- Body parts
- Heating elements
- Furnace lining and arrangement
- Heat insulation of the furnace
- Automation with temperature-time tasks
Having a muffle furnace in a home workshop, it is already possible to seriously master the heat treatment of metals, which is indispensable in the manufacture of knives, parts of mechanisms and tool technology. We offer a simple and inexpensive design for an automatic hardening furnace.
An electric hardening furnace can be easily built on the basis of a thin-walled container with a volume of 15 liters or more. For the most compact options, an ordinary galvanized bucket is suitable; for larger ones, you can use a washing machine tank, technical containers, or, for example, roll a sheet of roofing iron into a cylinder and fold the bottom with a single fold. The casing does not have to be fireproof, it is enough for it to withstand temperatures of 80–100 ° С.
The housing should be installed so that the clearance from the surface is about 100 mm, for which you need to attach the simplest legs. As these, threaded rods bent with a bracket, the edges of which are inserted into the holes in the body and tightened with nuts on both sides, will fit. The height of the legs must be immediately adjusted and the nuts must be securely locked.
An electrical connection hole must be made in the back of the case. It is best to cut a window with dimensions of 50×70 mm, and then install a fiberglass panel with dimensions of about 100×120 mm in this place. The connection of the heating elements is carried out on pins, while two or more pairs can be brought out to organize several stages or work from a three-phase network.
The first stage in the manufacture of the hardening furnace itself will be the calculation and search for heating elements with their subsequent assembly into a single fuel loop. This can be done in two ways: by selecting ready-made heating coils or by making them yourself..
Choosing spirals is not very difficult, you just need to ensure that they are made of the correct material and have a sufficient cross section. It is not recommended to use wire heating elements with wire thickness less than 0.4 mm for hardening furnaces. The optimal variant of the spiral material is fechral X27Yu5T or Kh23Yu5-N-VI. The most important rule when working with such alloys is not to heat them until the final formation and assembly of the fuel circuit..
The calculation of the heating elements must be carried out individually, taking into account the dimensions of the furnace and the corresponding heating power. As an example, you can take a furnace with a heating chamber dimensions of 150x100x300 mm. To heat such a space to a temperature of 1,100 ° C, a total power of the heating elements of about 4 kW will be required, however, heating will be most economical when the total power of the coils is 5.5–6 kW. When connected to a 220 V network, the current will be 28 A, and the total resistance of the heater is 7.86 Ohm.
Using this data, we can easily find the required wire length with a known electrical conductivity. The average resistivity of a fechral is 1.25 Ohmmm2/ m. If you use a wire with a diameter of 0.9 mm, its cross section will be 0.64 mm2, which means that the resistance of one meter will be equal to 0.8 ohms. Thus, it is required to create a heating element with a total wire length of 9.83 m. To twist the spiral, you need to use a mandrel rod, having previously calculated the length of one turn. If the coil has an outer diameter of 8 mm, the coil length will turn out to be slightly more than 25 mm, that is, the entire heating element will consist of 393 turns.
In a cross section, the perimeter of the chamber is 500 mm, with a normal packing density in a firebox with a depth of 300 mm, the spiral is placed in 5 rows with an offset of 40 mm from the front edge. Thus, the total length of the spiral is 2.5 meters, the coil must be evenly stretched to this length. If you calculate roughly, then after stretching the distance between the turns of the spiral will turn out to be a little more than 5 mm, which will ensure sufficient heating density. If the pitch turned out to be higher than 8 mm, the wire diameter would have to be reduced, with a turn pitch of less than 3 mm – to increase.
Furnace lining and arrangement
The very concept of a muffle furnace implies the presence of a muffle – an internal heat-resistant capsule that closes the heating circuit spiral, protecting it from small debris and scale. The muffle, as a rule, is removable in the furnaces, which allows repair and replacement of heating elements.
The main difficulty is to manufacture the muffle and the heating assembly body at the same time. This requires two types of heat-resistant ceramics: one for making a housing with grooves, the other for a thin-walled muffle. For a ceramic base, it is better to use a mixture of refractory clay with an aluminum oxide content of at least 30%. The clay should be diluted with excess water and left to swell for a day, then remove the settled water from above and leave only the swollen sediment.
The ceramic body of the heater is massive; therefore, it cannot be made of a pure binder; a filler is needed. As the latter, glass fiber, dry quartz sand or crushed chamotte are well suited. The total clay content in the solution should not be less than 50% by volume; as a result, the mixture acquires the consistency of a viscous plastic paste. If the composition turns out to be more liquid, the excess moisture is removed by adding small portions of stucco immediately before use..
