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Vacuum induction heating furnace

Vacuum induction heating furnace is a kind of eddy current heating furnace material produced in metal conductor by electromagnetic induction under vacuum condition. It has the characteristics of small volume of smelting chamber, short vacuum time and smelting cycle, easy temperature and pressure control, recoverable volatile elements and accurate control of alloy composition. Because of the above characteristics, it has been developed as an important equipment for the production of special steels, precision alloys, electrothermal alloys, superalloys and corrosion resistant alloys.
 1, what is vacuum?
 In an airtight container, because the number of gas molecules decreases, the pressure of gas molecules acting on a unit area decreases. At this time, the pressure inside the container is lower than normal pressure, which is called vacuum.
 2. The working principle of vacuum induction heating furnace?
 It mainly uses electromagnetic induction to produce electric current in the metal charge itself, and then relies on the resistance of the metal charge itself. According to Joule-Lenz law, electric energy is converted into thermal energy for smelting metal.
 3. How does the electromagnetic stirring of vacuum induction heating furnace come into being?
 The molten metal in the crucible produces electromotive force in the magnetic field produced by the induction coil. Due to skin effect, the eddy current produced by the molten metal is opposite to the current passing through the induction coil and repels each other. Because the inductor coil is short coil, both ends have short effect, so the corresponding electrodynamic force at both ends of the inductor coil becomes smaller, the distribution of electrodynamic force is small at both ends of the inductor coil, large in the middle. Under the action of this force, the liquid metal moves from the center to the axis of the crucible, and then upwards and downwards respectively. This phenomenon keeps circulating and forms the intense movement of the liquid metal. Actual smelting can be removed to see the liquid metal in the center part of the crucible up, up and down the phenomenon of tossing, which is electromagnetic stirring.
 
 4. What is the function of electromagnetic stirring?
 (1) It can accelerate the physical and chemical reaction speed in the smelting process; (2) make the composition of molten metal uniform; (3) the temperature of molten metal in the crucible tends to be uniform, leading to complete the reaction in the smelting process; (4) the stirring results overcome the effect of its own static pressure, and the melting bubble in the crucible is turned over to the liquid surface, facilitating gas discharge and reducing. The gas inclusion content of the alloy. _Intense stirring enhances the mechanical erosion of liquid metal on the crucible, affecting the life of the crucible; _Accelerates the decomposition of refractories in the crucible at high temperature, constituting a re-contamination of the molten alloy.
 5. What is vacuum?
 The degree of vacuum indicates the rarefaction of a gas below atmospheric pressure, usually expressed by pressure.
 6. What is the leakage rate?
 The leakage rate refers to the increase of pressure in the unit time after the vacuum equipment is closed.
 7, what is skin effect?
 Skin effect refers to the phenomenon that when alternating current passes through a conductor (smelting medium refers to furnace charge), the current distribution on the conductor section is not uniform. The higher the current density on the surface of the conductor, the smaller the current density toward the center.
 8, what is electromagnetic induction?
 Alternating current passes through a wire and produces alternating magnetic field around it. When a closed wire is placed in a changing magnetic field, alternating current is generated in the wire. This phenomenon is called electromagnetic induction.
 9, the calculation formula of leakage degree of vacuum induction furnace?
 P2-P1
 E = - - (Pa/min)
 Delta T
 10. What are the advantages of vacuum induction heating furnace?
 (1) no air and slag pollution, smelting alloy pure, high performance level;
 2. Smelting under vacuum creates good degassing conditions, and the steel and alloy produced by smelting are low in gas content.
 Under vacuum condition, metals are not easy to oxidize.
 (4) impurities (Pb, Bi, etc.) carried by raw materials can evaporate part of the material in vacuum, so that the material can be purified.
 (5) when vacuum induction furnace is smelted, carbon can deoxidize, deoxidize products are gas, and the purity of alloy is high.
 It can adjust and control the chemical composition accurately.
 You can use the return material.
 11. What are the disadvantages of true vacuum induction heating furnace?
 (1) the equipment is complex, the price is high, and the investment is large.
 Second, the maintenance is not convenient, the smelting cost is high and the cost is relatively high.
 During the smelting process, crucible refractory materials pollute metal.
 The production volume is small and the workload of inspection is large.

