Vacuum degassing is a procedure that is used to improve the internal cleanliness of steel after the initial melt has occurred. The principle of the process is to reduce the atmospheric pressure above the liquid steel in the vacuum remelt furnace. The pressure over the molten steel is reduced to approximately 1 mm of mercury (Hg). Generally, the vacuum degassing furnace is fitted with a vacuum-tight lid over the molten steel, and the degassing procedure is started.

Some units will consist of the main vacuum vessel into which there is the ladle of molten steel. The furnace will also be fitted with an electrical induction coil, which, when turned on, creates a stirring action within the liquid steel and, thus, agitates the steel within the ladle. This helps the homogeneity of the steel melt. The action will also help to release trapped gas particles within the molten-steel liquid and will ensure that the chemistry of the melt is homogenous.

This method of cleaning the steel will not remove all of the potential gaseous impurities present, but it will remove approximately 50% of the gaseous impurities down to a level of approximately 30 ppm of oxygen and down to approximately 2.5 ppm of hydrogen.

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