What is Argon Oxygen Decarburization? Steel Foundations
Argon oxygen decarburization, or ‘AOD’, is a process primarily used in the production of stainless steel or other high-grade steel alloys where the alloying elements, such as chromium and aluminum, are highly reactive with the oxygen in the atmosphere.
After initial melting in one of the primary steelmaking processes, the metal is transferred to an AOD vessel or converter, where it will be subjected to three refining steps: decarburization, reduction, and desulphurization.
The decarburization step is regulated by controlling the ratio of oxygen and an inert gas, like argon or nitrogen, to remove the carbon from the metal bath without removing the oxidizable alloying elements, such as chromium and aluminum. The gases are usually blown through a top lance (oxygen only) and tuyeres in the sides/bottom (oxygen with an inert gas shroud). The carbon is removed through several chemical reactions by combining oxygen and the carbon in the liquid steel to form carbon dioxide (CO) gas. To help drive the reactions that form the CO, the partial atmospheric pressure of CO is lowered by using argon or nitrogen to displace it around the liquid steel bath.
The reduction step uses elements with a higher affinity for oxygen than chromium, such as silicon or aluminum, to recover oxidized elements, such as chromium, that have been absorbed by the slag and return them to the molten steel. These additions will remove the oxygen from the chromium oxide molecules formed in the decarburization step and allow the chromium to go back into the liquid steel bath. Other additions such burnt lime (CaO) and fluorspar (CaF2) are added to the slag to keep it fluid, which speeds these chemical reactions. By the end of this process, around 97% of Chromium Cr is retained in the steel.
The desulfurization step is achieved by having a high lime concentration in the slag and a low oxygen content in the metal bath, where additions of lime dilute and reduce the sulfur content in the metal bath. Aluminum or silicon may also be added to remove oxygen, and additional alloys are added at the end to fine-tune the final steel chemistry.
Upon completion of the process, the metal is then poured back out of the AOD to a steel ladle for further refinement.