Melting metal at home. Induction furnaces for melting metal Do-it-yourself electric furnace for melting metal

An induction furnace is a heating device where the induction method is used to melt steel, copper and other metals (the metal is heated by currents excited by a non-alternating inductor field). Some consider it one of the types of resistance heaters, but the difference is energy transfer method heated metal. First, electrical energy becomes electromagnetic, then again electrical, and only at the very end it turns into heat. Induction stoves are considered the most perfect from all gas and electric (, steelmaking, mini stoves), thanks to its heating method. With induction, heat is generated within the metal itself, and the use of thermal energy is most efficient.

Induction furnaces are divided into two types:

with a core (channel);
without core (crucible).

The latter are considered more modern and useful (heaters with a core, due to their design, are limited in power). The transition from channel to crucible furnaces began in early 1900s. At the moment, they are widely used in industry.

Such types of electrical appliances as muffle melting furnace, steel melting furnace and arc steel melting furnace are quite popular. The former are very effective and safe to use. On the shelves there is a large assortment of muffle furnaces of this type. A very important role for metallurgy was played by such an invention as a steel furnace. With its help, it became possible to heat any materials.

However, at the moment, steel smelting is more often carried out using such a heating structure as, it uses the thermal effect for melting, and it is more convenient and practical.
With your own hands, you can make many simple heating structures. For example, very popular. If you decide to build a mini heating structure with your own hands, you need to know its device. There are many types of induction furnaces, but we will describe only a few of them. If necessary, you can use the necessary diagrams, drawings and video recordings.

Induction Furnace Components

For the simplest designs, there are only two main parts: an inductor and a generator. However, you can add something of your own, improve the unit, using the necessary schemes.
Inductor
The heating coil is the most important component. Absolutely the entire operation of the heating structure depends on it. For homemade stoves with low power, it is acceptable to use an inductor from a bare copper tube. with a diameter of 10 mm. The inner diameter of the inductor must be not less than 80 mm. and not more than 150 mm., the number of turns - 8-10. It must be taken into account that the turns should not touch, so the distance between them should be 5-7 mm. Also, no part of the inductor should touch its screen.
Generator
The second most important component of the furnace is the alternator. When choosing a generator circuit, you should in every possible way avoid blueprints, giving a hard current spectrum. As something that you DO NOT need to choose, we present a popular circuit on a thyristor key.

Crucible furnace device

Inside there is a melting crucible with a drain sock (“ collar“). On the outer sides of the structure, an inductor is located in a vertical position. Next comes a layer of thermal insulation, and at the top is a cover. One of the outer sides may have a supply current and cooling water. Below is a device for signaling the wear of the crucible.

The melting crucible is one of the most important components of the unit, it largely determines its operational reliability. Therefore, very stringent requirements are imposed on the crucible and other materials used.

How to make an induction oven

First you need to assemble the generator for the inductor. Here you will need the K174XA11 circuit. The transformer should be wound on a mini-ring with a diameter of 2 centimeters. The entire winding is carried out with a wire with a diameter of 0.4 centimeters and should be 30 turns. The primary winding is characterized by the presence exactly 22 turns of wire with a diameter of 1 millimeter, and the secondary should contain only 2-3 turns the same wire, but already folded four times. The inductor must be made of 3 mm. wire with a diameter of 11 mm. There should be exactly 6 turns. To adjust the resonance, it is best to set the normal or mini led.

A home induction furnace copes with the melting of relatively small portions of metal. However, such a hearth does not need a chimney or bellows that pump air into the melting zone. And the entire design of such a furnace can be placed on a desk. Therefore, heating by electrical induction is the best way to melt metals at home. And in this article we will consider the designs and assembly schemes of such furnaces.

How an induction furnace works - generator, inductor and crucible

In factory workshops, you can find channel induction furnaces for melting non-ferrous and ferrous metals. These installations have a very high power, which is set by the internal magnetic circuit, which increases the electromagnetic field density and the temperature in the furnace crucible.

However, channel structures consume large portions of energy and take up a lot of space, therefore, at home and in small workshops, an installation without a magnetic circuit is used - a crucible furnace for melting non-ferrous / ferrous metal. Such a design can be assembled even with your own hands, because the crucible installation consists of three main components:

A generator that produces alternating current with high frequencies, which are necessary to increase the density of the electromagnetic field in the crucible. Moreover, if the diameter of the crucible can be compared with the long wave frequency of the alternating current, then such a design will allow transforming up to 75 percent of the electricity consumed by the installation into thermal energy.
The inductor is a copper spiral created on the basis of an accurate calculation of not only the diameter and number of turns, but also the geometry of the wire used in this process. The inductor circuit must be tuned to gain power as a result of resonance with the generator, or rather with the frequency of the supply current.
The crucible is a refractory container in which all the melting work takes place, initiated due to the occurrence of eddy currents in the metal structure. In this case, the diameter of the crucible and other dimensions of this container are determined strictly according to the characteristics of the generator and inductor.

