A Guide to Duplex Stainless Steel

Duplex stainless steels are being used more and more often, becoming a standard offering from all major stainless steel mills. New grades are frequently announced and there is plenty of marketing activity surrounding these announcements. Here we look in more detail at duplex stainless steels.

The principles behind the making of duplex stainless steels date back to the 1920s and the first cast was made in Sweden in 1930. Despite this heritage, it is only in the last three decades that the idea has been significantly embraced, due to advances in the techniques of steelmaking (particularly thanks to improvements in the ability to control nitrogen content).

Standard austenitic steel (such as 304) and ferritic steel (such as 430) are, relatively speaking, easy to fabricate: this is because they consist mainly of a single phase. Although both are suited to a wide range of applications, both show technical weaknesses. Austenitic steel, for instance, has a low resistance to stress corrosion cracking and its high nickel content leads to volatility of price while ferritic steels have a relatively low strength, poor low temperature toughness and poor weldability in thick sections.

Duplex steels have a chemical composition with an approximately equal mixture of ferrite and austenite, a balance of phases that gives duplex stainless steel several advantages:

  • Higher strength. The range of 0.2% PS for the current duplex grades is from 400 – 550 MPa, leading to reduced section thicknesses and reduced weight. This represents major advantages for structural applications.
  • Good weldability in thick sections. Though not as straightforward as with austenitic stainless steel, duplex is far better than ferritics for thick-section welding.
  • Improved toughness. Particularly at low temperatures down to minus 80 degrees celsius, duplex stainless steels are much tougher.
  • Resistance to stress corrosion cracking. This is especially important for water tanks.
  • Lower price and better price stability.

Duplex stainless steel is ideal for manufacturing screws. Traditional screws made using 316 stainless steel are more likely to suffer from corrosion than the sheet they are screwed into, whereas duplex is much more resistant to this form of attack. In addition, 316 stainless steel screws are not as hard as their duplex counterparts, making them more difficult to use on sheet materials.

Duplex self-tapping screws, available from BS Stainless, are three times as hard as comparable screws made using 316 stainless steel. Their resistance to corrosion is far superior and they have a much higher PRE value. With hundreds of applications, particularly in the offshore environment, duplex self-tapping screws are as versatile as they are cost-effective.

Find out more about duplex stainless steel screws on the BS Stainless website.

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