Duplex stainless steel are extremely corrosion resistant, work hardenable alloys. Their microstructures consist of a mixture of austenite and ferrite phases. As a result, duplex stainless steels display properties characteristic of both austenitic and ferritic stainless steels.
Stainless Steel Grades Explained
This combination of properties can mean some compromise when compared with pure austenitic and pure ferritic grades. Duplex stainless steel are in most cases, tougher than ferritic stainless steel. Strengths of duplex stainless steels can in some cases be double that for austenitic stainless steels.
Whilst duplex stainless steel are considered resistant to stress corrosion cracking, they are not as resistant to this form of attack as ferritic stainless steel. However, the corrosion resistance of the least resistant Duplex stainless steel is greater than that for the most commonly used grades of stainless steel, i.e. 304 and 316.
Duplex stainless steel are also magnetic, a property that can be used to easily differentiate them from common austenitic grades of stainless.
ASTM A240/A240M
Property data given in this document is typical for bar products covered by EN 10088-3:2005. ASTM, EN or other standards may cover products sold. It is reasonable to expect specifications in these standards to be similar but not necessarily identical to those given in this datasheet.
Alloy Designations
Due to excellent corrosion resistance and high strength, the most widely used duplex stainless steel is 2205. The name 2205 comes from a typical composition of 22% Chromium and 5% Nickel. Other designations for 2205 are given in the table below.
UR52N+ is a popular “super duplex” grade that has high strength and exceptional corrosion resistance. This grade differs in composition to 2205 stainless steel in that it contains an addition of copper. This gives greatly improved resistance to hot chlorides and strong reducing acids, like sulphuric acid.
UR52N+ is the widely used registered trade name of the Usinor group. Other designations it is known by are given in the table below.
There are many other duplex stainless steel most of which are proprietary grades developed and produced by individual manufacturers.
Table 1. Some standard designations for duplex stainless steel alloys
|
Euronorm
|
UNS
|
BS
|
En
|
Grade
|
|
1.4462
|
S31803
S32205
|
318S13
|
-
|
2205
|
|
1.4507
|
S32520
S32550
|
-
|
-
|
UR52N+
|
Chemical Composition of Duplex stainless steel
Table 2. Typical chemical composition for two duplex stainless steel alloys
|
%
|
2205
|
UR52N+
|
|
C
|
0-0.03
|
0.03 max
|
|
Mn
|
2.0
|
1.5
|
|
Si
|
1.0
|
0.8
|
|
P
|
0-0.03
|
0.035
|
|
S
|
0-0.02
|
0.02
|
|
Cr
|
21-23
|
24-26
|
|
Mo
|
2.5-3.5
|
3.0-5.0
|
|
Ni
|
4.5-6.5
|
5.5-8.0
|
|
N
|
0.08-0.2
|
0.2-0.35
|
|
Cu
|
-
|
0.5-3.0
|
Properties of Duplex stainless steel
Mechanical Properties of Duplex stainless steel
Table 3. Typical mechanical properties of two duplex stainless steel alloys
|
Grade
|
2205
|
UR52N+
|
|
Tensile Strength (MPa)
|
620
|
770
|
|
Proof Stress 0.2% (MPa)
|
450
|
550
|
|
Elongation A5 (%)
|
25
|
25
|
Physical Properties of Duplex stainless steel
Table 4. Typical physical properties of two duplex stainless steel alloys
|
Property
|
2205
|
UR52N+
|
|
Density (g.cm3)
|
7.805
|
7.810
|
|
Modulus of Elasticity (GPa)
|
200
|
205
|
|
Electrical Resistivity (Ω.m)
|
0.85 x 10-6
|
0.85 x 10-6
|
|
Thermal Conductivity (W/m.K)
|
19 at 100 °C
|
17 at 100 °C
|
|
Thermal Expansion (m/m.K)
|
13.7 x 10-6 to 100 °C
|
13.5 x 10-6 to 200 °C
|
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 exhibit very high resistance to stress corrosion cracking.
The super duplex grades are even more resistant to corrosion.
Heat Resistance of Duplex stainless steel
The high chromium content of Duplex stainless steel that p rotects against corrosion, causes embrittlement at temperatures over about 300°C.
At low temperatures duplex stainless steels have better ductility than the ferritic and martensitic grades. Duplex grades can readily be used down to at least -50°C.
Fabrication of Duplex stainless steel
Fabrication of all stainless steel should be done only with tools dedicated to stainless steel materials. Tooling and work surfaces must be thoroughly cleaned before use. These precautions are necessary to avoid cross contamination of stainless steel by easily corroded metals that may discolour the surface of the fabricated product.
Heat Treatment of Duplex stainless steel
Duplex stainless steel cannot be hardened by heat treatment. They can however be work hardened.
Solution treatment or annealing can be done by rapid cooling after heating to around 1100°C.
Machinability
Although machinable, the high strengths of Duplex stainless steel makes machining difficult. As an example, machining of 2205 is around 20% slower than for 304.
Machining can be enhanced by using the following rules:
- Cutting edges must be kept sharp. Dull edges cause excess work hardening.
- Cuts should be light but deep enough to prevent work hardening by riding on the surface of the material.
- Chip breakers should be employed to assist in ensuring swarf remains clear of the work
- Low thermal conductivity of austenitic alloys results in heat concentrating at the cutting edges. This means coolants and lubricants are necessary and must be used in large quantities.
Welding of Duplex stainless steel
Duplex stainless steel have good weldability. All standard welding processes can be used. They are not quite as easily welded as the austenitic grades but low thermal expansion in duplex grades reduces distortion and residual stresses after welding. The recommended filler material for 2205 stainless steel is 2209.
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
DISCLAIMER
This Data is indicative only and must not be seen as a substitute for the full specification from which it is drawn. In particular, the mechanical property requirements vary widely with temper, product and product dimensions. The information is based on our present knowledge and is given in good faith. However, no liability will be accepted by the Company is respect of any action taken by any third party in reliance thereon.
As the products detailed may be used for a wide variety of purposes and as the Company has no control over their use; the Company specifically excludes all conditions or warranties expressed or implied by statute or otherwise as to dimensions, properties and/or fitness for any particular purpose.
Any advice given by the Company to any third party is given for that party’s assistance only and without liability on the part of the Company. Any contract between the Company and a customer will be subject to the company’s Conditions of Sale. The extent of the Company’s liabilities to any customer is clearly set out in those Conditions; a copy of which is available on request.
This information has been sourced, reviewed and adapted from materials provided by Aalco - Ferrous and Non-Ferrous Metals Stockist.
For more information on this source, please visit Aalco - Ferrous and Non-Ferrous Metals Stockist.