Duplex Pipes

Duplex pipes are stainless pipes containing the high amount of chromium and minimum amount of nickel.

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:

Steel Grade:

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

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, Max

Brinell

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)

Flaring Test

Reverse Flattening Test

Hydrostatic or Nondestructive Testing

Markings

Note:

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.

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:

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.

Delivery requirment:

Capped end stainless steel tube

  • Package: knitting strip bundle, wooden box or steel box
  • Mill test certificate: according to EN 10204 3.2
  • Inspection: Third party inspection, or by clints
Capped end stainless steel tube
Tubes putted in wooden boxes

Tubes putted in wooden boxes

  • Marking: SunnySteel, material grade, standard, specification, heat no.

The wooden boxes packing for stainless products

  • Surface Treatment: Bright annealed, polished outside and inside surface.
Stainless Steel Seamless Pipes
Stainless Steel Seamless Pipes

Warehouse

Pipe size range:

  • Outside diameter: range from 6mm to 530mm.
  • Wall thickness: SCH5S, SCH10S, SCH40S, SCH80S, SCH 120S
  • Legnth: unbending length or fix length

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



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