Duplex stainless steels have many attractive properties, with their high strength and excellent corrosion resistance achieved in a cost-effective way. As such, they have become widely used in a wide variety of applications. However, there are instances where their characteristics are not best suited and other grades may have to be used.
‘Duplex’ stainless steels are a family of stainless steels that contain a mixed microstructure of austenite and ferrite phases. Steels based upon a ferritic microstructure, whether that’s carbon steels or stainless steels, undergo a ductile-to-brittle transition as the temperature reduces. Somewhere between -50 to -100degC the impact toughness falls significantly, as the metal becomes more brittle in nature. That is not to say it becomes lower strength, just that it absorbs less energy when it does break. As duplex steels contain a proportion of ferrite grains, they have the same limitation, so are less likely to be selected for applications where very low temperature service is expected.
Duplex stainless steels tend to form the deleterious sigma microstructural phase if subjected to long periods at higher temperatures. Sigma is a chromium-rich intermetallic phase found in stainless steel. It forms when an alloy is cooled slowly through the range from around 1000degC to 550degC. The formation of these chromium-rich particles denudes the surrounding metal of chromium, thereby reducing its resistance to pitting corrosion. In addition, these particles significantly reduce the impact toughness of the alloy at lower temperatures. This is prevented during production by ensuring the quenching operation after heat treatment is performed effectively. However, sigma can also occur if the long-term operating temperature is above 250degC. Therefore, it is not recommended to use super duplex stainless steels above 250degC.
Whilst some people may feel that super duplex stainless steels are more complicated to fabricate and weld, they are widely processed without problem as long as the correct consumables and procedures are adopted. Machining can be more difficult given the high strength of these grades, but tooling and settings for super duplex stainless steels are widely available from vendors.
For more information about our range of alloys, please contact Langley Alloys today.