Duplex Stainless Steel
Duplex stainless steel is a kind of stainless steel composed of ferrite and austenite
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What is duplex stainless steel ?
Duplex stainless steel is a kind of stainless steel composed of ferrite and austenite. The ferrite content is about 50%, and the austenite content is about 50%. This kind of steel has the characteristics of both austenitic stainless steel and ferritic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has higher plasticity and toughness and better intergranular corrosion resistance, while maintaining the brittleness and high thermal conductivity of ferritic stainless steel at 475 ℃.
Compared with austenitic stainless steel, it has higher strength, corrosion resistance, and stress resistance. Duplex stainless steel has excellent corrosion resistance, high strength, and easy to manufacture.
Their physical properties are between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel. The corrosion resistance of duplex stainless steel is related to the content of Cr, Mo, and Ni.
Duplex stainless steel grades & types
The first type is a low alloy type, representing UNS S32304.
The steel contains no molybdenum, PREN: 24-25, and can be used instead of AISI 304 or 316 in stress corrosion resistance.
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The second type is medium alloy type, which represents UNS S31803. PREN: 32-33.
It has corrosion resistance between 316L and 6% Mo + n austenitic stainless steel.
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The third type is the high alloy type, which generally contains 25% Cr, Mo, and N, and some also contain copper and tungsten.
The standard grade is UNS S32550.
PREN: 38-39.
It has better corrosion resistance than 22% Cr duplex stainless steel.
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The fourth type is super duplex steel, containing high molybdenum and nitrogen.
The standard grade is UNS S32750, some also contain tungsten and copper.PREN > 40.
It can be used in a harsh environment and has good corrosion resistance and mechanical properties.
It is equivalent to super austenitic stainless steel.
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Equivalent of commonly used duplex stainless steel
| China | US | Sweden | Germany | France | Japan | |
| Low alloy type | 00Cr23Ni4N | UN23(SAF2304) | SS23(SAF2304) | W.Nr.1.4362 | UR35N | DP11 |
| Medium alloy type | 00Cr18Ni5Mo3Si2 00Cr22Ni5Mo3N | UNS S31500 UNS S31803 | SS2376(3RE60) SS2377(SAF2205) | W.Nr.1.4417 W.Nr.1.4462 | UR45N | DP1 DP8 |
| High alloy type | 0Cr25Ni5Mo2 00Cr25Ni7Mo3WCuN | UNS S32900 UNS S31260 | SS2324(10RE51) | W.Nr.1.4460 W.Nr.1.4501 | 329J1 329J2L | |
| Super Duplex Steel | 00Cr25Ni7Mo4N 00Cr25Ni6Mo3CuN | UNS S32750 UNS S32550 | SS2328(SAF2507) | W.Nr.1.4410 W.Nr.1.4507 | UR47N+ UR52N+ |
Duplex stainless steel properties
Corrosion resistance
The corrosion resistance of duplex stainless steel is equal to that of CA6NM stainless steel and higher than that of marine stainless steel, such as 6% Mo austenitic stainless steel. All duplex stainless steels have better stress and corrosion resistance than 300 series austenitic stainless steel. Their strength is much higher than that of austenitic stainless steel, and they have good plasticity and toughness. It has excellent corrosion resistance and is also a nickel saving stainless steel.
Mechanical property
Duplex steel has excellent mechanical properties. It has high strength and fatigue strength, and its yield strength is twice that of 18-8 austenitic stainless steel. The elongation of the solid solution is 25%, and the toughness value (AK) is above 100J.
Weldability
It can be welded with austenitic stainless steel or carbon steel without preheating before welding and heat treatment after welding.
Thermal conductivity
Compared with austenitic stainless steel, it has higher thermal conductivity, smaller linear expansion coefficient, and higher heat transfer efficiency
Brittleness
This alloy steel still has the brittleness tendency of high chromium ferritic stainless steel, so it is not suitable to be used in working conditions above 300 ° C.
Chemical composition
The main alloying elements of dual-phase steel are Cr, Ni, Mo, and n.
Cr and Mo are ferrite elements, while Ni and N are austenite elements.
Some steels also contain elements such as Mn, Cu, and W.
Cr, Ni, and Mo can improve the corrosion resistance.
| C | Cr | Ni | Mo | N | P | S | |
| SAF 2304 | 0.030 | 23.0 | 4.5 | – | 0.1 | 0.040 | 0.040 |
| SAF 2205 | 0.030 | 21.0-23.0 | 4.5-6.5 | 2.5-3.5 | 0.08-0.2 | 0.030 | 0.030 |
| SAF 2507 | 0.030 | 24.0-26.0 | 6.0-8.0 | 3.0-5.0 | 0.32 | 0.035 | 0.020 |
| 3RE60 | 0.030 | 18.0-19.0 | 4.25-5.25 | 2.0-2.5 | 0.05-0.1 | 0.030 | 0.030 |
| AISI 329 | 0.080 | 23.0-28.0 | 2.0-5.0 | 1.2-2.0 | – | 0.040 | 0.030 |