Duplex stainless steel is an engineering material with high strength, excellent corrosion resistance and economy. It is widely used in petrochemical, marine engineering, nuclear power, food processing and other fields. This blog will introduce the definition, characteristics, classification, application and material selection method of duplex stainless steel to help you better choose the appropriate stainless steel grade.
Basic characteristics of duplex stainless steel
The notable feature of duplex stainless steel is the coexistence of two crystal structures, ferrite (α-Fe) and austenite (γ-Fe), in the microstructure, and the two usually coexist in a volume ratio close to 1:1. This duplex structure is achieved by precisely controlling the ratio of elements such as chromium (Cr), nickel (Ni), and molybdenum (Mo): chromium is a ferrite forming element, and nickel is an austenite stabilizer. The balance of the two ensures that the ratio of the two phases is close to 50:50. The addition of nitrogen further stabilizes the austenite phase, while improving strength and corrosion resistance.
From a performance perspective, the ferrite phase gives the material high strength and resistance to chloride stress corrosion cracking, while the austenite phase provides toughness and acid resistance. This synergistic effect enables duplex stainless steel to achieve the best balance between strength, corrosion resistance and toughness. For example, ordinary austenitic stainless steel (such as 304) is prone to stress corrosion cracking in chloride ion environments, while high-chromium ferritic steel is more brittle. Duplex stainless steel overcomes the shortcomings of both types of materials. Test data shows that the yield strength of duplex stainless steel can reach 400-550 MPa, which is more than twice that of 304 stainless steel (about 205 MPa), while the elongation remains at 25%-40%, which is significantly higher than high-strength low-alloy steel.
In terms of corrosion resistance, the pitting resistance equivalent (PREN) of duplex stainless steel can be calculated by the formula:
PREN= %Cr + 3.3 ×%Mo + 16 ×%N
Take 2205 grade as an example, Cr 22%, Mo 3%, N 0.15%, its PREN value reaches 33-34, which is much higher than 304 stainless steel (PREN≈18) and 316 stainless steel (PREN≈24). More importantly, duplex stainless steel shows strong resistance to chloride stress corrosion cracking (SCC) – in a seawater environment containing chloride ions, ordinary austenitic stainless steel may crack within a few months, while duplex stainless steel can be used safely for decades.
Classification and typical grades of duplex stainless steel
Currently, duplex stainless steel is mainly divided into four generations. The characteristics and typical grades of each generation are as follows:
The first generation representative grade 2205 (S31803) contains Cr 22%, Ni 5%, Mo 3%, and N 0.15%. The classic formula balances corrosion resistance and strength and is widely used in chemical equipment and seawater pipelines. The second generation super duplex steel 2507 (S32750) contains Cr 25%, Ni 6%, Mo 3.5%, and N 0.25%. The PREN value is ≥40, and the corrosion resistance is stronger. It is suitable for offshore platforms and LNG equipment. The third generation 2507Cu adds Cu elements to 2507 to improve crevice corrosion resistance. It is suitable for high-concentration hydrochloric acid and papermaking industries. The fourth generation economical grade LDX 2101 contains low nickel (1.5%), high manganese (5.5%), Cr 21%, Mo 0.3%, and a PREN value of 26-28. It is used in construction and environmental protection equipment.
The key point is: the first and second generations are mainstream application grades. The second generation significantly improves corrosion resistance by increasing the content of chromium, molybdenum and nitrogen; the third generation is optimized for specific corrosive environments (such as high concentration hydrochloric acid); the fourth generation achieves cost control by reducing the nickel content.
Application and material selection of duplex stainless steel
The oil and gas industry is the main application area of duplex stainless steel. The deep-sea oil production platform manifold system uses duplex stainless steel (such as 2507) to withstand seawater corrosion (PREN≥40) and high pressure (≥600 MPa); the lining of the LNG storage tank uses 2205 or 2507, taking into account low-temperature toughness (-50℃) and resistance to hydrogen sulfide stress corrosion. In the chemical and energy fields, wet desulfurization devices and nuclear power coolant pipelines use 2205 or 2507 to resist sulfuric acid, phosphoric acid medium corrosion and high-temperature sulfide stress corrosion. In the construction and environmental protection fields, LDX 2101 is used for bridge guardrails and sewage treatment equipment, reducing maintenance costs with its low nickel cost (1.5%) and good resistance to atmospheric/sewage corrosion.
Material selection should focus on the corrosive environment (such as seawater, acid, chloride ion concentration), mechanical properties (pressure bearing, impact resistance), cost (nickel content and material usage) and processing and welding requirements. For example, 2507 with PREN ≥ 40 is preferred in seawater environment; chemical equipment needs to choose 2205 or 2507 according to the medium concentration; and economical LDX 2101 can be used for building guardrails. Typical problem solutions include: controlling heat input during welding to avoid embrittlement (using matching welding wire and multi-layer multi-pass welding process); surface treatment (Ra ≤ 0.8μm) and structural design (avoiding gaps) to prevent pitting.
Production process and quality points
The production of duplex stainless steel starts with smelting. The raw materials are melted in an electric arc furnace (EAF) and then transferred to an argon oxygen decarburization (AOD) furnace for decarburization, and the carbon content is controlled at ≤0.03% (super duplex steel requires ≤0.02%). Vacuum oxygen decarburization (VOD) further reduces impurities, and after continuous casting into slabs, hot rolling (1050-1150℃) and cold rolling are carried out, and finally annealing (1020-1100℃ water quenching) is performed to restore the duplex balance.
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Daxun Alloy Co., Ltd. provides high-quality duplex stainless steel to customers around the world, including plates, coils, tubes and bars, etc., with standard production processes and strict regulations. If your project requires it, please contact us immediately to get the best quotation.