Any radio amateur can assemble such an oven. To do this, he needs to find the right scheme and stock up on materials and parts. You can find a list of all this below.

What furnaces are assembled from - we select materials and parts

The design of a home-made crucible furnace is based on the simplest laboratory inverter Kukhtetsky. The scheme of this installation on transistors is as follows:

Based on this diagram, you will be able to assemble an induction furnace using the following components:

two transistors - preferably field type and brand IRFZ44V;
copper wire with a diameter of 2 mm;
two diodes brand UF4001, even better - UF4007;
two throttle rings - they can be removed from the old power supply from the desktop;
three capacitors with a capacity of 1 microfarad each;
four capacitors with a capacity of 220nF each;
one capacitor with a capacitance of 470 nF;
one capacitor with a capacitance of 330 nF;
one 1 watt resistor (or 2 resistors of 0.5 watts each), designed for a resistance of 470 ohms;
copper wire with a diameter of 1.2 mm.

In addition, you will need a couple of heatsinks - they can be removed from old motherboards or CPU coolers, and a battery with a capacity of at least 7200 mAh from an old 12 V uninterruptible power supply. Well, in this case, the crucible tank is actually not needed - in The furnace will melt bar metal, which can be held by the cold end.

Step-by-step instructions for assembly - simple operations

Print and hang a drawing of Kukhtetsky's laboratory inverter over your desktop. After that, lay out all the radio components by grades and brands and heat up the soldering iron. Attach the two transistors to the heatsinks. And if you work with the stove for more than 10-15 minutes in a row, fix coolers from the computer on the radiators by connecting them to a working power supply. The pinout diagram for transistors from the IRFZ44V series is as follows:

Take a 1.2 mm copper wire and wind it around the ferrite rings, making 9-10 turns. As a result, you will get chokes. The distance between the turns is determined by the diameter of the ring, based on the uniformity of the pitch. In principle, everything can be done "by eye", varying the number of turns in the range from 7 to 15 turns. Assemble a battery of capacitors by connecting all the parts in parallel. As a result, you should get a 4.7 microfarad battery.

Now make an inductor out of 2mm copper wire. The diameter of the turns in this case can be equal to the diameter of a porcelain crucible or 8-10 centimeters. The number of turns should not exceed 7-8 pieces. If during the testing process the power of the furnace seems insufficient to you, redo the design of the inductor by changing the diameter and number of turns. Therefore, in the first pair, it is better to make the inductor contacts not soldered, but detachable. Next, assemble all the elements on the PCB board, based on the drawing of Kukhtetsky's laboratory inverter. And connect a 7200 mAh battery to the power contacts. That's all.

For many years, people have been smelting metal. Each material has its own melting point, which can be reached only with the use of special equipment. The first furnaces for melting metal were quite large and were installed exclusively in the workshops of large organizations. Today, a modern induction furnace can be installed in small workshops when setting up the production of jewelry. It is small, easy to handle and highly efficient.

Operating principle

The melting unit of the induction furnace is used to heat a wide variety of metals and alloys. The classic design consists of the following elements:

Drain pump.
Water cooled inductor.
Frame in stainless steel or aluminium.
Contact area.
Hearth made of heat-resistant concrete.
Support with hydraulic cylinder and bearing assembly.

The principle of operation is based on the creation of eddy induced Foucault currents. As a rule, during the operation of household appliances, such currents cause failures, but in this case they are used to heat the charge to the required temperature. Almost all electronics start to heat up during operation. This negative factor in the use of electricity is used to its full potential.

Device advantages

The induction melting furnace has been used relatively recently. Famous open-hearth furnaces, blast furnaces and other types of equipment are installed at production sites. Such a metal melting furnace has the following advantages:

It is the latter advantage that determines the spread of the induction furnace in jewelry, since even a small concentration of foreign matter can adversely affect the result.

Depending on the design features, floor and desktop induction furnaces are distinguished. Regardless of which option was chosen, there are several basic rules for installation:

The device may become very hot during operation. That is why there should not be any flammable or explosive substances nearby. In addition, according to fire safety regulations, nearby should be installed fire shield.

