Duplex stainless steel

Duplex stainless steels are a family of stainless steels. These are called duplex grades because their metallurgical structure consists of two phases, austenite and ferrite in roughly equal proportions.
They provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as A2/304 or A4/316. The main differences in composition, when compared with austenitic stainless steel is that 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 give significant cost benefits. Duplex steels also have higher strength. For example, a Type 304 stainless steel has a 0.2% proof strength in the region of, a 22%Cr duplex stainless steel a minimum 0.2% proof strength of some and a superduplex grade a minimum of.
Duplex steels are used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc. and in the petrochemical industry for pipelines and pressure vessels.

Grades of duplex stainless steels

Duplex stainless steels are usually divided into three groups based on their pitting corrosion resistance, characterised by the pitting resistance equivalence number,.
; Standard duplex : Typically Grade EN 1.4462. It is typical of the mid-range of properties and is perhaps the most used today
; Super-duplex : Typically grade EN 1.4410 up to so-called hyper duplex grades 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. Applications include deepwater offshore oil production.
; Lean duplex grades : 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 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 Standard are given in the table below:

ISO Steel designation

EN Number

UNS equiv

C, max.

P, max.

S, max.

Other

X2CrNiN22-2

1.4062

S32202

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

S31500

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

S32304

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

3.5 to 5.5

X2CrMnNiN21-5-1

1.4162

S32101

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

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

S31803,
S32205

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

S31803,
S32205

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

S32520

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

S31200

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

S32750

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

S32760

0.03

≤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

S32906

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

S32707

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

S32304

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 :

ISO desig.	EN num.	0.2% proof stress, min	Ultimate tensile strength	Elongation, min
X2CrNiN23-4	1.4362			25
X2CrNiMoN22-5-3	1.4462			25
X3CrNiMoN27-5-2	1.4460			20
X2CrNiN22-2	1.4062			30
X2CrCuNiN23-2-2	1.4669			25
X2CrNiMoSi18-5-3	1.4424			25
X2CrMnNiN21-5-1	1.4162			25
X2CrMnNiMoN21-5-3	1.4482			25
X2CrNiMnMoCuN24-4-3-2	1.4662			25
X2CrNiMoCuN25-6-3	1.4507			25
X2CrNiMoN25-7-4	1.4410			25
X2CrNiMoCuWN25-7-4	1.4501			25
X2CrNiMoN29-7-2	1.4477			25
X2CrNiMoCoN28-8-5-1*	1.4658			25

*for thickness ≤
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

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 and Cr-rich nanophase, accompanied by G-phase precipitation, occurs, which makes the ferrite phase a preferential initiation site for micro-cracks.

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 which drastically reduce the impact resistance at room temperature as well as the corrosion resistance.
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.

Architecture
* Stockholm's waterfront building
* Louvre Abu Dhabi
* La Sagrada Familia
Infrastructure:
* Helix Bridge, Singapore
* Cala Galdana bridge
*Hong Kong–Zhuhai–Macau bridge and undersea tunnel
* sea walls, piers, etc.
* tunnels
Oil and gas:
* a wide range of equipment: flowlines, manifolds, risers, pumps, valves, etc.
Pulp and paper:
* digesters, pressure vessels, liquor tanks, etc.
Chemical engineering:
* pressure vessels, heat exchangers, condensers, distillation columns, agitators, marine chemical tankers, etc.
Water:
* desalination plants, large tanks for water storage, waste water treatment
renewable energy: Biogas tanks
Mobility: tramcars and bus frames, tank trucks, iron ore wagons
Engineering: pumps, valves, fittings, springs, etc.
Duplex and super-duplex stainless steels are also available in a variety of commercial product forms, including bars, pipes, plates, flanges, fittings, and fasteners. Frequently used grades, which are selected based on their mechanical strength and resistance to corrosion. These product forms and grades support a wide range of industrial applications, including chemical processing, marine, and offshore environments.