1. Definition of duplex stainless steel
The so-called duplex stainless steel is that the ferrite phase and the austenite phase account for half in its hardened structure, and the general minimum phase content may reach 30%.
2. Characteristics and advantages of duplex stainless steel
Due to the characteristics of the two-phase structure, through the correct control of chemical composition and heat treatment process, DSS has the advantages of both ferritic stainless steel and austenitic stainless steel.

Compared with austenitic stainless steel, the advantages of duplex stainless steel are as follows:
(1) The yield strength is more than double that of ordinary austenitic stainless steel, and it has sufficient plastic toughness required for forming. The use of duplex stainless steel to make a storage tank or [wiki]pressure vessel[/wiki] reduces the wall thickness by 30-50% compared to the commonly used austenite, which is beneficial to reduce costs.

(2) Has excellent resistance to stress[wiki]corrosion[/wiki] cracking, even the lowest alloyed duplex stainless steel has a higher resistance to stress corrosion cracking than austenitic stainless steel, especially in chlorine-containing ionic’s [wiki]environment[/wiki]. Stress corrosion is a prominent problem that ordinary austenitic stainless steel is difficult to solve.

(3) The corrosion resistance of the most commonly used 2205 duplex stainless steel in many media is better than that of ordinary 316L austenitic stainless steel, while the super duplex stainless steel has extremely high corrosion resistance, and in some media, such as acetic acid, Formic acid, etc. can even replace high-alloy austenitic stainless steel, and even corrosion-resistant alloys.

(4) It has good local corrosion resistance. Compared with austenitic stainless steel with equivalent alloy content, its wear corrosion resistance and fatigue corrosion resistance are better than austenitic stainless steel.

(5) The coefficient of linear expansion is lower than that of austenitic stainless steel, which is close to that of carbon steel. It is suitable for connecting with carbon steel and has important engineering significance, such as the production of clad plates or linings.

(6) Whether under dynamic load or static load conditions, it has a higher energy absorption capacity than austenitic stainless steel, which is suitable for structural parts to cope with sudden accidents such as collision, [wiki]explosion[/wiki], etc., duplex stainless steel The advantages are obvious and have practical application value.

Compared with austenitic stainless steel, the disadvantages of duplex stainless steel are as follows:
(1) The universality and versatility of application are not as good as that of austenitic stainless steel. For example, its use temperature must be controlled below 250 degrees Celsius.
(2) Its plasticity and toughness are lower than that of austenitic stainless steel, and its cold and hot working processes and forming properties are not as good as that of austenitic stainless steel.
(3) There is a medium temperature brittle zone, and it is necessary to strictly control the processing system of heat treatment and welding to avoid the appearance of harmful phases and damage the performance.
Compared with ferritic stainless steel, the advantages of duplex stainless steel are as follows:
(1) The comprehensive mechanical properties are better than that of ferritic stainless steel, especially the plasticity and toughness, which is not as sensitive to brittleness as ferritic stainless steel.
(2) Except for stress corrosion resistance, another localized corrosion resistance is better than ferritic stainless steel.
(3) The cold working process performance and cold forming performance are far superior to those of ferritic stainless steel.
(4) The welding performance is also much better than that of ferritic stainless steel. Generally, no preheating is required before welding, and no heat treatment is required after welding.
(5) The application range is wider than that of ferritic stainless steel.

Compared with ferritic stainless steel, the disadvantages of duplex stainless steel are as follows:
The content of alloying elements is high, the price is relatively high, and generally, ferrite does not contain nickel.
To sum up, we can see the general situation of the service performance and process performance of DSS. It has won the favor of users with its superior comprehensive mechanical and corrosion resistance properties and has become an excellent resistance to both weight and investment. corrosion engineering materials.

