In technical terms, Super Duplex pipes are made from a form of austenitic-ferritic stainless steel, based upon iron with alloying additions of chromium, nickel and molybdenum. As such, Super Duplex pipes are pipes made out of this material. Super duplex pipes combine the features of ferritic steel and of austenitic steel. This means this material possesses high strength from the ferritic structure and high corrosion resistance from the austenitic structure, plus the ability to resist stress corrosion cracking. They have good toughness, are readily available and more competitively priced than other corrosion resistant metals that use higher contents of expensive alloying additions.
Super Duplex Pipe Production
Super Duplex Seamless Pipe is commonly produced in a single piece by the extrusion process. This ensures excellent surface finish, dimensional accuracy and consistency of properties of every piece we supply to our clients.
The superior combination of strength and corrosion resistance achieved by super duplex stainless steels makes them an ideal choice for piping, flanges and fittings.
A wide range of scheduled sizes are available as seamless tubes, whilst heavy walled pipes and much larger diameter pipes can be produced from plate with a single longitudinal seam weld. By making use of our mill partnerships we can offer very competitive prices on duplex and super duplex pipe and tube, up to 8” outside diameter as seamless. In addition to this, we can either produce in-house, or source a complementary range of flanges and fittings to complete a project package.
Duplex stainless steel (DSS) is featured with structure of two phases (α and γ) and has characteristics of both ferritic and austenitic stainless steel. It has good corrosion resistance and high strength. It is alloyed with 18%-28% Cr and 3%-10% Ni in the condition of low C. Some steels also contain the alloy elements such as Mo, Cu, Nb, Ti and N, etc.
Standard:
- ASTM A789/SA789 – Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Tubing for General Service
- ASTM A790/SA790 – Standard Specification for Seamless and Welded Ferritic/Austenitic Stainless Steel Pipe
- ASTM A928 (ASME SA928) – Standard Specification for Ferritic/Austenitic (Duplex) Stainless Steel Pipe Electric Fusion Welded with Addition of Filler Metal
Steel Grade:
- Duplex 2507 (UNS S32750) is a super duplex stainless steel with 25% chromium, 4% molybdenum, and 7% nickel designed.
- Duplex 2205 (UNS S31803), or Avesta Sheffield 2205 is a ferritic-austenitic stainless steel.
- SUPER DUPLEX UNS S32760 – is a super duplex stainless steel, so-called due to its higher Chromium and Molybdenum content.
While Duplex 2205 is the most common duplex product we supply, we can also deliver Duplex 2304 and Duplex 2207 with reduced lead times as these alloys are stocked. Alloy 2205 (UNS S32305/S31803) is a 22% chromium, 3% molybdenum, 5-6% nickel, nitrogen alloyed duplex stainless steel pipe with high general, localized, and stress corrosion resistance properties in addition to high strength and excellent impact toughness.
Alloy 2205 provides pitting and crevice corrosion resistance superior to 316L or 317L austenitic stainless steel tube in almost all corrosive media. It also has high corrosion and erosion fatigue properties as well as lower thermal expansion and higher thermal conductivity than austenitic.
ASTM Referenced Standards
- A450 A450M Specification for General requirements for carbon, Ferritic Alloy, and Austenitic alloy steel tubes.
- A480 A480M Specification for general requirements for flat-rolled stainless and heat-resisting steel plate, sheet, and strip.
- A941 Terminology Relating to Steel, Stainless Steel, Related Alloys, and Ferroalloys.
- E527 Practice for numbering metals and alloys (UNS).
ASTM A789 A789M Covers Grades of nominal wall thickness, stainless steel tubing for services requiring general corrosion resistance, with particular emphasis on resistance to stress corrosion cracking. These steels are susceptible to embrittlement if used for prolonged periods at elevated temperatures.
