Vacuum treatment (generally called vacuum degassing) is a commonly used steelmaking process, used for removing dissolved gases (e.g. hydrogen) from the steel. In this process, the steel is exposed to a vacuum which promotes transfer of dissolved gases from the liquid steel to gas phase.

Exposure of steel to vacuum also promotes reaction between oxygen and carbon dissolved in the steel to produce carbon monoxide, by shifting the equilibrium conditions. Similar to ladle arc heating, this has the effect of chemically reducing the ladle system.

In this way, alumina found in refractories and the ladle top slag can be reduced to give dissolved aluminium in the steel:

In Vacuum:  Al₂O₃ ® 2Al + 3O

When the vacuum is removed, the dissolved aluminium can react with inclusions in the steel to increase their alumina content, and in some steels inclusions detrimental to product properties can be formed in this way. For such steels, a lower vacuum is used during degassing to avoid this problem.