Duplex stainless steel

The first type is a low-alloy type, representing the grade UNS S32304 (23Cr-4Ni-0.1N), the steel does not contain molybdenum, the PREN value is 24-25, and can be used instead of AISI304 or 316 in stress corrosion resistance.
The second category belongs to the medium alloy type, representing the brand UNS S31803 (22Cr-5Ni-3Mo-0.15N), the PREN value is 32-33, and its corrosion resistance is between AISI 316L and 6%Mo+N austenitic stainless steel.
The third class is a high alloy type, generally containing 25%Cr, also contains molybdenum and nitrogen, and some also contain copper and tungsten, standard grade UNSS32550 (25Cr-6Ni-3Mo-2Cu-0.2N), PREN value is 38-39, the corrosion resistance of this type of steel is higher than 22%Cr duplex stainless steel.
The fourth category is 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 medium conditions, with good corrosion resistance and mechanical comprehensive properties, comparable to super austenitic stainless steel.