Duplex stainless steel (Duplex Stainless Steel, referred to as DSS), refers to the ferritic and austenitic each accounted for about 50%, generally the content of the lesser phase of the stainless steel needs to reach a minimum of 30%. 2205 stainless steel (S32205) due to the austenitic stainless steel compared to the lower cost, better corrosion resistance, used as flue gas desulphurisation plant absorption tower materials. Early desalination projects, the use of duplex stainless steel evaporator can reduce the overall impact on the material marine environment. In the oil and gas industry, duplex stainless steel is mainly used in fluid pipes, process piping systems and equipment, such as separation devices, scrubbing devices and pumps; food and beverage industry used as food storage tanks.
Early grades were alloys of chromium, nickel and molybdenum and were originally developed to minimise intergranular corrosion problems. Later, both forged and cast duplex stainless steel grades were used in a variety of process industries, including vessels, heat exchangers and pumps. 1968 stainless steel refining and argon oxygen decarburisation (AOD) process was invented, so that a series of new stainless steel grades to produce possible.
Duplex stainless steel physical properties between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel.
Duplex stainless steel is divided into low alloy type, medium alloy type, high alloy type and super duplex stainless steel type. Low alloy type representative grade UNS S32304 (23Cr-4Ni-0.1N), the steel does not contain molybdenum, PREN value of 24-25. The medium alloy type representative grade is UNS S31803 (22Cr-5Ni-3Mo-0.15N), PREN value of 32-33; The high alloy type generally contains 25% Cr, but also contains molybdenum and nitrogen, and some contain copper and tungsten. The standard grade UNSS32550 (25Cr-6Ni-3Mo-2Cu-0.2N), PREN value of 38-39, the corrosion resistance of this type of steel is higher than that of the duplex stainless steel with 22% Cr. The super duplex stainless steel type containing high molybdenum and nitrogen, standard grade UNS S32750 (25Cr-7Ni-3.7Mo-0.3N), some also contain tungsten and copper, PREN value is greater than 40, can be applied to harsh media conditions with good corrosion resistance and mechanical properties.
Modern duplex stainless steel can be divided into five types
– Economical duplex stainless steels without added molybdenum such as 2304;
– Standard duplex stainless steels such as 2205, which is the main type of steel, accounting for more than 80% of duplex steel usage;
– 25Cr duplex stainless steels such as alloy 255, with a PREN value of less than 40;
– Super duplex stainless steels (PREN values of 40 to 45), containing 25% to 26% Cr and higher levels of molybdenum and nitrogen than 25Cr duplex stainless steels, such as grade 2507;
– Extra super duplex stainless steels, highly alloyed duplex stainless steels with PREN values above 45.
