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Duplex S31803

The most commonly employed Duplex grade is 2205. It offers high strength, hardness, and excellent corrosion resistance. It possesses these qualities partly through the chemical composition but largely as a result of carefully controlling the temperature during the production process. This results a a steel composed of a mix of austenitic ferritic forms - hence the name "Duplex".

There is often confusion with Duplex grades since they have evolved rapidly. This results in 2205 including both UNS S31803 and UNS S32205 - there are small differences in the quantities of both Molybdenum and Nitrogen required/permitted. In common with other duplex steels it offeres greater strength and hardness.
While duplex steels are generally more expensive per weight their increased strength allows for smaller sections to be used in fabrications resulting in savings in weight and often cost.

Duplex 2205 includes UNS S31803 and S32205. Where figures are given in general remarks they refer to UNS S31803. Precise figures for both grades are given in the tables.

2205 offers high tensile and yield strength and greater hardness than austenitic steels.

Duplex 2205 offers excellent corrosion resistance in moderate temperatures. Up to a temperature range of 150oC it is resistant to chloride stress corrosion cracking (SCC). At temperatures up to 300oC (the usual limit for the uses of duplex steels) it also offers excelent resistance to intergranular, pitting and crevice corrosion corrosion.

It can be welded using any of the standard methods but care must be taken not to overheat the surrounding metal. (See the notes below).

Its potential drawbacks are its limited service tempeature range; -50oC to 300oC.

While on a £/Kg it is typically 25% more expensive than 316, if you compare it on the basis of yield strength, it is 50% less expensive.

Its high strength and hardness coupled with its excellent corrosion resistance make useful in:

  • Oil and Gas Industry - both exploration and processing
  • Chemical processing storage and transport
  • The marine industry or other high chloride environments - transport industry in areas prone to freezing and therefore use of salt 
  • In digesters, liquor tanks, and the paper industry

Similar applications within the temperature range -50oC to 300oC.

Duplex 2205 offers greater tensile and yield strength, good corrosion resistance and heat resistance within temperature specifications and a high degree of hardness.

Strength

2205 has more than twice the yield strength of 316 - and significantly higher ultimate tensile strength. Yield strength for S31803 is 450MPa.

Hardness

Brinell hardness is 293 as compared with 123. While this offers significant advantages where hardness is required it also makes machining more difficult. sharp tools and a 20% slower cutting rate is advised

Corrosion Resistance

Corrosion resistance - within the temperature limitations - is excellent. It is better than Grade 316 in most environments, including the marine environment. 

Resistant to stress corrosion cracking, it also has good resistance to intergranular, pitting and crevice corrosion. Its PREN is in the range of 30 - 38 as compared with 316 which is in the range of 25 - 30.

Heat Resistance

Duplex 2205 resists oxidation at higher temperatures however it will be embrittled if held a temperature above 300oC for even a short period. Should this occur it can be corrected by full solution annealing. this requires heating to between 1020oC and 1100oC and cooling rapidly.

Cost

One of the major benefits of Duplex 2205 can be cost. Due to its greater yield strength, significantly thinner sections can be used to achieve the same strength. This offers weight savings which can also impact other areas of any fabrication. The consequence is that although the cost per kilo of 2205 is higher its use can significantly reduce the cost of a project. 

A common failing is to switch to 2205 for its superior corrosion resistance or hardness and fail to take advantage of its superior strength.

The primary limitations of Duplex 2205 are its restricted operating temperature ranges.

At temperatures below -50oC it rapidly becomes less ductile and significantly more brittle.

Above 300oC brittle micro constituents are preciptated significantly harming its corrosion restance and at temperatures above this ist duplex structure is compromised.

The qualities of high tensile and yield strength mean that bending and forming are difficult. Cold bending will demand higher capacity equipment than austenitic steels and the temperature contraints make it unsuitable for hot bending.

Welding must be done with care to ensure the surrounding material is not overheated. The danger is the production of excessive ferrite. This can generally be avoided by using filler metal and adding nitrogen to the shielding gas. Pre and post weld heating should be avoided. Having added these cautions it is worth noting that the lower co-efficient of thermal expansion results in less stress and distortion. 

The main constituents of Duplex 2205 stainless steel - other than iron - are Chromium and Nickel. However, it is the addition of 2.5 to 3.5% Molybdenum that provides the increased corrosion resistance.

2205 contains 21 - 23% Chromium (Cr) - significantly more than 316. Chromium is the essential chemical in all stainless steel and it is that which forms the thin passive layer that makes the metal "stainless"

2205 also contains 4.5-6.5% Nickel (Ni). This lower level of nickel helps keep price volatility down - nickel being subject to major price swings.

In addition, a number of other chemicals may be present but these are expressed as maximum permitted levels.

GradeCMnSiPSCrMoNiN
2205
S31803
min.
max.
-
0.03
-
2.0
-
1.0
-
0.03
-
0.020
21.0
23.0
2.5
3.5
4.5
6.5
0.08
0.20
2205
S32205
min.
max.
-
0.03
-
2.0
-
1.0
-
0.03
-
0.020
21.0
23.0
3.0
3.5
4.5
6.5.0
0.14
0.20

Physical Properties

 

Metric

English

Density

7.8 g/cc

0.282 lb/in³

               

Mechanical Properties

Hardness, Brinell

293

123

 

Tensile Strength, Ultimate

620 MPa

89923psi

 

Tensile Strength, Yield

450MPa

65267 psi

at 0.2% offset

Elongation at Break

25 %

25 %

in 50 mm

Modulus of Elasticity

200 GPa

   

Electrical Properties

Electrical Resistivity

8.5e-005 ohm-cm

7.2e-005 ohm-cm

at 20°C (68°F); 1.16E-04 at 650°C (1200°F)

Thermal Properties

CTE, linear 20°C

13.7 µm/m-°C

 

from from 0-100°C

CTE, linear 250°C

14.7.8 µm/m-°C

 

at 0-315°C (32-600°F)

CTE, linear 500°C

     

Specific Heat Capacity

0.45 J/g-°C

 

from 0-100°C (32-212°F)

Thermal Conductivity

19.0 W/m-K

 

at 0-100°C, 21.5 W/m°C 

Melting Point

1470 °C

2680 °F

 

 Forms Available

  • Tube
  • Pipe
  • Fittings
  • Flanges
  • Special Sections
  • Sheet
  • Plate
  • Flat Bar
  • Round Bar
  • Hollow Bar
  • Angles
  • i Beam
  • U Channel

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