Since the discovery of the two-phase organization, duplex stainless steel (referred to as "duplex steel") has been developed for three generations:
The first generation DSS is represented by 3RE60 steel developed in Sweden in the mid-1960s;
The second generation DSS is represented by 2205 developed by Sweden after the 1970s, and the PREN is 32~39;
The 3rd generation DSS is super duplex stainless steel (SDSS) of an ultra-low carbon, high molybdenum and high nitrogen content which developed in the late 1980s. Its representative grades are 2507, 2906, 2707, etc. The PRE value is large today, we mainly talk about the largest and most common 2205 duplex steel (international grades are UNS S31803 and UNS S32205). Generally, duplex steel refers to the 2205 grade without special instructions.
Advantages of austenite + ferrite two-phase duplex steel
Duplex steel is formed on the basis of 18-8 austenitic stainless steel, which increases the Cr content or adds other ferrite elements, so that the steel has austenite and ferrite biphasic structure, and saves nickel.
The theoretical ratio is 1:1, which is approximately 50%. Generally, a certain phase is 40~60%.
Under low stress, chromium-containing dual-phase steel has good resistance to chloride stress corrosion and good pitting corrosion resistance. In general, the pitting and crevice corrosion resistance of duplex steels exceeds 316L. Due to the excellent corrosion resistance and strength required. According to the statistics, the amount of 2205 has increased by at least 15 times in the past ten years.
The comprehensive mechanical properties are good, with high strength and fatigue strength, and the yield strength is twice that of 18-8 austenitic stainless steel. Therefore, the thickness of the material can be reduced in design.
Duplex pipe is widely used in various fields due to its unique advantages. It is summarized as follows:
Pulp and paper industry: The pulp and paper industry is the first processing industry to duplex steel. The use of dual-phase steel instead of 304 and 316 austenitic stainless steel sulphuric acid and sulphuric acid cooking pots improves the corrosion resistance and strength of the steamer, while reducing the wall thickness and reducing the total weight of the steamer. This is also an important market for the inventor of 2205 duplex steel, Outokumpu, Sweden.
Chemical processing industry: The chemical processing industry has a wide range of applications and complicated working conditions. Storage tanks for the preservation of industrial wet process phosphoric acid are usually made of 316L or 317L type austenitic stainless steel, but the walls are thick. With duplex steel 2205, the strength is almost double that of 316L and 317L, and the wall thickness can be reduced, thereby reducing the importance and cost of the tank. At the same time, in the industrial sulfuric acid environment, especially in the presence of chloride ions, 2205 has better local corrosion resistance than ordinary austenitic stainless steel.
Refining industry: The refining industry is one of the fields that use more duplex steels. It is mainly used in atmospheric and vacuum distillation, catalytic cracking hydrodesulfurization and other devices, and is used at home and abroad. In addition, duplex steels are increasingly used in the production of sour gas and oil, and are mainly used as pipe systems for pipe lining, onshore and offshore, and heat exchangers.
Petrochemical industry: The corrosive environment of petrochemical and chemical industries is characterized by high reaction temperature, and the medium often contains high or medium concentration of chloride, which is easy to induce stress corrosion cracking of stainless steel.
Food and Beverage Industry: Food and beverage industry production is typically processed in hot solutions containing chloride ions. Duplex steels have unique advantages in stress corrosion cracking.
Transportation: Due to the high strength and excellent corrosion resistance of duplex steel, it has great advantages when used in the production of chemical tankers. It reduces the weight of the ship and reduces the fuel consumption. In this respect, the most used is 2205 duplex steel. At the same time, it can meet the corrosion resistance of the materials required for the cargo.
Wall building industry: In the coastal areas, the air is filled with the sea atmosphere evaporated from the sea, where the chloride ion concentration is much higher than the inland air, so the seawater resistant material is needed. The corrosion resistance of duplex steels has this advantage, and its high strength also reduces weight. At present, many offshore oil platforms use duplex steel to make explosion-proof walls.
In addition, duplex steel has also been successfully used in water treatment heat exchangers, hot water pipes, and water heaters.
Methods and precautions of duplex steel pipe welding
Most commonly used are SMAW, GTAW, SAW.
In general, FCAW is not used for the best, it is ugly, difficult to control, and the heat input is large, and GMAW is also not used.
Good solderability and low thermal cracking tendency. Generally, there is no need to preheat before welding, no heat treatment after welding, and welding with dissimilar steel such as 18-8 austenitic stainless steel or carbon steel. In terms of process, the welding process of ordinary austenitic stainless steel is sufficient.
In order to obtain a considerable proportion of post-weld structure, it is critical to know the line energy when welding 2205 duplex steel. It should not be too big or too small. In order to avoid the grain growth too fast or coarse, control the heat input and try to take the welding current at a small point. The interlayer temperature is preferably controlled within 100 degrees.It’s suggest ed that the ferrite content should be measured during welding and after welding. If there is a technical agreement, it must be evaluated according to the technical agreement!
In a certain range, the current must be small. When welding, pay attention to the arc not too long, otherwise it will be easy to vent.
Duplex steels have various brittleness tendencies and should not be used under working conditions above 300 degrees.
The protective gas cannot be used with Ar+Co2. Since one of the most important characteristics of duplex steel is its nitrogen content, it is recommended to use Ar+1~3% nitrogen, and the back protective gas is recommended to use pure nitrogen.
About the selection of duplex steel welding consumables
But first of all, it must be stated that the specification of the electrode can not be selected or recommended, 5.0mm, 2.5mm~4.0mm is recommended, and the wire size is 1.2~3.2. (SAW is 3.2mm, don't use 4.0mm, the flux is best with the original ). It’s recommended to use welding wire ER2209, welding rod E2209.
When using E2209 welding rod, the drying temperature should be a little higher than the recommended temperature from the manufacturer.
There are more than a dozen of famous domestic and foreign producers of duplex steel welding consumables, the European is better!
In terms of cost, the difference between domestic and foreign is very small.