Any type of metal that you can think of will have impurities after the initial melting process. With the vacuum-degassing process, the atmospheric pressure above the liquid steel is reduced. More specifically, this pressure is reduced to less than 1 millibar (1000mbar considered to be the atmospheric pressure). In the process, the vacuum-degassing furnace is fitted with a very tight lid placed over the molten steel before the degassing begins.
The vacuum-degassing process will not remove impurities found in steel; however, it will remove most of the gaseous impurities and make the level go down to about 25ppm or lower of oxygen and less than 2ppm of hydrogen. With the vacuum degas process, sulfur is also greatly reduced.
The way the furnace is set up can vary but it is usually fitted with vacuum pumps and boosters. The liquid metal in the ladle is placed inside the vacuum chamber and then closed with an airtight lid so that there is no leakage . Then a vacuum is created inside the chamber and argon is purged from the bottom of the ladle. The argon creates the agitation inside the liquid metal and exposes it to vacuum, which removes the dissolved gases in steel and helps in removing oxide inclusion particles from the liquid steel. The agitation of the steel in the ladle also helps make the steel more even and uniform with homogeneous chemistry and bath temperature.
What Is the Significance of Vacuum Degassing?
During the process of making steel, molten steel stays in the ladle. Before it is poured, however, the degassing process has to take place. Why? For three main reasons:
- First: It is used in order to reduce or even eliminate any of the dissolved gases in there, in particular nitrogen and hydrogen.
- Second: It is used to improve ductility by reducing dissolved carbon.
- Third and last: When you refine stainless steel grades, you want to promote oxidation of the dissolved carbon over chromium.
In addition, during the steel smelting process, many unwanted gases are dissolved in the liquid. This can result in numerous defects and imperfections. Vacuum degassing helps to remove those undesirable gases. After the molten steel is no longer in the furnace but before it is poured into ingots or into a continuous caster, the process has to begin. Timing, in other words, is extremely important during the vacuum-degassing process when it comes to alloy steel or many other metals.
There are several forms of vacuum degassing, including ladle degassing and stream degassing. There are even recommendations regarding how to choose the best pump for vacuum degassing. The decision is usually based on argon inflow, the price of electricity and steam, and the leakage rate of air that gets into the vacuum chamber, among others.