This article describes the introduction, advantages and disadvantages of various types of electric furnaces.
Keywords: electric furnace; electric arc furnace
An 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 atmosphere in the furnace is easy to control and can even be evacuated; the material is heated quickly, the heating temperature is high, and the temperature is easy to control .
An 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 atmosphere in the furnace is easy to control and can even be evacuated; 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 hygiene conditions are good; the thermal efficiency is high; the product quality is good; and it is more environmentally friendly and is a good product to deal with the increasingly serious environmental problems. In the metallurgical industry, electric furnaces are mainly used for smelting, heating and heat treatment of steel, ferroalloys, non-ferrous metals, etc. Industrial-scale electric furnaces appeared at the end of the 19th century. 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 divided into resistance furnaces, induction furnaces, electric arc furnaces, plasma furnaces, electron beam furnaces, etc.
Resistance furnace
An electric furnace that uses Joule heat generated by electric current passing through a conductor as its heat source. According to the way electric heat is generated, resistance furnaces are divided into two types: direct heating and indirect heating. In the direct heating resistance furnace, the electric current passes directly through the material. Since the electric heating power is concentrated on the material itself, the material heats very quickly. It is suitable for processes that require rapid heating, such as the heating of forging billets. This kind of resistance furnace can heat materials to very high temperatures. For example, a carbon material graphitization electric furnace can heat materials to over 2500°C. The direct heating resistance furnace can be made into a vacuum resistance heating furnace or a protective gas resistance heating furnace. In powder metallurgy, it is often used for sintering tungsten, tantalum, niobium and other products. When using this kind of stove for heating, you should pay attention to:
① In order to heat the material evenly, the conductive cross-section and conductivity of each part of the material are required to be consistent;
② Since 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 transmission electrode and the material should be good to avoid arc burning of the material, and the resistance of the power transmission bus should be small to reduce circuit losses. ; Most resistance furnaces are indirect heating resistance furnaces, which are equipped with a resistor specifically used to achieve electric-heat conversion, called an electric heating element, which transfers heat energy to the materials in the furnace.
The furnace shell of this kind of electric furnace is made of steel plate, and the furnace is lined with refractory materials such as ceramic fiber and contains materials.
The most commonly used electric heating elements are iron-chromium-aluminum electric heating elements, nickel-chromium electric heating elements, silicon carbide rods and molybdenum disilicide rods, silicon carbon rods, and zirconium diboride ceramic composite heating elements. Depending on the needs, the atmosphere in the furnace can be a normal atmosphere, a protective atmosphere or a vacuum. The general power supply voltage is 220 volts or 380 volts, and an intermediate transformer with adjustable voltage is provided if necessary. Single-phase power supply for small furnaces (<10 kW) and three-phase power supply for large furnaces. For materials with a single variety and large batches, continuous furnace heating should be used. Most resistance furnaces with furnace temperatures below 700°C are equipped with blowers to enhance heat transfer in the furnace and ensure uniform heating. The resistance furnace used to melt fusible metals (lead, lead-bismuth alloy, aluminum, magnesium and their alloys, etc.) can be made into a crucible furnace; or it can be made into a reverberatory furnace with a molten pool, and an electric heating element is installed on the top of the furnace. An electroslag furnace is a resistance furnace that realizes electrothermal conversion by molten slag (see electroslag remelting).
Induction furnace
Introduction to induction furnace: An electric furnace that uses the induction electrothermal effect of materials to heat or melt them. The basic component of an induction furnace is an induction coil wound with copper tubes. AC voltage is applied to both ends of the induction coil to generate an alternating electromagnetic field. Conductive materials are placed in the induction coil. Due to electromagnetic induction, eddy currents are generated in the materials. Due to the action of resistance, the electrical energy is converted into thermal energy to heat the materials; therefore, it can also be considered as Induction electric heating is a direct heating resistance electric heating.
The characteristic of induction electric heating is that the electric heating power (current distribution) transformed in the heated material is very uneven, with the largest surface and smallest center, which is called the skin effect. In order to improve the electric heating efficiency of induction heating, the power supply frequency must be appropriate. Small smelting furnaces or high-frequency electricity is used for surface heating of materials, and large smelting furnaces or medium-frequency or industrial frequency electricity is used for deep penetration heating of materials. The induction coil is a load with considerable inductance, and its power factor is generally very low. In order to improve the power factor, the induction coil is generally connected in parallel with an intermediate frequency or high frequency capacitor, which is 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 tubes. The tube should be cooled by water. The gap between the turns of the induction coil should be as small as possible and the insulation should be good. Induction heating device is mainly used for heating and casting of steel, copper, aluminum and zinc. It has fast heating, low burning loss, high degree of mechanization and automation, and is suitable for configuration on automatic operation lines.
