What is vacuum induction melting?

Vacuum melting is a metal and alloy melting technique carried out in a vacuum environment.  

This technology can prevent rare metals from being contaminated by the atmosphere and refractory materials, and has the function of purification and purification. By vacuum melting, high-quality metals and alloys with low gas content, few inclusions, and small segregation can be obtained. This method is crucial for obtaining high-purity and high-quality metal materials, especially suitable for alloys or metals that are difficult to melt and require ultra-high purity. The methods of vacuum melting include electron beam melting, vacuum induction melting, vacuum arc furnace melting, and plasma furnace melting. For example, electron beam melting uses high-energy electron beams to bombard molten materials, rapidly converting them into thermal energy and melting them. This method is suitable for melting high difficulty and ultra-high purity alloys or metals.

In addition, vacuum melting also helps to improve the toughness, fatigue strength, corrosion resistance, high-temperature creep performance, and magnetic permeability of metal materials.

Vacuum induction furnace melting is a process of using electromagnetic induction to generate eddy currents in metal conductors under vacuum conditions to heat the furnace material. It has the characteristics of small melting chamber volume, short vacuum pumping time and melting cycle, convenient temperature and pressure control, recyclability of volatile elements, and accurate control of alloy composition. Due to the above characteristics, it has now developed into an important equipment for the production of special alloys such as special steel, precision alloys, electric heating alloys, high-temperature alloys, and corrosion-resistant alloys.

1. What is vacuum?

In  closed container, due to the decrease in the number of gas molecules, the pressure exerted by gas molecules on a unit area decreases. At this time, the pressure inside the container i lower than normal pressure. This type of gaseous space that is lower than normal pressure is called a vacuum.

2. What is the working principle of a vacuum induction furnace?

The main method is to apply electromagnetic induction to generate current in the metal charge itself, and then rely on the resistance of the metal charge itself to convert electrical energy into thermal energy according to the Joule Lenz law, which is used for melting metals.

3. How is electromagnetic stirring formed in a vacuum induction furnace?

The molten metal in the crucible generates electric force in the magnetic field generated by the induction coil. Due to the skin effect, the eddy currents generated by the molten metal are opposite to the direction of the current passing through the induction coil, resulting in mutual repulsion; The repulsive force on the molten metal always points towards the axis of the crucible, and the molten metal is also pushed towards the center of the crucible; Due to the fact that the induction coil is a short coil with short effects at both ends, the corresponding electric force at both ends of the induction coil decreases, and the distribution of electric force is smaller at the upper and lower ends and larger in the middle. Under this force, the metal liquid first moves from the middle towards the axis of the crucible, and then flows upwards and downwards towards the center. This phenomenon continues to circulate, forming a fierce movement of the metal liquid. During actual smelting, the phenomenon of metal liquid bulging upwards and flipping upwards and downwards in the center of the crucible can be eliminated, which is called electromagnetic stirring.

4. What is the function of electromagnetic stirring?

① It can accelerate the rate of physical and chemical reactions during the smelting process; ② Unify the composition of the molten metal liquid; ③ The temperature of the molten metal in the crucible tends to be consistent, resulting in complete completion of the reaction during melting; ④ The result of stirring overcomes the effect of its own static pressure, flipping the dissolved bubbles deep in the crucible onto the liquid surface, facilitating gas discharge and reducing the gas inclusion content of the alloy Intense stirring enhances the mechanical erosion of the molten metal on the crucible, affecting its lifespan; ⑥ Accelerate the decomposition of refractory materials in crucibles at high temperatures, resulting in re contamination of the molten alloy.

5. What is vacuum degree?

Vacuum degree represents the thinness of a gas below one atmospheric pressure, commonly expressed as pressure.

6. What is the leakage rate?

Leakage rate refers to the amount of pressure increase per unit time after the vacuum equipment is closed.

7. What is the skin effect?

The skin effect refers to the phenomenon of uneven current distribution on the cross-section of a conductor (referring to the furnace charge in smelting) when alternating current passes through it. The higher the surface current density of the conductor, the lower the current density towards the center.

8. What is electromagnetic induction?

Alternating current passes through a wire and generates an alternating magnetic field around it, while placing a closed wire in a changing magnetic field generates alternating current inside the wire. This phenomenon is called electromagnetic induction.

