Castings destined for Norsok-approved offshore applications must be made from feedstock produced by the argon oxygen decarburisation (AOD) process. We recently supplied a significant quantity of Sanmac® 2205 (F51/S32205) as feedstock for exactly this purpose - a customer producing castings for an offshore end client that required Norsok approvals.

Why the melting route matters

Around 75% of all stainless steel produced globally uses the AOD process. All large mills use it, because it produces cleaner, more consistent steel and allows a wider range of often lower-cost raw materials. The rest of production uses other process types, melting stainless steel scrap. Given the size and cost of AOD vessels, many stainless steel castings are produced by a non-AOD route, and so would not be allowable for Norsok-approved applications.

What AOD is

Argon oxygen decarburisation is used mainly in steelmaking to produce alloys containing oxidisable elements, such as the chromium in stainless steels. Scrap is first melted in a conventional electric arc furnace (EAF), then transferred to the AOD. There, excess carbon is "burned" out with an oxygen lance - but oxygen alone would also oxidise the chromium, losing a large proportion of that expensive element.

The AOD process therefore uses argon alongside oxygen to remove excess carbon preferentially, without removing the desirable chromium. Argon is usually bubbled up through the vessel from the bottom, which also mixes the molten metal and removes inclusions and impurities. After decarburisation, additions bring the composition to its final target: silicon or aluminium remove excess oxygen ("killing" the steel), and limestone takes sulphur into the slag. The result is considerably cleaner than air-melted product, with more consistent composition and fewer inclusions, giving improved mechanical properties and corrosion resistance.

Our stock as feedstock

All of our duplex stainless steel is produced by the AOD process. Foundries may be able to source similarly produced billet if they do not run their own AOD, but lead time and short-term availability can be a challenge. We carry an extensive stock of Sanmac 2205 duplex stainless steel. It is supplied with mechanical and ultrasonic testing as standard - not required for re-melting - but our cost competitiveness still makes it a relevant option. With several hundred tonnes of bar stock in this alloy, availability is never an issue.