Arc steel furnace: device, principle of operation, power, control system

An arc steel furnace (ASF) is a device that heats material by electrical bending.

Industrial appliances range in size from small units of approximately one ton capacity (used in foundries for making cast iron products) to 400 units per ton used for secondary steel processing. Arc steel furnaces and EAFs used in research laboratories may have a capacity of only a few tens of grams. Temperatures of industrial devices can reach 1800 °C (3272 °F), while laboratory installations exceed 3000 °C (5432 °F).

Arc steel furnaces (AFS) differ from induction furnaces in that the charged material is directly exposed to electrical bending, and the current at the terminals passes through the charged material.

Laboratory electric arc furnaces

In laboratories and workshops, small arc furnaces are used, powered by a welding transformer.
These installations are factory-made and home-made. You can assemble such an electric arc furnace with a capacity of 5-10 kg in a clay pot. Two electrodes are inserted through holes in the walls. The outside of the pot is covered with a layer of thermal insulation. The arc is ignited when the graphite rods are brought together and then spread apart. It is carried out manually or electrically.

The dimensions of the furnace, the volume of the melting chamber and the productivity of the furnace are determined by the distance between the electrodes, which depends on the choice of voltage of the output transformer:

• 25-30V – 100mm;
• 50-60V – 150mm.

For example, in a melting chamber measuring 100*65*50mm at a voltage of 30V, 70-80 grams of metal can be melted. For laboratory conditions this is considered a medium sized unit.

Transformer for laboratory arc furnace

The basic technical requirements and operating principle of such installations are the same as for industrial chipboards. Welding transformers meet these conditions. It is allowed to use conventional devices with a power of more than 1 kVA as electric furnaces. In the absence of such devices, two transformers are used, connected in parallel.

Connecting wires to reduce heating are used with a cross-section of 10 mm² in reliable, possibly double insulation.

If it is necessary to melt a small amount of metal, melting is carried out in a graphite crucible or in a recess made in graphite powder. One of the outputs of the welding transformer is connected to the crucible, and the second to the electrode. It uses rods from batteries or brushes from electric motors. It is clamped into a holder for manual electric welding and the entire process is carried out manually while being controlled through a welding shield.

Important! Only graphite brushes are used with copper feeders removed. Copper in copper-graphite brushes changes the composition of the melting metal

Features of melting in laboratory furnaces

When melting precious metals, they are placed in a glass flask, which protects the melt from burning out. After cooling, the glass covers the metal with an easily removable crust.

When melting dissimilar metals, the more refractory metal is melted first, and low-melting additives are introduced into the melt. For example, when making bronze, copper is melted first, and then tin is added to the melt.

One of the most used types of electric furnaces in heavy industry are resistance furnaces. Furnace transformers in their composition are used when electric furnace heaters are made of a material whose resistance has a strong dependence on temperature. Also, the use of a transformer makes it possible to use heaters with a larger cross-section, which helps to increase their service life.

The output voltage of most transformers feeding resistance electric furnaces is regulated stepwise, without excitation, by changing the number of turns of the primary winding. The power of furnace transformers as part of such electric furnaces, as a rule, ranges from 10 to 250 kVA.

Types and characteristics of electric arc furnaces

Modern arc furnaces come in a variety of sizes and have a distinctive set of features.

Indirect arc furnaces

Arc combustion in such furnaces occurs between electrodes that are located above the molten mass. Due to this, heat exchange occurs between the material and the source of energy transfer. The radiation emanating from the arc, as well as convection, allows the metal to be heated to the temperature necessary for its melting.

Indirect arc furnaces are equipped with the following electrical equipment:

• electric drive of the feed mechanism for consumable electrodes;
• transformer;
• adjusting device.

Such ovens come in capacities of 0.5 and 0.25 tons. The maximum power of the power transformer can be 600 KV/A.

Current flow from the transformer substation to the electrodes is carried out through flexible cables. Adjustment of the distance between electrical conductors is carried out through automated control.

Indirect electric arc furnaces have a low coefficient of waste emission and metal evaporation. A reduction in the yield of vaporous substances is achieved due to the high location of the eclectic arc from the material to be melted.

Indirect arc furnaces are used for remelting various non-ferrous metals and their alloys. Often such thermal equipment is used in the smelting of certain types of nickel and cast iron.

Indirect arc furnaces are relatively small and it is impossible to carry out all metal remelting processes in them, since some alloys require more power and higher temperatures.

Direct arc furnaces

In such furnace devices, an arc is formed between an electrical conductor and molten metal, which thereby heats up. Due to direct contact between the electrode and the material, high evaporation of the metal occurs.

Direct electric arc furnaces are quite powerful equipment that can operate on three-phase current. They are distinguished by their high productivity and are mainly used for smelting various refractory metals into ingots, including structural and high-alloy steels.

Direct electric arc furnace

The electric furnace is equipped with hydraulic or electromechanical drive mechanisms that allow tilting to drain molten steel, turning and lifting the roof, and moving the electrodes. Current is supplied to the conductor holders by air-cooled copper pipes or busbars.

The process of igniting the electrodes is carried out by lowering them to the molten metal. After this, an electric arc is formed during the lifting of the conductors.