2. The main representative grades of duplex stainless steel
DSS can generally be divided into four categories:
Low alloy type ━ representative grade is UNS S32304 (23Cr-4Ni-0.1N) PREN value 24～25
Medium alloy type? The representative grade is UNS S31803 (22Cr-5Ni-3Mo-0.15N), PREN value 32～33
High alloy type? The standard grade is UNS S32550 (25Cr-6Ni-3Mo-2Cu-0.2N), PREN value 38～39
Super duplex stainless steel type? Standard grades are UNS (25Cr-7Ni-3.7Mo-0.3N), PREN value> 40
(※ PREN pitting corrosion resistance index PREN=Cr%+3.3×Mo%+16×N%)
Low alloy type UNS S32304 does not contain molybdenum and can be used instead of AISI304 or 316 in terms of stress corrosion resistance.
The corrosion resistance of medium alloy type UNS S31803 is between AISI 316L and 6%Mo+N austenitic stainless steel.
High alloy type, generally containing 25% Cr, also containing molybdenum and nitrogen, and some also containing copper and tungsten, the corrosion resistance of this type of steel is higher than that of 22% Cr duplex stainless steel. Super duplex stainless steel type, containing high molybdenum And nitrogen, some also contain tungsten and copper, can be applied to harsh media conditions, has good corrosion resistance and mechanical comprehensive properties, can be compared with super austenitic stainless steel.
The main chemical composition representing the grade
Type UNS Grade Chemical Composition, %
C Cr Ni Mo Cu N
Low alloy type S32304 ≤0.03 23 4 0.05/0.20
Medium alloy type S31803 ≤0.03 22 5 3 0.08/0.20
Medium alloy type S32205 ≤0.03 22 5 3 0.14/0.20
High Alloy S32550 0.04 25 6 3 2 0.10/0.25
Super DSS S32750 ≤ 0.03 25 7 4 0.24/0.32
It can be seen from the table: S 32205 is a steel grade derived from S31803. In the ASTM A 240/240M-99a standard, it was only accepted in 1999. Its Cr, Mo and N elements have a relatively narrow range. It is easy to achieve the balance of the phases (that is, the two phases account for about half), which improves the strength, corrosion resistance and welding performance of the steel, and is mostly used for materials with high performance requirements and need to be welded, such as oil and gas pipelines.
4. The development trend of duplex stainless steel is worth paying attention to the development of low-alloy manganese-containing duplex stainless steel. In the past ten years, relevant countries such as the United States and South Africa have studied the development of manganese-nickel duplex stainless steel, but in addition to castings, the development of Most of the new steel grades have metastable austenite, and the strength is improved by the transformation of martensite after cold deformation. It is difficult to be used for welding parts, and it is also difficult to adapt to certain environments, such as the environment that will produce stress corrosion. Very limited. In recent years, the low-manganese and low-nickel duplex stainless steel developed in Sweden is relatively mature and has a clear goal. In order to save nickel to replace the widely used 304, it may even replace the 2304 duplex stainless steel whose price is comparable to 304 and is not widely used at present. , has the value of practical promotion, it is worth noting. The LDX 2101 duplex stainless steel (21.5%Cr, 5%Mn, 1.5%Ni, 0.22%N) developed by Avesta Polaris AB in Sweden is stable due to the increase of nitrogen in the steel. The austenite has good phase balance and microstructure stability and is not sensitive to the precipitation of intermetallic phases. At 650 °C, the most sensitive temperature for precipitation, the impact value only drops to 50J after 10h of heat preservation, and its microstructure stability is relatively high. 2205 steel is good. The steel has high strength, and corrosion resistance comparable to 304, can be welded, and its life cycle cost is lower than that of austenitic stainless steel. LDX 2101 steel has undergone industrial trial production, and it is refined in AOD/CLU furnace for continuous casting to produce long product castings Preforms and slabs, but they have not been widely used yet. In view of its good comprehensive performance, it has a wide range of potential uses, especially for structural parts. For example, it can replace 2205 duplex stainless steel for bridge structures. It can be used to make steel bars in concrete to solve the corrosion problem of materials in chloride-contaminated environments. In addition, it can replace 304 and 316 steel to make containers, towers, hot water storage tanks and heaters.
Judging from the consumption of stainless steel in the world, duplex stainless steel only accounts for a small share, and its price is still relatively high. From the perspective of price ratio: if 304, 304L is 1, then 316L is 1.3, and 2205 is 1.6, It shows that in order to increase the market share of duplex stainless steel, there should be cheaper new steel grades that can compete with the most commonly used 304 (304L) steel. Of course, there is still a lot of work to be done in this regard. Also mentioned is the development of super duplex stainless steel containing manganese, and high alloys containing 25%Cr, 6%Mn, 6%Ni, 3%Mo, 2%W and ~0.5%N, and Cu were developed in Germany. Cast steel, the steel is strengthened by solid solution, σ0.2 can reach 550MPa, which is nearly 70MPa higher than similar steel containing 0.15%N, the critical pitting corrosion temperature (CPT) reaches 70℃, and the wear and corrosion resistance is good. Austria has also developed BO A920 (26%Cr, 7%Ni, 3%Mn, 4%Mo, 0.35%N) super duplex stainless steel with PREN value of 45, σ0.2 to 600MPa, AK to 100 to 200J, It is suitable for making small-sized and high-speed parts. These two sheets of steel are still in the laboratory stage. Both of these steels rely on the addition of manganese to improve the solubility of nitrogen. Due to the high alloy content, the structural stability is relatively low, and it is easy to precipitate. brittle phase.