Chemical Composition:
Chemicals composition of grades from EN 10088-1 (2014) Standard are given in the table below:
Composition by weight (%)
| Steel designation | Number | C, max. | Si | Mn | P, max. | S, max. | N | Cr | Cu | Mo | Ni | Other |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| X2CrNiN22-2 | 1.4062 | 0.03 | ≤1.00 | ≤2.00 | 0.04 | 0.010 | 0.16 to 0.28 | 21.5 to 24.0 | – | ≤0.45 | 1.00 to 2.90 | – |
| X2CrCuNiN23-2-2 | 1.4669 | 0.045 | ≤1.00 | 1.00 to 3.00 | 0.04 | 0.030 | 0.12 to 0.20 | 21.5 to 24.0 | 1.60 to 3.00 | ≤0.50 | 1.00 to 3.00 | – |
| X2CrNiMoSi18-5-3 | 1.4424 | 0.03 | 1.40 to 2.00 | 1.20 to 2.00 | 0.035 | 0.015 | 0.05 to 0.10 | 18.0 to 19.0 | – | 2.5 to 3.0 | 4.5 to 5.2 | – |
| X2CrNiN23-4 | 1.4362 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 22.0 to 24.5 | 0.10 to 0.60 | 0.10 to 0.60 | 3.5 to 5.5 | – |
| X2CrMnNiN21-5-1 | 1.4162 | 0.04 | ≤1.00 | 4.0 to 6.0 | 0.040 | 0.015 | 0.20 to 0.25 | 21.0 to 22.0 | 0.10 to 0.80 | 0.10 to 0.80 | 1.35 to 1.90 | – |
| X2CrMnNiMoN21-5-3 | 1.4482 | 0.03 | ≤1.00 | 4.0 to 6.0 | 0.035 | 0.030 | 0.05 to 0.20 | 19.5 to 21.5 | ≤1.00 | 0.10 to 0.60 | 1.50 to 3.50 | – |
| X2CrNiMoN22-5-3 | 1.4462 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.10 to 0.22 | 21.0 to 23.0 | – | 2.50 to 3.50 | 4.5 to 6.5 | – |
| X2CrNiMnMoCuN24-4-3-2 | 1.4662 | 0.03 | ≤0.70 | 2.5 to 4.0 | 0.035 | 0.005 | 0.20 to 0.30 | 23.0 to 25.0 | 0.10 to 0.80 | 1.00 to 2.00 | 3.0 to 4.5 | |
| X2CrNiMoCuN25-6-3 | 1.4507 | 0.03 | ≤0.70 | ≤2.00 | 0.035 | 0.015 | 0.20 to 0.30 | 24.0 to 26.0 | 1.00 to 2.50 | 3.0 to 4.0 | 6.0 to 8.0 | – |
| X3CrNiMoN27-5-2 | 1.4460 | 0.05 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 25.0 to 28.0 | – | 1.30 to 2.00 | 4.5 to 6.5 | – |
| X2CrNiMoN25-7-4 | 1.4410 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.24 to 0.35 | 24.0 to 26.0 | – | 3.0 to 4.5 | 6.0 to 8.0 | – |
| X2CrNiMoCuWN25-7-4 | 1.4501 | 0.03 | ≤1.00 | ≤1.00 | 0.035 | 0.015 | 0.20 to 0.30 | 24.0 to 26.0 | 0.50 to 1.00 | 3.0 to 4.0 | 6.0 to 8.0 | W 0.50 to 1.00 |
| X2CrNiMoN29-7-2 | 1.4477 | 0.03 | ≤0.50 | 0.80 to 1.50 | 0.030 | 0.015 | 0.30 to 0.40 | 28.0 to 30.0 | ≤0.80 | 1.50 to 2.60 | 5.8 to 7.5 | – |
| X2CrNiMoCoN28-8-5-1 | 1.4658 | 0.03 | ≤0.50 | ≤1.50 | 0.035 | 0.010 | 0.30 to 0.50 | 26.0 to 29.0 | ≤1.00 | 4.0 to 5.0 | 5.5 to 9.5 | Co 0.50 to 2.00 |
| X2CrNiCuN23-4 | 1.4655 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 22.0 to 24.0 | 1.00 to 3.00 | 0.10 to 0.60 | 3.5 to 5.5 | – |
Mechanical properties
Mechanical properties from European Standard EN 10088-3 (2014)[8] (for product thickness below 160 mm):
Mechanical properties at room temperature of solution-annealed austenitic–ferritic stainless steels
| Material | Grade | 0.2% proof stress, min. (MPa) | Ultimate tensile strength (MPa) | Elongation, min. (%) |
|---|---|---|---|---|
| X2CrNiN23-4 | 1.4362 | 400 | 600 to 830 | 25 |