Resistance arc furnaces

A special feature of resistance furnaces is that the arc is formed inside the material being melted. The charge can be directed relative to the electrical discharge in parallel or in series.

Resistance arc furnaces do not have a tilt function. The molten mass passes through a special hole - a tap hole. The electrodes are located vertically in the structure. They are relatively large in size. Thanks to this, the unit can operate with high power and at a significant current.

In furnaces of this type, metals are melted with a high resistivity. Such equipment is used for smelting and reducing ore. Using resistance arc furnaces, it is possible to produce alloys of cast iron, carbide, abrasives, calcium, and nickel matte. Resistance thermal installations, unlike other types of arc furnaces, are capable of raising the temperature to extreme levels.

Vacuum arc furnaces

Such units belong to direct-acting equipment. The arc in vacuum furnaces burns in vapor or inert gas of the metal being melted. The process occurs at low pressure. There are two types of vacuum furnaces:

1. With consumable electrode. The arc in such devices burns between the electrical conductor being melted and a bath of liquid metal.
2. With non-consumable electrode. An electrical discharge occurs between the graphite electrical conductor and the metal, which is melted.

In both the first and second variants, melting is carried out in a vacuum chamber. All heating elements of such equipment are cooled with water. Thanks to this, various operations can be carried out in vacuum furnaces at fairly high temperatures.

Units with non-consumable electrodes are practically not used in industry. Their main purpose is to smelt small-sized ingots in laboratory conditions. They are a good tool for performing various analyses.

Example of an electric arc furnace

Vacuum arc furnaces with a consumable electrode are widely used for industrial purposes. In such devices, when working with metal, the following processes occur:

• melting;
• recovery;
• deoxidation;
• crystallization.

At the same time, at high temperatures, volatile gas impurities are removed, and unstable compounds decompose. Thanks to this, in vacuum arc furnaces it is possible to obtain material with a low content of non-metallic impurities and gases.

Vacuum furnaces are used for industrial purposes in industries such as rocketry and nuclear energy. Using such equipment it is possible to obtain ingots weighing more than 50 tons.

Plasma arc furnaces

In such installations, the metal is heated by an electric arc passing along with a jet of inert gas plasma. This process ensures the purity of the melted material, and also allows you to significantly increase the productivity of furnace equipment.

In plasma-arc furnaces, metals with a low oxygen content are smelted. The melting process is carried out in a neutral atmosphere, which allows creating all the conditions for maximum gas release. Metal smelting occurs at high speed.

Flame-arc furnaces are used to produce high-quality steel and alloys. Their use is much cheaper than metal smelting in vacuum furnaces.

REGULATION OF VOLTAGE OF ARC FURNACE

Possible ways to regulate the electrical mode:

Changing the input voltage

Change in arc resistance, i.e. changing its length

The automated workplace of arc furnaces must provide:

Automatic arc ignition

Automatic elimination of arc breaks and operational short circuits

Response time of about 3 seconds when eliminating arc breaks of an operational short circuit

Aperiodic nature of the regulation process

The ability to smoothly change the power input into the furnace within the range of 20-125% of the nominal and maintain it with an accuracy of 5%

Stopping the electrodes when the supply voltage disappears

Technical characteristics of DC arc furnaces

The above list of a series of electric arc furnaces does not exhaust all the capabilities of NPF COMTERM LLC; for additional information, please contact our specialists using the contact information listed on the website.

Oxygen elements

AC furnaces typically have a pattern of hot and cold spots around the perimeter of the fireplace, located between the electrodes. Modern ones install oxygen-fuel burners in the side wall. They are used to supply chemical energy to negative zones, which makes heating of the steel more uniform. Additional power is provided by introducing oxygen and carbon into the furnace. Historically done with lances (hollow mild steel tubes) in the slag door, it is now primarily done with wall-mounted injection units that combine oxy-fuel burners and air systems into one vessel.

A modern, average-sized steel furnace has a transformer rated at about 60,000,000 volt-amperes (60 MVA), with a secondary voltage of 400 to 900 and a current in excess of 44,000. In a modern shop, such a furnace would be expected to produce 80 metric tons of liquid steel approximately 50 minutes from cold scrap loading to release.

By comparison, basic oxygen furnaces can have a capacity of 150–300 tons per batch, or "heat up" and produce heat for 30–40 minutes. There are huge differences in the details of furnace design and operation, depending on the end product and local conditions, as well as the research being done to improve the efficiency of the installation.

The largest, scrap-only (in terms of tap mass and transformer power rating), is a DC unit exported from Japan, with a tap mass of 420 metric tons and fed by eight 32 MVA transformers for a total power of 256 MVA.

To produce a ton of steel in an electric arc furnace it takes approximately 400 kilowatt-hours short, or about 440 kWh metric. The theoretical minimum amount of energy required to melt scrap steel is 300 kWh (melting point 1520 °C / 2768 °F). Therefore, a 300 tonne 300 MVA EAF will require approximately 132 MWh of energy and have an on-time of approximately 37 minutes.

Steel production using an electric arc is only economically viable if there is sufficient electricity available with a well-developed network. In many locations, mills operate during off-peak hours when utilities have excess production capacity and meter prices are lower.

