DEGASSIFICATION IN CASTING: Principle of Degassification, Vacuum Degassing, Static Bath Treatement, Induction Degassing, Fractional Degassing, Stream Droplet Degassing, Gas Scavenging, Solid Degassing and Jet Degassing

DEGASSIFICATION IN CASTING

Degassification is the process of removing the gases from the melt. If it is impossible to prevent the absorption of gases completely, the melt is a solution to obtain sound casting.

Effect of dissolved gas can be reduced by removing or isolating one or more of the components of gas.

Principle of Degassification

Most degassing treatment are based on the equilibrium between molten metal and gas atmosphere.

If a gas is completely removed from the atmosphere around the molten metal, then the gas dissolved in molten metal tends to escape out.

Principle of Degassification: If a gas is completely removed from the atmosphere around the molten metal, then the gas dissolved in molten metal tends to escape out.

V = volume of molecular gas

P = pressure of gas around and above liquid metal

k = constant

Techniques

(i) Vacuum Degassing

– static bath treatment

– induction degassing

– fractional degassing

– stream droplet degassing

(ii) Gas scavenging including solid degasser and jet degassing.

Vacuum Degassing

In this method melting and pouring of metal is done in vacuum. This method of degassing is very effective and also very costly.

Static Bath Treatement

Metal is melted in air.

Before pouring it is placed within a chamber which can be sealed and evacuated.

In the second type, the laddle is sealed with a cover and molten metal degassed.

After vacuum degassing pouring is done in atmosphere.

Rate of degassing depends on

– surface area of liquid metal.

– mass of liquid metal.

– extent of agitation and stirring which accelerate transport of dissolved gas to nearest atmosphere.

– No circulation of melt is provided.

– Dagassing occurs by natural convection only, thus it is a slow process.

Static Bath Vacuum Degassing
Static Bath Vacuum Degassing

Induction Degassing

In this method the laddle containing molten metal is placed in a chamber which can be sealed and evacuated.

An induction stirring coil surrounds the laddle. It helps the gasses to evolve from the melt and the gas is drawn away by vacuum pump.

Advantages of Induction Degassing

(i) circulation of melt

(ii) induction heating

More efficient than static bath degassing

Induction Degassing
Induction Degassing


Fractional Degassing

In this degassing method a separate vacuum vessel is utilized.

The molten metal in the laddle is degassed by lowering and raising of the vessel alternately into the laddle.

It occurs due to the raising of molten metal in the vacuum vessel.

Fractional Degassing
Fractional Degassing

Stream Droplet Degassing

In this method the molten metal from the laddle is tapped into another preheated laddle placed in an evacuated chamber.

This falling stream of molten metal is exposed to vacuum and thus degassing occurs.

It is a very rapid method of dissolved gas extraction from molten metal (melt).

Vacuum degassing
Vacuum degassing

Gas Scavenging

It is the most effective technique of gas extraction from melt.

Inert gas is used for this purpose.

Inert gas is bubbled through the melt using graphite tube.

Pressure of dissolved gas within inert gas bubble is zero/negligible than in melt. So the dissolved gas migrate to the bubble. Then collected there and flushed out from melt.

Some allays and scavenging gases

Copper base –––––––––– N2

Aluminium base ––––––– N2, Cl2, Ar

Magnesium base ––––––– Cl2, Ar

Nickel base ––––––––– O2

Inert Gas Scavenging
Inert Gas Scavenging

Solid Degassing

In this method, inert gases are generated within the melt by decomposition of unstable compounds as C2Cl6 (hexachloro – ethane).

C2Cl6 is a solid degasser.

Tablets of hexachloroethance are plunged and held beneath the melt surface.

They decompose and flushing is accomplished.

Calcium carbonate is used as solid degasser to generate CO2 in copper base alloys.

Solid Degassing
Solid Degassing

Jet Degassing

It is one of the methods of gas scavenging.

In this process inert gas jet is impinged on surface of molten metal. For example argon jet is used to reduce hydrogen content in steel.

In steel preparation, carbon monoxide from carbon boil (FeO + C = CO + Fe) acts as scavenging gas and removes hydrogen from melt.

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