10. What are the advantages of vacuum induction furnace smelting?

① No air and slag pollution, the smelted alloy is pure and has a high level of performance;

② Vacuum smelting creates good degassing conditions, resulting in low gas content in the melted steel and alloy;

③ Under vacuum conditions, metals are not easily oxidized;

④ Impurities (Pb, Bi, etc.) brought in by raw materials can evaporate in a vacuum state, resulting in material purification;

⑤ During vacuum induction furnace smelting, carbon deoxidation can be used, and the deoxygenation product is gas, resulting in high alloy purity;

⑥ Can accurately adjust and control chemical composition;

⑦ Returned materials can be used.

11. What are the drawbacks of vacuum induction furnace smelting?

① The equipment is complex, expensive, and requires a large investment;

② Inconvenient maintenance, high smelting costs, and relatively high costs;

③ Metal contamination caused by refractory materials in crucibles during the smelting process;

④ The production batch is small, and the inspection workload is large.

12. What are the main basic parameters and meanings of vacuum pumps?

① Extreme vacuum degree: The minimum stable pressure value (i.e. the highest stable vacuum degree) that can be obtained after a long period of emptying when the inlet of a vacuum pump is sealed is called the maximum vacuum degree of the pump.

② Evacuation rate: The volume of gas extracted by a pump per unit time is called the pumping rate of a vacuum pump.

③ Maximum outlet pressure: The maximum pressure value at which gas is discharged from the exhaust port of a vacuum pump during normal operation.

④ Pre pressure: The maximum pressure value that needs to be maintained at the exhaust port of the vacuum pump to ensure safe operation.

13. How to choose a reasonable vacuum pump system?

① The pumping rate of a vacuum pump corresponds to a certain inlet pressure of the vacuum pump;

② Mechanical pumps, Roots pumps, and oil booster pumps cannot directly exhaust to the atmosphere and must rely on the front stage pump to establish and maintain the prescribed pre pressure in order to operate normally.

14. Why do capacitors need to be added to electrical circuits?

Due to the large distance between the induction coil and the metal furnace material, magnetic leakage is very serious, useful magnetic flux is very low, and reactive power is high. Therefore, in capacitive circuits, the current leads the voltage. To offset the influence of inductance and improve power factor, it is necessary to incorporate an appropriate number of electrical containers in the circuit, so that the capacitor and inductor can resonate in parallel, thereby improving the power factor of the induction coil.

15. How many parts are the main equipment of a vacuum induction furnace?

Melting chamber, pouring chamber, vacuum system, power supply system.

16. What are the maintenance measures for the vacuum system during the smelting process?

① The oil quality and oil level of the vacuum pump are normal;

② The filter screen is reversed normally;

③ The sealing of each isolation valve is normal.

17. What are the maintenance measures for the power supply system during the smelting process?

① The cooling water temperature of the capacitor is normal;

② The transformer oil temperature is normal;

③ The cooling water temperature of the cable is normal.

18. What are the requirements for crucibles in vacuum induction furnace melting?

① Has high thermal stability to avoid cracking caused by rapid cooling and heating;

② Has high chemical stability to prevent contamination of the crucible by refractory materials;

③ Having sufficient high fire resistance and high-temperature structural strength to withstand high temperatures and furnace material impacts;

④ The crucible should have a high density and a smooth working surface to reduce the surface area of contact between the crucible and the metal liquid, and to reduce the degree of adhesion of metal residues on the surface of the crucible.

⑤ Has high insulation properties;

⑥ Small volume shrinkage during sintering process;

⑦ Has low volatility and good resistance to hydration;

⑧ The crucible material has a small amount of gas release.

⑨ The crucible has abundant resources of materials and low prices.

19. How to improve the high-temperature performance of crucibles?

① Reduce the content of CaO and the ratio of CaO/SiO2 in MgO sand to reduce the amount of liquid phase and increase the temperature at which liquid phase is generated.

② Improve the stability of crystal grains.

③ To achieve a good recrystallization state in the sintered layer, to reduce porosity, reduce grain boundary width, and form a mosaic structure, forming a direct combination of solid and solid phases, thereby reducing the harmful effects of the liquid phase.

20. How to choose the appropriate geometric size of the crucible?

① The wall thickness of the crucible is generally 1/8 to 1/10 of the diameter of the crucible (formed);

② Steel liquid accounts for 75% of the crucible volume;

③ The angle of R is around 45 °;

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 crucibles?

① Organic matter: dextrin, pulp waste liquid, organic resin, etc;

② Inorganic substances: sodium silicate, brine, boric acid, carbonate, clay, etc.

22. What is the function of the adhesive (H3BO3) for knotting crucibles?

Boric acid (H3BO3) can remove all moisture by heating below 300 ℃ under normal circumstances, and is called boronic anhydride (B2O3).

