Vacuum Oxygen Decarburisation (VOD)

The vacuum oxygen decarburisation process was developed specifically for the production of stainless steel.  The blowing of oxygen under vacuum conditions together with the use of low cost ferroalloys offers a time and cost saving method of producing stainless, corrosion and heat resisting steel grades.
The ladle of liquid stainless steel is placed in a vacuum tank by the overhead crane. The ladle is stirred using inert gas through a porous plug.  Once the cover is placed on the vacuum tank, the vacuum tank is then evacuated by a set of powerful vacuum pumps connected to the tank.

At a predetermined pressure an oxygen lance which is located in the vacuum cover is lowered to a set distance above the liquid steel. Oxygen is then blown onto the surface of the liquid stainless steel at a controlled rate, whilst the liquid stainless steel is stirred by inert gas.

Because the reaction takes place under vacuum, the thermodynamics of the process favour the formation of a gaseous product which is continuously removed from the vacuum tank by the vacuum pumping system. In this case the oxygen blown onto the surface of the liquid stainless steel reacts with dissolved carbon to form carbon monoxide, rather than with chromium in the steel to form chromium oxide. By oxygen blowing under vacuum, much lower levels of carbon can be achieved with significantly lower chromium oxidation than would be possible at atmospheric pressure.

The vacuum oxygen decarburisation process therefore allows the use of high carbon ferrochromium in the furnace charge as opposed to the more expensive low carbon ferrochromium.

Without the application of secondary metallurgical technology it would be difficult to equal the cost effectiveness of this process.

Advantages:

• Vacuum decarburisation from virtually any practical initial carbon level.
• Controlled achievement of very low carbon, nitrogen, hydrogen and sulphur levels.
• Special grades, e.g. superferrite, can be produced.
• Small chromium losses.
• Narrow composition tolerances.