| X2CrNiMoN22-5-3 | 1.4462 | 450 | 650 to 880 | 25 |
| X3CrNiMoN27-5-2 | 1.4460 | 450 | 620 to 680 | 20 |
| X2CrNiN22-2 | 1.4062 | 380 | 650 to 900 | 30 |
| X2CrCuNiN23-2-2 | 1.4669 | 400 | 650 to 900 | 25 |
| X2CrNiMoSi18-5-3 | 1.4424 | 400 | 680 to 900 | 25 |
| X2CrMnNiN21-5-1 | 1.4162 | 400 | 650 to 900 | 25 |
| X2CrMnNiMoN21-5-3 | 1.4482 | 400 | 650 to 900 | 25 |
| X2CrNiMnMoCuN24-4-3-2 | 1.4662 | 450 | 650 to 900 | 25 |
| X2CrNiMoCuN25-6-3 | 1.4507 | 500 | 700 to 900 | 25 |
| X2CrNiMoN25-7-4 | 1.4410 | 530 | 730 to 930 | 25 |
| X2CrNiMoCuWN25-7-4 | 1.4501 | 530 | 730 to 930 | 25 |
| X2CrNiMoN29-7-2 | 1.4477 | 550 | 750 to 1000 | 25 |
| X2CrNiMoCoN28-8-5-1* | 1.4658 | 650 | 800 to 1000 | 25 |
*for thickness ≤ 5 mm
Tensile Requirements
| Grade | Tensile strength, min., ksi [MPa] | Yield strength, min., ksi [MPa] | Elongation in 2 in., or 50mm, min, % | Hardness, MaxBrinell |
| S31803 | 90 [620] | 65 [450] | 25 | 290 |
| S32205 | 95 [655] | 70 [485] | 25 | 290 |
| S31500 | 92 [630] | 64 [440] | 30 | 290 |
| S32550 | 110 [760] | 80 [550] | 15 | 297 |
| S31200 | 100 [690] | 65 [450] | 25 | 280 |
| S31260 | 100 [690] | 65 [450] | 25 | 290 |
| S32001 | 90 [620] | 65 [450] | 25 | 290 |
| S32304 | 100 [690] | 65 [450] | 25 | 290 |
| S32750 | 116 [800] | 80 [550] | 15 | 310 |
| S32760 | 109 [750] | 80 [550] | 25 | 300 |
| S32950 | 100 [690] | 70 [480] | 20 | 290 |
| S32520 | 112 [770] | 80 [550] | 25 | 310 |
Flange Test (for welded tubes)
- One test shall be made on specimens from one end of one tube from each lot of finished tubes.
Flaring Test
- One test shall be made on specimens from one end of one tube from each lot of finished tubes.
- The minimum expansion of the inside diameter shall be 10%.
Reverse Flattening Test
- For welded tubes, one reverse flattening test shall be made on a specimen from each 1500Ft or 450m of finished tubing.
Hydrostatic or Nondestructive Testing
- Each pipe shall be subjected to the nondestructive electric test or the hydrostatic test, the type of test to be used shall be at the option of the manufacturer, unless otherwise specified in the purchase order.
- The hydrostatic test shall be in accordance with specification A 450/A 450M, except that in the calculation of the hydrostatic test pressure 64000(441.2) shall be substituted for 32000(220.6).
Markings
- Marking prescribed in specification A450/A 450M,
- The marking shall indicate whether the tubing is seamless or welded.
Note:
- Mill test certificates will be issued according to EN10204.3.
- All tubes shall be supplied as per applicable ASTM A789 /A789M Specification.
Applications:
Duplex pipes are stainless pipes containing the high amount of chromium and minimum amount of nickel. Duplex pipes provide great strength and resistance to corrosive environments. Duplex pipes are used in desalination plants, heat exchangers, and marine processes.
