Duplex Stainless Steel (DSS) refers to stainless steel with ferrite and austenite each accounting for about 50%. Generally, the content of less phases needs to be at least 30%. In the case of low C content, the Cr content is 18% to 28%, and the Ni content is 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti, and N.

This type of steel has the characteristics of austenitic and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, and significantly improved intergranular corrosion resistance and welding performance, while still maintaining iron The 475℃ brittleness, high thermal conductivity, and superplasticity of element stainless steel. Compared with austenitic stainless steel, it has high strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel.

1. Material introduction

Performance characteristics

Due to the characteristics of the two-phase structure, through correct control of the chemical composition and heat treatment process, the duplex stainless steel has the advantages of both ferritic stainless steel and austenitic stainless steel. It combines the excellent toughness and weldability of austenitic stainless steel with iron The high strength and resistance of the element stainless steel. The cutting equipment has the advantages of fast speed, high precision, smooth cutting surface, stable operation, strong cutting ability and high efficiency

The chloride stress corrosion performance is combined together. It is these superior properties that make duplex stainless steel develop rapidly as a weldable structural material. Since the 1980s, it has become a side-by-side martensitic, austenitic and ferritic stainless steel. Steel category. Duplex stainless steel has the following performance characteristics:

(1) Duplex stainless steel containing molybdenum has good resistance to chloride stress corrosion under low stress. Generally, 18-8 type austenitic stainless steel is prone to stress corrosion cracking in neutral chloride solution above 60°C. Heat exchangers, evaporators and other equipment made of this type of stainless steel in trace chloride and hydrogen sulfide industrial media There is a tendency to produce stress corrosion cracking, but duplex stainless steel has good resistance.

(2) Duplex stainless steel containing molybdenum has good pitting corrosion resistance. When having the same pitting resistance equivalent value (PRE=Cr%+3.3Mo%+16N%), the critical pitting potential of duplex stainless steel is similar to that of austenitic stainless steel. The pitting corrosion resistance of duplex stainless steel and austenitic stainless steel is equivalent to AISI 316L. The pitting and crevice corrosion resistance of 25% Cr, especially nitrogen-containing high-chromium duplex stainless steel exceeds AISI 316L.

(3) It has good corrosion fatigue and wear corrosion resistance. Under certain corrosive media conditions, it is suitable for making pumps, valves and other power equipment.

(4) Good comprehensive mechanical properties. It has higher strength and fatigue strength, and its yield strength is twice that of 18-8 austenitic stainless steel. The elongation in the solid solution state reaches 25%, and the toughness value AK (V-notch) is above 100J.

(5) Good weldability and low thermal cracking tendency. Generally, preheating is not required before welding, and no heat treatment is required after welding. It can be welded with dissimilar types such as 18-8 austenitic stainless steel or carbon steel.

(6) The hot working temperature range of duplex stainless steel with low chromium (18%Cr) is wider than that of 18-8 austenitic stainless steel, and its resistance is small. It can be directly rolled to produce steel plates without forging. The hot working of duplex stainless steel containing high chromium (25%Cr) is slightly more difficult than austenitic stainless steel, and products such as plates, tubes and wires can be produced.

(7) The work hardening effect of cold working is greater than that of 18-8 type austenitic stainless steel. In the initial stage of deformation of the tube and plate, greater stress is required to deform.

(8) Compared with austenitic stainless steel, it has higher thermal conductivity and lower linear expansion coefficient, making it suitable for lining equipment and producing composite plates. It is also suitable for making heat exchanger tube cores, with higher heat transfer efficiency than austenitic stainless steel.

(9) There are still various brittleness tendencies of high-chromium ferritic stainless steel, and it is not suitable for working conditions higher than 300°C. The lower the chromium content in duplex stainless steel, the less harmful the brittle phases such as σ.

