Duplex Stainless Steels: Properties, Fabrication and Applications
Duplex stainless steel grades have a ferritic-austenitic microstructure, with a phase balance of approximately 50% ferrite and 50% austenite. This combination of properties can mean some compromise when compared with pure austenitic and pure ferritic grades. Duplex stainless steel are in most cases, tougher than ferritic stainless steel. Strengths of duplex stainless steels can in some cases be double that for austenitic stainless steels. Duplex stainless steel are extremely corrosion resistant, work hardenable alloys.
Whilst duplex stainless steel are considered resistant to stress corrosion cracking, they are not as resistant to this form of attack as ferritic stainless steel. However, the corrosion resistance of the least resistant Duplex stainless steel is greater than that for the most commonly used grades of stainless steel, i.e. 304 and 316.
Duplex stainless steel are also magnetic, a property that can be used to easily differentiate them from common austenitic grades of stainless.
Due to excellent corrosion resistance and high strength, the most widely used duplex stainless steel is 2205 (UNS S32205 / S31803). The name 2205 comes from a typical composition of 22% Chromium and 5% Nickel.
Duplex 2304 (UNS S32304) is a 23% chromium, 4% nickel, molybdenum-free duplex stainless steel whose structure is a balance of ferritic and austenitic. It has general corrosion resistance similar or better than Alloys 304L and 316L but with yield strength nearly double that of austenitic stainless steels.
Duplex 2507 (UNS S32750) is a super duplex stainless steel with 25% chromium, 4% molybdenum, and 7% nickel designed for demanding applications which require exceptional strength and corrosion resistance, such as chemical process, petrochemical, and seawater equipment.
LDX2101 (UNS S32101) is a duplex (austenitic-ferritic) stainless steel with relatively low contents of alloying elements. The grade has high mechanical strength, similar to that of other duplex grades. Its good corrosion resistance is on par with that of most standard stainless steel grades.
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.
There are many other duplex stainless steel most of which are proprietary grades developed and produced by individual manufacturers.
Corrosion Resistance of Duplex stainless steel
Duplex stainless steel are extremely corrosion resistant. They have high resistance to intergranular corrosion. Even in chloride and Sulphide environments, Duplex stainless steel exhibit 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 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 of Duplex stainless steel
Fabrication of all stainless steel 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 discolor 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.
Although machinable, the high strengths of Duplex stainless steel 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 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 stainless steel is 2209.
Applications of Duplex stainless steel
Duplex stainless steel 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
Duplex stainless steel are usually supplied in the following forms: