What Duplex Stainless Steel Is
Duplex stainless steel has a two phase microstructure of roughly equal proportions of ferrite and austenite. The best known grade is UNS S32205, covered by ASTM A240 for plate and sheet, ASTM A790 for seamless and welded pipe and EN 1.4462 in the European designation system, while S31803 and the super duplex grade S32750 cover lower and higher alloy duties. The two phase structure delivers a yield strength that is roughly double that of the standard austenitic grades, combined with markedly better resistance to chloride pitting, crevice corrosion and chloride stress corrosion cracking.
Property Data
| Property | UNS S32205 (2205) |
|---|---|
| Minimum yield strength | 450 MPa |
| Minimum tensile strength | 620 MPa |
| Pitting resistance equivalent number | 34 or above |
| Typical ferrite content | 40 to 60 percent in the annealed condition |
| Service temperature range | Up to about 300 degrees C, above which embrittlement risk rises |
The high chromium, molybdenum and nitrogen content raises the pitting resistance equivalent number to 34 or more for S32205, which is what allows the material to survive chloride bearing environments that cause pitting in type 316 stainless steel. Mechanical properties are developed by solution annealing followed by rapid cooling, and any welding or hot forming operation has to be followed by a correct thermal cycle to restore the phase balance.
Pressure Vessels and Process Equipment
Pressure vessel manufacture is a major application. Duplex plate and heads are used for reactors, separators, columns and storage vessels in the chemical, petrochemical and fertiliser industries, because the grade resists both general corrosion and chloride stress corrosion cracking while its higher strength allows a thinner wall than an austenitic alternative. Welded fabrication is straightforward with matching consumables, and the finished vessel is normally tested for ferrite content and, where required, for intergranular corrosion resistance.
Coastal Architecture and Curtain Walls
In coastal cities the salt laden atmosphere corrodes brackets, anchors and facade components made of type 304 or 316 stainless steel, which shows up as surface rust staining and, over time, as a safety concern for the fixing system. Duplex stainless steel components solve this problem at the source: the material keeps a passive surface under chloride deposition, offers the strength needed for slim structural fixings, and retains a clean appearance without frequent cleaning and re-treatment.
Seawater Desalination and Water Treatment
Multi stage flash and reverse osmosis desalination plants handle hot, aerated, high chloride seawater, which is one of the most demanding environments for metallic materials. Duplex stainless steel is used for high pressure reverse osmosis pipework, brine lines, pumps and heat exchanger components, and demand for the grade grows with plant capacity because larger daily seawater throughput means more wetted surface and a longer design life requirement.
Bridges, Reinforcement and Marine Structures
For cross sea bridges the chloride level in the splash and spray zone can reach about 20000 ppm, and offshore access makes maintenance extremely expensive. Duplex stainless steel reinforcement bar, supplied to the relevant stainless steel bar specification, offers high strength, good toughness and reliable corrosion resistance, and it is easy to prepare and fix on an island site. The same reasoning applies to jetty piles, immersion tunnel fixings, coastal guardrails and offshore platform components.
Selection and Fabrication Points
Confirm the chloride level, the temperature and the oxygen content of the environment before choosing between standard and super duplex grades.
Use matching duplex filler metal for welding, control heat input and interpass temperature, and avoid excessive dilution from the parent metal.
Do not apply post weld heat treatment to duplex steel; the phase balance is controlled by the welding thermal cycle and by the alloy content.
Verify ferrite content in the weld cap and root, and confirm that the chromium and nitrogen balance of the filler is appropriate for the joint.
Clean and pickle or passivate welded and heat affected surfaces, removing iron contamination to allow the passive film to reform.
Respect the upper service temperature limit of each grade, because exposure above it promotes sigma phase formation and loss of toughness.
FAQ
Q: What is duplex stainless steel used for?
Its main uses are pressure vessels and process equipment, coastal building facades and fixings, seawater desalination and water treatment plant, cross sea bridge reinforcement and general marine structures.
Q: How strong is duplex compared with type 316 stainless steel?
The minimum yield strength of UNS S32205 is 450 MPa against about 205 MPa for type 316, so duplex is roughly twice as strong and can often be used in a thinner section.
Q: Can duplex stainless steel be welded?
Yes, with matching duplex filler metal, controlled heat input and interpass temperature, and without post weld heat treatment. The finished weld is normally checked for ferrite content.
Q: Why is duplex preferred for seawater service?
Because its pitting resistance equivalent number of 34 or more gives strong resistance to chloride pitting, crevice attack and chloride stress corrosion cracking, which are the failure modes of austenitic grades in seawater.
Q: Is duplex stainless steel magnetic?
It is magnetic to a limited degree because of its ferrite content of roughly 40 to 60 percent, which is enough to allow magnetic lifting but far less than a fully ferritic or carbon steel.
Q: What temperature limit applies to duplex pipe?
Standard duplex grades are normally limited to about 300 degrees C in service, because higher temperatures accelerate the formation of embrittling intermetallic phases such as sigma phase.
