1.The second generation duplex stainless steel (commonly known as standard duplex stainless steel) has the characteristics of ultra-low carbon, low nitrogen, typical composition Cr5%Ni0.17%n and 2205 higher nitrogen content than the first generation duplex stainless steel, improving Improves the resistance to stress corrosion and pitting corrosion in acidic media with high chloride ion concentration. Nitrogen is a strong austenite-forming element. Adding nitrogen to duplex stainless steel not only improves the strength of the steel, but also does not significantly damage the plasticity and toughness of the steel, and inhibits the precipitation and retardation of carbides.
2.The second generation duplex stainless steel (commonly known as standard duplex stainless steel) has the characteristics of ultra-low carbon, low nitrogen, typical composition Cr5%Ni0.17%n and 2205 higher nitrogen content than the first generation duplex stainless steel, improving Improves the resistance to stress corrosion and pitting corrosion in acidic media with high chloride ion concentration. Nitrogen is a strong austenite-forming element. Adding nitrogen to duplex stainless steel not only improves the strength of the steel, but also does not significantly damage the plasticity and toughness of the steel, and inhibits the precipitation and retardation of carbides.
3.The yield strength and stress corrosion resistance of two-phase stainless steel are almost twice that of austenitic stainless steel, which can save materials under the same pressure level. The linear expansion coefficient of austenitic stainless steel is lower than that of austenitic stainless steel and close to that of low carbon steel. Making the connection between duplex stainless steel and carbon steel more suitable has important engineering significance. Both forging and cold stamping are inferior to austenitic stainless steel.
4.Weldability: Duplex stainless steel 2205 has good weldability and has low sensitivity to cold and hot cracks during welding. Usually there is no preheating before welding and no heat treatment after welding. Due to the high nitrogen content, the tendency of single-phase ferrite in the heat-affected zone is small. When welding materials are properly selected and the welding line energy is controlled at this time, it has good overall performance.
5.Hot cracking: Hot cracking sensitivity is much lower than that of austenitic stainless steel. This is because the nickel content is not high, the impurities that easily form low-melting-point eutectic are very low, and the low-melting-point liquid film is not easy to form. In addition, there is no danger of rapid growth of grains at high temperatures.
6.Embrittlement of the heat-affected zone: The main problem with duplex stainless steel welding is not the weld, but the heat-affected zone. Since the heat-affected zone is in a non-equilibrium state of rapid cooling under the action of welding heat cycles, more ferrite is always retained after cooling, which increases corrosion tendency and hydrogen-induced cracking (brittleness) susceptibility.
7.Welding metallurgy: During the welding process of duplex stainless steel, a series of changes occur in the microstructure of the weld metal and heat-affected zone under the action of thermal cycles. At high temperatures, the metallographic structure of duplex stainless steel is ferrite, and austenite precipitates during cooling. The amount of austenite precipitation is affected by many factors.
8.The mechanical properties and corrosion resistance of duplex stainless steel welded joints depend on whether the appropriate welded joint ratio can be maintained. Therefore, welding revolves around how to ensure a double-sided structure. When the amount of ferrite and austenite is close to 50%, the performance is good and the properties of the base materials are close to each other. Changing this relationship will reduce the corrosion resistance and mechanical properties of duplex stainless steel welded joints. The optimal ferrite content of 2205 duplex stainless steel is 45%. When the ferrite content is less than 25%, the strength and stress corrosion cracking ability decrease. Excess ferrite content greater than 75% will also affect the corrosion resistance and reduce the impact toughness. .
9.Comparison factors: The balance of ferrite and austenite in welded joints is not only affected by the content of alloy elements in the steel, but also by the filler metal, welding thermal cycle and shielding gas.
10.Influence of alloying elements: According to research and a large number of experiments, it is found that nitrogen, the base material, is very important. Nitrogen plays an important role in ensuring sufficient austenite formation in the weld metal and heat affected zone after welding. Nitrogen (such as nickel) forms austenite values and expands austenite elements, but the ability of nitrogen is also greater than that of nickel, which can prevent the appearance of single phases after welding and prevent the precipitation of harmful metal phases. Due to the influence of the welding thermal cycle, when the composition of autogenous welding or the filler metal is the same as that of the base metal, the ferrite content of the weld metal increases sharply, and even a pure ferrite structure appears. In order to suppress the excessive increase of ferrite in the weld, the welding metal with main austenite is the welding tendency of duplex stainless steel. Usually, the two methods are to add nickel or nitrogen to the welding material. Typically, the nickel content is 2% to 4% higher than the base metal, for example, the 2205 filler metal has a nickel content of up to 8% to 10%. The nitrogen-containing solder ratio can only improve the effect of nickel solder, but the addition of nitrogen can not only delay the precipitation between metals, but also improve the strength and corrosion resistance of the weld metal. At present, filling materials are usually based on adding nickel, and then adding nitrogen with the same content of the base metal.
11.For duplex stainless steel 2205, Sandvik 22.8.3L (ER2209) welding wire is used for TIG welding, and Avesta 2205AC/DC welding rod is used for TIG welding to meet the requirements of welding materials. These characteristics of duplex stainless steel 2205 and welding materials on alloy elements provide a certain range for the selection of welding process parameters, that is, welding line energy, which is very beneficial to welding.
12.Thermal cycle: The biggest feature of duplex stainless steel welding is that the welding heat cycle has an impact on the structure of the welded joint. Whether it is the weld seam or the hot film star zone, phase change will occur, which has a great impact on the performance of the welded joint. Therefore, multi-layer and multi-pass welding is beneficial. The subsequent welds have a heat treatment effect on the previous weld. The ferrite in the weld metal is further transformed into austenite, and the austenite in the heat affected zone adjacent to the weld is The bulk phase is also increased, which can refine the ferrite grains and reduce the precipitation of carbides and nitrides from crystals and grain boundaries, thereby significantly improving the structure and performance of the entire welded joint. Due to the influence of the welding thermal cycle, the weld beads in contact with the medium should be welded with duplex stainless steel, which is opposite to the requirements for the welding sequence of austenitic stainless steel.
13.Influence of process parameters: The number of welding processes, that is, the welding line energy, also plays a key role in the balance of the two-phase microstructure. Since duplex stainless steel is 100% ferrite at high temperatures, if the line energy is too small If the cooling rate of the heat-affected zone is too fast, excessive ferrite precipitated from austenite will continue to exist under the supercooling conditions of the greenhouse. If the line energy is too large and the cooling rate is too slow, although a sufficient amount of austenite can be obtained, it will also lead to the grain growth of ferrite in the heat-affected zone and the precipitation of harmful metal phases equal to 0, resulting in joint embrittlement. . To avoid the above situation, the best measures are to control the weld energy and interpass temperature and use filler metal.
14.Influence of process parameters: The number of welding processes, that is, the welding line energy, also plays a key role in the balance of the two-phase microstructure. Since duplex stainless steel is 100% ferrite at high temperatures, if the line energy is too small If the cooling rate of the heat-affected zone is too fast, excessive ferrite precipitated from austenite will continue to exist under the supercooling conditions of the greenhouse. If the line energy is too large and the cooling rate is too slow, although a sufficient amount of austenite can be obtained, it will also lead to the grain growth of ferrite in the heat-affected zone and the precipitation of harmful metal phases equal to 0, resulting in joint embrittlement. . To avoid the above situation, the best measures are to control the weld energy and interpass temperature and use filler metal.
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