Vacuum Arc Remelting (VAR) is a secondary melting process used in the production of metal ingots with a precise chemical and mechanical homogeneity for highly demanding applications. Ingots derived from the VAR process are typically utilized in the production of the critical components of jet engines and industrial gas turbines, as well as for military applications and heavy industry.
Vacuum Arc Remelting (VAR) Process
The VAR process is primarily used in applications where purity and homogeneity
are essential for the finished product quality. Therefore, the electrodes used
in the VAR process are arc melted in a water cooled crucible inside a vacuum
chamber to ensure they possess the physical characteristics to maintain the
required purity and remelting consistency for the production of creep resistant
steel.
In contrast to the ESR (Electro Slag Remelting) process, the refining of molten
steel cannot proceed unless the desired electrode characteristics have been
achieved. Figure 1 shows a schematic diagram of the VAR equipment.
Another comparison between VAR and ESR is the relative cooling rates between the
two processes. Molten steel cools down at a faster rate through the VAR process
yielding a higher quality product through superior solidification and reduced
segregation. As described there are some clear advantages of the VAR process in
the production of superalloys however due to the physical constraints of using a
vacuum chamber the capacity and therefore yields of the VAR process are limited
in comparison to ESR.
Figure 1: Schematic of the VAR furnace
Vacuum Arc Remelting (VAR) is a secondary melting process used in the
production of metal ingots with a precise chemical and mechanical homogeneity
for highly demanding applications. Ingots derived from the VAR process are
typically utilized in the production of the critical components of jet engines
and industrial gas turbines, as well as for military applications and heavy
industry. The nature of such applications often demand specific and precise
material properties to ensure the finished product specifications are met.
Common examples of such materials are Ni or Ti-based super alloys and highly
alloyed steels.
One of the most significant problems of Vacuum Arc Remelting (VAR) is the lack
of chemical homogeneity in the ingots that are produced. Previous mathematical
studies of the VAR process have demonstrated that the characteristics of the
mushy zone (the interaction of liquid and solid in the same zone) are the key
factors defining the severity and the characteristics of macro-segregation in
the produced ingots.
Thus, a new mathematical model of the complete VAR process was developed,
capable of representing two distinctive morphologies of the mushy zone. (1) A
rigid columnar structure and slurry of free floating equiaxed grains is
introduced. (2) A model accompanied by a simple criterion for the
columnar-to-equiaxed transition allows the capture of segregation defects
induced by motion and settling of equiaxed grains.
In addition to the described morphological models, the mathematical simulations
needed to include studies of two distinctive flow regimes in VAR. (1) Weak
Buoyancy driven flow and (2) Strong Lorentz driven flow. The results demonstrate
a swift transition from weak buoyancy driven flow to strong electromagnetically
driven flow in the presence of an increasing arc current.
The shift of flow regime to a strong electromagnetically driven state results in
a relative increase of macro-segregation and thus is not desirable. The key to
understanding the speed of the transition between flow regimes is in the
instabilities of the thermal stratification within liquid pool at the early
stages of the VAR process.
Mathematical models are also used for the study of macro-segregation evolution
during multiple VAR melts (when the ingot of the previous melt is used as a
source material for the next melt). The study results demonstrate that the
increase of micro-segregation during a sequence of multiple melts mostly occurs
due to the increase of ingot radius, and not the non-uniformity in electrode
composition. In some instances however, nonuniform electrode composition may
result in substantial additional buoyancy forces in the liquid pool, thus
affecting the final ingot macrosegregation
Vacuum Induction Melting (VIM) Process
The VIM process involves using a melting furnace to melt raw materials by
electromagnetic induction while under vacuum. Since refining does not result
from the VIM process, it is essential that the raw materials (ferroalloys and
metals depending on the application) should be of extremely high purity.
Although application to large steel forging products is quite limited, the
process coupled with VAR/ESR processes is indispensable for the production of
super alloys with exacting product specifications.
The VIM Process was specifically developed for the processing of
specialized and exotic alloys. As the requirement for specialized materials
rises this methodology is becoming more common throughout the industry. VIM was
developed to melt and cast super alloys and high-strength steels, many of which
require vacuum processing because they contain refractory and reactive elements
such as Ti, Nb and Al. It can also be used for stainless steels and other metals
when a high-quality initial melt is required.
As the name suggests, the process involves melting of a metal under vacuum
conditions in conjunction with electromagnetic induction. The Electromagnetic
induction process is used to create electrical eddy currents (via the induction
coil) in the metal and the subsequent result of the heated charge is to melt the
metal.
The furnace consists of an airtight, water-cooled steel jacket capable of
withstanding the required vacuum for processing. The metal is melted in a
crucible housed in a water-cooled induction coil, and the furnace is typically
equiped with a suitable refractory lining.
Metals and alloys that have a high affinity for gases, in particular nitrogen
and oxygen, are often melted/refined in vacuum induction furnaces to prevent
contamination with these gases.
Figure 2: Vacuum induction furnace