Around 75% of all stainless steel products produced globally will make use of the AOD process. All large steel mills will utilise this process as it allows them to produce cleaner steel with more consistent quality, as well as utilise a wider range of (often lower cost) raw materials. The balance of production will utilise several different process types, melting steel and stainless steel scrap. Given the size and cost of AOD vessels, then many stainless steel castings are produced using a non-AOD route, and would therefore not be allowable for Norsok approved applications.
What is AOD?
Argon oxygen decarburization is a process primarily used in steelmaking to produce alloys with oxidizable elements i.e stainless steels that contain chromium. Firstly, the steel scrap is melted in a conventional EAF (electric arc furnace) before being transferred to the AOD. Here, excessive levels of carbon can be removed by effectively ‘burning’ them out with an oxygen lance. However, this would also oxidise the chromium present too, thereby losing a significant proportion of this expensive raw material.
Therefore, the AOD process uses argon as well as oxygen to preferentially remove the excess carbon without removing the desirable chromium. The argon is normally bubbled through the vessel from the bottom, which also helps to mix the molten metal and remove inclusions and other impurities. After removal of the carbon, the AOD process will then use additions to achieve the final composition; silicon or aluminium are added to remove excess oxygen (‘kill the steel’) and then limestone is added to remove sulphur into the slag. As a result, AOD-produced steel is considerably cleaner than simple air-melted products, with more consistent composition and fewer inclusions contributing to improved mechanical properties and corrosion resistance.
All of Langley Alloys duplex stainless steel is produced via the AOD process. Foundries may be able to source billet that has been produced in a similar way, assuming they do not operate their own AOD, but lead-time or short-term availability may be a challenge. Langley Alloys carry an extensive stock of Sanmac™ 2205 duplex stainless steel. Although it will have been produced with mechanical and ultrasonic testing as standard, which is not required for re-melting applications, our cost competitiveness makes it a relevant option. With several hundred tonnes of bar stock in this alloy, availability is never an issue.
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