Vacuum Degassing
Vacuum Degassing is the process of removing dissolved or trapped gases form liquid steel. It is also used to produce ultra low carbon steel with carbon content of 30 ppm or less.[1]
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During the primary steelmaking process, gases like oxygen, hydrogen, and nitrogen dissolve or get trapped in the liquid steel. These gases affect both the mechanical and physical properties of steel. Excess hydrogen present in the liquid steel gets removed during solidification resulting in a pinhole formation making the steel porous. Lower levels of hydrogen in liquid steel causes blistering and loss of tensile ductility of the metal. Presence of excess nitrogen in steel causes formation of either blow holes or nitrides during solidification, embrittlement of heat affected zone during welding of steels and also impairs cold formability of steel. The desorption of gases or the degassing of steel is an important secondary steelmaking process.[1]
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Dissolved oxygen from liquid steel cannot be removed as molecular oxygen, but is removed through a process called deoxidation. The term degassing is used for the removal of hydrogen and nitrogen gases from the liquid steel. Since the degassing process of the liquid steel is carried out under vacuum, it is also known as vacuum degassing of liquid steel. Vacuum degassing processes are carried out in steel teeming ladles.[1]
Types of Vacuum degassing techniques
Vacuum degassing processes can be broadly classified into the following three types:
- Stream degassing practice or Ladle-to-mold degassing: It is a vacuum degassing method, in which a mold is placed in a vacuum chamber, to which molten steel is poured from a tundish attached to the cover of the chamber. The tundish, on the other hand is continuously filled with the melt poured from the ladle. The steel stream gets deoxidised when it is falling to the mold cavity in vacuum due to the deoxidation reaction [C] + [O] = {CO}. Hydrogen dissolved in the steel diffuses into the CO bubbles. The gas then gets removed by the vacuum pump.[2][3] Another variation of this method is a ladle-to ladle-degassing, in which a second ladle is located inside the vacuum chamber, in place of the mold.[3][4]
- Circulation degassing practice: This process involves recirculating molten steel in vacuum which tend top remove the trapped gases in the liquid steel. Examples of recirculating systems are Ruhrstahl Heraus, Ruhrstahl Heraus- Oxygen blowing, and Dortmund Hoerder etc.[1]
- Ladle or tank degassing practice In this process, degassing is carried out by placing the ladle containing molten steel under vacuum. The effectiveness of degassing decreases from top to bottom of the liquid steel bath. Bottom layers of steel are much less affected by vacuum since these layers are under ferrostatic pressure from the column of liquid steel above. Hence, bath agitation is often adopted to expose the entire content of liquid steel to the vacuum.[1][3]
References
- ↑ 1.0 1.1 1.2 1.3 1.4 "Ladle Metallurgy – IspatGuru". www.ispatguru.com. Retrieved 2024-04-13.
- ↑ "Ladle refining [SubsTech]". www.substech.com. Retrieved 2024-04-15.
- ↑ 3.0 3.1 3.2 International, VAC AERO (2017-12-05). "Vacuum Degassing of Steel". Vacaero. Retrieved 2024-04-15.
- ↑ Gupta, S.K. "Vacuum Degassing" (PDF). Cetapps. Retrieved 11 April 2024.
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