Super duplex stainless steel refers to duplex steel with PREN>40, containing 25% Cr and high molybdenum (>3.5%), and high nitrogen (0.22%~0.30%). The main grades are UNS S32550 (UR52N+), S32750 (SAF 2507), and S32760 (Zeron 100), which were developed by companies such as CLI in France, SANDVIK in Sweden, and MATER+LATT in the UK. Their deformed materials were introduced in 1990 and 1991 respectively and have entered the market in recent years. The composition of these three types of steel is similar, with the difference being the content of tungsten and copper.

Since the beginning of the year 2000, the development of modern duplex stainless steel has shown two major trends. On the one hand, while paying attention to the microstructure and thermal stability of steel, the chromium, molybdenum, and nitrogen contents in steel are further increased to obtain higher strength and better corrosion resistance, in order to meet the needs of modern industrial development and compete with super austenitic stainless steel. Special super duplex stainless steels with PRE values close to 50 have emerged, such as SAF2707 HD and SAF 3207 HD. On the other hand, there is a shift towards developing economical models with low nickel content and no precious element molybdenum or only a small amount of molybdenumbipolarstainless steelTo reduce the production cost and selling price of duplex stainless steel, and improve its hot workability and weldability. Thus increasing the use of duplex stainless steel and other types of universal stainless steel, especially 304 and 316 chromium nickel typesaustenitenorusty steelThe competitiveness. At present, there are earlier models such as SAF 2304 and AL 2003, LDX2101, Armo Nitronic 19D, ATI 2102, 2201, 2202, LDX2404, which have only been introduced since 2000. The large-scale production and application of economical duplex stainless steel have had a huge impact on the survival and development of the first generation 18Cr duplex stainless steel 3RE60, which contains about 3% molybdenum.

In biphasic and superBipolar notrusty steelDuring the development process, some physical and metallurgical problems encountered have attracted people's attention. For example, the Cr%+Mo% content in steel should be greater than or equal to 21% to prevent cold forming from causingMartensitic phasechangeAnd it leads to a decrease in the performance of steel. Domestic research on the effect of cold deformation on the formation of martensite and the corrosion resistance of improved 18-5 duplex stainless steel (with Cr+Mo content of 21% and N content of 0.07% and 0.10%) shows that the increase in martensite formation and quantity will reduce the stress corrosion resistance and pitting corrosion resistance of this steel. For example, in order to increase the corrosion resistance of duplex stainless steel, when increasing the chromium and molybdenum content in the steel, necessary preventive measures should be taken to prevent the deterioration of plasticity, toughness, hot workability, welding and corrosion resistance caused by the decrease in thermal stability of duplex stainless steel structure and equal precipitation between metals after Cr%+Mo% is greater than or equal to 35%. For example, nitrogen is a very beneficial and important element, but the addition of a large amount of nitrogen can cause the formation of chromium nitrides and precipitation along grain boundaries, leading to intergranular corrosion and other corrosion damage, as well as a decrease in toughness. The amount of nitrogen in steel exceeds the solid solution amount of steel, and during the solidification process of steel, the escape of nitrogen can cause a large amount of waste products; If a large amount of manganese is added to improve the solid solubility of nitrogen in steel, the harm caused by the formation of manganese sulfide should also be evaluated.

At present, the main grade of super duplex steel in China is 2507 (S32750)

2507（UNS S32750）Bipolar notrusty steelIt has strong strength and corrosion resistance, and is mainly used in chemical processing, petrochemicals, and underwater equipment. It has strong resistance to chloride corrosion, high thermal conductivity, and lowthermal expansioncoefficientThe high content of chromium, molybdenum, and nitrogen makes it highly resistant to spot corrosion, crack corrosion, and general corrosion. Its impact strength is also very high, and 2507 alloy is not suitable for use in temperature environments above 570 ℃. Long term exposure to such environments will reduce its toughness. Its chemical composition and mechanical properties are shown in Tables 1 and 2.

Table 1 Chemical Composition (%)