12. What are the basic parameters and meanings of vacuum pumps?
 (1) Limit vacuum degree: when the inlet of the vacuum pump is sealed, the lowest pressure stability value (that is, the highest stable vacuum degree) can be obtained after a long time of pumping, which is called the limit vacuum degree of the pump.
 2. Pumping rate: the volume of gas pumped by the pump in unit time is called the pumping rate of the vacuum pump.
 The maximum outlet pressure: the maximum pressure value of the gas discharged from the exhaust port when the vacuum pump works normally.
 Pre-pressure: In order to ensure the safe operation of the vacuum pump, the maximum pressure needed to maintain at the pump outlet.
 13, how to choose a reasonable vacuum pump system?
 (1) the pumping rate of the vacuum pump corresponds to the inlet pressure of a certain vacuum pump.
 (2) Mechanical pumps, roots pumps, oil booster pumps can not directly exhaust into the atmosphere, must rely on the establishment of the pump and maintain the pre-set pressure in order to work properly.
 14. Why should capacitors be added to the electrical circuit?
 Because the distance between induction coil and metal burden is very large, magnetic leakage is very serious, useful magnetic flux is very few, reactive power is large, so in capacitive circuit, current ahead of voltage, in order to offset the influence of inductance, improve the power factor, it is necessary to incorporate a proper number of capacitors in the circuit, so that capacitor and inductance in parallel harmonic. Vibration, thereby improving the induction coil power factor.
 15. How many parts are there in the main equipment of vacuum induction heating furnace?
 Smelting room, casting room, vacuum system, power supply system.
 16. Maintenance of vacuum system during smelting process?
 1. Vacuum pump oil and oil level are normal.
 (2) the filter screen is replaced normally.
 The isolation valves are sealed normally.
 17, maintenance of power system during smelting process?
 (1) the cooling water temperature of the capacitor is normal.
 2. Transformer oil temperature is normal.
 3. The cooling water temperature of the cable is normal.
 18. What are the requirements for crucible in vacuum induction furnace?
 1. It has high thermal stability to avoid quenching and hot cracking.
 (2) high chemical stability to prevent refractory materials from polluting the crucible.
 (3) high enough refractoriness and high temperature structural strength to withstand high temperature and charge impact.
 The crucible should have high density and smooth working surface, so as to reduce the surface area of the contact between the crucible and liquid metal, and reduce the adhesion of metal residue on the surface of the crucible.
 High insulation;
 The volume shrinkage is small during sintering.
 It has low volatility and good hydration resistance.
 The crucible material has a smaller discharge volume.
 Crucible materials are rich in resources and low in price.
 19, how to improve the high temperature performance of crucible?
 (1) Reducing the content of CaO and CaO in MgO sand and the ratio of CaO to SiO2 can reduce the amount of liquid phase and increase the temperature of producing liquid phase.
 2. Improve crystal grain stability.
 (3) To make the sintered layer reach a good recrystallization state, so as to reduce the porosity, reduce the width of grain boundary, and form a mosaic structure, forming a direct combination of solid phase and solid phase, thereby reducing the harmful effect of liquid phase.
 20. How to choose the proper size of the crucible?
 The thickness of crucible is generally 1/8 to 1/10 of the diameter of the crucible (Molding).
 2. Molten steel accounts for 75% of the volume of the crucible.
 (3) the angle of R is about 45 degrees.
 4. The thickness of the furnace bottom is generally 1.5 times that of the furnace wall.
 21. What are the commonly used adhesives for knotting crucible?
 (1) organic matter: dextrin, pulp waste liquid, organic resin, etc.
 Inorganic substances: sodium silicate, brine, boric acid, carbonate, clay, etc.
 22. What is the function of the binder (H3BO3) for knotting crucible?
 Boric acid (H3BO3) in general, by heating below 300 degrees Celsius, you can remove all the moisture, known as boric anhydride (B2O3).
 (1) At low temperatures, some MgO and Al2O3 can be dissolved into liquid B2O3 to form a series of transition products, which accelerates the solid phase diffusion of MgO. Al2O3 and promotes the recrystallization, so that the sintering layer of the crucible begins to form at lower temperatures, thus reducing the sintering temperature.
 (2) Thicken the semi-sintered layer or increase the strength of the crucible before secondary sintering by the melting and bonding of boric acid at medium temperature.