Induction furnace series heating furnace features
(1) Fast heating speed, high production efficiency, less oxidation and decarburization, saving material and forging die costs.
Since the principle of medium frequency induction heating is electromagnetic induction, the heat is generated in the workpiece itself. Ordinary workers using the medium frequency electric furnace can continue the forging task ten minutes after going to work. There is no need for professional furnace workers to burn and seal the furnace in advance. . There is no need to worry about the waste of billets heated by the coal furnace due to power outages or equipment failures. Due to the fast heating rate of this heating method, there is very little oxidation. Compared with coal-fired furnaces, each ton of forgings can save at least 20-50 kilograms of steel raw materials, and its material utilization rate can reach 95%. Because this heating method heats evenly and the temperature difference between the core and the surface is extremely small, it also greatly increases the life of the forging die in terms of forging, and the surface roughness of the forging is less than 50um.
(2) The working environment is superior, improves the working environment of workers and the company’s image, is pollution-free, and has low energy consumption.
Compared with coal furnaces, induction heating furnaces no longer subject workers to the baking and smoke of coal furnaces under the scorching sun. They can also meet various index requirements of the environmental protection department, and at the same time establish the company’s external image and the future development trend of the forging industry. . Induction heating is the most energy-saving heating method among electric heating furnaces. Heating a ton of forgings from room temperature to 1100°C consumes less than 360 degrees of electricity.
(3) Heating is uniform, the temperature difference between the core and the surface is extremely small, and the temperature control accuracy is high.
Induction heating furnaces have the advantages of small size, light weight, high efficiency, excellent thermal processing quality and favorable environment. They are rapidly replacing coal-fired furnaces, gas furnaces, oil-fired furnaces and ordinary resistance furnaces. They are a new generation of metal heating equipment.
Induction melting furnaces used in industry include crucible furnaces (coreless induction furnaces) and trench furnaces (core induction furnaces). Crucibles are made of refractory materials or steel and have capacities ranging from a few kilograms to tens of tons. Its melting characteristic is that the melt in the crucible is affected by electrodynamic force, forcing the liquid level of the molten pool to bulge, and the melt flows from the center of the liquid level to the surroundings, causing circular flow. This phenomenon is called the electrodynamic effect, which can make the melt composition uniform. The disadvantage is that the slag is biased to the periphery and has poor coverage. Compared with the trench furnace, the crucible furnace has flexible operation and high melting temperature, but has a low power factor and high power consumption. The inductor of the molten trench furnace is composed of an iron core, an induction coil and a molten trench furnace lining. The molten trench is one or two strip-shaped annular trenches, which are filled with melt connected to the molten pool. In principle, the trench furnace can be regarded as an iron core transformer with only one turn of the secondary coil and a short circuit. Induced current flows in the molten trench melt to achieve electrothermal transformation.
During production, after each furnace of metal is smelted, the molten pool cannot be emptied, otherwise it will easily dry up. A part of the melt must be retained as the starting melt for the next furnace. The temperature of the molten trench is higher than that of the molten pool, and it is also subject to the erosion of the melt flow, so the furnace lining of the molten trench is easily damaged. To facilitate maintenance, the sensors of modern furnaces are made into easy-to-replace assemblies. The capacity of the trench furnace ranges from a few hundred kilograms to more than a hundred tons. The molten trench furnace supplies power at industrial frequency. Since the iron core made of silicon steel sheets is used as the magnetic path, the electrical efficiency and power factor are very high. The trench furnace is mainly used for melting cast iron, copper, zinc, brass, etc. It can also be used as a mixed melter to store and heat the melt.
Electric arc furnace
An electric furnace that utilizes arc heating effects to smelt metals and other materials. It is divided into three types according to the heating method: ①Indirect heating electric arc furnace. The arc is generated between the two electrodes without contacting the material, and relies on heat radiation to heat the material. This kind of stove is noisy and has low efficiency, so it is gradually being eliminated. ② Direct heating of electric arc furnace. An electric arc is generated between the electrode and the material to directly heat the material; the three-phase electric arc furnace for steelmaking is the most commonly used direct heating arc furnace (see electric arc furnace steelmaking). ③Submerged arc electric furnace, also known as reduction electric furnace or submerged arc electric furnace. One end of the electrode is buried in the material layer, forming an arc in the material layer and heating the material by using the resistance of the material layer itself; often used for smelting ferroalloys (see ferroalloy electric furnace)
Vacuum electric arc furnace
It is an electric furnace that uses an electric arc to directly heat smelting metal in a evacuated furnace body. The gas in the furnace is thin, and the arc mainly relies on the vapor of the molten metal. In order to stabilize the arc, DC power is generally supplied. According to the smelting characteristics, it is divided into metal remelting furnace and casting furnace. According to whether the electrode is consumed (melted) during the smelting process, it is divided into consumable furnaces and non-consumable furnaces. Most of them used in industry are consumable furnaces. Vacuum electric arc furnaces are used for melting special steels, reactive and refractory metals such as titanium, molybdenum, and niobium (see vacuum metallurgy).