Only two types of furnaces are widely used: crucible and channel. They have similar advantages and disadvantages, the differences are only in the method of work used:

The crucible variety of induction furnaces is more popular. This is due to their high performance and ease of operation. In addition, a similar design, if necessary, can be made independently.

Homemade versions are quite common.. To create them you need:

Generator.
Crucible.
Inductor.

An experienced electrician, if necessary, can make an inductor with his own hands. This structural element is represented by a winding of copper wire. The crucible can be purchased at the store, but a lamp circuit, a do-it-yourself battery of their transistors or a welding inverter is used as a generator.

Using a welding inverter

A do-it-yourself induction furnace for melting metal can be created using a welding inverter as a generator. This variant is the most widely used since the efforts made concern only the manufacture of the inductor:

Thin-walled copper tube is used as the main material. The recommended diameter is 8-10 cm.
The tube is bent according to the desired pattern, which depends on the features of the body used.
Between the turns there should be a distance of no more than 8 mm.
The inductor is placed in a textolite or graphite case.

After the creation of the inductor and its placement in the housing, it remains only to install the purchased crucible in its place.

Such a circuit is rather complicated in execution, it involves the use of resistors, several diodes, transistors of various capacities, a film capacitor, copper wire with two different diameters, and rings from chokes. Assembly recommendations are as follows:

The created circuit is placed in a textolite or graphite case, which are dielectrics. Scheme, involving the use of transistors, is quite difficult to implement. Therefore, it is necessary to undertake the manufacture of such a furnace only if there are certain work skills.

Furnace with lamps

Recently, lamp stoves have been created less and less, as they require care when handling. The applied circuit is simpler in comparison with the case of using transistors. Assembly can be carried out in several stages:

The lamps used must be protected from mechanical impact.

Equipment cooling

When creating an induction furnace with your own hands, the most problems arise with cooling. This is due to the following points:

During operation, not only the molten metal is heated, but also some elements of the equipment. That is why effective cooling is required for long-term operation.
The method based on the use of air flow is characterized by low efficiency. In addition, it is not recommended to install fans near the oven. This is due to the fact that metallic elements can affect the generated eddy currents.

As a rule, cooling is carried out with water supply. Creating a water cooling circuit at home is not only difficult, but also uneconomical. Industrial versions of the furnace have an already built-in circuit, to which it is enough to connect cold water.

Safety

When using an induction furnace, certain safety precautions must be observed. Key recommendations:

When installing the equipment, consideration should be given to how the charge will be loaded and the molten metal will be extracted. It is recommended to allocate a separate prepared room for the installation of an induction furnace.

A crucible is a vessel for melting metal. As a rule, pig metal is melted in crucibles; already brought to the required degree of quality for casting into a mold or refining (deep cleaning from impurities). The general line of development of large-scale metallurgy is to reduce the number of redistributions, up to the production of conditioned metal immediately from the melting furnace, but crucible melting still retains significant importance in industry, and dominates in handicrafts and jewelry.

The crucible is not just a sufficiently heat-resistant vessel. Its chemical composition and design must correspond to the type of metal being melted and the melting mode. This article describes how to make a crucible with your own hands and what conditions it must meet for use at home or in a small workshop. Based on beginner metallurgists, you will first have to touch on the process of metal smelting itself, because. requirements for the crucible are determined mainly by its conditions.

A little about melting

In a high vacuum, high-purity remelted metal can be heated exactly to the melting point or slightly higher, and kept at it for some time, so that tiny, literally a few atoms, remnants of crystallites melt. Then the metal can be allowed to cool slightly below the melting point - it will remain liquid, like a supersaturated solution without a seed crystal. If now the metal is poured, also in vacuum, into a mold made of a chemically absolutely inert material, in which a seed crystal of the same metal is placed, then, observing all the subtleties of this technology, we will obtain a single-crystal casting with unique properties.

In amateur conditions, vacuum melting, alas, is not feasible. In order to properly make a crucible for melting metal yourself, you need to take into account a number of features of melting in a non-inert chemically gaseous environment. The remelted metal, firstly, interacts with air, which is why part of it is lost to the formation of oxide, which is especially important when remelting precious metal scrap: at its melting temperature (1060 degrees Celsius), even gold is noticeably oxidized. To compensate for oxidation to some extent, the crucible must create a reducing environment for the melt or be chemically inert if the metal is melted with a clean open flame, see below.