 3. In the magnesia containing CaO, the crystal change of 2CaO? SiO2 was inhibited by binding agent below 850 C.
 23, crucible has several molding methods, what are they?
 2 ways.
 (1) Prefabrication outside the furnace; after mixing the billet (fused magnesia or alumina-magnesia spinel refractory) with a certain proportion of particle size and choosing a suitable binder, the billet of the crucible is formed in the crucible mold by shaking, isostatic pressing process, and the crucible body is baked and processed into a prefabricated crucible at a maximum firing temperature of (>1700 *8h) in a high temperature tunnel kiln.
(2) ramming directly in furnace; adding a certain amount of solid binder, such as boric acid, in the appropriate particle size ratio, after mixing evenly, ramming is adopted to achieve compact filling, and different microstructure is formed depending on the temperature of each part during sintering.

24. How can the sintering structure of the crucible be divided into several layers and how will it affect the quality of the crucible?
 The crucible sintering structure is divided into three layers, namely, sinter layer, semi sinter layer and loose layer.
 Sintering Layer: The granularity recrystallizes in the oven. Except for the middle granularity of sand at the low temperature, the original proportion can not be seen at all, but the structure is uniform and fine, the grain boundary is very narrow, and the impurities are redistributed on the new grain boundary. The sintering layer is a hard shell located at the bottom of the crucible wall. It contacts the smelted metal directly and bears various forces. Therefore, this layer is very important to the crucible.
 Loose layer: when sintered near the insulation part of the temperature is low, magnesia can neither be sintered, nor can be glass phase bonding, in a completely loose state. This layer is located at the outermost part of the crucible, and plays the following roles: first, because of its loose structure and poor heat conductivity, the heat from the inside of the crucible to the outside is less, reducing heat loss, playing a role of insulation, improving the thermal efficiency of the crucible; second, the loose layer is a protective layer. Because the sintered layer has become a shell, contact with the liquid metal directly, easy to produce cracks, once it produces cracks, molten liquid metal from the cracks to seep out, and loose layer due to loose structure, not easy to produce cracks, from the inner leakage of liquid metal is blocked by him, to protect the induction coil; The layer is also a buffer. Because the sintered layer has become a hard shell, the volume expansion and shrinkage of the crucible will occur when the sintered layer is heated and cooled. The loose structure of the loose layer will buffer the volume change of the crucible.
 Semi-sintered layer (also known as transition layer): located between the sintered layer and loose layer, divided into two parts, near the sintered layer part, impurities melt and redistribute or bond magnesia, magnesia occurs partial recrystallization, large sand particles appear particularly dense; near the loose layer part, near the loose layer part is completely bonded by adhesive. The semi sintered layer has the function of sinter layer and loose layer.
 25, how to choose the oven process system?
 (1) Maximum oven temperature: when the thickness of insulating layer of knotted crucible is 5-10 mm, the sintering layer only accounts for 13-15% of the thickness of crucible when the electric fused magnesia is baked at 1800%. The temperature of the oven is 24-27% at 2000 C. Considering the high temperature strength of the crucible, it is better to raise the temperature of the oven, but it is not easy to be too high. The honeycomb structure is formed when the temperature is higher than 2000 C because of the sublimation of magnesia or the reduction of magnesia by carbon and the intense recrystallization of magnesia. Therefore, the highest oven temperature should be controlled below 2000 C.
 (2) Heating rate: In the early stage of heating, in order to effectively remove moisture in refractories, should be fully preheated, generally in the temperature below 1500 degrees Celsius to slow; furnace temperature above 1500 degrees Celsius when the sintering of fused magnesia began, at this time should be used high-power, in order to quickly warm up to the highest expected oven temperature.
 (3) Holding time: When the furnace temperature reaches the highest oven temperature, it is necessary to keep temperature at this temperature. Holding time varies according to the type and material of the oven, such as 15-20 minutes for small crucible, 30-40 minutes for large crucible and medium crucible.
 Therefore, the heating rate of the oven and the baking time at the highest baking temperature are summed up as follows: low temperature slow baking, high temperature fast sintering, with a certain holding time as a basic experience to master.