2. Purpose

Used in heat exchangers and cold showers and devices for oil refining, fertilizers, papermaking, petroleum, chemical industries, etc., resistant to seawater, high temperature and concentrated nitric acid. Used in shipbuilding, metallurgy, boiler vessels, petroleum pipes, food machinery, textile machinery, electrical cabinets, metal manufacturing, engineering machinery and other industries. 

Three, structure and type

Duplex stainless steel has the characteristics of austenitic stainless steel and ferritic stainless steel due to its austenite + ferritic duplex structure and the content of the two phase structures is basically the same. The yield strength can reach 400Mpa ~ 550MPa, which is twice that of ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has high toughness, low brittle transition temperature, and significantly improved intergranular corrosion resistance and welding performance. At the same time, it retains some of the characteristics of ferritic stainless steel, such as brittleness and heat at 475°C. High conductivity, low linear expansion coefficient, superplasticity and magnetism. Compared with austenitic stainless steel, the strength of duplex stainless steel is higher, especially the yield strength is significantly improved, and the properties of pitting corrosion resistance, stress corrosion resistance, corrosion fatigue resistance and other properties are also significantly improved.

Duplex stainless steels are classified according to their chemical composition and can be divided into four types: Cr18, Cr23 (without Mo), Cr22 and Cr25. For Cr25 type duplex stainless steel, it can be divided into ordinary type and super duplex stainless steel. Among them, Cr22 type and Cr25 type are more widely used. The duplex stainless steels used in my country are mostly produced in Sweden. The specific grades are: 3RE60 (Cr18 type), SAF2304 (Cr23 type), SAF2205 (Cr22 type), SAF2507 (Cr25 type).

Four, classification

Duplex stainless steel

The first category is a low-alloy type, representing the brand UNS S32304 (23Cr-4Ni-0.1N), the steel does not contain molybdenum, and the PREN value is 24-25. It can be used instead of AISI304 or 316 in terms of stress corrosion resistance.

The second category is a medium alloy type, the representative grade is UNS S31803 (22Cr-5Ni-3Mo-0.15N), the PREN value is 32-33, and its corrosion resistance is between AISI 316L and 6%Mo+N austenitic stainless steel between.

The third category is a high-alloy type, generally containing 25% Cr, also containing molybdenum and nitrogen, and some also containing copper and tungsten. The standard grade is UNSS32550 (25Cr-6Ni-3Mo-2Cu-0.2N), and the PREN value is 38-39 The corrosion resistance of this type of steel is higher than that of 22% Cr duplex stainless steel.

The fourth category is super duplex stainless steel type, containing high molybdenum and nitrogen, standard grade UNS S32750 (25Cr-7Ni-3.7Mo-0.3N), some also contain tungsten and copper, PREN value is greater than 40, suitable for harsh Medium conditions, with good corrosion resistance and comprehensive mechanical properties, comparable to super austenitic stainless steel.

stainless steel

There are many types of stainless steel with different properties, and it has gradually formed several major categories in the development process.

According to the organizational structure, it is divided into four categories: martensitic stainless steel (including precipitation hardening stainless steel), ferritic stainless steel, austenitic stainless steel and austenitic plus ferritic duplex stainless steel;

According to the main chemical composition of steel or some characteristic elements in steel, it is classified into chromium stainless steel, chromium nickel stainless steel, chromium nickel molybdenum stainless steel, low carbon stainless steel, high molybdenum stainless steel, high purity stainless steel, etc.;

According to the performance characteristics and use of steel, it is divided into nitric acid resistant stainless steel, sulfuric acid resistant stainless steel, pitting corrosion resistant stainless steel, stress corrosion resistant stainless steel, high strength stainless steel, etc.;

According to the functional characteristics of steel, it is divided into low-temperature stainless steel, non-magnetic stainless steel, free-cutting stainless steel, superplastic stainless steel and so on. The commonly used classification method is to classify according to the structural characteristics of steel and the characteristics of chemical composition of steel, as well as the combination of the two. Generally divided into martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, duplex stainless steel and precipitation hardening stainless steel, etc., or divided into two categories: chromium stainless steel and nickel stainless steel.

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