Vacuum Degassing

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.

Vacuum Degassing for Steel CastingsAbstract: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

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 energyThe 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 furnacesMetallurgical 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.

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.