Features of the use of arc furnaces for steel melting

Electric arc furnaces are used to produce the following types of steels:

1. Highly alloyed;
2. Instrumental;
3. Structural;
4. Special weapons and other alloys.

Due to the ability to maintain a high melting temperature, arc furnaces are capable of melting almost any steel and alloy. The main feature of electric arc furnaces is their method of converting electrical energy into heat. It is carried out by an electric arc. It is due to the arc that it becomes possible to achieve such high temperature conditions.

DC electric arc furnaces are the most popular equipment in modern steel mills. The design features of electric arc furnaces make it possible to obtain homogeneous alloys with a minimum amount of non-metallic inclusions. In addition to steel mills, electric arc furnaces are used in specialized laboratories. Laboratory models are compact in size, but by design they are full-fledged arc furnaces. They are used for various kinds of physical and chemical research.

Advantages and disadvantages

The use of electric arc furnaces for steel smelting is widely used in the metallurgical industry. The main advantages of using such equipment are the ability to carry out the following operations:

• melting the charge regardless of its composition;
• rapid heating of metal in a furnace;
• temperature control;
• deoxidation of the metal and resulting in a material with a low content of impurities.

When remelting steel in a furnace unit, all conditions are created to reduce the waste of alloying components. This ensures that metal losses due to oxidation at high temperatures are reduced.

Electric arc units are widely used for industrial purposes for melting various metals. With their help you can obtain high-quality strong steel alloys. The efficiency of an arc furnace largely depends on the quality of the heating device. Therefore, you should purchase reliable equipment from well-known and trusted manufacturers.

Melting gold and silver

A gold smelting furnace is easy to make at home. It is also applicable for smelting silver.

The operating procedure is as follows:

• A fireclay brick is taken and cut into 2 parts. Using a Pobedit drill with a diameter of 48 mm, a through hole is made in one half, in the middle. And in the second hole is drilled to half the height.
• A spiral is drawn through the hole and both halves are tightened with bolts, holes for which are drilled from the sides.
• A graphite crucible is installed on top.
• A metal frame is made and both halves are inserted into it.
• All side gaps are covered with clay.
• Voltage is applied to the removed ends of the spiral.
• Pieces of gold or silver are thrown into the crucible.
• During the heating process, the non-ferrous metal melts.

Making furnaces for melting metal with your own hands is a complex process, but doable. To do this, you need to study the characteristics of the types of equipment. Decide which one is most preferable to the given conditions. Manufacturing costs will quickly pay for themselves.

At large enterprises of the Vtorchermet system, it is economically profitable to carry out not only the primary processing of scrap metal, but also its melting with subsequent rolling of the resulting semi-finished products - ingots. This increases the profitability of the operation of such enterprises, since the operating costs of purchasing equipment are recouped by the quick sale of products. Subject to compliance with technical production standards, it turns out to be of almost the same quality as steel produced at specialized metallurgical plants. The first link in this chain is furnaces for smelting metal from scrap metal.

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Overflow

The vessel is then taken to the smelting shop, the furnace roof is opened and the material is loaded into it. Overfilling is one of the most dangerous operations for operators. A lot of potential energy is released by tons of falling metal. Any liquid substance in the furnace is often forced upward and outward by the solid scrap and grease. Dust on the metal will ignite if the furnace is hot, causing a fireball to ignite.

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In some double-shell devices, scrap is loaded into the second while the first is melting, and is preheated by the exhaust gas from the active part. Other operations include continuous loading and temperature control on a conveyor belt, which then discharges the metal into the furnace itself. Other devices may be loaded with hot material from other operations.

Next stage

The scrap is loaded into large buckets called baskets with clamshell doors for the base. It is necessary to ensure that the scrap is in the basket to ensure good operation of the oven. A strong melt is laid on top with a light layer of protective shred, on top of which another part is laid. All of them must be present in the oven after loading. At this time, the basket can go into a scrap preheater, which uses the plant's hot, waste gases to melt and recover energy, increasing efficiency.

Burning impurities

Once the scrap metal is completely melted and a flat pool has been achieved, another bucket can be loaded into the furnace. After the second charge is completely melted, refining operations are carried out to check and adjust the chemical composition of the steel and to superheat the melt above its freezing point in preparation for tapping. More slag formers are introduced and a lot of oxygen enters the bath, burning impurities such as silicon, sulfur, phosphorus, aluminum, manganese and calcium and removing their oxides into the slag.

Carbon harvesting occurs after these elements burn out first, as they are more similar to oxygen. Metals that have a lower affinity than iron, such as nickel and copper, cannot be removed by oxidation and must be controlled only by chemistry. This is, for example, the introduction of direct reduced iron and cast iron, mentioned earlier.

Foamy slag persists everywhere and often overflows the furnace to spill out of the door into the designated pit. Temperature measurements and chemical sampling are carried out using automatic copies. Oxygen and carbon can be measured mechanically with special probes that are immersed in the steel.

Vacuum melting furnaces

The scope of application of such units is quite wide. They are used in mechanical engineering, construction and many other industries. Used for melting high-quality steels and heat-resistant alloys based on iron, nickel and cobalt, as well as non-ferrous metals and alloys. In addition, these furnaces can be used for zone cleaning, heat treatment of metal parts, glass melting, and production of single crystals. An ideal furnace for melting steel, it can produce low-carbon steel that is not contaminated by nitrogen, hydrogen and oxygen.

