Compared with ordinary
austenitic stainless steel, the main difference is that duplex stainless steel has a higher chromium content of 20-28%, and a higher content of molybdenum, up to about 5%; the nickel content is lower, 9 %, the nitrogen content is around 0.05-0.5%.
Both the low nickel content and high strength of duplex stainless steels (enabling the use of thinner sections) are clearly cost-effective. Therefore, they are widely used in pipeline systems in the offshore oil and gas industry, and are increasingly popular in the pipeline and pressure vessel industry in the petrochemical industry.
Duplex stainless steel not only has higher corrosion resistance than 300 series stainless steel, but also has higher strength. For example 304 stainless steel has a 0.2% yield strength in the region of 280N/mm2, 22% chromium duplex stainless steel has a minimum 0.2% yield strength of about 450N/mm2, and super duplex grades have a minimum 0.2% yield strength of 550N/mm2 .
Although duplex stainless steels are highly resistant to corrosion and oxidation, they cannot be used at high temperatures. This is because the ferrite phase in it can form brittle phases at lower temperatures, which can have a catastrophic effect on the toughness of duplex stainless steels. As a result, the ASME Pressure Vessel Code limits the service temperature of all grades to 315°C, with other codes specifying even lower service temperatures, perhaps as low as 250°C for super duplex steels.
Duplex alloys can be divided into three categories, poor duplex, 22% Cr duplex and 25% Cr super duplex, and even higher alloyed super duplex steel grades. This division is mainly based on the alloying of duplex stainless steel Levels, such as equivalent pitting resistance numbers, measure the alloy's resistance to pitting corrosion. PREN is calculated by a simple formula: PREN = %Cr + 3.3% Mo + 16% N, sometimes a factor of 1.65 for W can be considered. Duplex steels have PRENs less than 40; supercomposites have PRENs between 40-45, and super complex PRENs are over 45, while leaner grades usually have lower nickel and therefore are less expensive.
