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Comparison of duplex stainless steel, austenitic and ferritic stainless steel
The so-called duplex stainless steel is half of the ferrite phase and the austenite phase in the solidified structure. Generally, the minimum phase content may reach 30%.
Due to the characteristics of the two-phase structure, DSS combines the advantages of ferritic stainless steel and austenitic stainless steel by properly controlling the chemical composition and heat treatment process.
The advantages of duplex stainless steel compared to austenitic 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 molding. The thickness of the tank or pressure vessel made of duplex stainless steel is 30-50% lower than the commonly used austenite, which is beneficial to reduce the cost.
(2) It has excellent resistance to stress corrosion cracking. Even the duplex alloy with the lowest alloy content has higher resistance to stress corrosion cracking than austenitic stainless steel, especially in the environment containing chloride ions. Stress corrosion is a prominent problem that is difficult to solve in ordinary austenitic stainless steels.
(3) The most common 2205 duplex stainless steel used in many media has better corrosion resistance than ordinary 316L austenitic stainless steel, while super duplex stainless steel has high corrosion resistance. In some media, such as acetic acid, Formic acid and the like can even replace high-alloy austenitic stainless steels and even corrosion-resistant alloys.
(4) It has good local corrosion resistance, and its wear resistance and fatigue corrosion resistance are better than that of austenitic stainless steel compared with austenitic stainless steel with equivalent alloy content.
(5) The austenitic stainless steel has a low coefficient of linear expansion and is close to carbon steel. It is suitable for connection with carbon steel and has important engineering significance, such as producing composite plates or linings.
(6) Compared with austenitic stainless steel, it has higher energy absorption capacity under dynamic or static load conditions. This pair of structural parts can cope with unexpected accidents such as collision and explosion. Duplex stainless steel has obvious advantages and has practical application value. .
Compared with austenitic stainless steel, the weakness of duplex stainless steel is as follows:
(1) The universality and versatility of the application are inferior to that of austenitic stainless steel. For example, the temperature of use must be controlled below 250 degrees Celsius.
(2) Its plastic toughness is lower than that of austenitic stainless steel, cold, hot processing and forming properties 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 occurrence of harmful phases and impair performance.
The advantages of duplex stainless steel compared to ferritic stainless steel are as follows:
(1) The comprehensive mechanical properties are better than ferritic stainless steel, especially plastic toughness, and are not as sensitive to brittleness as ferritic stainless steel.
(2) In addition to stress corrosion resistance, other local corrosion resistance is superior to ferritic stainless steel.
(3) Cold working process performance and cold forming performance are much better than 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 weakness of duplex stainless steel is as follows:
The alloying element content is high and the price is relatively high. Generally, the ferrite does not contain nickel.
In summary, the overview of the performance and process performance of DSS can be seen in general terms. It has won the favor of users with its superior mechanical and corrosion resistance performance, and has become an excellent resistance to both weight saving and investment saving. Eclipse engineering materials.
304 steel will also be magnetic, there is no magnetism can not judge the pros and cons of stainless steel
People often think that magnets adsorb stainless steel, verify its advantages and disadvantages and authenticity, do not attract non-magnetic, think it is good, the goods are genuine; if the absorber is magnetic, it is considered a counterfeit fake. In fact, this is an extremely one-sided, unrealistic way of distinguishing mistakes.
There are many types of stainless steel, which can be divided into several categories according to the organizational structure at room temperature:
1. Austenitic type: such as 304, 321, 316, 310, etc.;
2, martensite or ferrite type: such as 430, 420, 410, etc.;
The austenite type is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic.
Most of the stainless steels used as decorative tube sheets are austenitic type 304 materials, which are generally non-magnetic or weakly magnetic. However, magnetic properties may occur due to fluctuations in chemical composition or processing conditions due to smelting, but this cannot be considered as Counterfeit or unqualified, what is the reason?
It is mentioned above that austenite is non-magnetic or weakly magnetic, and martensite or ferrite is magnetic. Due to segregation or improper heat treatment during smelting, a small amount of martensite or ferrite in austenitic 304 stainless steel is caused. Body organization. Thus, 304 stainless steel will have weak magnetic properties.
In addition, 304 stainless steel is cold-worked, and the microstructure is also transformed into martensite. The greater the cold work deformation, the more martensite transformation and the greater the magnetic properties of the steel. Like a batch of steel strips, it produces Φ76 tubes with no obvious magnetic induction and produces Φ9.5 tubes. Due to the large deformation of the bending, the magnetic induction is obvious. The rectangular tube produced is larger than the round tube, especially the angled part, and the deformation is more intense and the magnetic is more obvious.
In order to completely eliminate the magnetic properties of the 304 steel caused by the above reasons, the austenite structure can be restored by high-temperature solution treatment to eliminate the magnetic properties.
In particular, the magnetic properties of 304 stainless steel caused by the above reasons are not at the same level as the magnetic properties of other materials such as 430 and carbon steel. That is to say, the magnetic properties of 304 steel always show weak magnetic properties.
This tells us that if the stainless steel strip is weakly magnetic or not magnetic at all, it should be judged as 304 or 316 material; if it is magnetic similar to carbon steel, it shows strong magnetism, because it is judged not to be 304 material.
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