Oxidation of impurities

An important part of steelmaking is the formation of slag, which floats on the surface of the molten steel. It is typically composed of metal oxides and also acts as a collection site for oxidized impurities, as a thermal blanket (stopping excessive heat loss), and also helps reduce erosion of the refractory lining

For a basic refractory furnace producing carbon steel, common slag formers are calcium oxide (CaO in the form of burnt lime) and magnesium oxide (MgO in the form of dolomite and magnesite.). These substances are either loaded with scrap or blown into the furnace during melting.

Another important component is iron oxide, which is formed when steel is burned with introduced oxygen. Later, when heated, carbon (in the form of coal) is injected into this layer, reacting with iron oxide to form metal and carbon monoxide. This causes the slag to foam, resulting in greater thermal efficiency. The coating prevents damage to the roof and sides of the stove from radiant heat.

Arc steel furnace design

In this type of furnace, the body is mainly made of metal, which functions as a casing and has a spherical bottom. The inside of this chamber has lining layers, the material of which must be fire-resistant. The oven container has an upper lid, the shape of which follows a hemisphere. This vault consists of refractory bricks laid in a ring of the required diameter. In order to be able to observe the processes occurring inside the furnace, an observation window is mounted in the wall, the glass of which must also be fireproof. There is a hole in the lower part of the wall for draining slag. As for the viewing window, it serves not only to control the ongoing processes, but also as a loading compartment through which slag-forming substances, ore, ferroalloys can be added to the material being processed, as well as samples of metal and slag can be taken.

In order for the stove to be tilted either for draining or for adding other elements, it is installed on a special cradle, the mechanism of which is designed like a swing. Naturally, the tilting of the furnace is not done manually, but using a pneumatic or electromechanical drive.

Tapping and other operations

The arc steel-smelting furnace EAF 50 is built on an inclined platform, so that liquid steel can be poured into another container for transportation. The tilting operation to transfer molten steel is called tapping. Initially, all steel-making vaults of an arc furnace had an outlet chute covered with refractory, which was washed out when it was tilted.

But often modern equipment has an eccentric bottom outlet valve (EBT) to reduce the inclusion of nitrogen and slag in the liquid steel. These ovens have a hole that runs vertically through the hearth and shell and is offset from the shape of an egg. It is filled with fireproof sand.

Modern plants may have two shells with one set of electrodes that are transferred between them. The first part heats the scrap, and the other is used for melting. Other DC furnaces have a similar arrangement, but have electrodes for each shell and one set of electronics.

Features of operation

Working on such furnaces first of all requires compliance with safety and labor protection rules. The entire cycle of work is carried out in several main stages:

• First of all, before starting work, the condition of the refractory bricks on the roof and bottom of the furnace is inspected. All affected or damaged areas are repaired. It is mandatory to check the serviceability of the ventilation and water cooling systems.
• Filling the charge. For filling modern production plants, an overhead system is used using loading buckets or a special filling machine with a bucket. This technique is used to introduce alloying additives or necessary components to adjust the composition of the metal during the smelting period. Small scrap is placed at the bottom of the hearth, this way it is possible to avoid damage to the refractory bricks during this operation.
• For early formation of slag and protection of the bath from harmful gases, 2% lime from the full chamber load is added to the mixture.
• The furnace is covered with a roof with electrodes and power is supplied to them.
• During the steel melting stage, sudden failure of one or more electrodes may occur. This mainly occurs when there is insufficient passage of electric current, when the minimum gap from the tip of the electrode to the upper edge of the charge is not observed.
• The power and melting speed can be adjusted by changing the position of the heating element. Then the length of the electric arc changes. The load also changes when the incoming current load increases or decreases.
• The charge melted, a layer of slag and molten metal formed. Slag is removed through a special channel throughout the entire period of operation of the furnace. This helps remove harmful substances from the steel. To do this, the slag layer is foamed with carbon-containing materials, which interrupt the operation of the electric arc.
• Samples are taken periodically and laboratory analysis is carried out to determine the composition and readiness of the steel. At home, this operation will have to be performed by eye. If necessary, you can visually monitor the progress of work through a special cavity, which can also serve as a tap hole for adding and adjusting the quality of steel or other types of metals or alloys.
• After the material is ready, it is discharged through special channels into a steel ladle or discharge is made by tilting the furnace body.
• After finishing work, the power is turned off. The work and production of one batch of charge is completed. The surfaces are cleaned of plaque and defects after the furnace has cooled, and only then can the next melt be performed.

This type of work must be performed in any facility, regardless of furnace size or volume.

In conclusion, pechnoy.guru once again recalls the main idea of ​​the article:

Don't try to save money by buying expensive equipment. Never contact unknown suppliers and do not buy equipment at a promotion or sale on an unfamiliar website - a fly-by-night website. This way you will not only save money, but also get high-quality and durable equipment.

Melting metal and cast iron

A furnace for melting metal is a body made of fireclay bricks. The binding element is clay. The firebox is designed to burn coal. There is a hole at the bottom through which the air is blown into the inferno. At the bottom there is a cast iron grate called a grate. Coke or coal is laid out on it. It can be removed from the old stove. Sometimes refractory bricks are laid on edge when forming the body. The finished structure is fastened from the outside with a metal belt.

