Duplex stainless steels combine high strength and excellent corrosion resistance in a cost-effective way, which is why they are so widely used. There are, however, two situations where they are not the best choice: very low temperature service, and prolonged exposure to higher temperatures.
Low-temperature toughness
Duplex stainless steels contain a mixed microstructure of austenite and ferrite phases. Any steel based on a ferritic microstructure, whether carbon steel or stainless, undergoes a ductile-to-brittle transition as temperature falls. Somewhere between -50 and -100degC the impact toughness drops significantly as the metal becomes more brittle. It does not lose strength; it simply absorbs less energy when it breaks. Because duplex steels contain a proportion of ferrite grains, they share this limitation and are less likely to be chosen for very low temperature service.
Sigma phase at higher temperatures
Duplex stainless steels tend to form the deleterious sigma phase if held for long periods at higher temperatures. Sigma is a chromium-rich intermetallic phase that forms when an alloy cools slowly through the range from around 1000degC to 550degC.
These chromium-rich particles strip chromium from the surrounding metal, reducing its resistance to pitting corrosion, and they significantly reduce impact toughness at lower temperatures. In production this is prevented by ensuring the quench after heat treatment is done effectively. Sigma can also form if the long-term operating temperature is above 250degC, so we do not recommend using super duplex stainless steels above 250degC.
Fabrication and machining
Some believe super duplex stainless steels are harder to fabricate and weld, but they are processed without problem as long as the correct consumables and procedures are used. Machining can be more difficult given the high strength of these grades, but suitable tooling and settings are widely available from vendors.