1. Introduction to duplex stainless steel
Duplex stainless steel refers to a stainless steel in which the solid solution structure is composed of a ferrite and austenite duplex, and one of the comparative examples is about 45% to 55%.
Due to the characteristics of the two-phase structure, the duplex stainless steel has the advantages of both ferritic stainless steel and austenitic stainless steel. Compared with the ferritic stainless steel, it has high toughness, low brittle transition temperature, resistance to intergranular corrosion and good welding performance. The advantages of high thermal conductivity and small expansion coefficient of ferritic steel are retained. Compared with austenitic stainless steel, its yield strength is twice that of austenitic stainless steel. The resistance to chloride stress corrosion cracking is significantly higher than that of 300 series austenitic stainless steel. The resistance to pitting and crevice corrosion is similar to that of 316. stainless steel. Duplex stainless steel is widely used in oil and gas, petrochemical, fertilizer, bridge, construction and chemical tankers due to its excellent mechanical properties and corrosion resistance.
2. Progress in foreign research of duplex stainless steel
The development of duplex stainless steel began in the 1930s. In 1936, the steel grade named Uranus50 obtained the first patent for duplex stainless steel in France. So far, duplex stainless steel has been developed for three generations.
The first generation of duplex stainless steel is represented by Swedish 3RE60 and American AISI329. 3RE60 is a duplex stainless steel specially developed to improve the resistance to chloride stress corrosion cracking. AISI 329 duplex stainless steel is widely used in heat exchanger tubes for nitric acid plants. The first generation of duplex stainless steel has good performance characteristics, but it has limitations in the welding state. The heat affected zone of the weld has low toughness due to excessive ferrite and corrosion resistance is significantly lower than that of the base metal. These limitations have limited the use of first-generation duplex stainless steels to specific applications in non-welded conditions.
In 1968, the stainless steel refining process, the invention of argon-oxygen decarburization, made it possible to produce a series of new stainless steel grades. One of the many advances brought by AOD is the addition of alloying elemental nitrogen. The addition of nitrogen to the duplex stainless steel allows the toughness and corrosion resistance of the heat affected zone to be close to that of the base metal in the welded state, and the nitrogen also reduces the rate of formation of the harmful intermetallic phase. Nitrogen-containing duplex stainless steel is known as second-generation duplex stainless steel. 2205 is a representative steel of the second generation of dual-phase steel and is widely used in offshore oil platforms, chemical, paper and other fields.
The super duplex stainless steel developed in the late 1980s belongs to the third-generation duplex stainless steel, and the representative grades are SAF2507, UR52N, Zeron100, etc. These steels are characterized by low carbon content, high molybdenum and high nitrogen. This type of steel has excellent pitting resistance. Pitting equivalent) is greater than 40.
From the current research and production level, Europe and the United States are at the leading level in the research and development and production of duplex stainless steel and the recognition of users. The representative companies include Sweden’s Outokumpu, Sandvik, and the US KA, KL. In Asia, Japan and South Korea are major producers of duplex stainless steel. The level of steel production is somewhat different from that of Europe and the United States. Take 2205 production as an example. The main brands produced in Japan and South Korea are S31803, while European and American production. The main grade is S32205.
3. Domestic research progress of duplex stainless steel
The research on domestic duplex stainless steel began in the mid-1970s. The Beijing Iron and Steel Research Institute first started research work in this area. The 00Cr18Ni5Mo3Si2 developed by the company has been included in the national standard. Baosteel Special Steel Co., Ltd. has produced 2507 super duplex stainless steel pipes. Baosteel Pugang has produced 2205 plates. However, its production volume is small, and the finished product rate is low, mainly for special purposes. Taigang and Baosteel are currently available in China for commercial products. Taigang started to develop duplex stainless steel 2205 from the late 1990s, and introduced its products to the market in 2005. The 2205 produced by TISCO reached the standard level of S31803. The TISCO patent discloses a low nickel duplex stainless steel which has the advantages of low cost and high strength. Baosteel No. 1 Steel Co., Ltd. also started to develop Cr22 duplex stainless steel in 2006. Now it can produce 4~8mm coil plate and 14-30mm medium plate 2205. Baosteel’s patent discloses a high-nitrogen low-nickel duplex stainless steel with excellent room temperature impact resistance and corrosion resistance, which can be widely used in coastal construction, petrochemical and other fields. It is worth noting that some small and medium-sized stainless steel companies have also begun research on duplex stainless steel.
4. Future development direction of duplex stainless steel
In recent years, with the rising and volatile fluctuations in Ni prices, duplex stainless steel has shown a trend of two developments. First, give full play to the advantages of resource saving of duplex stainless steel, and develop economical duplex stainless steel with 2101. Its development direction is to replace the traditional 300 series austenitic stainless steel. Its design idea is to replace Ni, Mo, etc. with Mn and N elements. Precious metal. Second. It is to fully utilize the characteristics of high-strength and good corrosion resistance of duplex stainless steel, and develop super duplex stainless steel represented by 2507 and 2906 to partially replace super austenitic stainless steel 904L, 254Mo and so on.
It can be seen from the development of duplex stainless steel at home and abroad that duplex stainless steel is a resource-saving high-performance stainless steel product, and its production and application are in line with the future development of stainless steel. At the same time, duplex stainless steel is a high-tech, high value-added product. Its development and production not only reflects the technical level of stainless steel production enterprises, but also creates more economic benefits for enterprises. It is expected that the proportion of duplex stainless steel will be further expanded in the future, and the development of duplex stainless steel has a bright future.
Source: China Duplex Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)