 26. Why do we wash the stove?
After the crucible is baked, there is a large amount of liquid phase on the grain boundary of the sintering layer, and a part of the liquid steel enters the melting process; there are many pores on the surface of the crucible, and there are many gases and impurities; the cooling rate is very fast after sintering, and a considerable amount of impurities with high melting point solidified in the periclase grain or glass phase can not precipitate before the sintering layer reaches its best state. 4. The temperature at the mouth of the furnace and tapping is low.

27. What are the reactions between molten metal and crucible refractories under vacuum?
 (1) under vacuum condition, the oxides in refractories will decompose.
 MgO (solid) =Mg (gas) +1/2O2 (gas)
 AL2O3 (solid) =2AlO (gas) +1/2O2 (gas)
 SiO2 (solid) =SiO (gas) +1/2O2 (gas)
 2. The following reactions will occur between molten metal and refractories.
 Fe (liquid) +MgO (solid) =Mg (gas) +[FeO]
 Fe (liquid) +SiO2 (solid) =SiO (gas) +[FeO]
 (3) With the decrease of pressure, the reduction ability of C in molten metal increases. The refractories of crucible are eroded by the reduction of C. The following reactions occur:
 MgO (solid) +[C]=Mg (gas) +CO (gas)
 AL2O3 (solid) +3[C]=2 [Al] +3CO (gas)
 SiO2 (solid) +2[C]=[Si]+2CO (gas)
 Besides, other elements in molten metal also have reductive action.
 MgO (solid) +[Si]=Mg (gas) +SiO (gas)
 SiO2 (solid) +[Ti]=[Si]+TiO2 (gas)
 28, what should we pay attention to crucible dry knot?
 First, check the insulation of the induction coil and whether the water cooling system is normal.
 2. Knotting tools are clean, no rust and no dirt.
 (3) crucible iron sheet (Shi Moxin) is perpendicular to the bottom, and several layers of yellow board are wrapped outside, which are easy to pull out.
 4. The height of the bottom of the furnace must be changed to the bottom of the circle.
 5. Attention should be paid to the quality of knot at the transition point between the bottom of crucible and the furnace wall to prevent cracks.
 _Before each layer is knotted, the first layer must be loosened by steel brazing for 3-5mm, and then knotted with materials to prevent delamination and reduce particle size segregation.
 29. What are the requirements of the particle size ratio of the fused Magnesium Oxide + Al Mg spinel?
 The fused magnesia is 50-60% of the total amount, which is divided into coarse, medium and fine particles, and the alumina-magnesia spinel is 40-50% of the total weight, which is divided into coarse and fine powder.
 30. What is the task of melting period?
 Chemicals, degassing, removing harmful metal impurities and non-metallic inclusions at low melting point, adjusting the temperature of the alloy liquid and putting it under sufficient vacuum, create conditions for refining.
 31. What are the characteristics of the melting period?
 (1) The burden melts gradually, the molten steel forms gradually, the relative surface area of molten steel is very large, and the molten pool is relatively shallow, which is very conducive to degassing and removal of non-metallic inclusions. The removal of inclusions accounts for more than 70% of the total inclusions.
 (2) Most of the gas can be removed during the melting period, 70-80% of hydrogen, 60-70% of nitrogen and 30-40% of oxygen can be removed.
 3. During the heating and melting process, a large amount of gas is released to reduce the vacuum degree.
 (4) During the melting process, the temperature of the metal around the crucible wall is the highest (especially in the middle and lower part), which is the first to melt. Because of the combined effect of eddy current heat, radiation heat and conduction heat, the whole burden gradually sinks automatically and the temperature of molten steel is stable.
 32, what preparations should be made before melting?
 The equipment must be inspected before melting, including furnace body, vacuum, electrical appliances, machinery, water cooling, temperature measurement, pouring, etc.
 (2) the furnace must have the appropriate vacuum and leakage rate during smelting.
 Raw materials meet the requirements of technical regulations, and the ingredients are accurate.
 Crucible has good knotting and sintering quality.
 (5) develop appropriate processes.
 33. What is the principle of vacuum induction heating furnace charging?
 Tighten up and loose, reasonable cloth, prevent bridging, refractory metal is installed in high temperature area.
 34, what is the task of refining period?
Tempering, alloying (adjusting ingredients), degassing, removing impurities and even ingredients.

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