Alloys made of duplex stainless steel are extremely corrosion resistant and work hardenable. There is a mixture of austenite and ferrite phases in their microstructures. The result is that stainless steels made from duplex alloys have properties characteristic of both austenitic and ferritic materials.

A combination of properties like these can lead to some compromises when compared with grades that are pure austenitic or pure ferritic. Stainless steels made from duplex types tend to be tougher than those made from ferritic types. It is possible for duplex stainless steels to possess a strength twice that of austenitic stainless steels in some cases.

In spite of the fact that duplex stainless steel is considered to be resistant to stress corrosion cracking, it is not as resistant as ferritic stainless steel. While Duplex stainless steel is the least corrosion resistant of stainless steels, its corrosion resistance is greater than the most commonly used grades of stainless steel, 304 and 316.

Stainless steel grades that contain duplex elements are magnetic as well, which can be used to distinguish them from austenitic grades.

Corrosion Resistance of Duplex stainless steel

Duplex stainless steel are extremely corrosion resistant. They have high resistance to intergranular corrosion. Even in chloride and sulphide environments, Duplex stainless steel 2205 exhibit very high resistance to stress corrosion cracking. The super duplex grades are even more resistant to corrosion.

Heat Resistance of Duplex stainless steel

A high chromium content in Super Duplex stainless steel protects it against corrosion, but embrittles at temperatures above 300°C.

The duplex stainless steels have improved ductility at low temperatures compared to the ferritic and martensitic varieties. In most cases, duplex 2205 grades are suitable for use at temperatures down to -50 °C.

Heat Treatment of Duplex stainless steel

The duplex stainless steel cannot be hardened by heat treatment. Although they cannot be hardened, they can be worked hardened. After heating to around 1100°C, rapid cooling can be used to do solution treatment or annealing.

Machinability

Despite its machinability, Duplex 2205 stainless steel has high strengths, making machining difficult. The machining of 2205, for example, takes around 20% longer than that of 304.

The following rules can be used to improve machining:

• It is important to keep cutting edges sharp. A dull edge causes excess work hardening.
• To prevent work hardening by riding on the surface of the material, cuts should be light but deep enough.
• In order to prevent swarf from entering the work area, chip breakers should be used
• The low thermal conductivity of austenitic alloys causes heat to concentrate at the cutting edges. It is therefore necessary to use large quantities of coolants and lubricants.

Welding of Duplex stainless steel

The weldability of duplex stainless steel is good. It is possible to use all standard welding processes. In duplex grades, residual stresses and distortion are reduced after welding due to low thermal expansion. They are not as easy to weld as austenitic grades, but they are not as easy to weld as austenitic grades either.

 

Applications of Duplex stainless steel

Duplex stainless steel are typically used in:

• Chemical processing, transport and storage
• Oil and gas exploration and offshore rigs
• Oil and gas refining
• Marine environments
• Pollution control equipment
• Pulp & paper manufacturing
• Chemical process plant

Supplied Forms

Aalco typically supplies 2205 duplex stainless steel in the following forms:

• Fittings & Flanges
• Pipe
• Plate
• Sheet
• Tube
• Bar

Mechanical Properties

 

		0.2% proof stress, min	Ultimate tensile strength	Elongation, min (%)
X2CrNiN23-4	1.4362	400 MPa (58 ksi)	600 to 830 MPa (87 to 120 ksi)	25
X2CrNiMoN22-5-3	1.4462	450 MPa (65 ksi)	650 to 880 MPa (94 to 128 ksi)	25
X3CrNiMoN27-5-2	1.4460	450 MPa (65 ksi)	620 to 680 MPa (90 to 99 ksi)	20
X2CrNiN22-2	1.4062	380 MPa (55 ksi)	650 to 900 MPa (94 to 131 ksi)	30
X2CrCuNiN23-2-2	1.4669	400 MPa (58 ksi)	650 to 900 MPa (94 to 131 ksi)	25
X2CrNiMoSi18-5-3	1.4424	400 MPa (58 ksi)	680 to 900 MPa (99 to 131 ksi)	25
X2CrMnNiN21-5-1	1.4162	400 MPa (58 ksi)	650 to 900 MPa (94 to 131 ksi)	25
X2CrMnNiMoN21-5-3	1.4482	400 MPa (58 ksi)	650 to 900 MPa (94 to 131 ksi)	25
X2CrNiMnMoCuN24-4-3-2	1.4662	450 MPa (65 ksi)	650 to 900 MPa (94 to 131 ksi)	25
X2CrNiMoCuN25-6-3	1.4507	500 MPa (73 ksi)	700 to 900 MPa (100 to 130 ksi)	25
X2CrNiMoN25-7-4	1.4410	530 MPa (77 ksi)	730 to 930 MPa (106 to 135 ksi)	25
X2CrNiMoCuWN25-7-4	1.4501	530 MPa (77 ksi)	730 to 930 MPa (106 to 135 ksi)	25
X2CrNiMoN29-7-2	1.4477	550 MPa (80 ksi)	750 to 1,000 MPa (109 to 145 ksi)	25
X2CrNiMoCoN28-8-5-1*	1.4658	650 MPa (94 ksi)	800 to 1,000 MPa (120 to 150 ksi)	25

 

Chemical Properties

 

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

–