Special Piping Materials

The metallurgy of duplex stainless steel

The metallurgy of duplex stainless steel is very interesting indeed. It is made from two different grades of metal (ferritic and austenitic), and it therefore benefits from the properties of both microstructures.

The metallurgy of duplex stainless steel can be characterised by the fact that it is a Fe-Ni-Cr alloy, and it has a two-phase ferritic-austenitic stainless-steel microstructure when it is at room temperature.

Duplex stainless steels are characterised by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels. The most used duplex stainless steels are the 2205 (22% Chromium, 5% Nickel) and the 2507 (25% Chromium, 7% Nickel); 2507 is known as “super duplex” due to its higher resistance to corrosion.

Duplex stainless steel is highly sought after in heavy industries, like oil and gas nuclear and chemical processing.

The metallurgy of duplex stainless stee

The metallurgy of duplex stainless steel

The origin of duplex steels can be traced back to the 1920s, with the first cast being made in Sweden in 1930. The popularity of duplex started to rise in the 1990s when steelmaking technology became more advanced.

The production of a metal with a ferritic-austenitic microstructure was ground-breaking as it meant that it had higher strength, good weldability, good toughness and resistance to stress corrosion cracking.

Compared with austenitic grades, duplex steels characteristically have high chromium content but a rather low nickel content. Molybdenum and nitrogen are added to the metal in order to improve corrosion resistance and balance the microstructure. Nitrogen also increases the mechanical strength.

Varieties of Duplex Stainless Steel

Duplex stainless steels are typically divided into three groups based on their pitting corrosion resistance. The three groups are:

Properties of Duplex Stainless Steel

Duplex stainless steel has a variety of useful properties including:

 

This array of properties means that duplex stainless steel can be found in many different applications such as:

 

Weldability of Duplex Stainless Steel

Welding duplex stainless steel is not particularly difficult but it has to be approached differently to other steels. It is generally thought that the weldability and welding characteristics of duplex stainless steels are better than those of ferritic steels, although maybe not as good as austenitic steels. Interestingly, modern duplex steels with improved nitrogen content are readily weldable.

It is important to note that the properties of duplex steel can be affected by welding parameters such as heat input. This is because the duplex microstructure is more sensitive compared to standard austenitic grades.

Rapid cooling from the high temperatures used during the welding process result in high ferrite levels in the weld metal and adjacent base metal. To combat this, filler metals are used which have been specially designed with higher nickel contents to produce a phase balance like that of the base material.

Consequently, this means that welders should be sure to carry out correct welding procedures to obtain the acceptable weldment structure and properties.

 

Special Piping Materials is well known for the high-quality duplex products that we source and stock for our discerning clients around the world.  We can provide a whole range of different products in duplex, including specialist fittings & flanges, pipe, plate, sheet, tube and bar.

For more information about the metallurgy of duplex stainless steel and the products we stock, please contact your local Special Piping Materials team.

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