Duplex Steels are composed of a mix of Austenitic an Ferritic types. They offer:
- Higher strength leading to weight saving
- Greater corrosion resistance particularly stress corrosion cracking
- Better price stability
- Lower price
The basic idea of duplex is to produce a chemical composition that leads to an approximately equal mixture of ferrite and austenite. This balance of phases provides the following:
- Higher strength – The range of 0.2% PS for the current duplex grades is from 400 – 550 MPa. This can lead to reduced section thicknesses and therefore to reduced weight. This advantage is particularly significant for applications such as:
- Pressure Vessels and Storage Tanks
- Structural Applications e.g. bridges
- Good weldability in thick sections – Not as straightforward as austenitics but much better than ferritics.
- Good toughness – Much better than ferritics particularly at low temperature, typically down to minus 50 deg C, stretching to minus 80 deg C.
- Resistance to stress corrosion cracking – Standard austenitic steels are particularly prone to this type of corrosion. The kind of applications where this advantage is important include:
- Hot water tanks
- Brewing tanks
- Process plant
- Swimming pool structures
The combination of high strength, corrosion resistance and moderate weldability has many benefits but also bring disadvantages and limitations.
High strength is a disadvantage when forming and machining. The higher strength also means that the metal is less ductile than austenitic grades. This means that these steels are not good when the goods being produce require any degree of complex forming.
It is also worth bering in mind that evemn if the item can be formed in duplex steel greater forces are required.
The metallurgy of duplex stainless steels is much more complex than for austenitic or ferritic steels whith the result that they are more complex and therefore expensive to produce. Howver the lower quantity of nickel in them as compared to austenitic grades does help to keep the cost down and reduce price volatility.