- Duplex stainless steel 2205 has below capability:
- High intensity and good impact toughness
- It can bear stress corrosion well
- Good ability to avoid patch and crack
- Low heat expand modulus and better heat transmit
- High pressure work
- Weldable
The basic idea of duplex is to produce a chemical composition that leads to an approximately equal mixture of ferrite and austenite. This balance of phases provides the following:
- Higher strength – The range of 0.2% PS for the current duplex grades is from 400 – 550 MPa. This can lead to reduced section thicknesses and therefore to reduced weight. This advantage is particularly significant for applications such as:
- Pressure Vessels and Storage Tanks
- Structural Applications e.g. bridges
- Good weldability in thick sections – Not as straightforward as austenitics but much better than ferritics.
- Good toughness – Much better than ferritics particularly at low temperature, typically down to minus 50 deg C, stretching to minus 80 deg C.
- Resistance to stress corrosion cracking – Standard austenitic steels are particularly prone to this type of corrosion. The kind of applications where this advantage is important include:
- Hot water tanks
- Brewing tanks
- Process plant
- Swimming pool structures
The combination of high strength, corrosion resistance and moderate weldability has many benefits but also bring disadvantages and limitations.
High strength is a disadvantage when forming and machining. The higher strength also means that the metal is less ductile than austenitic grades. This means that these steels are not good when the goods being produce require any degree of complex forming.
It is also worth bering in mind that evemn if the item can be formed in duplex steel greater forces are required.
The metallurgy of duplex stainless steels is much more complex than for austenitic or ferritic steels whith the result that they are more complex and therefore expensive to produce. Howver the lower quantity of nickel in them as compared to austenitic grades does help to keep the cost down and reduce price volatility.
Principle of Duplex Stainless Steels
Duplex stainless steels are becoming more common. They are being offered by all the major stainless steel mills for a number of reasons:
- Higher strength leading to weight saving
- Greater corrosion resistance particularly stress corrosion cracking
- Better price stability
- Lower price
The idea of duplex stainless steels dates back to the 1920s with the first cast being made at Avesta in Sweden in 1930. However, it is only in the last 30 years that duplex steels have begun to “take off” in a significant way. This is mainly due to advances in steelmaking techniques particularly with respect to control of nitrogen content.
The standard austenitic steels like 304 (1.4301) and ferritic steels like 430 are relatively easy to make and to fabricate. As their names imply, they consist mainly of one phase, austenite or ferrite. Although these types are fine for a wide range of applications, there are some important technical weaknesses in both types:
- Austenitic – low strength (200 MPa 0.2% PS in solution annealed condition), low resistance to stress corrosion cracking
- Ferritic – low strength (a bit higher than austenitic, 250 MPa 0.2% PS), poor weldability in thick sections, poor low temperature toughness
- In addition, the high nickel content of the austenitic types leads to price volatility which is unwelcome to many end users.
- The basic idea of duplex is to produce a chemical composition that leads to an approximately equal mixture of ferrite and austenite.This balance of phases provides the following:
- Higher strength – The range of 0.2% PS for the current duplex grades is from 400 – 550 MPa. This can lead to reduced section thicknesses and therefore to reduced weight. This advantage is particularly significant for applications such as:
- Pressure Vessels and Storage Tanks
- Structural Applications e.g. bridges
- Good weldability in thick sections – Not as straightforward as austenitics but much better than ferritics.
- Good toughness – Much better than ferritics particularly at low temperature, typically down to minus 50 deg C, stretching to minus 80 deg C.
- Resistance to stress corrosion cracking – Standard austenitic steels are particularly prone to this type of corrosion. The kind of applications where this advantage is important include:
- Hot water tanks
- Brewing tanks
- Process plant
- Swimming pool structures
How the Austenite/Ferrite Balance is Achieved
To understand how duplex steels work, first compare the composition of two familiar steels austenitic 304 (1.4301) and ferritic 430 (1.4016).
