Vacuum Arc Remelting furnace (VAR furnace)

Introduction:

This is a consumable electrode remelting process performed inside a vacuum chamber. Exposure of molten metal droplets to vacuum reduces free gas content in the steel providing a cleaner product with little segregation. Consistently higher yields and reproducible metals are assured through fully automated computer/load cell control throughout the remelt process eliminating the need for operator intervention. The end product is a dense ingot with minimum segregation and very low hydrogen and oxygen levels.

The VAR process involves the melting of an electrode by a controlled DC arc in a vacuum environment. The arc is established between the negative consumable electrode (cathode) and the crucible base (anode) and quickly forms a molten pool of metal. The gap between the melting electrode and metal pool (arc gap) is precisely maintained and a controlled melt rate is established.

An advanced computer control system maintains the melt rate and arc gap utilizing custom designed algorithms taking into account measurements from the load beam weighing system, measurements of the furnace voltage, and measurements of the number of drip (drop) shorts.

Refining of the metal occurs as the metal droplets falling through the arc gap are exposed to the vacuum environment and the extreme temperatures of the arc zone. This causes removal of dissolved gasses, vaporization of tramp elements, and improvement in oxide cleanliness.

Because of the water-cooled crucible, the molten pool of metal formed by the metal droplets is solidified in a directional fashion. When the melt rate and arc gap are correctly controlled this controlled solidification prevents macrosegregation and reduces the amount of microsegregation thereby enhancing the material properties of the solidified ingot.

Toward the end of the process the power is gradually reduced providing a controlled hot top maximizing the yield of useful product.

VAR vacuum arc remelting - Advantages
The primary benefits of remelting a consumable electrode under vacuum are:
• Removal of dissolved gases, such as hydrogen, nitrogen and CO;
• Reduction of undesired trace elements with high vapor pressure;
• Improvement of oxide cleanliness;
• Achievement of directional solidification of the ingot from bottom to top, thus avoiding macro-segregation and reducing micro-segregation.

Oxide removal is achieved by chemical and physical processes. Less stable oxides or nitrides are thermally dissociated or are reduced by carbon present in the alloy and are removed via the gas phase. However, in special alloys and in high-alloyed steels the non-metallic inclusions, e.g. alumina and titanium-carbonitrides, are very stable. Some removal of these inclusions takes place by flotation during remelting. The remaining inclusions are broken up and evenly distributed in the cross-section of the solidified ingot.

VAR vacuum arc remelting - Features:
• Ingot diameters up to 1,500 mm;
• Ingot weights up to 50 tons;
• Electrode is melted by means of a DC arc under vacuum (electrode negative, melt pool positive);
• Remelting currents up to 40 kA;
• Vacuum range: 1– 0.1 Pa (some applications up to 1000 Pa);
• Electrode weighing system;
• Stable or free-standing gantry design;
• Coaxial high current feeding system;
• Computer controlled remelting process according to remelting recipes (arc gap control, melt rate control, data acquisition system, print-out of melt records.

Application fields:

• Super alloys for aerospace;
• High strength steels for rocket booster rings and high pressure tubes;
• Ball-bearing steels;
• Tool steels (cold and hot work steels) for milling cutters, drill bits, etc.
• Die steels;
• Melting of reactive metals (titanium, zirconium and their alloys) for aerospace, chemical industry, off-shore technique and reactor technique