VAR is widely used to improve the cleanliness and refine the structure of standard air-melted, vacuum induction melted or ESR remelted ingots. VAR steels and superalloys as well as titanium, zirconium and their alloys are used in a great number of high integrity applications where cleanliness, homogeneity, improved fatigue and fracture toughness of the final product are essential. Aerospace, power generation, defense, medical and nuclear industries rely on the properties and performance of these advanced remelted materials.
VAR Advantages
Removal of dissolved gases, such as hydrogen, nitrogen and Carbon dioxide
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
Lowest energy input of all remelting processes
Total ceramic free melting process
VAR is the continuous remelting of a consumable electrode by means of an arc under vacuum. DC power is applied to strike an arc between the electrode and the baseplate of a copper mold contained in a water jacket. The intense heat that is generated by the electric arc, melts at the tip of the electrode and a new ingot is progressively formed in the water-cooled mold. A high vacuum is maintained throughout the remelting process.
The basic design of the VAR furnace has been improved continuously over the years particularly in computer control and regulation with the objective of achieving a fully-automatic remelting process. This in turn has resulted in improved quality and reproducibility of the metallurgical properties of the products. Close control of all remelting parameters is required for reproducible production of homogeneous ingots, which are free of macro segregation and show a controlled solidification structure and superior cleanliness.
To fulfill today’s most stringent material quality specifications, VAR furnaces make use of computer controlled process automation. Logic control functions, continuous weighing of the consumable electrode, closed loop control of process parameters (e. g. remelting rate, arc gap based on arc voltage or drip short pulse rate), data acquisition and management are handled by dedicated computer systems. An operator interface PC (OIP) acting hierarchically as master of the automatic melt control system (AMC) is utilized as the interface between operator and VAR process. The OIP provides process visualization, parameter indications, graphic displays, soft keys for operator commands, editing and handling of remelting recipes, data acquisition and storage as well as for generation of melt records. Optionally the OIP can be equipped with an Ethernet network interface which may be utilized for data transfer to other computers connected to the local area network (e. g. supervisory PC, customer’s main frame, etc.).
ALD’s automatic melt control system (AMC) is unsurpassed in the world for its versatile features and ease of operation to ensure accuracy and reproducibility of control.
Oxide removal is achieved by both chemical and physical processes. Less stable oxides or nitrides are thermally dissociated or reduced by the 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.
The laboratory Vacuum Arc Remelting furnace is primarily used to produce small ingots, button samples and precision cast parts. These products are used to examine the mechanical, physical and chemical properties of steel, reactive metals and their alloys.
VAR Features
Chemical and mechanical homogeneity of Ingots
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 48 kA
Vacuum range: 1 – 0,01 Pa
Electrode weighing system
Positioning system to centerline the electrode
Stable and free-standing gantry design
Coaxial high current feeding system
Computer controlled remelting process recipes (arc gap control, melt rate control, data acquisition system, print-out of melt records)
Process gas supply to enhance heat transfer to the crucible
Process gas supply to stabilize the vacuum level
External magnetic field on crucible area for stirring
Laboratory VAR Systems for Ti, Ta, Zr, Fe- and Ni-based and Cu-alloys for Ingot diameters up to 200 mm
Laboratory VAR LK6 Systems with non-consumable electrode technique for button sample melting
VAR Applications
Superalloys for aerospace
Melting of reactive metals (titanium, zirconium, tantalum and their alloys) for aerospace, chemical, oil and gas, nuclear and electronic industries
Copper and copper alloys for high voltage circuit breakers
High strength steels for rocket booster rings, landing gear and high pressure tubes
Ball-bearing steels
Tool steels (cold and hot work steels) for milling cutters, drill bits, etc.
Die steels
For more information please contact ACC.
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