Do you really know about duplex stainless steel?

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

1. Why does stainless steel not rust?
The corrosion resistance of stainless steel generally increases as the chromium content increases. When the chromium content in the steel is ≥12%, a very thin but dense oxide film (commonly known as the passivation film) will be formed on the surface of the steel. The substrate is no longer oxidized or corroded, so stainless steel has excellent corrosion resistance.
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2. Composition of duplex stainless steel
Duplex Stainless Steel (DSS) refers to stainless steel with ferrite and austenite each accounting for about 50%. Generally, the content of the smaller phase needs to be at least 30%. In the case of low C content, the Cr content is 18%~28% and the Ni content is 3%~10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti, and N.

3. Classification of duplex stainless steel

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

The third type is a high alloy type, generally containing 25% Cr, 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, containing high molybdenum and nitrogen. The standard grade is UNS S32750 (25Cr-7Ni-3.7Mo-0.3N). Some also contain tungsten and copper. The PREN value is greater than 40 and can be used in harsh environments. medium conditions, it has good corrosion resistance and mechanical properties, comparable to super austenitic stainless steel.

4. Duplex stainless steel VS austenitic & ferritic stainless steel
Due to the characteristics of the two-phase structure, duplex stainless steel has the advantages of both ferritic stainless steel and austenitic stainless steel through correct control of chemical composition and heat treatment process.

Compared with austenitic stainless steel, the advantages of duplex stainless steel are as follows:
(1) The yield strength is more than twice that of ordinary austenitic stainless steel, and it has sufficient plastic toughness required for forming. The wall thickness of storage tanks or pressure vessels made of duplex stainless steel is 30-50% lower than that of commonly used austenite, which is beneficial to reducing costs.

(2) It has excellent resistance to stress corrosion cracking. Even duplex stainless steel with the lowest alloy content has a higher resistance to stress corrosion cracking than austenitic stainless steel, especially in environments containing chloride ions. Stress corrosion is a prominent problem that is difficult to solve for ordinary austenitic stainless steel. (3) The corrosion resistance of 2205 duplex stainless steel, which is the most commonly used in many media, is better than that of ordinary 316L austenitic stainless steel, while super duplex stainless steel has extremely high corrosion resistance. 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 the same alloy content, its wear corrosion resistance and fatigue corrosion resistance are better than austenitic stainless steel.

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

(6) No matter under dynamic or static load conditions, it has higher energy absorption capacity than austenitic stainless steel. This structural part can cope with unexpected accidents such as collisions, explosions, etc. Duplex stainless steel has obvious advantages and has practical application value. . Compared with austenitic stainless steel, the disadvantages of duplex stainless steel are as follows:
(1) The universality and versatility of applications are not as good as austenitic stainless steel. For example, its use temperature must be controlled below 250 degrees Celsius.

(2) Its plastic toughness is lower than that of austenitic stainless steel, and its cold and hot processing technology and forming performance are not as good as austenitic stainless steel.

(3) There is a medium-temperature brittle zone, and the process system of heat treatment and welding needs to be strictly controlled to avoid the appearance of harmful phases and damage to performance.
 
Compared with ferritic stainless steel, the advantages of duplex stainless steel are as follows:
  (1) The comprehensive mechanical properties are better than those of ferritic stainless steel, especially the plasticity and toughness, and are not as sensitive to brittleness as ferritic stainless steel.

(2) In addition to stress corrosion resistance, other local corrosion resistance properties are better than ferritic stainless steel.

(3) Cold working process performance and cold forming performance are far superior to 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 and the price is relatively high. Generally, ferrite does not contain nickel.