Duplex flange materials: ASTM A182 UNS S31803 and UNS S32205 (2205) chemical composition, pressure rating, and mechanical properties.

 

Duplex steel (ASTM A182 2205) is an extremely corrosion resistant, work hardenable stainless steel, whose microstructure consists of a mixture of austenite and ferrite phases.

Due to this particular chemical and physical composition, duplex stainless steel UNS S31803 features the properties characteristic of both types of stainless steel materials (ferritic and austenitic).

Generally speaking, duplex stainless steel is way tougher than ferritic stainless steels, has a superior strength than austenitic steels (series 300 and 400) and has a superior resistance to corrosion when compared to SS304 and SS316 (high intragranular corrosion, also in chloride and sulfide environments).

Whilst austenitic stainless steels are non-magnetic, duplex stainless steel shows magnetic properties.

DUPLEX FLANGE CHEMICAL COMPOSITION

%

Duplex 2205
(ASTM A182 UNS S31803 – UNS S32205)

Super Duplex
ASTM A182 UNS S32750 – 32760)

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

 

The most widely used grade for Duplex flange is 2205, due to its superior resistance to corrosion and mechanical strength. The designation “2205” is related to the chemical composition of this material, which features 22% of chromium and 5% of nickel.

Super Duplex flange show an even superior strength and a higher corrosion resistance than standard duplex steel (and austenitic grades, of course). The main difference between a duplex and a super duplex grade is the addition of copper to the alloy (in addition to increased amounts of Chromium, Moly, and Nickel).

The addition of copper gives Super Duplex Stainless Steel an improved resistance to hot chlorides and strong reducing acids, like sulphuric acid, compared to a standard Duplex grade.

Whilst UNS S31803, UNS 32205 (duplex) and UNS S32750, UNS S32760 are standard designations, most manufacturers of superalloys attribute proprietary names to these steels (example UR52N+ is a Usinor/Arcelor Super Duplex steel, Ferralium, Zeron, Sandvik SAF 2205 22Cr, etc).

 

DUPLEX FLANGE MECHANICAL PROPERTIES

TENSILE, YIELD STRENGTH AND ELONGATION

Mechanical Properties

Duplex 2205
(ASTM A182 UNS S31803 – UNS S32205)

Super Duplex
ASTM A182 UNS S32750 – 32760)

Tensile Strength (in MPa)

620

770

Proof Stress 0.2% (in MPa)

450

550

A5 Elongation (in %)

25

25

 

PHYSICAL PROPERTIES

Physical Attribute

Duplex 2205
(ASTM A182 UNS S31803 – UNS S32205)

Super Duplex
ASTM A182 UNS S32750 – 32760)

Density (g.cm3)

7.805

7.810

Modulus of Elasticity (GPa)

200

205

Electrical Resistivity (Ω.m)

0.085×10-6

0.085×10-6

Thermal Conductivity (W/m.K)

19 at 100°C

17 at 100°C

Thermal Expansion (m/m.K)

13.7×10-6 to 100°C

13.5×10-6 to 200°C

The significant addition of Chromium in Duplex Steel grades, that protects the alloy against corrosion, is a source of steel embrittlement at temperatures over about 300°C. However, at lower temperatures duplex steels show better ductility properties than ferritic and martensitic stainless steels (they can easily be used at a temperature below -50 C°).