Author:Site Editor Publish Time: 2019-02-15 Origin:Site
Duplex stainless steel consists of ferrite phase and austenite phase in its quenched structure, and the content of the minimum phase may reach 30%. Duplex stainless steel is formed on the basis of 18-8 austenitic stainless steel by increasing the content of Cr or adding other ferrite elements, which makes the steel have two-way structure of austenite and ferrite, and saves Ni alloy.
Duplex stainless steel has the advantages of both ferritic stainless steel and austenitic stainless steel by correctly controlling chemical composition and heat treatment process due to the characteristics of two-phase structure of bidirectional stainless steel. Duplex stainless steel reduces the brittleness and crystal growth tendency of high Cr voxel steel due to the presence of austenite, while maintaining its excellent toughness and weldability. The existence of ferrite improves the yield strength, intergranular corrosion resistance and chloride stress corrosion resistance of austenite.
I. Properties of Duplex Stainless Steel
(1) Advantages of Duplex Stainless Steel
1. Compared with austenitic stainless steel, duplex stainless steel has the following advantages:
(1) Yield strength is more than twice as high as that of common austenitic stainless steel, and it has enough plasticity and toughness for forming. The wall thickness of tanks or pressure vessels made of duplex stainless steel is 30% - 50% less than that of austenite commonly used, which is conducive to reducing costs.
(2) Dual-phase stainless steel with the lowest alloy content has higher stress corrosion cracking resistance than austenitic stainless steel, especially in the environment containing chloride ions. Stress corrosion is an outstanding problem that ordinary austenitic stainless steel is difficult to solve.
(3) The corrosion resistance of 2205 duplex stainless steel is better than 316L austenitic stainless steel, which is most commonly used in many media. Super duplex stainless steel has very high corrosion resistance. In some media, such as acetic acid, formic acid, etc., it can even replace high alloy austenitic stainless steel, even corrosion resistant alloy.
(4) It has good local corrosion resistance. Compared with austenitic stainless steel with the same alloy content, its wear resistance and fatigue corrosion resistance are better than those of austenitic stainless steel.
(5) The linear expansion coefficient of austenitic stainless steel is lower than that of austenitic stainless steel, which is close to that of carbon steel. It is suitable for joining with carbon steel and has important engineering significance, such as producing composite plates or linings.
(6) Dual-phase stainless steel has higher energy absorption capacity than austenitic stainless steel under both dynamic and static loads. It has obvious advantages in dealing with unexpected accidents, such as collision and explosion, and has practical application value.
2. Compared with ferritic stainless steel, duplex stainless steel has the following advantages:
(1) The comprehensive mechanical properties of ferritic stainless steel are better than that of ferritic stainless steel, especially its plasticity and toughness. It is not as sensitive to brittleness as ferritic stainless steel.
(2) Except the stress corrosion resistance, the local corrosion resistance of ferritic stainless steel is better than that of ferritic stainless steel.
(3) Duplex stainless steel is much better than ferritic stainless steel in cold processing and cold forming properties.
(4) Welding performance is much better than that of ferritic stainless steel. Generally, there is no need for preheating before welding and no need for heat treatment after welding.
(5) The application range is wider than that of ferritic stainless steel.
(2) Defects of Duplex Stainless Steel
1. Compared with austenitic stainless steel, the disadvantage of duplex stainless steel is as follows:
(1) The universality and versality of the application are not as good as that of austenitic stainless steel, and the heat resistance is lower. The service temperature of the austenitic stainless steel should be controlled below 300 C.
(2) Its plastic toughness is lower than that of austenitic stainless steel, and its cold working hardening effect is greater than that of austenitic stainless steel. In the initial stage of tube and plate deformation, it needs to exert a greater force to deform, so that the internal stress of tube and plate is larger. Duplex stainless steel containing 25% Cr is more difficult to hot work than austenitic stainless steel.
(3) There is a medium temperature brittle zone, so the heat treatment and welding process should be strictly controlled to avoid the appearance of harmful phases and damage the properties.
2. Compared with ferritic stainless steel, duplex stainless steelhas the following disadvantages: high content of alloy elements, relatively high price, and generally ferrite does not contain nickel.
Classification and Classification of Duplex Stainless Steel
So far, the development of duplex stainless steel has gone through three generations. At present, all countries in the world have their own brands. Typical duplex stainless steels can be classified into four categories:
The first type belongs to low alloy type. The representative brand is UNS S32304 (23Cr-4Ni-0.1N). The steel does not contain molybdenum. The pitting corrosion equivalent PRE value is 24-25. It can replace AISI304 (0Cr18Ni9) or 316 (OCr17Ni12Mo2) in stress corrosion resistance.
The second type belongs to medium alloy type. The representative brand is UNS S31803 (22Cr-5Ni-3Mo-0.15N) and the PRE value is 32-33. Its corrosion resistance is between AISI316L (OOCr17Ni14Mo2) and 6% Mo+N austenitic stainless steel.
The third type belongs to high alloy type, which generally contains 25% Cr, molybdenum and nitrogen, and some also contain copper and tungsten. The standard grade is UNS S32550 (25Cr-6Ni-3Mo-2Cu-0.2N), and the PRE value is 38-39. The corrosion resistance of this kind of steel is higher than that of duplex stainless steel with 22% Cr.
The fourth type belongs to super duplex stainless steel with high molybdenum and nitrogen. The standard grades are UNS S32750 (25Cr-7Ni-3.7Mo-0.3N), and some contain tungsten and copper. The PRE value is more than 40. It can be used in harsh medium conditions. It has good corrosion resistance and mechanical properties, and can be comparable with super austenitic stainless steel.
Application of Duplex Stainless Steel
Duplex stainless steel is widely used in various fields because of its unique advantages:
Pulp and paper industry: Pulp and paper industry is the first processing industry to use duplex stainless steel. The use of duplex stainless steel instead of 304 and 316 austenitic stainless steel in sulfuric acid and sulfite cooking pots improves the corrosion resistance and strength of the cooking pots, and reduces the wall thickness of the cooking pots.