Vacuum degassing is commonly used in steel production to remove gases like hydrogen and nitrogen from liquid steel. It works by exposing the steel to vacuum conditions, which allows the gases to be readily removed. Specifically, vacuum degassing lowers the levels of dissolved gases to parts per million and improves the quality of the final cast product by preventing cracking defects. It is a critical process that improves both productivity and quality in continuous steel casting.

1. Vacuum Degassing

4. VACUUM TREATMENT
Exposing steel to vacuum conditions has a profound effect on all metallurgical reactions involving gases.
First, it lowers the level of gases dissolved in liquid steel. Hydrogen, for example, is readily removed in a
vacuum to less than two parts per million. Nitrogen is not as mobile in liquid steel as hydrogen, so that
only 15 to 30 percent is typically removed during a 20-minute vacuum treatment.
Another important process is vacuum decarburization and deoxidation. In theory, oxygen and carbon,
when dissolved in steel, react to form carbon monoxide until they reach equilibrium at the following
relationship:
This means that, under vacuum conditions (when there are only small amounts of carbon monoxide in the
surrounding gas and therefore little carbon monoxide pressure), carbon and oxygen will react vigorously
until they reach equilibrium at very low levels. For instance, liquid steel at 1 atmosphere pressure may
contain 0.043 percent carbon and 0.058 percent oxygen, but, if the pressure is lowered to 0.1 atmosphere,
the two elements will react until they reach equilibrium at 0.014 percent carbon and 0.018 percent
oxygen. Under a pressure as low as 0.01 atmosphere, equilibrium will be reached at 0.004 percent carbon
and 0.006 percent oxygen. In practical operation, the obtainable levels of carbon and oxygen are far above
equilibrium conditions, because the movement of carbon and oxygen atoms in liquid steel is time-
consuming and treatment time is limited. In addition, the steel is continuously reoxidized by multiple
sources of oxygen. Nevertheless, it is common practice to produce ultralow-carbon steel, containing less
than 0.003 percent carbon, in 20 minutes at a vacuum treatment station under pressure of one torr. (In
vacuum technology, pressures are often expressed in torr, which is equivalent to the pressure of a column
of one millimetre of mercury. One atmosphere equals 760 torr.)
There are several types of vacuum treatment, their use depending on steel grade and required production
rates. In the tank degasser (shown in B in the figure), the ladle is placed in an open-top vacuum tank,
which is connected to vacuum pumps. The vacuum pumping system often consists of two or three
mechanical pumps, which lower the pressure to about 0.1 atmosphere, and four or five stage steam
ejectors, which bring the pressure to under 1 torr, or 0.0013 atmosphere. Practical treatment time is 20 to
30 minutes. The ladles used in tank degassing stations are large and, when filled with steel, retain about
one metre of freeboard in order to contain the melt during a vigorous boil.
A modification of the tank degassers is the vacuum oxygen decarburizer (VOD), which has an oxygen
lance in the centre of the tank lid to enhance carbon removal under vacuum. The VOD is often used to
lower the carbon content of high-alloy steels without also overoxidizing such oxidizable alloying
elements as chromium. This is possible because, in the pressure-dependent carbon-oxygen reaction
outlined above, oxygen reacts with carbon before it combines with chromium. The VOD is often used in
the production of stainless steels.
There are also tank degassers that have electrodes installed like a ladle furnace, thus permitting arc
heating under vacuum. This process is called vacuum arc degassing, or VAD.
For higher production rates (e.g., 25 ladles treated per day) and large ladles (e.g., 200 tons), a
recirculation degasser is used, as shown in C in the figure. This has two refractory-lined snorkels that are
part of a high, cylindrical, refractory-lined vacuum vessel and are immersed in the steel. As the system is
evacuated, atmospheric pressure pushes the liquid steel through the snorkels and up into the vessel. One
atmosphere lifts liquid steel about 1.3 metres. Injecting argon into one of the snorkels then circulates the
steel through the vessel, continuously exposing a portion of the steel to the vacuum. Recirculation
facilities are often very elaborate, using fast vessel-exchange systems or even two operating vessels at one
station to achieve high production rates. Some units also inject oxygen during vacuum treatment, through
either the side or the top of the vessel. This is done to speed up decarburization or, by simultaneously
adding aluminum, to increase the steel temperature. Some shops apply a similar system but use a vacuum
vessel with only one snorkel. Here, a portion of the steel in the ladle flows in and out of the vacuum
vessel and is exposed to the vacuum by a continuous raising and lowering of either the vessel or the ladle.

