The concept of duplex stainless steel dates back to the 1920s, the first Shuanggang is made in Avesta in Sweden in 1930. However, in the past 30 years, Double steel has begun to take off in a significant manner. This is mainly due to the progress of steelmaking technology, especially for nitrogen content.
Standard austenite steels such as 304 (1.4301) and ferrite steel like 430, relatively easier to manufacture and assemble. As their names are meant, they are mainly composed of single-phase austenite or ferrite. Although these types apply to a wide variety of applications, these two types have some important technical weaknesses:
Austenite - low strength (200MPa 0.2% solution annealing state), low stress corrosion cracking.
Ferrite - low strength (slightly higher than the austenite, 250 MPa 0.2%), the thick weldingability is very weak, and the low temperature is poor.
In addition, the austenitic type of high-nickel content leads to many price fluctuations, which is unacceptable to many end users.
The basic concept of duplex stainless steel is to produce a chemical component, which makes the ferrite and austenite mixture. The balance of these two phases provides the following:
① higher strength. The current duplex stainless steel score is 0.2% PS ranges from 400-550 MPa, which can reduce weight by reducing cross section thickness. This advantage is a bridge, such as a sticker, and a bridge, such as a storage tank.
② Thick cross section, having good weldability. Although not as simple as Austenites, there are a lot more than ferrite.
③ Good toughness, low temperature than ferritic, especially to zero 50 degrees Celsius, stretch to zero-50 degrees Celsius.
Resistance corrosion rupture - Standard Austenitic steel is particularly prone to this type of corrosion. Some important applications that use this advantage include: hot water tank, brewing tank, refinery, swimming pool structure.
How does Austenitic / ferrite achieve balance?
Important ingredients in stainless steel can be divided into ferrite and austenite, each element facilitating a structure or another.
The 430 level has an advantage of ferrites, and the ferrite is also the same. Level 304 can become austenite, mainly by using about 8% nickel. In order to achieve approximately 50% of the double steel structure per stage, balance must be maintained between the austenite and ferriticles. This explains why bilateral nickel content is usually lower than the austenite.
Stress corrosion cracking (SCC)
SCC is a corrosion form in combination with a variety of factors: tensile tension and corrosion environment; sufficiently high temperatures. Under normal circumstances, in a particular environment, especially the swimming pool, it can occur at a temperature of about 25 degrees.
Unfortunately, standard austenite steels such as 304 (1.4401) and 316 (1.4401) are most susceptible to SCC. Some of the materials below generally do not easily appear SCC: ferritic stainless steel, duplex stainless steel, high nickel population.
Due to the resistance to SCC, the duplex stainless steel can accommodate many applications allowed at high temperatures, but it is worth noting that hot water boilers, brewing tanks, seawater dilution treatment.
It is well known that the stainless steel structure of the swimming pool is very prone to SCC phenomenon. Standard austenitic stainless steels are prohibited in this application, such as 304 and 316 stainless steel. The best steel that can be used in the swimming pool is high nickelous steel, such as 6% molybdenum. However, in some cases, 2205 (1.4462) and ultra-duplex stainless steel grade can also be considered.
Barriers during the process of biphasic stainless steel
High intensity, extensive corrosion resistance, moderate weldability, etc. It is really very attractive combination, which seems to have a huge potential for increasing the market share of duplex stainless steel. However, it is important to understand the limitations of duplex stainless steel, and why they can always become "niche market participants".
The advantages of high strength will be very disadvantageous when considering the formation and machining. High strength also has lower ductility than the austenite level. Therefore, any application that requires a high degree of formatting, for example, a sink, is excluded from duplex stainless steel. Even if it is appropriate, a higher force is also required to form a material, such as when the tube is curved. There is only one exception, that is, a poor machining capacity, the level is 1.4162.
The metallurgy of duplex stainless steel is much more complicated than the austenitic or ferrite. This is why three-day meeting discussions can focus on duplex stainless steel. This factor means that it is more difficult to produce and manufacture these products in the factory.
In addition to ferrite and austenite, if there is no correct treatment method, remove a series of unwanted stages, duplex stainless steel can also be formed, especially heat treatment.
The formation of SIGMA phase is most likely when the cooling rate is not fast during production or welding. The higher the degree of alloying of steel, the higher the probability of Sigma phase. Therefore, superb steel is most prone to this problem.
The formation of 475 extent is due to a stage called α'. Although the worst temperature is 475 DEG C, it still can still be formed at a temperature of less than 300 degrees, which results in the limit of the highest temperature of Double steel. This limitation further reduces the potential range of the application.
On the other end of the balance, the low temperature use of duplex stainless steel is limited compared to the austenite level. Unlike austenic steel, duplex stainless steel exhibited ductile brittle transitions in impact tests. A typical test temperature is - 46 DEG C, the application for marine oil and gas, the negative 80 degree DEG C is the lowest temperature usually encountered by duplex stainless steel.
The following summary is the characteristics of duplex stainless steel:
① The design intensity is twice the austenite and ferrite stainless steel.
② Corrosion resistance is given a wide range of applications.
③ Good toughness, can be lowered to zero 80 degrees Celsius, not true low temperature applications.
④ Special anti-stress corrosion cracking.
⑤ Welcomability of the wall thickness portion.
⑥ Forming and machining, it is more difficult than the austenite.
⑦ The maximum limit of temperature is to 300 degrees Celsius.