The heater body is made on a volumetric template of corrugated cardboard, the dimensions of which should be 15–20 mm larger than the planned dimensions of the firebox on each side. Previously, a cord or silicone hose of the appropriate diameter must be wound on the template, forming the required number of grooves for the spiral. After that, the template with a cord must be glued over with clay from all sides, avoiding the formation of voids and achieving a wall thickness of at least 40 mm. The addition of alabaster helps the piece to maintain its shape prior to firing. Carefully remove the cardboard insert from the dried ceramic body and pull the cord out of the grooves.
For the lining of the furnace, ceramics made of higher quality kaolin are used. The enriched clay of the KFN-2 brand is optimal; it is better to use crushed chamotte of high purity with a content of about 20-25% by volume of dry components as a filler. The mixture is sealed as described above and used to form the inner lining..
In order to easily separate the muffle, spirals are inserted into the ceramic body in advance. Then the inner surface is pasted over with shreds of newsprint according to the papier-mâché principle. You should get at least 8-10 layers, while the inner surface should contain as little roughness as possible. After the paper has dried, a refractory lining compound is applied from the inside. It is better to do this in several passes, allowing time for excess moisture to evaporate; as a result, the wall of the muffle should reach a thickness of 15–20 mm. In this state, the entire assembly is dried for several days until the complete loss of plasticity and the appearance of a ringing sound when tapped.
After drying, primary firing is performed – voltage is applied to the spirals and the hot product is kept for 4–6 hours. During the firing process, fechral passes the crystallization threshold and, becoming more fragile, takes the form of channels. The clay in the ceramic insert and muffle is baked and glassy, gaining resistance to cyclic temperature changes. Well, the paper and glue residues simply burn out, while forming a minimum technological gap for easy removal and installation of the muffle.
With this manufacturing method, you can use some tricks. For example, to form a ceramic body on a tapered template to facilitate removal of the muffle. It would also not be superfluous to lengthen the front of the assembly where there are no heaters, or place a small spiral at the bottom of the chamber. Themselves as muffles for one furnace, you can immediately make several copies.
Heat insulation of the furnace
As a result of the described actions, an almost finished furnace of the hardening furnace is obtained, it only needs to be placed in the case, securely fastened and minimized heat loss. For this, a pre-made container with legs will come in handy..
The internal volume of the container must be filled with mineral wool with a density of 45-50 kg / m3. Cotton wool needs to be rolled up in a spiral, first laying it under the outer walls and gradually moving towards the center. The packing density should be as high as possible, but the cotton itself should not be damaged. As a result, the complete firebox must be placed in the central fold. If the density of the cotton wool is sufficient, the heating part will not crush the insulation with its weight. All spiral leads must be carefully wrapped with fiberglass, inserted between them spacers made of scraps of mineral wool, and then brought out through the back wall, connected to the reverse side of the studs and put the panel in place.
To securely fix the firebox and install the door, the cotton wool must be crushed and sunk 6–8 cm deeper than the sides. The surface of the insulation must be sprinkled with alabaster milk several times so that the cotton wool hardens and stops intensively absorbing moisture. After that, the front part of the furnace is filled with a mixture of alabaster, sand and mineral fiber. While the composition is not frozen, a furnace door or mortgages for fixing it are embedded in it.
Automation with temperature-time tasks
There are three types of hardening furnace automation. The cost of organizing the control of the oven rises along with the comfort of use. The simplest option is a simpler thermostat with a thermocouple as a temperature sensor. This device will simply maintain the set temperature with a hysteresis of about 30-50 ° C. The holding time is manually controlled, as is the moment when the temperature point is reached.
More advanced automation is being developed specifically for electric ovens. Thermocontrollers of the Autonics TCN4 type have a proportional power control function, providing an adjustable hysteresis down to 1 ° C. Also, the device is equipped with additional functions, such as an alarm about reaching a preset temperature. If desired, the alarm channel can be used to activate a time delay relay in series with the thermostat.
The most advanced in this regard are automation devices for furnaces of the “Profile-M” type. They differ not only in the built-in power relays, but also in the possibility of more flexible configuration. In such controllers, a timer is preset, and it is also possible to set up heat treatment with a complex temperature profile, sequentially setting the length of time intervals in which a certain temperature should be maintained.