What is Vacuum Degassing? Steel Foundations
When liquid steel is produced, it often has dissolved gases, too much carbon, or too much oxygen. Vacuum Degassing removes these harmful elements quickly by creating an environment with low atmospheric pressure.
There are two main types of vacuum degassing equipment called the Ruhrstahl-Heraeus (RH) degasser and the Vacuum Tank Degasser (VTD). The RH degasser draws liquid steel up into the vacuum chamber from the ladle using two snorkels. The snorkels protect the liquid steel from the atmosphere during the transfer, with one snorkel bringing the steel up in to the vacuum chamber and the other bringing it back down into the ladle. Alternatively, The Vacuum Tank Degasser either sets the entire ladle into a large vacuum chamber or seals the top of the ladle, so the ladle itself becomes the vacuum chamber. Since the liquid steel stays in the ladle during the VTD process, it does not require additional protection from the atmosphere.
When liquid steel is produced from the Basic Oxygen Furnace or Electrical Arc Furnace, it is usually in a high-oxygen state (O2). It may also have other detrimental dissolved gases such as hydrogen (H2) and Nitrogen (N2). Additionally, steel can have too much carbon for the intended application. While these gases and elements can be removed from the liquid steel under regular atmospheric conditions, it simply takes too long without the degassing process.
However, when the liquid steel is in the reduced atmospheric pressure of the vacuum chamber of the degasser, the time it takes to degas (remove) the O2, H2, and N2 gases from the liquid steel is dramatically reduced. The low atmospheric pressure of the vacuum chamber also speeds the degassing of carbon monoxide (CO) and carbon dioxide (CO2) gases, which removes carbon from the steel by allowing the carbon and oxygen in the liquid steel to combine much faster. In the RH degassing process, the carbon content can be reduced from 0.04% to as low as 0.001%.
In addition to quickly reducing the dissolved gases and amount of carbon in the liquid steel, the degassing process also:
- Homogenizes the composition and temperature of the steel bath. In the RH degasser, this comes from bringing the liquid steel up and down from the vacuum chamber, while mixing in the VTD process comes from agon bubbling.
- Removes impurities, such as inclusions and metal oxides. The mixing in these processes enhances the probability of the impurities being absorbed into the slag layer.
- Produces conditions that are favorable for final removal of sulfur.
The degassing process is typically required for higher-end steels like ultra-low carbon steels, stainless steels, and advanced alloys.