Background

Duplex stainless steels 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. This combination of properties can mean some compromise when compared with pure austenitic and pure ferritic grades.

Duplex stainless steels are in most cases, tougher than ferritic stainless steels. Strengths of duplex stainless steels can in some cases be double that for austenitic stainless steels.

Whilst duplex stainless steels are considered resistant to stress corrosion cracking, they are not as resistant to this form of attack as ferritic stainless steels. However, the corrosion resistance of the least resistant duplex stainless steels is greater than that for the most commonly used grades of stainless steels, i.e. 304 and 316.

Duplex steels 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 flat rolled products covered by ASTM A240/A240M. ASTM, EN or other standards may cover products sold by Aalco. 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 steels 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

Table 2. Typical chemical composition for two duplex stainless steel alloys

%


2205


UR52N+

C


0.03 max


0.03 max

Mn


2.0


1.5

Si


1.0


0.8

P


0.03


0.035

S


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
Mechanical Properties

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

Hardness Rockwell C


31


-
Physical Properties

Table 4. Typical mechanical 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

Duplex stainless steels are extremely corrosion resistant. They have high resistance to intergranular corrosion. Even in chloride and sulphide environments, duplex stainless steels exhibit very high resistance to stress corrosion cracking.

The super duplex grades are even more resistant to corrosion.
Heat Resistance

The high chromium content of duplex stainless steels that protects 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

Fabrication of all stainless steels 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

Duplex stainless steels 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 steels 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

Duplex stainless steels 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

Duplex stainless steels 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:

· Round bar

· CCP plate

· Quarto plate