The high corrosion resistance and the excellent mechanical properties combination of duplex stainless steels can be explained by their chemical composition and balanced ('duplex') microstructure of approximately equivalent volume fractions of ferrite and austenite. Firstly, the chemical composition based on high contents of Cr and Mo, improves intergranular and pitting corrosion resistance, respectively. Moreover, additions of nitrogen can promote structural hardening by interstitial solid solution mechanism, which raises the yield strength and ultimate strength values without impairing toughness. Secondly, the two-phase microstructure guarantees higher resistance to pitting and stress corrosion cracking in comparison with conventional stainless steels. The duplex stainless steels comprise a family of grades with a range in corrosion performance depending on their alloy content. Modern duplex stainless steels are often addressed in four groups:
Lean duplex, such as 2304 (S32304), which contain little or no deliberate Mo addition;
2205 (S32205), the workhorse grade accounting for more than 80% of duplex use;
25 Cr duplex, such as 255 (S32550) and S31260;
Superduplex, with 25-26 Cr and increased Mo and N, such as 2507 (S32750).
The table lists the duplex stainless steels covered in ASTM specifications for plate, sheet, and bar products. Although few of the duplex grades are patented, many of these grades are closely associated with particular steel producers as proprietary developments. This situation is often reflected in the naming of the grades. Only one duplex stainless steel has an AISI designation, Type 329, a first generation duplex stainless steel without intentional nitrogen addition. Many of the grades have become commonly known by a number that reflects their typical chromium and nickel contents, e.g., 2205 with 22% Cr and 5% Ni. These composition-based names, used by many producers, have recently been added to A 240 and are being added to the ASTM specifications for other product forms. Trademarks of individual producers are not permitted in the ASTM specifications. All grades in the ASTM specifications are all listed by their designations in the Unified Numbering System (UNS). It is important to reference the UNS number when ordering to the standard specifications in order to get the desired grade.
Composition of Duplex Stainless Steelsa
a Weight percent, maximum unless otherwise noted. b Unless otherwise indicated, a common name, not a trademark, widely used, not associated with any one producer, as listed in ASTM A 240. c W 0.50-1.00; Cr+3.3Mo+16N=40 min. d AISI designation