A furnace for melting metals must have a crucible. It can be an enameled or cast iron cauldron. The location of the crucible is next to the burning coke. To improve airflow, a fan is installed nearby. The equipment is used for steel smelting, but can be used as a furnace for cast iron smelting.

Voltage

After charging, the roof folds back over the furnace and melting begins. The electrodes are dropped onto the scrap metal, an arc is created, and they are then set so that they spread into a layer of crumbs at the top of the device. Low voltages are chosen for this operation to protect the roof and walls from excessive heat and arc damage.

Once the electrodes have reached the heavy melt at the base of the furnace and the waves are shielded with a crowbar, the voltage can be increased and the electrodes raised slightly, extending and increasing the power for the melt. This allows the molten pool to form more quickly, reducing tapping time.

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Oxygen is blown into the scrap metal, burning or cutting the steel, and additional chemical heat is provided by wall burners. Both processes accelerate the melting of the substance. Supersonic nozzles allow oxygen jets to penetrate the foaming slag and reach the liquid bath.

Manufacturers

The market for steel-making electric arc equipment is flooded with offers of counterfeit, homemade models at low prices. Therefore, when planning a purchase, find the website of manufacturers of similar equipment on the Internet and order stoves directly or through official dealers. By purchasing units from an unverified seller, you risk purchasing a low-quality and short-lived design, albeit for little money.

Here is a short list of companies producing electric arc furnaces:

1. The group produces steel-smelting units of various designs and capacities for laboratory research and the metallurgical industry. High temperature furnaces can melt in a vacuum or at atmospheric pressure. In all cases, careful monitoring of the air condition in the chamber with the melt is carried out. The technique is used not only in the steelmaking industry, it works successfully in growing single crystals and for producing optical fiber.
2. The Russian company produces electric steel-melting furnaces for industrial installations under the “DPPT” and “ESHP” labels. DPPT models are one of the most powerful industrial steel-smelting electric arc furnaces. They contain the entire supply of designers and operators when developing a project for melting devices operating on alternating electric current. Powerful DC converters work for servicing stoves, chipboard models.
3. Arc steel furnace from Taiwan is a popular brand of manufacturers on the Russian market. Thyristor electric melting furnaces work not only with metals. The simplicity and reliability of the design allows the devices to be manufactured in 2 versions. They can be supplied with a hydraulic or gear version. Perfectly suited for small and medium-sized steelmaking enterprises with a 3-level productivity of over 2000 tons per year.
4. Since 1989, a scientific and technical company has been successfully operating on the Russian market, specializing in the production of thermal industrial equipment. Scientific methods used in the development and design of furnaces allow the company to compete with foreign manufacturers, especially in terms of price and product quality. AC and DC arc furnaces can be used to produce aluminum alloys and other metals.
5. Chinese furnaces provide fast melting at a reduced cost of operating the unit. Minimum dimensions with a large loading volume, excellent heat dissipation characteristics and a minimal level of environmental pollution when fully loaded and melting the charge - this is not a complete list of the advantages of this technology. Models “KGPS 200kW” are used for the production of stainless steel, cast iron, all types of alloy, heat-resistant and structural steels. Can be used to obtain non-ferrous and precious metals.

There is no point in indicating prices; they change very quickly. Therefore, we advise you to contact the manufacturers directly and find out the cost of the equipment you are interested in.

Which melting furnace to choose and where to buy it

When choosing such equipment, you need to focus on exactly what tasks you plan to perform with its help.

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Today you can purchase equipment for the metallurgical industry from major suppliers or directly from manufacturers. As a rule, both of them offer delivery, commissioning, maintenance, repairs and other services.

Principle of operation

An electric furnace for melting metal can work successfully in steel production and in a home workshop. The operating principle of any design using an electric arc is divided into 3 stages:

The process of melting the charge material. At this stage, the surface of the melt is covered with a film that prevents the entry of various harmful gases. Phosphorus, sulfur and other chemical elements that affect the quality of steel and alloys are absorbed. Oxidation of metals. At this stage, the content of harmful substances in the metal is adjusted. The maximum level of phosphorus or sulfur should not exceed 0.15% of the total mass

To form a steel grade, it is important to ensure that the content of nitrogen and hydrogen in it is adjusted. The temperature level in the furnace at this stage is maintained above the melting limit of the base substance by 1200

An oxygen or scale layer is used as an oxidizing agent. Recovery stage. During this period, sulfur inclusions are removed, and the metal structure is brought to a specified level in terms of the content of alloying additives and carbon.

This is the general principle of operation of furnaces, but depending on the type of appliances, the furnace will operate according to a certain pattern. Let's look at this question in more detail.

Direct current

DC electric arc furnaces are devices for use in the foundry and metallurgical industries. By maintaining the arc in the center, the service life of the inner layer of refractory bricks in the metal heating chamber is increased. Such work leads to energy savings and increased furnace productivity. Such devices consist of:

• outer housing of the metal heating chamber;
• vault made of fireproof material;
• heating electrode, which is mounted in the roof;
• 2 electrodes are installed in the bottom of the chamber;
• three powerful electromagnets for adjusting the position of the electric arc;
• control systems for the operation of the installation. It includes temperature sensors, thermocouples and other process control equipment. Thermocouples are installed in the upper cavity of the roof, above the upper limit of the molten metal, at a minimum distance of 500 mm;
• electromagnet control unit;
• An additional current source with a voltage of 24 V is installed.

