Duplex stainless steel has two kinds of metal phase structure, so the duplex stainless steel can combine the properties of ferritic stainless steel and austenitic stainless steel by properly controlling the chemical composition and heat treatment process, that is, the good toughness and welding of austenitic stainless steel. Performance combined with higher strength and chloride stress corrosion resistance of ferritic stainless steel. It is these superior properties that make duplex stainless steel develop rapidly as a weldable structural material. Typical grades in duplex stainless steel are 2205 duplex stainless steel and 2507 duplex stainless steel.
Development and application of duplex stainless steel
1.1.1 Overview of the development of duplex stainless steel at home and abroad
The development of duplex stainless steel began in the 1930s, France won the first patent in 1935, and duplex stainless steel has been developed for three generations. The first generation of duplex stainless steel is represented by 329 steel developed in the United States in the 1940s. It contains high chromium and molybdenum and has good local corrosion resistance, but has a high carbon content (≤0.1% c). Therefore, the loss of phase balance during welding and the precipitation of carbides along the grain boundary result in a decrease in corrosion resistance and toughness, which must be subjected to heat treatment after welding, and is generally used only for casting and forging, and is limited in application. In the 1950s, the 08X2lH5T and 08x2lH6M2T steels with stabilizing elemental titanium were developed. At the same time, Germany developed 1.458Z steel, France developed Urnnus50 steel, Britain successfully developed Ferriumlium255 steel, and Japan reduced it based on US 329 steel. The carbon content was presented at 329J. Steel grades, these duplex stainless steels can be used as weldable structural parts. Later, in the mid-1960s, Sweden developed the famous 3RE60 steel, which is the representative steel of the first generation of duplex stainless steel. It is characterized by ultra-low carbon, 18% chromium content, good welding and forming properties, and can be widely used to replace AIsl304L. 316L is used as a material resistant to chloride ion stress corrosion. Since the 1970s, with the emergence and popularization of secondary refining technologies such as AOD and VOD, ultra-low carbon steels can be easily smelted, and nitrogen is found as an austenite forming element to improve corrosion resistance of duplex stainless steel. Important role, improved the shortcomings of the first generation of duplex stainless steel, thus creating a second generation of new nitrogen-containing duplex stainless steel and developed a new application field of duplex stainless steel. The second-generation duplex stainless steels, whether 18Cr or 22Cr or 25cr, are mostly ultra-low carbon and have been included in the ASTM A789 and A790 standards in the United States. In addition, France has uRANUS series, Britain has ZERON cast steel series, Germany also has a series of grades of steel grades. The super duplex stainless steel (SuperDss) developed in the late 1980s belongs to the third-generation duplex stainless steel, and the grades are SAF2507, UR52N+, zeronl00, etc. These steels are characterized by extremely low carbon content (C 0.01% to 0.02). %), containing high molybdenum and high nitrogen (Mo 1% ~ 4%, N 0.1% ~ 0.3%), the ferrite content of steel is 40% ~ 50%, such steel has excellent pitting corrosion resistance The pitting resistance equivalent value (PRE=Cr%+3.3×Mo%+16×N%) is greater than 40⋯. South Korea’s 1996 duplex stainless steel (patent application number: 96190623) has superior thermoplasticity, high temperature oxidation resistance, corrosion resistance and impact toughness. China has developed duplex stainless steel since the mid-1970s. The Beijing Iron and Steel Research Institute first carried out research work in this area. The 00Crl 8Ni5M03Si2 duplex stainless steel developed by the company has been included in the national standard GBl220, GB3280, GB4237. In addition, on the basis of analyzing the development of foreign duplex stainless steel, the No. 5 Research Institute successfully developed a new rare earth duplex stainless steel SG52 with an anti-pitting equivalent of PRE≥40. The steel is modified with rare earth and has nickel, which has good mechanical properties, process performance and corrosion resistance.
1.1.2 Classification and composition of duplex stainless steel
Duplex stainless steel can be mainly divided into two categories, one is Cr-Ni duplex stainless steel, the composition (mass fraction) is generally: Cr 17% ~ 25%, Mn 3% ~ 10%, Mo 0 ~ 6%, Si ≤ 0.5% to 6%, Ni 3% to 7%; Cu 0 to 3%, N 0 to 0.4%, containing a small amount of carbon, the main feature is that it contains higher chromium and lower nickel; Is Cr-Mn-N duplex stainless steel, into
The sub-ranges are: Cr 18% to 25%, Mn 8% to 16%, N 0.2% to 0.4%, Mo 0 to 3%, Si ≤ 2%, C ≤ 0.20%, and its composition characteristics It is low in nickel and contains high nitrogen.
1.1.3 Performance characteristics of duplex stainless steel
Since the duplex stainless steel has a two-phase structure, the duplex stainless steel can combine the properties of ferritic stainless steel and austenitic stainless steel by properly controlling the chemical composition and the heat treatment process, that is, the good toughness and weldability of the austenitic stainless steel. Combined with higher strength and chloride stress corrosion resistance of ferritic stainless steel. It is these superior properties that make duplex stainless steel develop very rapidly as a weldable structural material. In a nutshell, duplex stainless steel has the following main features:
- (1) Duplex stainless steel containing 18% to 22% Cr has good resistance to neutral chloride stress corrosion under low stress. 18-8 type austenitic stainless steel applied to a neutral chloride solution at 70 ° C or higher is prone to stress corrosion cracking on heat-dissipating parts such as heat exchangers and evaporators used in industrial media such as chlorides and sulfides.
- (2) Duplex stainless steel containing molybdenum has good pitting corrosion resistance, and the pitting corrosion resistance of duplex stainless steel containing 18% Cr is equivalent to that of AISl316L steel. The corrosion resistance and crevice corrosion resistance of duplex stainless steel containing 25% Cr, especially nitrogen-containing duplex stainless steel, exceeds that of AISl316L steel.
- (3) Duplex stainless steel has high strength and toughness, good overall performance, and yield strength twice that of austenitic stainless steel 18-8.
- (4) Good weldability, low thermal cracking tendency, no preheating before welding, no heat treatment after welding, and welding with dissimilar steel such as 18-8 austenitic stainless steel or carbon steel. *Includes SAF2304, 2205, SAF2507
- (5) The hot working temperature range is wider than that of the 18-8 type austenitic stainless steel, and the deformation resistance is small. The duplex stainless steel containing low chromium (18% Cr) can be directly rolled to produce profiles and steel sheets without forging.
- (6) The cold working performance is larger than that of the 18-8 type austenitic stainless steel. In the initial stage of deformation of the tube and the plate, a large stress is required to deform.
- (7) Various brittleness tendencies of high-chromium ferritic stainless steel, and should not be used under working conditions higher than 350 °C. The lower the chromium content in duplex stainless steel, the less the hazard of phase brittleness.
- (8) Compared with austenitic stainless steel, it has high thermal conductivity and small coefficient of linear expansion, and is suitable for use as a lining for equipment and for producing composite panels.
1.1.4 Main application of duplex stainless steel pipe
After decades of research and development, duplex stainless steel has won a place in the chemical, petroleum, paper, energy, shipbuilding, military and other industrial fields, and its application range is also expanding.
Source: China Duplex Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)