Duplex Stainless Steel

Duplex stainless steel (DSS) refers to stainless steel in which ferrite and austenite each account for about 50%. Generally speaking, the content of the lesser phase 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 molybdenum, copper, niobium, titanium and nitrogen.

This steel has the characteristics of austenitic and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved resistance to intergranular corrosion and welding performance, while still maintaining the brittleness of ferrite, high thermal conductivity and elemental stainless steel Superplasticity.

Compared with austenitic stainless steel, it has high strength and significantly improves the resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel.

Historical development

Duplex stainless steel has developed to the third generation since its birth in the United States in the 1940s.

Its main feature is that the yield strength can reach 400-550MPa, which is twice that of ordinary stainless steel, so it can save materials and reduce equipment manufacturing costs.

In terms of corrosion resistance, especially under the conditions of harsh media environment (such as sea water, high chloride ion content), the resistance to pitting, crevice corrosion, stress corrosion and corrosion fatigue of duplex stainless steel is significantly better than that of ordinary austenite Stainless steel is comparable to high-alloy austenitic stainless steel.

Material introduction Performance characteristics

Due to the characteristics of two-phase structure, through correct control of chemical composition and heat treatment process, duplex stainless steel has both the advantages of ferritic stainless steel and austenitic stainless steel. It combines the excellent toughness and weldability of austenitic stainless steel and iron, and the higher strength and resistance of valve body stainless steel

Combines chloride stress corrosion performance. 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) Molybdenum-containing duplex stainless steel has good chloride stress corrosion resistance under low stress.

Generally speaking, 18-8 austenitic stainless steel is prone to stress corrosion cracking in neutral chloride solution above 60°C. In the industrial medium of trace chloride and hydrogen sulfide, heat exchangers, evaporators and other equipment made of this stainless steel have a tendency to produce stress corrosion cracking, but duplex stainless steel has good corrosion 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 resistance of duplex stainless steel and austenitic stainless steel is equivalent to AISI 316L. 25%Cr, especially the pitting and crevice corrosion resistance of nitrogen-containing high-chromium duplex stainless steel exceeds AISI 316L.

(3) It has good corrosion fatigue and wear and corrosion resistance. Under certain corrosive media conditions, it is suitable for manufacturing 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 greater than 100J.

Duplex stainless steel

(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 different types of materials such as 18-8 austenitic stainless steel or carbon steel.

(6) The hot working temperature range of low chromium (18%Cr) duplex stainless steel is wider than that of 18-8 type austenitic stainless steel, and its resistance is lower. The steel plate can be directly rolled 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, pipes 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 tube sheet deformation, deformation requires greater stress.

(8) Compared with austenitic stainless steel, it has higher thermal conductivity and lower linear expansion coefficient, and is suitable for lining equipment and production of composite plates. It is also suitable for manufacturing heat exchanger tube cores, whose heat exchange efficiency is higher than that of austenitic stainless steel.

(9) High-chromium ferritic stainless steel still has various brittleness tendencies and is not suitable for use under working conditions higher than 300°C. The lower the chromium content in duplex stainless steel, the less harmful the brittle phase (such as σ).

use

Used in heat exchangers, cold water showers, oil refining, fertilizer, paper, petroleum, chemical and other equipment.

Structure and type

Duplex stainless steel has the characteristics of austenitic stainless steel and ferritic stainless steel, and its austenite + ferrite duplex structure and two-phase structure content are 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 higher toughness, lower brittle transition temperature, and significantly improved resistance to intergranular corrosion 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 coefficient of linear expansion, superplasticity and magnetism. Compared with austenitic stainless steel, duplex stainless steel has higher strength, especially the yield strength is significantly improved, and the properties of pitting corrosion, stress corrosion resistance, and corrosion fatigue resistance are also significantly improved.

Duplex stainless steel is divided into four categories according to chemical composition: Cr18 type, Cr23 type (no molybdenum), Cr22 type and Cr25 type. Cr25 type duplex stainless steel can be divided into ordinary type and super duplex stainless steel. Among them, Cr22 and Cr25 are widely used. The duplex stainless steel used in our country is mainly produced in Sweden. The specific grades are: 3RE60 (Cr18 type), SAF2304 (Cr23 type), SAF2205 (Cr22 type), SAF2507 (Cr25 type).

Classification Duplex stainless steel

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

The second category is the 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 high alloy steel, which usually contains 25% chromium, also contains molybdenum and nitrogen, and some also contains 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 steel is higher than that of 22% chromium duplex stainless steel.

The fourth category is super duplex stainless steel, containing high molybdenum and high 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 media conditions , Has good corrosion resistance and comprehensive mechanical properties, comparable to super austenitic stainless steel.

Stainless steel

Stainless steel has many different properties. It gradually formed several categories during its development. According to the organization structure, it is divided into four categories:

martensitic stainless steel (including precipitation hardening stainless steel), ferritic stainless steel, austenitic stainless steel and austenitic + ferritic duplex stainless steel;

According to the main chemical composition of steel or some characteristic elements in steel, it is divided 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 uses of the steel, it is divided into nitric acid-resistant stainless steel, sulfuric acid-resistant stainless steel, pitting-resistant stainless steel, stress-corrosion-resistant stainless steel, and high-strength stainless steel. ；

According to the functional characteristics of steel, it can be 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 based on the structural characteristics of the steel and the chemical composition of the 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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