Duplex Stainless Steel (DSS) is a type of stainless steel that contains approximately equal parts of ferrite and austenite, with the lesser phase generally making up at least 30%. DSS typically has a chromium content between 18% and 28% and a nickel content between 3% and 10%. Some duplex stainless steels also contain alloying elements such as molybdenum (Mo), copper (Cu), niobium (Nb), titanium (Ti), and nitrogen (N).
This category of steel combines the characteristics of both austenitic and ferritic stainless steels. Compared to ferritic stainless steels, DSS has higher plasticity and toughness, lacks room temperature brittleness, and shows improved intergranular corrosion resistance and weldability. At the same time, it retains the 475°C brittleness and high thermal conductivity of ferritic stainless steels and exhibits superplasticity. Compared to austenitic stainless steels, DSS has higher strength and significantly better resistance to intergranular and chloride stress corrosion. DSS also possesses excellent pitting corrosion resistance and is considered a nickel-saving stainless steel.
Structure and Types
Due to its dual-phase structure of austenite and ferrite, with each phase accounting for roughly half, DSS exhibits the characteristics of both austenitic and ferritic stainless steels. The yield strength of DSS ranges from 400 MPa to 550 MPa, which is twice that of ordinary austenitic stainless steels. DSS has higher toughness, lower brittle transition temperature, and significantly improved intergranular corrosion resistance and weldability compared to ferritic stainless steels. It also retains some ferritic stainless steel properties, such as 475°C brittleness, high thermal conductivity, low thermal expansion coefficient, superplasticity, and magnetism. Compared to austenitic stainless steels, DSS has higher strength, especially yield strength, and improved resistance to pitting, stress corrosion, and corrosion fatigue.
DSS can be classified into four types based on its chemical composition: Cr18, Cr23 (Mo-free), Cr22, and Cr25. The Cr25 type can be further divided into standard and super duplex stainless steels. Among these, Cr22 and Cr25 types are more commonly used. In China, the majority of DSS grades adopted are produced in Sweden, including 3RE60 (Cr18 type), SAF2304 (Cr23 type), SAF2205 (Cr22 type), and SAF2507 (Cr25 type).
Types of Duplex Stainless Steel
1. Low-Alloy Type: Represented by UNS S32304 (23Cr-4Ni-0.1N), this steel does not contain molybdenum and has a Pitting Resistance Equivalent Number (PREN) of 24-25. It can replace AISI 304 or 316 in stress corrosion resistance applications.
2. Medium-Alloy Type: Represented by UNS S31803 (22Cr-5Ni-3Mo-0.15N), with a PREN of 32-33. Its corrosion resistance is between AISI 316L and 6% Mo+N austenitic stainless steels.
3. High-Alloy Type: Typically contains 25% Cr along with molybdenum and nitrogen, sometimes copper and tungsten. Represented by UNS S32550 (25Cr-6Ni-3Mo-2Cu-0.2N), with a PREN of 38-39, this steel has better corrosion resistance than 22% Cr DSS.
4. Super Duplex Stainless Steel: Contains high levels of molybdenum and nitrogen, represented by UNS S32750 (25Cr-7Ni-3.7Mo-0.3N), sometimes also containing tungsten and copper, with a PREN above 40. It is suitable for harsh media conditions, offering excellent corrosion and mechanical properties, comparable to super austenitic stainless steels.
Grades of Duplex Stainless Steel in China
The new Chinese standard GB/T 20878-2007 "Stainless and Heat-Resistant Steel Grades and Chemical Composition" includes many DSS grades, such as 14Cr18Ni11Si4AlTi, 022Cr19Ni5Mo3Si2N, and 12Cr21Ni5Ti. Additionally, the well-known 2205 duplex steel corresponds to the Chinese grade 022Cr23Ni5Mo3N.