Secondly, so that the metal in the crucible does not freeze until it is brought to the casting mold, so that the remains of the original crystallites do not spoil the casting, and the melt acquires sufficient fluidity, the metal in the crucible is overheated. For example, the melting temperature of zinc is 440 degrees, and its foundry is 600. Aluminum, respectively, 660 and 800. Since overheating of the metal after melting takes some time, degassing of the melt also occurs, this is third.

Recovery

In metallurgy, mainly atomic carbon C, carbon monoxide CO (carbon monoxide) and hydrogen H are used as reducing agents. for this purpose it is too active and is absorbed by metals, without forming chemical compounds with them, in large quantities, which spoils the casting material. For example, solid platinum at room temperature can absorb up to 800 volumes of hydrogen. A platinum blank in a hydrogen atmosphere literally swells before our eyes, cracks and breaks into pieces. If they are taken out of the hydrogen chamber and heated, hydrogen will be released back.

Note: in a similar way, but in smaller quantities, metals absorb / emit other gases, for example. nitrogen. That is why degassing of the melt is required, see also below.

A significant proportion of hydrogen reduction occurs when heated by an open flame of a gas burner, when it comes into contact with a less heated surface. It does not come to damage to the metal - the absorbed hydrogen is further released and burned during the melting process. But, if the material of the crucible is also prone to gas absorption, it can crack and burst during melting, this must be borne in mind.

CO recovery is noticeable if the metal in the crucible is melted by an open flame of a liquid (gasoline, kerosene, diesel) burner, for the same reasons. Liquid fuel burns much slower than gas, and the zone of its afterburning extends several cm from the burner nozzle. Reduction with carbon monoxide is the cleanest, from the point of view of the metal: it does not spoil the metal and does not produce by-products with a strong excess of the reducing agent. Therefore, the reduction of CO is widely used in metallurgy when smelting metal from ore, but no one has yet figured out how to make a crucible furnace (see below), in which oxidation compensation would be completely provided by CO.

The atomic carbon reducing agent is energetic enough to compensate for the oxidation. It is also easy to create a reducing environment in a crucible using C: it is enough to introduce free carbon in one or another allotropic modification into the composition of its material or to make the entire crucible from a heat-resistant and mechanically sufficiently strong allotrope C; such is graphite. When C is reduced, there is a danger of carburizing the melt, but graphite releases very little atomic carbon when heated. If you heat the metal in a graphite crucible with a gas flame, then excess C will immediately find a more “tasty” H for it and the danger of carburization will be reduced to zero. And for other heating methods (see below), you can choose the dimensions, configuration of the crucible and the addition of graphite to its material so that there will simply be no excess C under any conceivable melting mode. This is a very valuable property of graphite, also keep in mind.

Note: the coefficient of thermal expansion of graphite TKR is negative, which significantly compensates for the thermal expansion of the crucible, increases its durability and increases the service life. It's also a valuable asset.

Excerpt

So, why the melt in the crucible needs to be overheated and held is understandable. Although metal casting is a completely different topic, it still needs to be mentioned here that the melt holding time must be observed quite accurately. Chemically pure metals are almost never used in practice, for example. gold 9999 wears out very quickly; the exception is electrical copper and zinc for zinc, the purer the better. Most often use the so-called. eutectic alloys; e.g. steel is a eutectic of iron with carbon, and duralumin is a complex eutectic of several components. If the melt is allowed to stand, the structure of the eutectic in the casting will change and the finished product will be spoiled. The exposure time is especially critical for bronze and brass: they must be poured immediately, as soon as the play of the melt in the crucible apparently changes, it becomes calmer. Do you remember how the engineer Telegin in A. N. Tolstoy's "Walking Through the Torments" was worried that the bronze would not over-age?

With regard to the manufacture of a self-made crucible, degassing of the melt during holding is significant in that at this time it (the crucible) experiences significant dynamic loads from bubbles of evolving gases and/or the play of the melt itself. That is, to make the crucible withstand a large number of thermal deformations and, if required, reducing it, it is not enough. Its material must also be sufficiently viscous to withstand shock waves from bursting bubbles and shocks from melt jets. It is this circumstance that explains the low durability and reliability of self-made graphite crucibles (see below).

What to do

Melting crucibles are made (see figure below):

ceramic chemically neutral;
ceramic graphitized;
graphite;
cast iron;
steel.

