Vacuum Degassing of Steel
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 re-melt furnace. The air pressure over the molten steel is reduced to approximately 1mm 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 and 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 in the steel, but it will remove approximately 50% of the gaseous impurities down to a level of approximately 30ppm of oxygen and down to approximately 2.5ppm of hydrogen.
So it can be seen that the procedure will clean up most of the steels impurities (certainly as far as gas is concerned. Sometimes sulfur is removed (or reduced) by the vacuum degas method.
Any steel will contain some minute impurities when being made. Particularly sulfur can be a troublesome impurity. Especially when there is a high manganese content present in the steel. The sulfur can form what is known as plastic inclusions and are generally considered to be non metallics. The sulfur plastic inclusions are seen as (FeMn)S. The strength of the formation of these sulfides will determine the amount of manganese sulfides. The presence of the sulfides in the steel in significant quantities will begin to influence the mechanical properties of the steel and has the potential to reduce the fatigue strength and the transverse ductility and to some extent, the elongation value.
Sulfur can also have a positive effect (when low manganese percentages are present) on the machining characteristics and is now becoming a preferred method of making a ‘free machining steel.’ The older method of assisting the machining characteristics of the steel was to add Lead (Pb) The use of this element (lead) has now almost diminished as a free machining steel because of the toxicity of lead. (In addition, the lead will begin to evaporate and leech out of the steel surface on carburizing steels when carburizing. The resulting effect is a surface porosity of the carburized component containing lead.