① At low temperatures, some MgO and Al2O3 can dissolve into liquid B2O3 to form a series of transition products, accelerating the solid phase diffusion of MgO · Al2O3 and promoting recrystallization, causing the sintering layer of the crucible to form at lower temperatures, thereby reducing the sintering temperature.

② By relying on the melting and bonding effect of boric acid at medium temperature, the semi sintered layer can be thickened or the strength of the crucible before secondary sintering can be increased.

③ In magnesia sand containing CaO, the use of binders can suppress the crystal transformation of 2CaO · SiO2 below 850 ℃.

23. What are the various molding methods for crucibles?

Two ways.

① Prefabrication outside the furnace; After mixing the raw materials (electric fused magnesium or aluminum magnesium spinel refractory materials) with a certain particle size ratio and selecting appropriate adhesives, they are formed in the crucible mold through vibration and isostatic pressure processes. The crucible body is dried and processed into a prefabricated crucible in a high-temperature tunnel kiln with a maximum firing temperature of ≥ 1700 ℃ × 8 hours.

② Directly pounding inside the furnace; Add an appropriate amount of solid adhesive, such as boric acid, to the appropriate particle size ratio, mix evenly, and use tamping to achieve dense filling. During sintering, different microstructures are formed by varying temperatures of each part.

24. How many layers is the sintering structure of the crucible formed, and what is the impact on the quality of the crucible?

The sintering structure of the crucible is divided into three layers: sintering layer, semi sintering layer, and loose layer.

Sintering layer: During the oven process, the particle size undergoes recrystallization. Except for the medium sand particle size at the low temperature end, the original proportion cannot be seen at all, and a uniform and fine structure is presented. The grain boundaries are very narrow, and impurities are redistributed on the new grain boundaries. The sintered layer is a hard shell located at the innermost part of the crucible wall, which directly contacts the melted metal and bears various forces, so this layer is very important for the crucible.

Loose layer: During sintering, the temperature near the insulation layer is low, and magnesium sand cannot be sintered or bonded by the glass phase, remaining in a completely loose state. This layer is located at the outermost part of the crucible and serves the following purposes: firstly, due to its loose structure and poor thermal conductivity, the heat transferred from the inner wall of the crucible to the outside is reduced, reducing heat loss, providing insulation, and improving the thermal efficiency inside the crucible; Secondly, the loose layer is also a protective layer. Because the sintered layer has formed a shell and comes into direct contact with the liquid metal, it is prone to cracking. Once it cracks, the molten liquid metal will seep out from the crack, while the loose layer is less prone to cracking due to its loose structure. The metal liquid seeping out from the inner layer is blocked by it, providing protection for the sensing ring; Thirdly, the loose layer is still a buffer. Due to the fact that the sintered layer has become a hard shell, overall volume expansion and contraction occur when heated and cooled. Due to the loose structure of the loose layer, it plays a buffering role in the volume change of the crucible.

Semi sintered layer (also known as transition layer): located between the sintered layer and the loose layer, divided into two parts. Near the sintered layer, impurities melt and redistribute or bond with magnesium sand particles. Magnesium sand undergoes partial recrystallization, and large sand particles appear particularly dense; The parts near the loose layer are completely bonded together by adhesive. The semi sintered layer serves as both a sintered layer and a loose layer.

25. How to choose the oven process system?

① Maximum oven temperature: When the insulation layer thickness of the knotted crucible is 5-10mm, for electric fused magnesia, the sintered layer only accounts for 13-15% of the crucible thickness when baked at 1800 ℃. When baked in a 2000 ℃ oven, it accounts for 24-27%. Considering the high-temperature strength of the crucible, it is better to have a higher oven temperature, but it is not easy to get too high. When the temperature is higher than 2000 ℃, it forms a honeycomb like structure due to the sublimation of magnesium oxide or the reduction of magnesium oxide by carbon, as well as the intense recrystallization of magnesium oxide. Therefore, the maximum oven temperature should be controlled below 2000 ℃.

② Heating rate: In the early stage of heating, in order to effectively remove moisture from refractory materials, sufficient preheating should be carried out. Generally, the heating rate should be slow below 1500 ℃; When the furnace temperature reaches above 1500 ℃, the electric fused magnesia sand begins to sinter. At this time, high power should be used to quickly heat up to the expected maximum oven temperature.

③ Insulation time: After the furnace temperature reaches the highest oven temperature, insulation needs to be carried out at that temperature. The insulation time varies depending on the furnace type and material, such as 15-20 minutes for small electric melting magnesium crucibles and 30-40 minutes for large and medium electric melting magnesium crucibles.

Therefore, the heating rate during the oven and the baking at the highest baking temperature should be adjusted accordingly

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