Their comparative characteristics are as follows:

Ceramic neutral - used for the remelting of scrap jewelry with the preservation of the sample, because. with indirect heating (see below), the properties of the metal do not change. You can do it yourself, but it’s difficult (see below) and is it worth it? A 50 g crucible for gold costs up to 100 rubles in a jewelry store. Suitable for melting in an induction furnace (see below) without any problems. almost do not absorb the energy of the electromagnetic field (EMF). Resource - 10-30 swimming trunks.
Ceramic graphite - suitable for melting any metal; at home up to 1.5-2 kg at a time. For use in an induction furnace, its power for the same amount of metal will have to be increased by 1.5-2 times due to the absorption of electromagnetic fields by conductive graphite. You can do it yourself, see below. Resource - up to 50 or more melts.
Graphite - suitable for remelting old, oxidized scrap of non-ferrous and precious metals, because. create a strong restorative environment. The melting of silver with an open gas flame in a graphite crucible makes it possible to almost completely restore the initial weight of the oxidized metal. Do not do it yourself, see below. Resource - more than 100 swimming trunks.
Cast iron - are used mainly for remelting red copper into oxygen-free, because. actively absorb oxygen. The resource is up to 30 heats, and then amorphous carbon leaves the cast iron and the crucible degrades.
Steel - a home-made cheap option for melting small amounts of aluminum and magnesium alloys and other metals that are chemically inert in the melt. May be used to melt small amounts of lead into fishing sinkers, etc.

Note: graphite, cast iron and steel crucibles for use in induction furnaces (see below) are completely unsuitable, because. completely absorb EMF energy.

About Graphite Crucibles

Graphite crucibles are either machined from massive natural graphite (expensive), or sintered at high temperature from graphite powder (cheaper, but still not very cheap). Amateurs often try to make "graphite" crucibles from ground graphite on a binder of kaolin, etc., but these are not graphite, but excessively graphitized ceramic crucibles - fragile, withstanding no more than 10 melts and spoiling the metal due to excessive release of atomic carbon by finely dispersed graphite . A more or less rational way to use ground graphite in amateur crucible melting is to make it into a desktop mini crucible furnace for ceramic neutral crucibles, see fig.

Cold welding for assembling this furnace should be used at a temperature not lower than 800 degrees - the cheeks that conduct electricity well during one melt do not heat up above 400. Graphite powder will not heat up much more without a crucible, but when the crucible is pressed into it, it will turn out to be hot spot over 1000 degrees due to compaction of the powder under the crucible.

If gold is melted, then after the end of melting and cooling of the furnace, graphite powder is poured out and shaken up, because. he bakes. To melt silver and cupronickel, the powder is removed and shaken after 3-5 melts, so the furnace heats up faster. In any case, to keep the reducing environment, the furnace is covered with a mica cover during melting.

Heating methods

If it is required to melt more than 150-200 g of metal at a time, then a crucible furnace will also need to be built to the crucible, otherwise it will be very difficult to achieve melt homogeneity and high casting quality. The exception is low-melting and easily recoverable lead: up to 20-30 kg can be melted at a time at home. A relative exception is zinc for hot-dip galvanizing, its melt in a crucible without a furnace can be up to 2-2.5 kg, but borax must be poured over it so that the melt mirror is completely covered with its fluidized layer. Steel fasteners are thrown into the melt through a layer of borax.

The optimal way in all respects to heat the crucible in the furnace is with gas, pos. 1 in Fig., but the gas crucible furnace is a rather complex structure, although it can be made independently. The most suitable crucible for a gas furnace is ceramic graphitized, because. its material has a fairly high thermal conductivity. For especially high requirements for metal purity, it is better to use a ceramic neutral crucible. When lowered for low-melting metals - cast iron, as it conducts heat better and thereby saves fuel. Graphite crucibles are placed in a gas furnace only if a strong reduction of the old oxidized metal is required, and the risk of carburization is insignificant, for example, when remelting silver extracted from the earth for refining

For fusible metals, the most economical is often an electric crucible furnace, pos. 2; it may be the so-called. ohmic (with heating by a nichrome spiral) or induction, with heating from an electromagnetic oscillation generator, see below. In the induction furnace, only ceramic neutral or, to a limited extent, graphite crucibles are applicable.

If the crucible is more than 2-2.5 kg of metal, then the crucible furnace, according to safety rules, must be made tipping (pos. 3), because and 1 kg of melt spilled on the floor is already a big problem. Metal in small jewelry crucibles, on the contrary, is preferably heated without a furnace, directly with a burner flame, pos. 4. In this case, the crucible is held all the time during melting with a special spring grip, pos. 5 and 6.

Note: silver and its alloys, as well as lead on sinkers, can be melted at home in an amount of up to 15-20 g, using instead of a crucible ... a stainless steel spoon, see fig. on right. For safety, then it is necessary to make gaskets with longitudinal cuts under the spoon handle to the vise jaws. The flame is exclusively gas; gasoline can burn a spoon.

