Vacuum Arc Remelting (VAR)
VAR is widely used to improve the cleanliness and refine the structure of standard air-melted or vacuum induction melted ingots, then called consumable electrodes. VAR steels and superalloys as well as titanium and zirconium and its alloys are used in a great number of highintegrity 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.
Process Technology and Process Characteristics
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 generated by the electric arc melts the tip of the electrode and a new ingot is progressively formed in the water-cooled mold. A high vacuum is being 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 reproducibility of the metallurgical properties of the products.
1. 12 ton VAR furnace, 2. 30 ton VAR furnace
Metallurgy of the Vacuum Arc Remelting Process
The VAR ingot’s solidification structure of a given material is a function of the local solidification rate and the temperature gradient at the liquid/solid interface. To achieve a directed dendritic primary structure, a relatively high temperature gradient at the solidification front must be maintained during the entire remelting process. The growth direction of the cellular dendrites conforms to the direction of the temperature gradient, i.e., the direction of the heat flow at the moment of solidification at the solidification front. The direction of the heat flow is always perpendicular to the solidification front or, in case of a curved interface, perpendicular to the respective tangent. The growth direction of the dendrites is thus a function of the metal pool profile during solidification. As pool depth increases with the remelting rate, the growth angle of the dendrites, with respect to the ingot axis, also increases. In extreme cases, the growth of the directed dendrites can come to a stop. The ingot core then solidifies non-directionally, e.g., in equiaxed grains, leading to segregation and micro-shrinkage. Even in the case of directional solidification, micro-segregation increases with dendrite arm spacing.
A solidification structure with dendrites parallel to the ingot axis yields optimal results. However, a good ingot surface requires a certain level of energy input, resulting in respective remelting rates. Optimal melt rates and energy inputs depend on ingot diameter and material grade, which means that the necessary low remelting rates for large diameter ingots cannot always be maintained to achieve axis-parallel crystallization.
In spite of directional solidification, defects such as “tree ring patterns”, “freckles” and “white spots” can occur in remelted ingots. These defects can lead to rejection of the ingot, particularly in the case of special alloys.
Tree ring patterns can be identified in a macro-etched transverse section as lightetching rings. They usually represent a negative crystal segregation. Tree ring patterns seem to have little effect on material properties. They are the result of a wide fluctuation of the remelting rate. In modern VAR plants, however, the remelting rate is maintained at the desired value by precise computer control of the electrode weight diminution and electrode speed of feed, so that the remelting rate exhibits no significant fluctuation unless caused by electrode defects.
Freckles and white spots have a much greater effect on material properties as compared to tree ring patterns. Both defects can represent a significant cause for premature failure of turbine disks in aircraft engines. Freckles are dark etching circular or nearly circular spots that are generally rich in carbides or carbide forming elements. The formation of freckles is usually a result of a high metal pool depth and sometimes of a rotating pool. The liquid pool can be set in rotation by stray magnetic fields. Freckles can be avoided by maintaining a low pool depth and by eliminating disturbing magnetic fields through coaxial current feeding on the VAR furnace. White spots are typical defects in VAR ingots. They are recognizable as light etching spots on a macro-etched surface. They are lower in alloying elements, e.g., titanium and niobium in Inconel 718.
There are several mechanisms that could account for the formation of white spots:
• Residues of unmelted dendrites of the consumable electrode in the ingot;
• Pieces of arc splatter that fall into the metal pool and are not dissolved or remelted and get embedded in the ingot;
• Pieces of the ingot shelf region transported into the solidifying interface of the ingot.
All three of the above-mentioned mechanisms, individually or combined, can be considered as possible sources for white spots. This indicates that white spots cannot be avoided completely during vacuum arc remelting, as they are inherent in the process. To minimize their frequency of occurrence, the following conditions should be observed:
• Use of maximum acceptable remelting rate permitted by the ingot macrostructure;
• Use of short arc gap to minimize crown formation and to maximize arc stability;
• Use of homogeneous electrode substantially free of cavities and cracks;
• Use of proper melting power supply to reduce excessive current spikes during drop shorts.
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 drop short pulse rate), data acquisition and management are handled by dedicated computer systems. These computer systems communicate via field bus or specific interfaces. 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 serves for process visualization, featuring parameter indications, graphic displays and 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.).
Established remelting parameters are stored as remelting recipes on hard disk and are available for subsequent VAR production of respective ingot size/material grade combinations to assure reproducibility of the metallurgical ingot quality.
ALD’s automatic melt control system (AMC) is unsurpassed in the world for its inherent features, ease of operation and last but not least its accuracy and reproducibility of control.
Schematic of the VAR furnace
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.
• 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.
• Superalloys 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.
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