We at Chemipetro produce a wide range of pipe fittings and flanges manufactured from duplex stainless steels, But what exactly are these types of steels and why use duplex steel rather than ordinary steels?

To answer these questions it is necessary to understand what exactly stainless steel is.

Ordinary carbon steel is strong and as such has many applications. However it has one draw back, it rusts when exposed to air and moisture. Under these conditions a film of iron oxide is formed which causes the corrosion to accelerate and more iron oxide to form.

To prevent this happening, chromium is add to produce an alloy. This was first developed in 1821 by Pierre Berthier, a French metallurgist. However these first stainless steels were too brittle to be of much practical use. It was not until the early 1900’s that a process was developed for producing a practical stainless steel.

Stainless steel resists corrosion by the chromium forming a protective layer of chromium oxide. This layer is impervious to water and air and so protects the metal beneath.

Stainless steels are categorised by their crystalline structure, the most common being austenitic steels which contain nickel or manganese as well as the chromium. However these steels can be expensive especially with the higher grades which also contain molybdenum.

Duplex steels have a mixed structure of austenite and ferrite, giving them improved strength over austenitic stainless steels and improved resistance to local corrosion. This means that for a given strength, castings can have reduced thickness and weight, thus making them cheaper.