Characteristics of Duplex Stainless Steel
Due to its dual-phase structure, by properly controlling the chemical composition and heat treatment process, DSS combines the advantages of both ferritic and austenitic stainless steels. It inherits the excellent toughness and weldability of austenitic stainless steels and the high strength and chloride stress corrosion resistance of ferritic stainless steels. These superior properties have made DSS rapidly develop as a weldable structural material since the 1980s, becoming comparable to martensitic, austenitic, and ferritic stainless steels. DSS has the following characteristics:
1. Chloride Stress Corrosion Resistance: Molybdenum-containing DSS exhibits excellent resistance to chloride stress corrosion at low stress levels. While 18-8 austenitic stainless steels tend to suffer from stress corrosion cracking in neutral chloride solutions above 60°C, DSS performs well in environments containing trace amounts of chlorides and hydrogen sulfide, making it suitable for heat exchangers and evaporators.
2. Pitting Corrosion Resistance: DSS has excellent pitting corrosion resistance. With the same Pitting Resistance Equivalent (PRE=Cr%+3.3Mo%+16N%), DSS and austenitic stainless steels show similar critical pitting potentials. DSS's pitting and crevice corrosion resistance, particularly in high-chromium, nitrogen-containing types, surpasses that of AISI 316L.
3. Corrosion Fatigue and Wear Corrosion Resistance: DSS performs well in certain corrosive environments, making it suitable for pumps, valves, and other power equipment.
4. Mechanical Properties: DSS has high strength and fatigue strength, with a yield strength twice that of 18-8 austenitic stainless steels. In the solution-annealed state, its elongation reaches 25%, and its toughness value AK (V-notch) exceeds 100 J.
5. Weldability: DSS has good weldability with low hot cracking tendencies. Preheating is generally not required before welding, and post-weld heat treatment is unnecessary, allowing for welding with 18-8 austenitic stainless steels or carbon steels.
6. Hot Working: Low-chromium (18%Cr) DSS has a wider hot working temperature range and lower resistance than 18-8 austenitic stainless steels, allowing for direct rolling into plates without forging. High-chromium (25%Cr) DSS is slightly more challenging to hot work but can be produced into plates, pipes, and wires.
7. Cold Working: DSS exhibits greater work hardening during cold working than 18-8 austenitic stainless steels, requiring higher initial stress for deformation during pipe and plate forming.
8. Thermal Conductivity and Expansion: DSS has higher thermal conductivity and lower thermal expansion coefficients compared to austenitic stainless steels, making it suitable for lining equipment and producing composite plates. It is also ideal for heat exchanger tube cores, with higher heat exchange efficiency than austenitic stainless steels.
9. Brittleness: DSS retains the brittleness tendencies of high-chromium ferritic stainless steels and is unsuitable for use at temperatures above 300°C. The lower the chromium content in DSS, the less prone it is to brittle phases such as sigma phase.
Womic Steel's Production Advantages
Womic Steel is a leading manufacturer of duplex stainless steel, offering a comprehensive range of products including pipes, plates, bars, and wires. Our products adhere to major international standards and are ISO, CE, and API certified. We can accommodate third-party supervision and final inspection, ensuring the highest quality standards are met.
Womic Steel's duplex stainless steel products are known for their:
High-Quality Raw Materials: We use only the finest raw materials to ensure superior product performance.
Advanced Manufacturing Techniques: Our state-of-the-art production facilities and experienced team enable us to produce duplex stainless steel with precise chemical compositions and mechanical properties.
Customizable Solutions: We offer a wide range of sizes and specifications to meet the specific needs of our customers.
Stringent Quality Control: Our rigorous quality control processes ensure that every product meets the highest standards of excellence.
Global Reach: With a robust export network, Womic Steel supplies duplex stainless steel to customers worldwide, supporting various industries with reliable and high-performance materials.
Choose Womic Steel for your duplex stainless steel needs and experience the unmatched quality and service that sets us apart in the industry.
Post time: Jul-29-2024