Argon oxygen decarburization
A refining process for stainless steel and high-grade alloys.
Argon oxygen decarburization (AOD) is a refining method mainly used to make stainless steel and other high-grade alloys that contain easily oxidized elements like chromium and aluminum. After an initial melt, the metal is moved into an AOD vessel, where it goes through three refining stages: decarburization, reduction, and desulfurization. The process was created in 1954 by the Lindé Division of Union Carbide Corporation, later known as Praxair from 1992.
The AOD process typically has three main steps: decarburization, reduction, and desulfurization.
Before decarburization, de-siliconization is important for protecting the refractory lining and aiding further refinement. The decarburization step is controlled by adjusting the ratio of oxygen to argon or nitrogen, which removes carbon from the molten metal. These ratios can be changed in several phases to drive the reaction. Gases are blown through a top lance (oxygen only) and through side or bottom tuyeres (oxygen mixed with an inert gas shroud). The blowing stages remove carbon as oxygen and carbon combine to form carbon monoxide gas.
4 Cr(bath) + 3 O2 → 2 Cr2O3(slag) Cr2O3(slag) + 3 C(bath) → 3 CO(gas) + 2 Cr(bath)
To push the reaction toward forming CO, the partial pressure of CO is lowered using argon or nitrogen. Since the AOD vessel has no external heating, the blowing stages also control temperature—burning carbon raises the bath temperature. By the end of this step, about 97% of the chromium remains in the steel.
Once the desired carbon level and temperature are reached, the process moves to reduction. This step recovers oxidized elements like chromium from the slag. Alloys with a higher affinity for oxygen than chromium—such as silicon alloys or aluminum—are added. The reduction mix also includes lime (CaO) and fluorspar (CaF2), which help drive the reduction of Cr2O3, manage the slag, keep it fluid, and minimize its volume.
Desulfurization relies on a high lime concentration in the slag and low oxygen activity in the metal bath.
S(bath) + CaO(slag) → CaS(slag) + O(bath)
Lime is added to dilute sulfur in the metal bath, and aluminum or silicon may be added to remove oxygen. Other trimming alloys might be added at the end of this step. Once sulfur levels are met, the slag is removed from the AOD vessel, and the metal bath is ready for tapping. The tapped bath is then sent either to a stir st
- field
- Steelmaking and metallurgy
- inventor
- Lindé Division of The Union Carbide Corporation (later Praxair)
- year_invented
- 1954
- primary_application
- Stainless steel and high grade alloys
- key_steps
- Decarburization, reduction, desulfurization
Lore & Background
The AOD process is usually divided in three main steps: decarburization, reduction, and desulfurization. Prior to decarburization, de-siliconization is an important factor for refractory lining and further refinement. The decarburization step is controlled by ratios of oxygen to argon or nitrogen to remove carbon from the metal bath, with gases blown through a top lance and tuyeres. The stages of blowing remove carbon by the combination of oxygen and carbon forming CO gas, and by the end of this process around 97% of chromium is retained in the steel.
After a desired carbon and temperature level have been reached, the process moves to reduction, which recovers oxidized elements such as chromium from the slag using alloy additions with elements that have a higher affinity for oxygen than chromium, such as silicon or aluminium. The reduction mix also includes lime and fluorspar to help drive the reduction and manage slag fluidity. Desulfurization is achieved by having a high lime concentration in the slag and a low oxygen activity in the metal bath, with additions of lime to dilute sulfur and aluminium or silicon to remove oxygen.
Reader's Guide
The AOD process has a significant place in the history of steelmaking, introducing a transformative method for refining stainless steel and shaping the industry's landscape. Its development began in the 1960s as an alternative to traditional steelmaking methods, initially introduced by American chemical companies. In the late 1960s, the process gained recognition for its ability to remove carbon efficiently, achieving lower carbon levels than other refining methods. During the 1970s, steel companies in Europe and the United States increasingly adopted the AOD method, attracted by its flexibility and ability to produce high-quality stainless steel. By the 1980s, the AOD process became widely accepted as a standard refining method for stainless steel worldwide, offering advantages such as high metallic yields, precise control over chemical composition, carbon control, desulfurization capabilities, and cleaner metal production. Today, the AOD process remains a prominent method in the stainless steel industry, offering steelmakers greater flexibility in raw material selection and enabling increased production capacity with relatively small capital investments compared to conventional electric furnace methods.
Did You Know?
- The AOD process was invented in 1954 by the Lindé Division of The Union Carbide Corporation, which became known as Praxair in 1992.
- By the end of the decarburization step, around 97% of chromium is retained in the steel.
- AOD slag has shown promising potential for usage as a carbon-capture construction material due to its high capacity for CO2 and its low cost.
- The desulfurization step is usually the first step of the process.
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