Electromagnets hold the arc at the center of the chamber. They are installed so that the angle of deviation along the axes does not exceed 1200.

Alternating current

AC arc furnaces - their operating principle is based on the penetrating effect of an alternating magnetic flux that passes through a closed circuit of the chamber. It contains materials that melt under the influence of a magnetic field. The inner chamber is enclosed in a metal casing made of heat-resistant steel. The entire internal space is filled to a certain level with molten metal with alloying additives.

The steel is brought to a certain temperature, goes through all three stages given above, and after the melting process is completed, it is discharged into a separate channel. When the metal is released from the furnace, the current is interrupted and the molten, finished steel is poured into ladles.

Advantages of electric arc furnaces

Electric arc furnaces have long dominated steel production due to the advantages this smelting method provides. The advantages of arc steel furnaces are as follows :

1. Due to the design features of electric furnaces, it is possible to use structural parts that are capable of supplying heat to the melting site, due to which it is possible to create a high melting temperature there;
2. Electric arc furnaces do not need to create a special atmosphere in their working area. As a result of this, there is no need for various additional systems that create a certain atmosphere, which significantly reduces economic costs in the production of high-quality high-alloy steel;
3. It is possible to quickly reach the melting temperature of the metal, which allows you to quickly add the required amount of alloying additives to the molten mass;
4. Makes it possible to introduce non-oxidizing slags, which helps reduce the percentage of waste from alloying components;
5. The melting temperature can be quickly and smoothly adjusted;
6. It is possible to smelt steels with a minimum amount of oxide non-metallic inclusions.

Most modern models of electric arc furnaces are used for melting scrap iron, from which high-quality steel is obtained. The finished product is transferred to the next stage of processing. The capabilities of an arc steel-smelting furnace make it possible to smelt a huge number of types of steel and cast iron.

FEATURES OF OVEN TRANSFORMER

Large rated current on the low voltage side, amounting to tens of thousands of amperes

Increased inductive resistance of the windings, necessary to limit short-circuit currents to 2.5-3.5 times the rated current, since steel-smelting furnaces operate with frequent short circuits of the electrodes to the charge when the arc is ignited and the collapse of the charge during the melting period

Increased mechanical strength of fastening windings and taps designed for frequent current surges and short circuits

Ability to regulate voltage under load over a wide range.

Technical characteristics of AC arc furnaces

Who invented it?

An electric arc furnace, namely the effect of melting metal using an electric arc, was first shown by the domestic scientist Popov at the beginning of the 19th century. Such experiments have shown that using an electric arc installation it is possible not only to melt metal and steel, but also to restore new materials from oxides when heated together with carbonaceous reducing agents. These experiments became the ancestor of electric arc welding.

But in parallel with Popov, research was also carried out by foreign scientists. Already in 1810, Davy Humphrey showed the first experimental installation of arc combustion, and in 1853 an attempt was made to build the first smelting furnace by Pichon. 1878 is the year when Wilhelm Siemens received a patent for the invention of the first electric arc furnace. But the world's first arc steel-smelting plant appeared only in 1899. Therefore, disputes about who invented this device continue to this day.

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Widespread use of such devices in the steelmaking industry began after the end of World War 2.

Several photos of electric arc furnaces:

Electric arc furnace in refractory brick

To create it, you will need a refractory brick, in which two holes are drilled: one for a graphite rod (from a battery), and the second for a graphite crucible. An electric arc will occur between these elements. The rod and crucible are connected through clamps to insulated copper wires and connected in series, with resistance, to the network. The metal is placed in the crucible, the arc is ignited manually, like on a welding machine, and closed. This DIY stove option is suitable for small volumes of metal melting.

Construction of industrial arc furnaces

These furnaces are an effective replacement for blast furnaces and open-hearth furnaces, as well as furnaces for metal remelting in foundries. The capacity of these installations is up to 400 tons at temperatures up to 3000°C.

In AC furnaces, melting is carried out by an electric arc that occurs between three graphite electrodes. The molten metal collects on the bottom of the furnace and flows down a special chute.

The electrodes are powered using a step-down transformer for the electric arc furnace connected to the high voltage line. The electrical connection diagram of the furnace transformer allows you to change the voltage on the electrodes:

• at the beginning of melting it is higher to ensure a stable arc when there is a lack of molten metal;
• in the middle and end of the process, the required voltage is lower to prevent the current from exceeding the value.

For greater stability and controllability of the smelting process when graphite burns out and the charge level decreases, the electrodes are made movable, and their drive is connected to automation systems.

Information! Using electric arc furnaces in the chemical industry, phosphorus, calcium carbide and other products are produced.

Arc furnace power supply circuit

Electric arc furnaces are powerful consumers of electricity. To reduce cable losses, transformers and current control systems are located near the furnaces. The section from the substation to the electrodes is called a “short network”.

