Duplex steels are grades of the metal that combine the features of stainless chrome steels and stainless chrome nickel steels. They have remarkable advantages such as increased strength, good weldability and high resistance to various types of corrosion. In the last 30 years duplex has become more popular largely to advances in steel techniques.
Duplex and super duplex grades are formed from around a 50/50 mixture of ferritic and austenitic steels. This is in order to combine the strength and improved corrosion resistance of ferritic steels with the toughness of austenitic stainless steels. Therefore duplex steels have advanced properties when compared to ferritic and austenitic steels alone. This means duplex steels are suited to a variety of industries including desalination, waste water treatment and storage for biofuels.
Duplex steel was originally developed in the early 1930’s in France and Sweden. This early version of the steel had good performance characteristics but became limited after it had been welded.
In the 1960’s and 1970’s, advances in the vacuum and argon oxygen decarbonisation process meant that a low carbon content, high chromium and high nickel content could be made. This produced a grade of steel with the properties we know today – a good resistance to corrosion, including resistance to stress corrosion cracking, high yield strength and fatigue strength.
Additional benefits can include the ability of some duplex stainless steels to be used at very cold temperatures. The developments in duplex steels during this time coincided with offshore growth, especially in the offshore oil industry. As a result the steel industry flourished as duplex could easily handle these tough environments.
In the 1980’s the oil industry demanded even higher alloyed duplex materials as they went to greater depths. Super duplex stainless steel was produced where the grades are made of 25% chromium or more and a pitting resistance equivalent number (PREN) higher than 40. The PREN is an industry recognised scale, with steels normally falling into the high 30’s category. Since it is more highly alloyed, super duplex has an even higher resistance to pitting, crevice and stress corrosion cracking. In terms of other benefits, the high allowable design stress of this alloy offers excellent castability, weldability and machinability.
Overall the versatility and price stability of duplex steels means that they are a popular choice in a range of industries. More and more they are recognised as a material with low maintenance and a long shelf life. With an ability to perform consistently over this time it proves that duplex and super duplex steels are here to stay.