Electric heating

Resistive crucible furnaces are mainly used for smelting lead or tin. For more refractory metals, they turn out to be uneconomical, but up to 20 kg of lead can be melted at a time in a domestic crucible electric furnace; how to make an electric crucible for melting lead, see for example. video:

Video: electric crucible for melting lead

Melting of aluminum in a crucible turns out to be more profitable than induction due to its high electrical conductivity, but this trick no longer works with copper - its temperature and latent heat of melting are much higher. In the induction method of melting, the metal is heated by Foucault eddy currents, for which the crucible with it is placed in the EMF of a coil of thick copper wire, fed by alternating current from an electromagnetic oscillation generator. How to make a do-it-yourself generator for inductively heating small amounts of metal, for example, for trinkets, is described in other materials, or, for example, see the following. video guide.

Video: do-it-yourself induction heating

With an increase in the amount of remelted metal, not only the required power of the generator increases, but also its optimal frequency decreases, this affects the so-called. surface effect (skin effect) in metal. If 100-200 g of aluminum can be melted into an EMF from any home-made generator for, then the installation of 1.5-2 kg of duralumin or magnesium alloy is already a solid structure, see fig. on right. If you intend to work with aluminum, then think carefully - is it worth it to fence something like that? Wouldn't it be easier to get a mini gas furnace for melting small amounts of aluminum alloys, see for example. video clip

Video: mini aluminum melting furnace

Making crucibles

Now it's time to make your own melting crucible. From the foregoing, it is clear that it makes sense to make crucibles with your own hands:

Steel;
Ceramic neutral;
Ceramic graphite.

There is nothing special to say about steel crucibles - it's just a steel vessel with a welded handle. Steel crucibles are used for remelting low-melting metals; sometimes - zinc for hot galvanizing with quality up to 3+. Steel crucibles for lead, tin and zinc are only suitable for melting one specific metal, because after 1-2 melts they themselves are covered with it from the inside.

Ceramic neutral

The composition of the mixture for the formation of a ceramic neutral crucible is 7 parts of fireclay clay, 1 part of finely ground fireclay (up to a fraction<1,5 мм) и 10 ст. ложек жидкого стекла (силикатного канцелярского клея) на 1 л сухой смеси. Молотый шамот в небольших количествах можно получить из кусков шамотного кирпича, растолченных в фаянсовой ступке (продаються в магазинах хозяйственных, медицинского оборудования и некоторых аптеках). Не жалко денег на крутизну – можно в сувенирном купить агатовую, они более стойкие. Если же вы собираетесь лить металл регулярно и довольно много, или делать тигли на продажу, то, возможно, лучше будет сделать для размола шамота цепную или шариковую мельницу.

Chamotte mill

Finely ground chamotte is a raw material for forming both neutral and graphitized crucibles, and the quality and durability of the crucible largely depend on it, and crushing chamotte by handicraft methods is very laborious and does not give a completely benign material. The device of a chain mill for mineral raw materials is shown in fig. on right. Material - steel. Chains - 4; they are suspended crosswise so that they sag horizontally by approx. 1/3 of the tank diameter. Option instead of chains for 1 broken fireclay brick - 2-3 handfuls of balls from the bearing. New purchased ones will cost more than chains, but old ones from broken bearings are quite suitable. Drive any: manual, electric. Both chain and ball mills are capable of grinding chamotte into dust like cement; to obtain certain fractions, the mill is stopped earlier. The mouth of the tank, so as not to get dusty, is covered with something for the time of grinding. It is enough to throw a brick for grinding from a height onto a hard floor and load the resulting pieces into a mill.

Preparation of the molding material

We mix dry clay with ground fireclay until complete homogeneity (homogeneity). The ideal option is to scroll 15-20 times in the same mill; if it is spherical, then the balls can not be thrown into the tank. We unload the mixed mass and add a little water (1.5-2.5 parts), mixing by hand, until the consistency: clenched in a fist, sticks together into a lump, but does not stick to the skin and does not squeeze between the fingers. We add liquid glass, also stirring until completely homogeneous, this is the most time-consuming step.

Deairing

Only one air bubble remaining in the mass for a ceramic crucible can cause the crucible to burst from heating. Therefore, air must be knocked out of the mass. To do this, a clean film is laid on a hard floor; newspaper, as advised in some manuals, is not necessary - the mass will be filled with paper fibers.