| Structure | Grade | EN Number | C | Si | Mn | P | S | N | Cr | Ni | Mo |
| Ferritic | 430 | 1.4016 | 0.08 | 1.00 | 1.00 | 0.040 | 0.015 | – | 16.0/18.0 | – | – |
| Austenitic | 304 | 1.4301 | 0.07 | 1.00 | 2.00 | 0.045 | 0.015 | 0.11 | 17.5/19.5 | 8.0/10.5 | – |
The important elements in stainless steels can be classified into ferritisers and austenitisers. Each element favours one structure or the other:
Ferritisers – Cr (chromium), Si (silicon), Mo (molybdenum), W (tungsten), Ti (titanium), Nb (niobium)
Austenitisers – C (carbon), Ni (nickel), Mn (manganese), N (nitrogen), Cu (copper)
Grade 430 has a predominance of ferritisers and so is ferritic in structure. Grade 304 becomes austenitic mainly through the use of about 8% nickel. To arrive at a duplex structure with about 50% of each phase, there has to be a balance between the austenitisers and the ferritisers. This explains why the nickel content of duplex steels is generally lower than for austenitics.
Here are some typical compositions of duplex stainless steels:
| Grade | EN No/UNS | Type | Approx Composition | ||||||
| Cr | Ni | Mo | N | Mn | W | Cu | |||
| 2101 LDX | 1.4162/ S32101 | Lean | 21.5 | 1.5 | 0.3 | 0.22 | 5 | – | – |
| DX2202 | 1.4062/ S32202 | Lean | 23 | 2.5 | 0.3 | 0.2 | 1.5 | – | – |
| RDN 903 | 1.4482/ S32001 | Lean | 20 | 1.8 | 0.2 | 0.11 | 4.2 | – | – |
| 2304 | 1.4362/ S32304 | Lean | 23 | 4.8 | 0.3 | 0.10 | – | – | – |
| 2205 | 1.4462/ S31803/ S32205 | Standard | 22 | 5.7 | 3.1 | 0.17 | – | – | – |
| 2507 | 1.4410/ S32750 | Super | 25 | 7 | 4 | 0.27 | – | – | – |
| Zeron 100 | 1.4501/ S32760 | Super | 25 | 7 | 3.2 | 0.25 | – | 0.7 | 0.7 |
| Ferrinox 255/ Uranus 2507Cu | 1.4507/ S32520/ S32550 | Super | 25 | 6.5 | 3.5 | 0.25 | – | – | 1.5 |
In some of the recently developed grades, nitrogen and manganese are used together to bring the nickel content to very low levels. This has a beneficial effect on price stability.
At present, we are still very much in the development phase of duplex steels. Therefore, each mill is promoting its own particular brand. It is generally agreed that there are too many grades. However, this is likely to continue until the “winners” emerge.
Corrosion Resistance of Duplex Steels
The following table shows how the duplex steels compare with some austenitic and ferritic grades.
| Grade | EN No/UNS | Type | Typical PREN |
| 430 | 1.4016/ S43000 | Ferritic | 18 |
| 304 | 1.4301/ S30400 | Austenitic | 19 |
| 441 | 1.4509/ S43932 | Ferritic | 19 |
| RDN 903 | 1.4482/ S32001 | Duplex | 22 |
| 316 | 1.4401/ S31600 | Austenitic | 24 |
| 444 | 1.4521/ S44400 | Ferritic | 24 |
| 316L 2.5 Mo | 1.4435 | Austenitic | 26 |
| 2101 LDX | 1.4162/ S32101 | Duplex | 26 |
| 2304 | 1.4362/ S32304 | Duplex | 26 |
| DX2202 | 1.4062/ S32202 | Duplex | 27 |
| 904L | 1.4539/ N08904 | Austenitic | 34 |
| 2205 | 1.4462/ S31803/ S32205 | Duplex | 35 |
| Zeron 100 | 1.4501/ S32760 | Duplex | 41 |
| Ferrinox 255/ Uranus 2507Cu | 1.4507/ S32520/ S32550 | Duplex | 41 |
| 2507 | 1.4410/ S32750 | Duplex | 43 |
| 6% Mo | 1.4547/ S31254 | Austenitic | 44 |
The range of duplex steels allows them to be matched for corrosion resistance with the austenitic and ferritic steel grades. There is no single measure of corrosion resistance. However, it is convenient to use the Pitting Resistance Equivalent Number (PREN) as a means of ranking the grades.
PREN = %Cr + 3.3 x %Mo + 16 x %N