Super Duplex
Super Duplex stainless steels have a ‘duplex’ two-phase microstructure consisting of both austenitic and ferritic grains that gives them a combination of attractive properties. Whilst duplex alloys were developed around a 22% Cr addition level, super duplex alloys utilise a higher 25%Cr base composition to increase the level of corrosion resistance achievable.
In general, they are twice as strong as either austenitic or ferritic stainless steels. They achieve good toughness and ductility, somewhere between the two. Their corrosion resistance is also very good, assuming comparable levels of chromium, molybdenum and nitrogen in selected compositions. By increasing the level of chromium additions, combined with significant levels of molybdenum and nitrogen, super duplex stainless steels achieve a Pitting Resistance Equivalent number (PREN) >40, which will match or beat all but the most highly alloyed austenitic stainless steels and often provide comparable performance to some nickel alloys also. One further advantage over austenitic stainless steels is their resistance to stress corrosion cracking.
The addition of copper to these grades results in additional corrosion resistance in use not directly recognised by the formula used to calculate PREN. It has been shown that the presence of copper can inhibit the formation of pitting corrosion, and it also contributes to exceptional resistance to sulphuric acid. Resistance to sulphuric, nitric and phosphoric acid makes these products attractive for a number of applications in the chemical processing industry. Ferralium® has an appreciable higher copper content than other super duplex stainless steels, imparting superior corrosion resistance.
Super duplex stainless steels offer a cost effective option for many demanding applications – as their design foregoes high nickel additions which are expensive and subject to price volatility. If the application can utilise the high strength levels of these grades then components can be designed with smaller sections, using less material. This can benefit as a direct material cost saving, but can also contribute to overall design savings if the application involves suspended loads i.e. marine sensors, down-hole tooling.