To knock out the air, the whole lump of mass is thrown to the floor with force many times. Practically - after the bubbles stopped popping out of the flopping mass, at least 10 more times.

Storage

For storage, the beaten mass is placed in a glass dish with a hermetically sealed lid. In plastic, and even more so wrapped with several layers of film, the mass dries out in a few weeks and cannot be restored, and in glass in a cool place it is stored for more than six months.

Usage

Crucibles from the resulting mass are simply molded by hand or molded in a destructible plaster mold or in a collapsible form, as described below. The molded crucible is dried, and, which is absolutely necessary for a given mass, after drying it is annealed in a muffle furnace for an hour or two at a temperature of 800 degrees. It is at this temperature that liquid glass will melt and firmly bind the remaining components. Below - the crucible will collapse during the first melting; higher - during annealing. This is a very significant drawback of this technology, because. muffle furnace equipment is not cheap and not simple, though. The maximum operating temperature of the obtained crucibles is up to 1600 degrees; resource, with high-quality grinding of fireclay - up to 30 heats.

graphite

Manufacturing technology of graphite crucibles for melting any metals, incl. black scrap, with any heating method, is well described in an article by the author A. Ramir from 2006 (see dendrite-steel.narod.ru/stat-ramir-3.htm). A. Ramir, apparently, is self-taught, but all the more honor to him - his products are quite consistent with good industrial designs. However, firstly, his article was rewritten many times by rewriters who obviously did not cast metal in their lives. Secondly, you can’t always get to it in the search, and for some reason the drawings are not downloaded, although they seem to be in free distribution. Thirdly, there is something to add to the materials of A. Ramir, no offense to him. One of the rules of technology says: in a good design there is always something to improve. Therefore, we repeat and supplement the main points of this publication.

Drawings of crucibles from the mentioned article are given in Fig.:

In kg, the maximum weight of the remelted steel is indicated; for another metal it needs to be recalculated. The main difficulty in this case is the manufacture of the flask - the round shell of the mold. Its inner surface is conical, otherwise the finished crucible cannot be removed after molding, so A. Ramir used turned flasks.

Meanwhile, a flask for any of these forms can be made from a piece of plastic pipe. It is intercepted with screw clamps in 3 places, at the bottom, in the middle and at the top, and heated from the inside with a hairdryer. By tightening the clamps, the surface is not completely conical, but the flask will be removed from the crucible. You only need to use worm clamps (see the figure on the right) or their homemade counterparts. Any other clamp deforms the pipe across. The flask from it will most likely come off the crucible, but it will not last long or will crack at the first melting.

The composition of the mixture used by the author is 7 volume parts of ground chamotte, 3 parts of pottery or oven clay and 1 part of ground graphite. A. Ramir also gives a recipe with 2 parts of graphite, but in terms of reducing ability this is a clear overkill, and the probability of crucible cracking from a mixture of 7:3:1 will be reduced to zero if fireclay is crushed into dust in a mortar or ground in a mill (see above) .

Soaking fireclay bricks, as advised by A. Ramir, is necessary only before crushing in the artisanal way described by him. Dry components are mixed until completely homogeneous in the specified sequence (fireclay, clay, graphite) and mixed with water with continuous stirring until the consistency is as described above. There is no need to knock air out of this mass, because. it is de-aired during the molding process. The mixture is not stored, so it must be prepared immediately before making the crucible.

To form the inner surface of the crucible, it is necessary to carve a block out of hard wood (filled with gray in pos. 1-5 of the figure), sand it and, it is highly desirable, walk over it with leather until the surface is completely smooth. In the center of the block surface that forms the bottom of the crucible, a blind hole is drilled and a toothpick or, better, a round, smooth plastic stick from an ear picker is inserted into it. The match used by A. Ramir is not the best option - it often breaks when pulled out, and the product goes into marriage from this.

Note: the use of any lubricants during the formation of the crucible is unacceptable - they will be absorbed into its material, and the crucible will burst from heating.

The form is filled with the mixture in layers of 15 mm, and each layer is rammed with a wooden tamper. This is the most critical stage: bubbles and uneven compaction of the mixture are unacceptable. When approx. 12 mm, the mixture is compacted with an already turned lid with a hole for the rod in the center, pos. 2. The mixture is added in layers of 1-2 mm until the gap between the very tightly pressed lid and the upper edge of the flask reaches 1-1.5 mm, pos. 3. If the gap is larger, part of the mixture can be taken away. Next, the lid is removed and the rod is carefully pulled out of the blockhead with pliers, the lid is put back and the mold is turned over. A handle is attached to the bottom of the blockhead with self-tapping screws and, carefully turning it back and forth, they pull it out of the casting.

