Duplex Stainless Steel
Features
Duplex stainless steel combines the advantages of ferrite and austenite. Chromium, nitrogen, of high molybdenum content, these steels have good corrosion resistance. Duplex helps organizations achieve high strength and high resistance to stress corrosion cracking performance. Duplex stainless steel also has good weldability.
- Excellent corrosion resistance, high resistance to stress corrosion cracking and corrosion fatigue properties
- High mechanical strength, good wear resistance and fatigue resistance
- Low thermal expansion
| Material | Cr | Ni | Mo | N | PRE | Y.S. MPa | T.S. MPa | E.L. % | HB |
| UNS S32101 | 21 | 1 | 0.3 | 0.2 | 26 | 530 | 700 | 30 | 230 |
| UNS S32304 | 23 | 4 | 0.3 | 0.1 | 26 | 400 | 600 | 25 | 225 |
| UNS S32205 | 22 | 5 | 3 | 0.2 | 35 | 450 | 655 | 25 | 250 |
| UNS S32750 | 25 | 7 | 4 | 0.25 | 42 | 550 | 795 | 15 | 255 |
Equivalent Grades in different countries:
| Material | Grade | USA | Japan | Germany | Europe |
| UNS | AISI/SAE | JIS | DIN | EN | |
| S32900 | 329 | 329 | SUS329J1 | X3CrNiMoN27-5-2 | 1.4460 |
| S31803 | F51/2205 | - | SUS329J3L | X2CrNiMoN22-5-3 | 1.4462 |
| S32205 | F60 | - | SUS329J3L | X2CrNiMoN22-5-3 | 1.4462 |
| S32550 | F61 | - | SUS329JL2 | X2CrNiMoCuN25-6-3 | 1.4507 |
| S32750 | F53/2507 | - | - | 25Cr-7Ni-4Mo-0.28N-Cu | 1.4410 |
| S32760 | F55 | - | - | X2CrNiMoCuWN25-7-4 | 1.4501 |
Metallography
Duplex Stainless Steel can get almost the same amount of Ferritic and Austenite under annealed condition. Duplex Stainless Steel can generate phase separation phenomenon easier than Austenite, embrittlement and redusce the corrosion resistant. Embrittlement occurred when the temperature is on 600～950℃ and 350～525℃. Therefore, it should be avoided under this temperature for a long time. In the operation of normal and heat treatment, the risk for embrittlement is low.
Corrosion Resistant
Pitting Resistance Equivalent PRE =%Cr + 3 , 3x % Mo + 1 6x % N. This value provides a simple way to identify the corrosion resistant. Larger value shows the better corrosion resistant. Duplex Stainless Steel provides a very wide range of corrosion resistant in varios environments. It is generally considered to have a better corrosion resistant if the corrosion is less than 0.2mm/year. Due to the higher chromium content of duplex stainless steel, it has good corrosion resistant in many corrosive catalyst.
Pitting and crevice corrosion can be increased with the content of chromium, molybdenum, and nitrogen.Salt spray test can get the critical pitting temperature - CPT.Pitting occurred urder this temperature.ASTM g48 method F - the measurement temperature in crevice corrosion temperature CPT under 6% FeCl3 + 1% HC - CCT. Different products and different surface treatments may show the different CCT value from the following figure.
Austenitic Stainless Steel may occur stress corrosion cracking -SSC in high temperature oxidiferous.Duplex Stainless Steel is difficult to have this kind of corrosion. The test result is shown on the CCT figure. It's obvious that the Duplex Stainless Steel is better than 304L, 316L, and Austenitic Stainless Steel.
It will increase the risk of SSC when hydrogen sulfide and chlorides are in the environment.This environment will exizt in oil well drilling. S31803 has been proved to have good resistant of SSC. NACE MR0175 （National Associattion of Corrosion Engineers- the sulfide stress cracking resistant and stress corrosion of metal in high sulfur content oil field environment )has approved this material.
Duplex structure and low carbon content result in good intergranlur corrosion resistant of Duplex Stainless Steel. The composition ensures the newly formed Austenitic phase in heat affted area after the welding. This will greatly decrease the risk of carides and nitrides at the grain boundary.
Combining the high hardness and good corrosion resistant, Duplex Stainless Steel has excellent erosion resistant. Therefore, it has good benefit if applying in the fluid containing particles of erosion in wear and tear on teastructure, such as carrying the sand and salt crystals in the piping system. High mechanical strength and excellent corrosion resistant provid the extremly high corrosion fatigue resistant. Accoring to 2205 and 316L S-N curve in artifical seawater shows the corrosion fatigue resistant in 2005 is much higher than in 316L.
DUPLEX 2205 Stainless Steel
Chemical Compositions:
UNS | C | Si | Mn | P | S | Cr | Ni | Mo | N |
S31803 | ≤0.030 | ≤1.00 | ≤2.00 | ≤0.030 | ≤0.020 | 21.0-23.0 | 4.5-6.5 | 2.5-3.5 | 0.08-0.20 |
S32205 | ≤0.030 | ≤1.00 | ≤2.00 | ≤0.030 | ≤0.020 | 22.0-23.0 | 4.5-6.5 | 3.0-3.5 | 0.14-0.20 |
Mechanical Properties (ASTM A182):
UNS | Yield Strength | Tensile Strength | Elongation | R.A. % | Hardness |
S31803(F51) | ≥450MPa | ≥620MPa | ≥25% | ≥45% | -- |
S32205(F60) | ≥450MPa | ≥655MPa | ≥25% | ≥45% | -- |
Characteristics:
UNS S31803 and UNS S32205 are also called Duplex 2205, including almost the same percentage of Ferrite and Austenite mixed structure. That's why Duplex comes from. In more than 70 years ago, it was developed by Sweden. In Sulfite paper industry, it is used to overcome the corrosion problem in bearing cold water and other corrosive chemicals. Then, taking Nitrogen as alloying agent deliberately, the Nitrogen quickly became the mainstream alloy composition in Duplex Stainless Steel.
When talking about Duplex 2205, it mens UNS S31803/F51 or UNS S32205/F60. The UNS S32205/F60 is corresponding to the higher range of corrosion resistant components. It means that the UNS S32205 has higher contents of chromium, nitrogen, and molybdenum. This ensures the better corrosion resistant for UNS S32205. Usually, the mill will produce the UNS S32205 to match the chemical composition reauirement for both UNS S31803 and UNS S32205.
Applications
- Chemical, paper, oil refining industry
- Desalination equipment, water systems
- Chemical tankers and cargo piping system
- Offshore platform fire and blast walls
- Bridges, structural design elements
- Pressure vessels, heat exchangers
- Turbine blades and drive shaft