What is vacuum induction melting

What is vacuum induction melting?

Vacuum induction melting, referred to as VIM for short, refers to the metallurgical method of smelting by using electromagnetic induction to generate eddy current heating charge in metal conductor under vacuum conditions. VIM process has the characteristics of small smelting chamber volume, short vacuum pumping time and smelting cycle, convenient temperature and pressure control, recyclable volatile elements, accurate control of alloy composition and so on. Due to the above characteristics, VIM has developed into one of the important processes for the production of special alloys such as special steel, precision alloy, electrothermal alloy, superalloy and corrosion-resistant alloy.

20210809133421 86287 - What is vacuum induction melting

Structural diagram of vacuum induction melting furnace (1 – vacuum system; 2 – furnace turnover mechanism; 3 – feeding mechanism; 4-crucible; 5 – inductor; 6 – Sampling and tamping device; 7 – temperature measuring device; 8-movable furnace body)

The vacuum pump works continuously to extract air and gaseous products to maintain a vacuum environment.

Advantages of vacuum environment:

  • ① It is easy to remove nitrogen, hydrogen, oxygen and carbon dissolved in steel and alloy to a level much lower than that of smelting at atmospheric pressure.
  • ② Impurity elements (copper, zinc, lead, antimony, bismuth, tin and arsenic, etc.) with higher vapor pressure than the base metal at melting temperature can be removed by volatilization.
  • ③ The composition of active elements such as aluminum, titanium, boron and zirconium to be added to the alloy is easy to control.

Under vacuum conditions, the alternating current acts on the induction coil to produce alternating magnetic field. The alternating magnetic field induces alternating current – “eddy current” on the charge, and the charge is heated and melted into liquid state by “eddy current”, so as to achieve the purpose of smelting metal.

Advantages of electromagnetic induction heating:

  • ① Without the graphite electrode required for arc heating, the possibility of electrode carburization is eliminated, and the steel and alloy with very low carbon content can be melted.
  • ② Electromagnetic stirring with certain strength in the molten pool can promote the uniformity of molten steel composition and temperature, and the inclusion in the steel can merge, grow and float up.
  • ③ The specific surface area of molten pool is small. It is easy to control the atmosphere in the smelting process. There is no arc and high temperature area under the arc. The alloy elements have less burning loss and less suction, which is conducive to composition control, low gas content and shortening the smelting time.
  • ④ Less smoke and dust, less environmental pollution. There is basically no flame and no combustion products in the smelting process.

The metal materials melted by vacuum induction can obviously improve the toughness, fatigue strength, corrosion resistance, high temperature creep performance and permeability of magnetic alloys.

① Raw materials should be carefully prepared.
The chemical purity, cleanliness, lumpiness and dryness of raw materials used in vacuum induction furnace smelting shall meet the technical requirements. The charge shall be fine and the preparation shall be careful.
② Keep the melting process smooth.
The melting speed of the charge shall be gradually accelerated and the heating power shall be gradually increased to ensure the smooth melting and full degassing of the charge. In the middle and late stage of melting, the heating power can be appropriately increased, the melting speed can be accelerated, and the melting time can be shortened.
③ Control the boiling strength of liquid steel and give play to the refining effect of boiling.
At the end of melting and the initial stage of refining, the role of boiling deoxidation, denitrification, dehydrogenation and inclusion removal should be brought into full play. However, the boiling intensity should be controlled by argon to reduce the scouring of crucible and prevent liquid steel overflow.
④ Maintain a high vacuum in each period.
After the charging is completed and the furnace is closed, the electric heating can be sent only after the high vacuum degree is ≤ 10Pa. A high vacuum should be maintained before melting.
During the refining period, high temperature and high vacuum shall be maintained to complete deoxidation and remove harmful impurities.
During alloying, argon should be filled under high fidelity to reduce the oxidation loss of easily oxidized elements.
⑤ During alloying, argon should be filled to cool down and easily oxidized elements should be added.
When the vacuum degree is 1.0 ~ 0.5pa, fill the furnace with argon. Cut off the power to reduce the temperature, add alloy elements, and immediately stir the molten steel after melting to make the alloy elements evenly distributed.
⑥ Magnesium addition at the end of alloying.
When adding magnesium, the operation shall be carried out in strict accordance with the regulations of magnesium addition process. After the furnace type, smelting process and magnesium addition conditions are determined, the operation shall be carried out in strict accordance with the specified argon pressure, liquid steel temperature and holding time.
⑦ Implement medium temperature pouring.
The tapping temperature and pouring temperature of molten steel are related to the metallurgical quality of finished products. The pouring temperature shall be controlled at 50 ~ 80 ℃ above the liquid phase point. Moderate temperature pouring can reduce the shrinkage depth of ingot, obtain dense ingot and improve the yield.

Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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