Arc electric heating can be considered as arc resistance electric heating. Arc (arc resistance) stability is a necessary condition for normal furnace production. AC electric arc furnaces usually use power frequency. In order to stabilize the arc, there must be appropriate inductive reactance in the furnace power supply circuit. However, the presence of inductive reactance will reduce the power factor and electrical efficiency. Reducing the current frequency is the way to develop AC electric arc furnace. The arc resistance is quite small. In order to obtain the necessary heat, the furnace requires a considerable operating current. Therefore, the resistance of the furnace short network 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 to avoid unbalanced load of the three phases.
Plasma furnace
An electric furnace that uses plasma generated when the working gas is ionized for heating or smelting. The device that generates plasma is usually called a plasma gun, and there are two types: arc plasma gun and high-frequency induction plasma gun. Pour the working gas into the plasma gun. The gun has a device that generates an arc or a high-frequency (5-20 MHz) electric field. The working gas is ionized after being acted upon, generating a plasma composed of electrons, positive ions, and a mixture of gas atoms and molecules. . After the plasma is ejected from the nozzle of the plasma gun, a high-speed and high-temperature plasma arc flame is formed, and the temperature is much higher than that of a general arc. The most commonly used working gas is argon, which is a monatomic gas that is easily ionized and an inert gas that can protect materials. The working temperature can be as high as 20,000℃; used for melting special steel, titanium and titanium alloys, superconducting materials, etc. Furnace types include water-cooled copper crystallizer furnaces, hollow cathode furnaces, plasma furnaces with induction heating, plasma furnaces with refractory linings, etc. (see plasma metallurgy).
Electron beam furnace
An electric furnace that bombards materials with high-speed electrons to heat and melt them. In the vacuum furnace shell, a low-voltage electric filament is used to heat the cathode, causing it to emit electrons. The electron beam is accelerated by the high-voltage electric field accelerating the anode, and bombards the metal material located at the anode, converting the electrical energy into heat energy. Because the electron beam can be highly concentrated through the electromagnetic focusing device, it can generate very high temperatures in the parts where the material is bombarded. Electron beam furnaces are used for melting special steels, refractory and reactive metals.
Electric furnaces used in industry are classified into two categories: periodic operating furnaces and continuous operating furnaces.
Periodic operation furnaces are divided into: box furnaces, sealed box furnaces, pit furnaces, bell furnaces, trolley furnaces, and dumping drum furnaces.
Continuous operating furnaces are divided into: kiln car furnaces, push rod furnaces, roller hearth furnaces, vibrating hearth furnaces, rotating hearth furnaces, walking furnaces, traction furnaces, continuous roller furnaces, conveyor belt furnaces, etc. Among them, conveyor belt furnaces can be divided into: mesh belt furnaces, stamped chain plate furnaces, cast chain plate furnaces, etc…
Electric furnace
The electric heating furnace can use metal heating elements or non-metal heating elements to generate heat sources. Its main features are simple structure and wide range of uses. It can be widely used in annealing, normalization, quenching, tempering, carburizing and carburizing and nitriding. wait. The main metal heating elements include Ni-Cr heating wires (the most common, which can be heated up to 1200°C), Mo-Si alloys, and pure metals such as W and Mo; non-metallic heating elements include SiC (the most common, which can be heated up to 1600°C). ), LaCrO3 and graphite rods (can be heated to 2000°C under vacuum or protective atmosphere).
advantage:
1) It is easier to get high temperature than fuel furnace.
2) The material can be heated from within.
3) Easy to use in controlled atmosphere furnaces and vacuum furnaces.
4) Electric furnaces do not have the exhaust heat loss of fuel furnaces, so they have high thermal efficiency.
5) Easy to control the temperature, remote control and fine adjustment.
6) Capable of rapid heating.
7) Good operating performance and does not pollute the environment.
Disadvantages:
1) The cost of power distribution equipment needs to be increased.
2) The cost of electricity is high.
3) When the resistance heating temperature exceeds 1000°C, the refractory material may conduct electricity, so attention must be paid to insulation issues.