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Super Duplex Stainless Steel Pipe UNS S31803 S/ UNS S32750 / 2205 / 2507 Duplex Stainless Pipe
Duplex stainless steels have a microstructure which is 50% ferritic and 50% austenitic. This composition gives them a better strength than either ferritic or austenitic steels. Further details about this topic are given below (see ASTM A790 pipes).
A duplex pipe features, at the same time, the qualities of a ferritic SS pipe (high resistance to corrosion cracking and tensile strength) and of an austenitic SS pipe (easy manufacturing and good resistance to corrosion and erosion). Duplex pipes 2205 (UNS S31803/UNS S32205) are used for applications below 600 degrees (F).
Duplex and super duplex stainless steels are weldable and have moderate formability. These types of stainless steels are magnetic but to a lower extent than ferritic, martensitic and precipitation hardening grades due to the 50% austenitic content.
The common grades under this family are:
The ASTM A790 specification covers seamless duplex and super duplex pipes for demanding applications, with high corrosion, temperature, and pressure. Electro-fusion welded duplex and super duplex welded pipes are covered, instead, by the ASTM A928 specification.
Duplex and super duplex can be considered as ‘higher grade’ stainless steel pipes, and are available on the market in the same dimensional range of conventional A312 stainless steel pipes (size range covered by the ASME B36.19 specification).
There are a few differences between duplex/super duplex pipes and conventional stainless steel pipes, which are explained below.
The term “duplex” refers to a family of stainless steels that have double structure – i.e. they are neither a fully austenitic SS structure, like 304 stainless, nor a standard ferritic, such as the 430 series.
Duplex stainless steel (UNS S31803/UNS S32205) has been developed to overcome the issues of the 300 series stainless steels as 316L and 317L (chloride stress corrosion cracking, under tensile stress in cases where the metal is exposed to fluids containing chlorides or by steel exposure to high temperatures).
The microstructure of duplex steels consists of austenite pools surrounded by a continuous ferrite phase (approx 40-50% ferrite).
Therefore, duplex pipes combine the qualities of ferritic and austenitic stainless steel materials.
UNS 2205 duplex steel finds application in marine projects, oil & gas exploration, closed loops water systems, desalinization plants, hydrocarbons storage and transportation, downstream refining, pulp, and paper.
Duplex are resistant to stress corrosion cracking. While standard duplex steels have corrosion resistance comparable to standard austenitic steels (but show better strength and resistance to stress-corrosion-cracking), the so-called super duplex steels feature enhanced strength and resistance to any type of corrosion compared to standard austenitic steels.
Duplex stainless steel shows a superior corrosion resistance to grade 316 for most applications (excellent resistance to localized corrosion including intergranular, pitting and crevice corrosion; the CPT of 2205 is generally at least 35ºC). Duplex is also resistant to chloride corrosion cracking (SCC) at temperatures up to about 150ºC.
Grade 2205 duplex will often perform well in environments which cause premature failure of the standard austenitic grades of series 300 stainless steel. It has also stronger resistance to seawater corrosion, compared to stainless steel 316.
Note that, despite Duplex stainless steel shows good high-temperature oxidation resistance, 2205 suffers from embrittlement if kept at temperatures above 300ºC for some period of time. (a situation that can be rectified only by a full solution annealing treatment).
For this reason, Duplex stainless steels are never used for temperatures in excess of 300ºC. Duplex pipes match the following fittings materials: ASTM A815 (butt weld fittings) and ASTM A182 F51 / F53 / F55 (flanges / forged socket weld – threaded fittings).
| UNS | Chemical Composition | Min.Tensile (KSI) | Min.Yield (KSI) | Elongation % |
|---|---|---|---|---|
| UNS S31803 | C 0.30 max Cr 21.0-23.0 Mn 2.00 max Mo 2.50-3.50 N 0.08-0.20 Ni 4.50-6.50 P 0.030 max S 0.020 max Si 1.00 max | 90 | 65 | 25 |
| UNS S32205 | C 0.30 max Cr 22.0-23.0 Mn 2.00 max Mo 3.00-3.50 N 0.14-0.20 Ni 4.50-6.50 P 0.030 max S 0.020 max Si1.00 max | 95 | 65 | 25 |
Duplex pipes are manufactured according to the ASME B36.19 standard.
Super duplex stainless steel is a variation of the duplex steel alloy which is used in the oil & gas, chemical, power generation, mechanical and desalination industries:
The higher chromium and molybdenum content of Super Duplex vs. Duplex makes it extremely resistant to uniform corrosion by organic acids like formic and acetic acid.
Super Duplex also provides excellent resistance to inorganic acids, especially those containing chlorides.
The pitting Resistance equivalent of Super Duplex, calculated by PREN = Cr + 3.3Mo + 16N, will exceed 40 in most material forms.
| UNS | Chemical Composition | Min.Tensile (KSI) | Min.Yield (KSI) | Elongation % |
|---|---|---|---|---|
| UNS S32750 | C 0.030 max Cr 24.0-26.0 Cu 0.5 max Mn 1.20 max Mo 3.0-5.0 N 0.24-0.32 Ni 6.0-8.0 P 0.035 max S 0.020 max Si 0.8 max | 116 | 80 | 15 |
| UNS S32760 (“Sandvik”) | C 0.03 max Cr 24.0-26.0 Cu 0.5-1.0 Mn 1.0 max Mo 3.0-4.0 N 0.2-0.3 Ni 6.0-8.0 P 0.03 max S 0.01 max Si 1.0 max W 0.5-1.0 |
