VACUUM DE-GASING OF METALS

1. VACUUM DE-GASING OF METALS

2. 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

3. 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

4. The following gases are being entrapped in liquid steel Hydrogen Nitrogen Following oxide inclusions are also present MnO Cr₂O ₃ Al₂O ₃ TiO ₂ Inclusions & Gases in steel

5. 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

6. 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

7. 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 Cr₂O ₃ are reduced in vacuum completely . Strong inclusion such as Al₂O ₃ or TiO ₂ can be reduced in a very high vacuum. Removal of oxide inclusions

8. Methods of Vacuum Degassing Following methods are employed Vacuum degassing in Ladle . Electric arc

9. 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

10. 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 ……….

11. 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

12. 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

13. 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

14. 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

15. 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