Most often, arc steel furnaces (ASFs) are connected to a high voltage line through their own high-voltage substation, which contains oil switches and disconnectors. The supply voltage is 6-110 kV. In large workshops where several chipboards and other equipment are installed, the substation and switch system are common to all switches.

Power

The power of furnace transformers reaches 300 MVA with an output voltage from 50 V in small-power furnaces to 1200 V in the largest and most productive devices. The output voltage is regulated during operation by switching the terminals of the primary winding.

The melting power is also regulated by moving the electrodes - when the length of the arc changes, its resistance and current change. This adjustment is carried out by a software-adaptive controller connected to the graphite rod drive. There are two types of drive:

• Electromechanical – movement is carried out by electric motors. It is not very common due to the delay in starting while the electric motor is accelerating and the impossibility of instantly stopping and reversing it.
• Hydraulic – more modern and inertia-free. The movement of the rods is carried out by a hydraulic system, in which oil pressure is maintained throughout the melting process.

Elements

The electrical circuit of an electric arc smelter consists of the following elements:

• suitable high voltage cables;
• a step-down transformer;
• choke that reduces short circuit current;
• switching equipment that controls melting voltage and current;
• short network;
• graphite electrodes with a drive system;
• measuring and protective devices.

During operation, to ensure process stability, the voltage on the electrodes and the current must be adjusted. This is done in two ways - by changing the output voltage of the transformer and by mechanically moving the electrodes and changing the arc length.

Peculiarities

The design features of furnace transformers and the control circuit must ensure normal operation in the following situations:

• initial ignition of the arc and start of melting;
• re-ignition of the arc within 3 seconds if it breaks;
• eliminating short circuits that occur when the charge collapses;
• smooth change in power in the range of 20-125% of the nominal.

To reduce short circuit current and power surges, a choke is connected in series with the electrodes. It is installed next to the furnace step-down transformer with the required technical characteristics and together with it is placed in a tank with oil for better cooling.

DC Arc Furnaces

AC melting plants have a number of disadvantages:

• low productivity;
• increased consumption of electrodes and waste of charge;
• large, up to 100 dB, noise;
• emissions of harmful gases.

DC installations are free from these disadvantages. Direct current arc furnaces (DCAFs) have one central graphite cathode mounted on the roof and several metal anodes mounted on the hearth. To reduce waste, the anodes are cooled by channels containing water inside them.

Charge materials

§ 34.

Metals, alloys, special alloys, slag-forming additives and other materials that are used to prepare various alloys are called charge materials or charge in the foundry industry. The charge includes: fresh materials (blast furnace cast iron of various grades, copper, aluminum, zinc, nickel, etc.), which come to foundries from metallurgical plants; scrap of ferrous alloys and scrap of non-ferrous alloys, which are processed industrial wastes; special ferroalloys and alloys (intermediate alloys of more refractory elements with low-melting elements) coming from metallurgical plants; waste from foundries and machine shops (sprues, profits, defective parts and briquetted shavings). The quantitative ratio of various materials in the charge depends on the quality of the starting materials and on the requirements for the manufactured alloys.

§ 35.

Main types of melting furnaces

In foundry production, melting furnaces operating on solid, liquid or gaseous fuels (coke, oil, fuel oil, gas) and electric furnaces are used. The first type of furnaces includes cupola furnaces and crucible furnaces, the second type includes electric arc furnaces and electric induction furnaces. The most widely used furnaces for melting cast iron are shaft-type furnaces—cupola furnaces. Gray cast iron produced in these furnaces is used for casting parts of various complexity. Electric furnaces melt steel, alloy cast iron, and white cast iron, which is then processed into malleable cast iron. The cupola furnace diagram is shown in Fig. 35. The cupola is a shaft furnace, the basis of which is a welded metal casing 1, lined on the inside with refractory bricks 2. The gap between the casing and the lining is filled with dry quartz sand 3. There is a loading window in the upper part of the cupola 4. Part of the cupola shaft located below the loading window , is lined with cast iron hollow bricks 5, which protect it from destruction when loading the charge 7.

Load the cupola using a skip hoist or jib crane. The upper part of the cupola ends with spark arrester 6.

To maintain combustion in the cupola furnace, air (blown) is supplied through special holes 8, called tuyeres, and blown by a fan. Molten cast iron along the hearth 9, located in the lower part of the shaft, flows through a special hole and chute into the reservoir 10. At the beginning of work, a layer of coke 500-1500 mm high is loaded into the cupola and ignited. This layer of coke is called blank shell. Then the working coke shell, flux and the first portion of the metal charge are loaded onto the idle shell. After loading the materials, the air necessary for combustion of the fuel is supplied through the tuyeres. In the melting belt, cast iron and slag are melted and flow into the cupola furnace. The resulting gases, rising upward, heat the metal charge and fuel, and then go into the pipe.

As the coke burns and the cast iron melts, the charge loaded into the cupola falls down, and new portions of charge materials are loaded in its place. During the melting process, liquid iron accumulates in the cupola furnace. The slag floats to the surface of the cast iron and is periodically released through the slag tap hole. The accumulated cast iron is drained through a tap hole along a chute into a special storage tank, and then into a ladle. The productivity of cupola furnaces is 0.5–30 tons of cast iron per hour.

