An ingot of 2507 duplex stainless steel

Duplex stainless steels^{[1][2][3][4][5]} are a family of stainless steels. These are called duplex (or austenitic-ferritic) grades because their metallurgical structure consists of two phases, austenite (face-centered cubic lattice) and ferrite (body centered cubic lattice) in roughly equal proportions. They are designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as type A2/304 or A4/316. The main differences in composition, when compared with an austenitic stainless steel is that the duplex steels have a higher chromium content, 20–28%; higher molybdenum, up to 5%; lower nickel, up to 9% and 0.05–0.50% nitrogen. Both the low nickel content and the high strength (enabling thinner sections to be used) give significant cost benefits. They are therefore used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc. and in the petrochemical industry in the form of pipelines and pressure vessels. In addition to the improved corrosion resistance compared with the 300 series duplex stainless steels also have higher strength. For example, a Type 304 stainless steel has a 0.2% proof strength in the region of 280 MPa (41 ksi), a 22%Cr duplex stainless steel a minimum 0.2% proof strength of some 450 MPa (65 ksi) and a superduplex grade a minimum of 550 MPa (80 ksi).^[6]

Grades of duplex stainless steels

Microstructures of four kinds of duplex stainless steel in each direction

Duplex stainless steels are usually divided into three groups based on their pitting corrosion resistance, characterised by the pitting resistance equivalence number, PREN = %Cr + 3.3 %Mo + 16 %N.^[7]

Standard duplex (PREN range: 28–38)

Typically Grade EN 1.4462 (also called 2205). It is typical of the mid-range of properties and is perhaps the most used today

Super-duplex (PREN range: 38–45)

Typically grade EN 1.4410 up to so-called hyper duplex grades (PREN: >45) developed later to meet specific demands of the oil and gas as well as those of the chemical industries. They offer a superior corrosion resistance and strength but are more difficult to process because the higher contents of Cr, Mo, N and even W promote the formation of intermetallic phases, which reduce drastically the impact resistance of the steel. Faulty processing will result in poor performance and users are advised to deal with reputable suppliers/processors.^[8] Applications include deepwater offshore oil production.

Lean duplex grades (PREN range: 22–27)

Typically grade EN 1.4362, have been developed more recently for less demanding applications, particularly in the building and construction industry. Their corrosion resistance is closer to that of the standard austenitic grade EN 1.4401 (with a plus on resistance to stress corrosion cracking) and their mechanical properties are higher. This can be a great advantage when strength is important. This is the case in bridges, pressure vessels or tie bars.

Chemical compositions

Chemicals composition of grades from EN 10088-1 (2014) Standard are given in the table below:^[9]

Mechanical properties

Mechanical properties from European Standard EN 10088-3 (2014)^[9] (for product thickness below 160 mm):

*for thickness ≤ 5 mm (0.20 in)

The minimum yield stress values are about twice as high as those of austenitic stainless steels.

Duplex grades are therefore attractive when mechanical properties at room temperature are important because they allow thinner sections.

475 °C embrittlement

Electron backscatter diffraction map of 128 hrs age hardened duplex stainless steel with the ferrite phase forming the matrix and austenite grains sporadically spread. The ferrite phase volume fraction is 58%.^[11]

EBSD map with austenite grains excluded (white). The scale bar is 500 µm. Colours denote the crystal orientation and are taken from the inverse pole figure at the lower right corner.^[12]

EBSD map with austenite grains excluded (white). The scale bar is 500 µm. Colours denote the crystal orientation and are taken from the inverse pole figure at the lower right corner.Duplex stainless is widely used in the industry because it possesses excellent oxidation resistance but can have limited toughness due to its large ferritic grain size, and they have hardened, and embrittlement tendencies at temperatures ranging from 280 to 500 °C, especially at 475 °C, where spinodal decomposition of the supersaturated solid ferrite solution into Fe-rich nanophase (${\displaystyle {\acute {a}}}$) and Cr-rich nanophase (${\displaystyle {\acute {a}}{\acute {}}}$), accompanied by G-phase precipitation, occurs,^[13][14][15] which makes the ferrite phase a preferential initiation site for micro-cracks.^[16]

Heat treatment

Duplex stainless steel grades must be cooled as quickly as possible to room temperature after hot forming to avoid the precipitation of intermetallic phases (Sigma phase in particular) which drastically reduce the impact resistance at room temperature as well as the corrosion resistance.^[17]

Alloying elements Cr, Mo, W, Si increase the stability and the formation of intermetallic phases. Therefore, super duplex grades have a higher hot working temperature range and require faster cooling rates than the lean duplex grades.

Applications of duplex stainless steels

Duplex stainless steels are usually selected for their high mechanical properties and good to very high corrosion resistance (particularly to stress corrosion cracking).

• Architecture
  • Stockholm's waterfront building ^[18]
  • Louvre Abu Dhabi^[19]
  • La Sagrada Familia^[20]
• Infrastructure:
  • Helix Bridge, Singapore ^[21]
  • Cala Galdana bridge ^[22]
  • Hong Kong–Zhuhai–Macau bridge and undersea tunnel ^[23]
  • sea walls, piers, etc.
  • tunnels
• Oil and gas:
  • a wide range of equipment: flowlines, manifolds, risers, pumps, valves, etc.^[24]
• Pulp and paper:
  • digesters, pressure vessels, liquor tanks, etc.^[25]
• Chemical engineering:
  • pressure vessels, heat exchangers, condensers, distillation columns, agitators, marine chemical tankers, etc.^[26]
• Water:
  • desalination plants, large tanks for water storage, waste water treatment ^[27]
• renewable energy: Biogas tanks
• Mobility: tramcars and bus frames, tank trucks, iron ore wagons ^[5]
• Engineering: pumps, valves, fittings, springs, etc.

Further reading

• TMR Stainless. Practical Guidelines for the Fabrication of Duplex Stainless Steels. 3rd ed. International Molybdenum Association (IMOA); 2014.

See also

• Age-hardened duplex stainless steel
• Spinodal decomposition

References