The Argon Oxygen Decarburization Process
Argon oxygen decarburization (AOD) is a unique refinement process for iron, nickel, and cobalt alloys whereby huge quality and efficiency gains can be made.
By loading the furnace charge into the AOD vessel the impurities in the charge can be significantly reduced and at the same time improvements can be made relating to the properties of the resulting end product.
Argon Oxygen Decarburization, frequently referred to as “AOD”, is an advanced technology for the refinement of iron, nickel, and cobalt based alloys. By melting a charge in a furnace, and then placing it into the AOD vessel, the metal producer is able to utilize lower cost charge materials, improve mechanical properties, recover nearly 100% of metallic charge elements, and remove dissolved gases, as well as most of the carbon and sulfur. No other process offers similar capabilities in the foundry.
In 1960’s, the Argon Oxygen Decarburization (AOD) was invented in USA. From then on, this kind of refining equipment rapidly substituted the position of the EAF in the field of stainless steel-making because of its higher efficiency on the decarburization and its simple parts. Up to the end of 20th century, more than 70 percent of the stainless steel was produced by the AOD in the world. The fundamental idea of the AOD to make efficient decarburization is diluting the CO with the inert gas (Ar or N2) which is the outcome of the decarburization chemical reaction in the steel to decrease the partial pressure of the CO. By this method, the carbon in the stainless steel can be removed efficiently without excessive chromium oxidation.
The AOD process (Argon, Oxygen, Decarburization) is the first production process for stainless steel grades in which the treatment time in a converter under atmospheric conditions corresponds to the casting time of a slab caster for many steel grades. On this way an essential prerequisite for the economically efficient production of stainless, acidand heat-resistant steel grades could be fulfilled, Figure 1.
Figure 1: Typical process lines for EAF stainless steelmaking and refining of ferroalloys
Generally, the AOD is combined with the EAF to produce the stainless steel and the raw materials of the EAF are only the scrap and the ferro-alloy. In this way, the power consumption of the EAF (about 450 kwh/t) is higher and the melting duration of the EAF (75 min to 80 min) is longer. To further decrease the power consumption of the EAF and to further shorten the melting duration of the EAF, one feasible solution is to use the hot metal as one of the raw material of the EAF. But higher carbon content in the premetal tapped from the EAF requires quicker decarburization in the AOD. So, from the end of the 20th century, many manufacturers of the stainless steel in the world tried to introduce the top lance in the AOD to supply more oxygen to the steel bath.