For the purposes of fire safety and protection from pollution of the surrounding area, cupola furnaces are equipped with spark arresters, which are also dust collectors.

For melting steel, foundries use open-hearth and electric arc furnaces with basic and acid linings, as well as induction crucible furnaces.

In Fig. Figure 36 shows a diagram of an electric arc furnace. The heat source in this furnace is an electric arc that occurs between the melt 3 located in the bath of the furnace 4 and three graphite electrodes 1 (passing through the roof of the furnace 6), through which an electric current is supplied. The capacity of such furnaces is 1.5-10 tons. Melting duration is 1.5-4 hours. The prepared metal is poured when the furnace is tilted, carried out by a special mechanism, through chute 5. The charge is loaded into the furnace through window 2 or through the furnace roof, raised and rotated by a special mechanism.

Melting steel in an electric arc furnace consists of the following operations: filling the electric furnace, filling the charge, melting the charge and casting the finished steel.

An induction furnace (Fig. 37) consists of a frame 6 made of non-magnetic material, inside of which there is an inductor (coil) made of turns 7 of a copper tube through which cooling water flows. The turns are separated from each other by insulation '8. The melting crucible 5 in this furnace is made of a stuffed lining. The upper parts of the lining 1 and 3 and its lower part 4 are made of shaped refractory bricks, layer 2 is made of refractory coating. The heat source in these furnaces is an induction current excited in the charge loaded into the crucible when an alternating current of high frequency is passed through an inductor.

Rice. 38. Single-phase electric arc furnace for melting copper alloys

Melting of non-ferrous alloys is carried out in crucible furnaces with oil or gas heating, in electric resistance furnaces, as well as in arc or induction electric furnaces.

Copper alloys are melted in crucible, flame and electric furnaces. The most widely used are single-phase electric arc furnaces of the DMK type (Fig. 38). The furnace is a metal drum 1, lined with refractory bricks 2. The heat required to melt copper is created by an electric arc arising between two horizontally located electrodes 3. Mounted on rollers 4, drum 1 can be rotated to a certain angle by a motor and a gear drive.

Charge materials are loaded through a working window equipped with a chute through which the finished melt is released. For the smelting of copper alloys, the charge materials are pigs, machine scrap, waste from own production, etc. During the smelting process of copper, zinc and lead, harmful gases and vapors are released, so smelting furnaces are equipped with powerful exhaust ventilation.

Aluminum alloys are melted in crucible and flame furnaces, electric resistance furnaces and induction furnaces. In Fig. 39 shows a gas-heated crucible furnace for melting aluminum alloys. The furnace has a refractory lining 1, inside which a cast iron crucible 2 is inserted. Gas is supplied to the burner 4 and burns in the space between the lining and the crucible. Hole 3 is provided for releasing metal when the crucible burns out. Combustion products and gases from the alloy are removed by an exhaust hood 5. The furnace is suspended by axles on side supports and can be tilted using a steering wheel and a worm gear.

Operating principle of steel-smelting electric arc units

The main function of arc furnaces is to release heat to the arc due to the high accumulation of electricity. Thanks to this, metal is melted at a significant heating rate.

An arc can burn both in the vapor of the processed material and in a normal atmosphere. The most popular in the industrial sector are electric arc steel-smelting furnaces. To produce steel, secondary raw materials – scrap – are consumed. The melting process consists of several stages:

• the vault rises;
• the charge is loaded into the furnace using a special crane;
• the arch is fixed in place;
• electrical power is supplied to the electrodes;
• electrical conductors touch the scrap loaded into the unit;
• an interphase short circuit is formed;
• automatic lifting of holders with electrodes is triggered;
• an electric arc ignites.

Thus, the operation of the furnace begins, which occurs at a high power temperature. It consists of the following main stages:

1. Melting scrap metal. The heated charge is covered with a protective film, which blocks the access of harmful gases to the material. At the same time, various substances that have a bad effect on the quality of the metal are absorbed.
2. Oxidation process. Harmful elements are corrected. At this time, the temperature in the unit rises. Its value becomes 120 degrees higher than the limit established for metal melting. Phosphorus and sulfur should occupy no more than 0.15 percent in the total composition. Hydrogen and nitrogen levels are also monitored.
3. Recovery. Sulfur elements are removed from the material, and the composition of the metal is brought to standard values.

The operating process of a furnace device largely depends on its design and functional features.

Electric arc furnace in clay pot

Another option for creating an electric arc stove with your own hands is using pots. To create a stove, you will first need two pots of different diameters 20 cm and 5 cm, in which holes of the same diameter are drilled at the same distance. circuit includes two carbon electrodes (from the arc lamp), which, from opposite sides, pass through prepared openings through the walls of two pots and meet in the cavity of the smaller one.

Sand is poured into a large pot and a small pot is placed, at the bottom of which another small hole is drilled.

Carbon electrodes are covered along the entire length with fire-resistant clay and inserted into the prepared passages. The drilled areas are well covered with clay so that heat is not lost. A layer of sand is poured over the insulated electrodes and compacted with a small amount of water.

The stove lid can be made from a pot stand, equipped with handles. A glass tube is screwed to one electrode to adjust the operation, the other remains in a fixed position. Electrodes with brass or copper clamps are connected by wires through a fuse to the network.