Duplex Stainless Steel Castings

The family of Duplex and Super Duplex stainless steels offer significant advantage over the 304 and 316 stainless steels.

The 50:50 ferrite:austenite structure gives improved corrosion and abrasion resistance together with much improved tensile and yield properties which can result in significant design and weight savings - often making duplex a cost effective alternative.

Resistance to SCC (Stress Corrosion Cracking) and pitting corrosion makes duplex an excellent choice for seawater applications.

The addition of nitrogen in the 316LN and duplex grades improves the tensile properties by solid solution strengthening, raises the pitting and crevice corrosion temperatures and particularly in duplex stainless steels lowers the rate of the brittle sigma phase formation.

AOD refining of duplex stainless steels:

Stainless, duplex and nickel alloys can only be refined in an AOD or VOD vessel. The ability to refine and readily control gas contents does not exist when dead melting in an induction furnace.

Goodwin carries out refining in an AOD vessel which removes volatile trace elements and metalloids, reduces carbon and sulphur; it produces a cleaner metal by removing non-metallic inclusions. The 'blow' can purge the melt of gases and metallic oxides to lower levels than are attainable by vacuum induction melting. Nitrogen can be added or removed at will.
In normal induction melting the essential nitrogen is added via expensive nitrogen bearing ferro-alloys, in the AOD the nitrogen is added through the tuyeres as a gas. This is a very economical process route.

Nitrogen in duplex stainless steel:

The nitrogen solubility is increased by chromium, molybdenum and manganese and therefore high levels of nitrogen can be maintained in duplex stainless steel.

However the nitrogen has to be balanced in order to not to exceed the solubility level and give rise to internal porosity