Duplex stainless steel is extremely corrosion resistant, work hardenable alloys. Their microstructures consist of a combination of austenite and ferrite phases. As a result, duplex stainless steels display properties sign of both austenitic and ferritic stainless steels. This combination of properties can often mean some compromise when compared to pure austenitic and pure ferritic grades.
Duplex stainless is in many instances, tougher than ferritic stainless. Strengths of duplex stainless steels can sometimes be double that for austenitic stainless steels.
Whilst duplex stainless-steel is known as resistant against stress corrosion cracking, they are not as resistant to this form of attack as ferritic stainless-steel. However, the corrosion resistance in the least resistant Duplex metal is greater than that for widely used grades of metal, i.e. 304 and 316.
Duplex metal is additionally magnetic, a home you can use to easily differentiate them from common austenitic grades of stainless.
Duplex stainless is in many instances, tougher than ferritic stainless. Strengths of duplex stainless steels can sometimes be double that for austenitic stainless steels.
Whilst duplex stainless-steel is known as resistant against stress corrosion cracking, they are not as resistant to this form of attack as ferritic stainless-steel. However, the corrosion resistance in the least resistant Duplex metal is greater than that for widely used grades of metal, i.e. 304 and 316.
Duplex metal is additionally magnetic, a home you can use to easily differentiate them from common austenitic grades of stainless.
ASTM A240/A240M
Property data given in this document is typical for flat rolled products covered by ASTM A240/A240M. ASTM, EN or other standards may cover products sold by Yaang. It is reasonable to expect specifications in these standards to be similar but not necessarily identical to those given in this article.
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)
|
0.085x10-6
|
0.085x10-6
|
Thermal Conductivity (W/m.K)
|
19 at 100°C
|
17 at 100°C
|
Thermal Expansion (m/m.K)
|
13.7x10-6 to 100°C
|
13.5x10-6 to 200°C
|
Corrosion Resistance of Duplex stainless steel
Duplex stainless steel is extremely corrosion resistant. They have high resistance to intergranular corrosion. Even in chloride and sulphide environments, Duplex stainless steelexhibit 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 steelmaterials. 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 duplex 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 duplex stainless steel is 2209.
Applications of Duplex stainless steel
Duplex stainless steel is 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
Yaang typically supplies 2205 duplex stainless steel in the following forms:
- Duplex stainless steel Fittings & Duplex stainless steel Flanges
- Duplex stainless steel Pipe
- Duplex stainless steel Plate
- Duplex stainless steel Sheet
- Duplex stainless steel U Tube
- Duplex stainless steel Bar
Source: Yaang
For more information on this source please visit http://www.yaang.com/ .
