Argon oxygen decarburization: Difference between revisions

Revision as of 02:30, 28 December 2013

Revision as of 02:36, 28 December 2013

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{{Refimprove|date=November 2010}}

[[File:AOD (Argon, oxygen decarburisation) vessel.jpg|thumb|Refining of a 9.5%CrMoWVNbN steel in an argon, oxygen decarburisation (AOD) vessel]]

'''Argon oxygen decarburization''' ('''AOD''') is a process primarily used in [[stainless steel]] [[steel making|making]] and other high grade alloys with oxidizable elements such as [[chromium]] and [[aluminum]]. After initial melting the metal is then transferred to an AOD vessel where it will be subjected to three steps of refining; [[decarburization]], [[Reduction (chemistry)|reduction]], and [[desulphurization]]. AOD was invented in 1954 by the Lindé Division of The Union Carbide Corporation <ref>{{cite journal|title=The linde argon-oxygen process for stainless steel; A case study of major innovation in a basic industry|journal=Metallurgical Transactions|year=1973|volume= 4|issue =6|pages=1439–1447|author=Krivsky, W. A.|url=http://wwwlink.springerlinkspringer.com/content/c6g66r71262g0158article/10.1007%2FBF02667991|doi=<!-- broken --> }}</ref> (which became known as [[Praxair]] in 1992).<ref>[http://www.unioncarbide.com/history/index.htm History]. Unioncarbide.com (1917-11-01). Retrieved on 2013-12-28.</ref>

==Process==

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The decarburization step is controlled by ratios of oxygen to argon or nitrogen to remove the carbon from the metal bath. The ratios can be done in any number of phases to facilitate the reaction. The gases are usually blown through a top lance (oxygen only) and tuyeres in the sides/bottom (oxygen with an inert gas shroud). The stages of blowing remove carbon by the combination of oxygen and carbon forming CO gas.

:4 Cr<mathsub>\mathrm{4Cr_{(bath)}</sub> + 3 O_2O₂ \longrightarrow→ 2Cr_2O_{32 Cr₂O<sub>3(slag)}}</mathsub>

:Cr<mathsub>2</sub>O_{\mathrm{Cr_2O_{3 (slag)}} + 3 C _{_{(bath)}\longrightarrow} → 3 CO _{_(gas)} + 2 Cr _{<sub>(bath)}}</mathsub>

To drive the reaction to the forming of CO, the partial pressure of CO is lowered using argon or nitrogen. Since the AOD vessel isn't externally heated, the blowing stages are also used for temperature control. The burning of carbon increases the bath temperature.

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Desulphurization is achieved by having a high lime concentration in the slag and a low oxygen activity in the metal bath.

:S<mathsub>\mathrm{S _{(bath)}</sub> + CaO _{_(slag)} \longrightarrow→ CaS_{CaS_(slag)} + O _{<sub>(bath)}}</mathsub>

So, additions of lime are added to dilute sulfur in the metal bath. Also, aluminum or silicon may be added to remove oxygen. Other trimming alloy additions might be added at the end of the step. After sulfur levels have been achieved the slag is removed from the AOD vessel and the metal bath is ready for tapping. The tapped bath is then either sent to a stir station for further chemistry trimming or to a caster for casting.