Duplex Steel

Duplex 2205® Stainless Steel Bars

UNS S31803/UNS S32205

UNS S31803 and UNS S32205 are commonly referred to as Duplex 2205® material, whose “Duplex” name is derived from the mixed microstructure which contains about equal portions of ferrite and austenite. Developed more than 70 years ago in Sweden precisely for the sulfite paper industry where the duplexes were used to combat corrosion problems caused by chloride-bearing cooling waters and other aggressive chemical products. Since then there have been advancements in the composition such as the deliberate addition of nitrogen as an alloying agent, which afterward quickly made Duplex the “workhorse” of stainless steels.

Benefits of Duplex include:

good weldability and workability, high resistance to corrosion fatigue, stress corrosion cracking (especially chloride stress corrosion cracking), erosion, and high energy absorption. Other advantages of Duplex are higher heat conductivity and low thermal expansion compared to austenitic stainless steels, high strength, good resistance to sulfide stress corrosion, and high resistance to pitting and crevice to corrosion. Duplex 2205® accounts for over 80% of the total usage of Duplex stainless steels throughout the world because of its superior workability as well as corrosion resistance.

Industries that use Duplex include:

• Chemical
• Desalination
• Material handling
• Oil and Gas extraction
• Pulp and Paper
• Petrochemical

Deriving the Duplex name from the presence of both Austenite and Ferrite in similar proportions, Duplex Stainless Steels combine excellent corrosion resistance with high strength. The typical mechanical properties would be double that of a single phase austenitic grade such as 316, while, for example, offering superior stress corrosion cracking resistance in chloride solutions.

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 steels are also magnetic, a property that can be used to easily differentiate them from common austenitic grades of stainless.

S31803 / S32205 Properties

Chemical Analysis (% max or range)

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.

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.

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.

Weldability

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.