A Simple Guide to Duplex Stainless Steel

The super duplex stainless steel is coming to be more popular. Most stainless steel mills now offer it because it is higher strength, which saves on weight, has greater resistance to corrosion, has a more stable price, and generally has a lower price. Every two to three years there is a conference about duplex which leads to the presentation of many technical papers and marketing activity. This has led to the creation of many new grades of duplex at a frequent rate. Despite all of this, market share for duplex is only between one and three percent. Here are a few more key things to know about duplex.

Principle

Duplex stainless steels are not a new invention. In the 1920s the first cast of this metal was made in Avesta, Sweden. It didn’t become widely used until the last 30 years. Its increased popularity is a result of advanced in steelmaking techniques specifically as they relate to controlling nitrogen content. The other, standard types of austenitic and ferritic steels are made more easily, as they consists mainly of one stage. They are both however, low strength. Austenitic also has the disadvantage of having a low resistance when it comes to corrosion cracking and having a high price volatility due to its high nickel content, while ferritic does not perform well in low temperature conditions, and has low weldability as it relates to thick sections. Duplex, as the name implies, is made of an equal mixture of ferrite and austenite.

Advantages of Duplex

Duplex is higher strength as a result of its composition. This allows for reduced thickness and less weight. This is especially useful for pressure vessels, storage tanks and structural applications. It has higher weldability than ferritic steel in thicker sections, although it is admittedly less weldable than austenitics. It is also tougher than ferritic steel, especially at low temperatures. Duplex also resists stress corrosion cracking, which is of particular use in hot water and brewing tanks, processing plants, and structures in swimming pools.

How It’s Made

Ferritisers include chromium, niobium, tungsten, titanium, molybdenum and silicon. Austenitisers include carbon, copper, nickel, nitrogen and manganese. Due to the need for an equal balance of these elements in duplex, the nickel content is generally lower. More modern grades use nitrogen and manganese together resulting in very low nickel content, resulting in greater price stability. There are many, in fact maybe too many grades due to the fact that we are still in the development stage of duplex, and this will remain the case until eventually some grades come out on top.

Barriers

While it may seem that duplex is the only choice at this point, it does have its disadvantages. Due to its high strength, duplex falls short when it comes to machinability and formability. As a result, it is not ideal for applications that require a high degree of formability, as sinks would. Duplex is also more difficult to produce and fabricate because of its complex metallurgy. It is also not immune to forming unwanted phases if it is not processed correctly, especially in heat treatment.