5. Vacuum Degassing for Steel Castings
Abstract:
Processes connected to handling liquid metals are extremely important in managing operational costs and
productivity but provide most gain in the opportunities available to impact overall quality of the cast
component and the related refining activities.
Vacuum degassing is most commonly used to remove hydrogen and nitrogen, which in the finished
product can lead to cracking defects in the cast.
Melting and handling liquid metals are two of the most critical components in the overall metal casting
operation. The manner in which the metal is melted, the way the metal is transferred into the castings, and
the whole liquid metal handling process have a significant impact on productivity, on the cost of
operations, and certainly on the quality of the resultant cast component.
Processing the metal in its molten state is the activity wherein the most gains can be achieved. Molten
metal processing is an opportunity for refining and quality enhancement. For example, processes such as
alloying, degassing, filtration, fluxing, and grain refinement and modification in aluminum are usually
carried out in the liquid metal prior to casting. The mass transfer rates and the kinetics are such that these
reactions are carried out much more effectively in the melt.
In the continuous casting of steel, vacuum degassing of the liquid steel is often performed to remove
hydrogen and nitrogen which can lead to cracking defects in the cast. Degassing effectively prevents this
low ductility from occurring and provides steel producers a wider “margin for error” in their caster
operation. However, there are significant energy costs associated with degassing that prohibit steel
producers from using it for all types of production. Degassing must be used intelligently to achieve a
combination of acceptable product quality and energy costs.
The Vacuum Induction Degassing (VID) furnace concept has been developed for special applications in
the ferrous and non-ferrous metals industry for charge weights up to 30 tons. Whenever pouring under
vacuum is not specified or not required for metallurgical reasons, the bell type furnace with open air
teeming is recommended for its favourable economics. Smaller steel shops and foundries will be able to
produce with the VID furnace, within one step, high quality vacuum treated steels, whereas larger shops
have to realize these qualities employing a conventional LF/VD/VOD production line. The temperature
losses during degassing treatment are compensated by induction heating.
Table 1: Product application and quality improvement in different processes

6. Vacuum Induction Melting (VIM) is one of the most commonly used processes in secondary metallurgy
applied for refining treatment in the liquid state and the adjustment of chemical composition and
temperature. To achieve the increasing quality demands on the resulting material and at the same time to:
• saving of raw materials such as alloying elements due to higher yield
• saving energy
The application of vacuum in the induction melting process is a must for many specialized materials. For
example, vacuum induction melting is indispensable in the manufacture of special alloys, which must be
melted under vacuum or in an inert gas atmosphere because of their reactivity with atmospheric oxygen.
The process is suitable for the production of high-purity metals within an oxygen-free atmosphere. This
limits the formation of non-metallic oxide inclusions.
Vacuum induction melting makes effective degassing of the melt possible and extraordinarily precise
adjustment of alloy composition, since the temperature, vacuum, gas atmosphere, pressure and material
transport (e.g., through stirring of the bath) can be adjusted independently of one another. Besides the
exact concentration of alloying elements, the content of trace elements is also important for many alloys.
Figure 1: Current processing route for products cast from VIM/VIDP furnaces
Metallurgical Advantages of Vacuum Degassing:
 Melting in an oxygen-free atmosphere, this limits formation of non-metallic oxide inclusions and prevents
oxidation of reactive elements;
 Achievement of very close compositional tolerances and gas contents;
 Removal of undesired trace elements with high vapor pressures;
 Removal of dissolved gases e.g. oxygen, hydrogen, nitrogen;
 Adjustment of precise and homogeneous alloy-composition and melt temperature.

7. Vacuum Oxygen Decarburization (VOD):
VOD system is consisting of…
i. Vacuum tank ( with Oxygen lancing Facility)
ii. Ladle furnace (with Stirring facility)
Process flow = EAF  VOD IC or CC
 The ladle has a free board of about 1 to 1.5 meter to contain violent (vigorous) agitation of the bath
during lancing.
 The charge ingredients are similar to the AOD process. The charge is melted in EAF and transferred
to the VOD system.
 Oxygen blowing from lid of the vacuum tank and argon bubbling from the ladle bottom are started
when required vacuum is established.
 Argon stirring is essential otherwise decarburization is delayed due to lack of mass transport of
carbon from bottom portion to the surface where carbon oxygen reaction is greatly achievable.
 The carbon can be lowered to around 0.02% at around 16-18%Cr.
 At the end of the refining the vacuum is broken and the heat/bath is deoxidized with Al/Fe-Si – or Fe-
Si/Cr-Si(for chromium recovery)
 Then de-sulphurisation is carried out by putting synthetic slag (Cao,SiO2,Al2O3 or CaF2 = e.g. 40: 40:
20) to the molten steel surface of about 2-3% by weight of the molten steel and argon Stirring of the
melt through the porous bottom plug results in deep desulfurization of the steel.
 The total VOD cycle is around 2-2.5Hours.
Benefits of Vacuum Oxygen Decarburization (VOD):
 -Deep carbon removal (ultra low carbon steel can produced
 -Low losses of chromium in treatment of stainless steels;
 -Hydrogen removal (degassing);
 -Sulfur removal (desulfurization);
 -Precise alloying;
 -Reheating; -Non-metallic inclusions (oxides and nitrides) removal;
 -Temperature and chemical homogenizing.
 Since many steels are required to be vacuum treated to decrease the gas content, the vacuum system
can be easily done with out extra additional investment.