, DC is generally supplied. According to the characteristics of smelting, it is divided into metal remelting furnace and casting furnace. According to whether the electrodes are consumed (melted) during the smelting process, they are divided into self-consumption furnaces and non-consumer furnaces. Most of the industrial applications are self-consumption furnaces. Vacuum arc furnaces are used to smelt special steel, reactive and refractory metals such as titanium, molybdenum and niobium (see Vacuum Metallurgy).
Arc electric heating can be considered as arc resistance electric heating. Stabilization of the arc (arc resistance) is a necessary condition for the normal production of the furnace. AC arc furnaces usually use power frequency. In order to stabilize the arc, the furnace power supply circuit should have appropriate inductive reactance, but the inductive reactance will reduce the power factor and electrical efficiency. Reducing the current frequency is the way to develop AC arc furnaces. The arc resistance is quite small. In order to obtain the necessary heat, the furnace needs a considerable working current, so the resistance of the short grid of the furnace should be as small as possible to avoid excessive circuit loss. For a three-phase electric arc furnace, the impedance of the three phases should be close to the same, so as to avoid the imbalance of the three-phase load.
Plasma furnace
An electric furnace that performs heating or melting by using plasma generated when the working gas is ionized. Plasma generating devices, commonly referred to as plasma guns, have arc plasma guns and high frequency induction plasma guns. The working gas is introduced into the plasma gun, and the gun has a device for generating an electric arc or a high-frequency (5-20 ohm) electric field. The working gas is ionized by the action, and a plasma composed of a mixture of electrons, positive ions, and gas atoms and molecules is generated. . After the plasma is ejected from the plasma gun nozzle, a high-speed, high-temperature plasma arc flame is formed, which is much higher than the general arc. The most common working gas is argon, which is a monoatomic gas that is easily ionized and is an inert gas that protects the material. Working temperature can be up to 20,000 □; used for melting special steel, titanium and titanium alloys, superconducting materials, etc. The furnace type includes a water-cooled copper crystallizer furnace, a hollow cathode furnace, a plasma furnace equipped with induction heating, a plasma furnace with a refractory furnace lining, etc. (see plasma metallurgy).
Electron beam furnace
The electric furnace is bombarded with high-speed electrons to heat the molten electric furnace (illustrated by the electron beam furnace of Fig. 4). In the vacuum furnace shell, the cathode is heated by a low-voltage electric filament to emit electrons, and the electron beam is accelerated by the action of the high-voltage electric field of the accelerating anode, bombarding the metal material located at the anode, and converting the electric energy into heat energy. Since the electron beam can be highly dense by the electromagnetic focusing device, it can generate a high temperature at the site where the material is bombarded. Electron beam furnaces are used to melt special steels, refractory and reactive metals.
The electric furnaces used in the industry are classified into two types: a periodic operation furnace and a continuous operation furnace.
The cycle type working furnace is divided into: box type furnace, sealed box type furnace, pit type furnace, bell cover furnace, trolley furnace, and dumping drum furnace.
Continuous operation furnaces are divided into: kiln type furnace, push rod type furnace, roller bottom furnace, vibrating bottom furnace, rotary hearth furnace, stepping furnace, traction furnace, continuous drum furnace, conveyor belt furnace and so on. The conveyor belt furnace can be divided into: mesh belt furnace, stamping chain plate furnace, cast chain plate furnace, etc...
Electric furnace
The electric heating furnace can use a metal heating element or a non-metal heating element to generate a heat source, and has a simple structure and a wide application. It is widely used in annealing, normalization, quenching, tempering, carburizing and carburizing and nitriding. Wait. The main metal heating elements include Ni-Cr heating wires (most common, up to 1200 ° C), Mo-Si alloys and pure metals such as W and Mo; non-metallic heating elements including SiC (most common, up to 1600 ° C) ), LaCrO3 and graphite rods (heated to 2000 ° C under vacuum or protective atmosphere).
advantage:
1) It is easy to obtain high temperature compared to a fuel furnace.
2) It can be heated from the inside of the material to heat it up.
3) Easy to use in controlled atmosphere furnaces and vacuum furnaces.
4) The electric furnace does not have the heat loss of the exhaust gas of the fuel furnace, so the heat efficiency is high.
5) It is easy to control the temperature, which is convenient for remote control and fine adjustment.
6) Can be heated quickly.
7) Good operation performance and no pollution to the environment.
Disadvantages:
1) Need to increase the cost of power distribution equipment.
2) High power costs.
3) When the resistance heating temperature exceeds 1000 °C, the refractory material may be electrically conductive, and it is necessary to pay attention to the insulation problem.
