VACUUM DE-GASING OF METALS

VACUUM DE-GASING OF METALS

Removal of entrapped gases from liquid metal is called de-gassing.

Before using this technique we should know following terms Dissolution Solubility

WHAT IS DE-GASING

Dissolution The process of entering of gases in steel or

metals in the form of molecular compound like hydrides, nitrides, oxides.

Solubility Ability of any metal solute to be dissolved into

some solvent is termed as Solubility

Dissolution

The following gases are being entrapped in liquid steel Hydrogen Nitrogen

Following oxide inclusions are also present MnO Cr₂O₃ Al₂O₃ TiO₂

Inclusions & Gases in steel

Hydrogen is soluble in liquid steel but not combined.

It may cause cracking during solidification due to its entrapment.

When a vacuum is created on the melt surface equilibrium 2[H] = H₂ shift to the right

Hydrogen has a high mobility in molten steel It can be removed very quickly by vacuum

treatment

Hydrogen

It is soluble in liquid iron. The equilibrium reaction is 2[N] = N₂ shifts to

right due to vacuum. It is less mobile in metal so its removal is

difficult than hydrogen under vacuum. In order to remove it completely we have to

maintain vacuum for a longer time.

Nitrogen

Steel always contain a certain quantity of carbon. During vacuum treatment [C] + [O] = CO shift to

right Many non metallic oxide inclusions are presents

in steel. These inclusions partially or completely

decompose by reacting with carbon. Less strong oxide inclusion such as MnO or Cr₂O₃

are reduced in vacuum completely . Strong inclusion such as Al₂O₃ or TiO₂ can be

reduced in a very high vacuum.

Removal of oxide inclusions

Methods of Vacuum Degassing

Following methods are employed

1. Vacuum degassing in Ladle .

2. Electric arc

One of the simplest method of degassing. The ladle with metal is placed in a vacuum

chamber. The mechanism of gases removal is already

explained. This method give better results if metal is

not de oxidized. If oxygen present it will react with dissolved

carbon. Dissolved hydrogen, and partially nitrogen

also evolves from metal vigorously and bath boils.

Vacuum Degassing in Ladle

To prevent overflow of metal to vacuum chamber metal level in ladle should be 1-1.5 m below the ladle top.

At the end de-oxidant and alloying addition are introduced into the metal,

Contnd……….

Low efficiency. Why?????????????????????? Uneven composition of metal in the ladle

upon addition of de-oxidant alloying elements due to weak stirring of metal.

Chilling of metal. Why??????????????????????

Disadvantages Vacuum Degassing in Ladle

In this method metal is heated by electric arcs.

in such plants metal can be held under vacuum for a longer time usually for two hours.

This ensures a high degree of cleaning from impurity.

Sometime certain quantity of slag former are added during the process.

Electric arc method

Co is the gaseous product of following reaction C + O CO

The equilibrium constant of reaction is: k = pco/[C][O]

Oxygen content can be reduced by lowering the total pressure.

Due to formation of CO Oxygen content can be reduced.

The CO gas is removed easily by vacuum treatment.

Vacuum carbon de-oxidation

It has following advantages It decreases concentration of dissolved

oxygen It deceases the concentration of H and N in

the metal It decreases the content of non metallic

inclusion Because of evolution of gas bubble metal

becomes compositionally homogeneous if there are non ferrous impurities like Lead,

Tin, Zinc etc.A noticeable portion of them evaporate

Advantages

Many useful metals lost in this process. In this treatment many useful impurities like

Mn evaporates. Iron may also be lost during this process

Disadvantages