Note: if the rod is not inserted into the bottom of the block, it will not be possible to remove it without destroying the castings - the vacuum under the block will not work.

The formation of a crucible with a flat bottom (which is 1.2 kg) has some peculiarities - you can’t just pull out its blockhead. Therefore, when the rammed mass rises to the flat top of the blockhead, a circle of toilet or filter paper is placed on it.

Now the hole from the rod and small defects in the inner surface of the crucible are closed with the same mass. It must be perfectly smooth, otherwise the probability of destruction of the crucible during melting is quite high, so after correcting the defects, it must be smoothed out. The best way to do this is to line it with toilet paper (pos. 4), insert a block (pos. 5), and turn it several times.

It remains to remove the flask. To do this, it, together with the crucible, is turned over again to the working (for the crucible) position, a round wooden chock is substituted and the flask is carefully pulled together, pos. 5 and 6. If the flask is plastic, then its protruding upper edge is slightly bent outward in several places with the fingers; most likely, the flask after that will come off like clockwork.

And, finally, the finished casting is dried. Equipment - cooker with oven. The casting is placed upside down on a baking sheet and placed in the oven. They heat for half an hour on the lowest gas, then another half an hour on medium (the temperature according to the built-in thermometer is about 150 degrees) and another 2 hours on full. After that, the fire is turned off and the casting is left in the oven to cool until tomorrow morning. Do not open the oven during the drying time!

The crucible must be checked for hidden cracks before use. To do this, holding it with your fingertips at the bottom, tap your nails in a circle from top to bottom. Each knock should be answered by ringing. If somewhere it doesn’t ring - marriage, you can’t melt with this. Annealing for a crucible made using this technology is not required. It rings everywhere - you can immediately melt in it.

What for?

A reader who is interested in home metallurgy “for general development” may have a question: why all this trouble? Not everyone wanders around with a metal detector in the forest after the rain, not everyone is keen on smelting damask steel at home, and not everyone has in mind centners of old electronics, from which tens of grams of gold, platinum, and palladium can be extracted.

For melting metal on a small scale, some kind of device is sometimes necessary. This is especially acute in the workshop or in small production. The most effective at the moment is a furnace for melting metal with an electric heater, namely induction. Due to the peculiarity of its structure, it can be effectively used in blacksmithing and become an indispensable tool in the forge.

Induction furnace device

The oven consists of 3 elements:

1. Electronic-electrical part.
2. Inductor and crucible.
3. inductor cooling system.

In order to assemble an operating furnace for melting metal, it is enough to assemble a working electrical circuit and an inductor cooling system. The easiest option for melting metal is shown in the video below. Melting is carried out in the counter electromagnetic field of the inductor, which interacts with the induced electro-eddy currents in the metal, which keeps a piece of aluminum in the space of the inductor.

In order to effectively melt the metal, currents of large magnitude and high frequency of the order of 400-600 Hz are required. The voltage from an ordinary 220V household outlet has sufficient data to melt metals. It is only necessary to turn 50 Hz into 400-600 Hz.
Any scheme for creating a Tesla coil is suitable for this. I liked the following 2 schemes on the lamp GU 80, GU 81 (M). And powering the lamp with an ILO transformer from a microwave.

These circuits are designed for a Tesla coil, but an induction furnace is excellent from them, it is enough instead of the secondary coil L2 to place a piece of iron in the interior of the primary winding L1.

The primary coil L1 or inductor consists of a copper tube rolled into 5-6 turns, at the ends of which a thread is cut to connect the cooling system. For levitational melting, the last turn should be done in the opposite direction.
Capacitor C2 on the first circuit and identical to it on the second sets the frequency of the generator. At a value of 1000 pF, the frequency is about 400 kHz. This capacitor must be high-frequency ceramic and designed for high voltage of the order of 10 kV (KVI-2, KVI-3, K15U-1), other types are not suitable! Better to put K15U. You can connect capacitors in parallel. It is also worth considering the power for which the capacitors are designed (this is written on the case), take it with a margin. the other two capacitors KVI-3 and KVI-2 heat up during prolonged operation. All other capacitors are also taken from the KVI-2, KVI-3, K15U-1 series, only the capacitance changes in the characteristics of the capacitors.
Here's a schematic of what it should look like. Framed 3 blocks.

The cooling system is made of a pump with a flow of 60 l / min, a radiator from any VAZ car, and I put a regular home cooling fan in front of the radiator.

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