1 The second-generation duplex stainless steel is generally called standard duplex stainless steel. Its composition is ultra-low carbon and nitrogen. Its typical composition is 22% cr + 5% ni + 0.17% n, compared with the first generation duplex stainless steel. 2205 further increases the nitrogen content and enhances the stress corrosion resistance and pitting resistance in an acidic medium with a high chloride ion concentration. Nitrogen is a strong austenite forming element and is added to the duplex stainless steel to improve the strength of the steel without significantly impairing the plastic toughness of the steel, and inhibiting the precipitation and retardation of the carbide.
2 Tissue characteristics: Duplex stainless steel accounts for about half of the austenite and ferrite in the solid solution in the greenhouse, and has two-phase structure characteristics. It retains the characteristics of small ferritic stainless steel guide, small pitting resistance, crack and chloride stress corrosion, austenitic stainless steel with good toughness, low brittle transition temperature, resistance to intergranular corrosion, mechanical properties and welding. Good performance.
3 The performance is outstanding in yield strength and stress corrosion resistance. Duplex stainless steel is nearly twice as high as the austenitic stainless steel. Under the same pressure rating conditions, materials can be saved. The austenitic stainless steel has a low coefficient of linear thermal expansion and is close to that of low carbon steel. It makes the connection of duplex stainless steel and carbon steel suitable, which has great engineering significance. Forging and cold forming are not as good as austenitic stainless steel.
4 Weldability: Duplex stainless steel 2205 has good weldability and is less sensitive to weld cold cracks and hot cracks. Usually no preheating before welding, no heat treatment after welding. Due to the high nitrogen content, the single-phase ferrite tendency of the heat-affected zone is small. When the welding material is selected reasonably, the welding line energy control, the welding head has good comprehensive performance.
5 Thermal cracking: The sensitivity of hot cracks is much smaller than that of austenitic stainless steels. This is because the amount of nickel is not high, and impurities which easily form a low-melting eutectic are extremely small, and it is difficult to produce a low-melting liquid film. In addition, the crystal grains do not have a sharp increase in temperature at high temperatures.
6 Heat-affected zone embrittlement: The main problem of duplex stainless steel welding is not in the weld, but in the heat affected zone. Because the heat-affected zone is in a fast-cooling non-equilibrium state under the thermal cycle of welding, more ferrite is always retained after cooling, thereby increasing the corrosion tendency and hydrogen-induced crack (brittle) sensitivity.
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 cycling. At high temperatures, the metallurgical structure of all duplex stainless steels is entirely composed of ferrite, which is precipitated during cooling. The amount of austenite precipitation is affected by many factors.
8 Comparative requirements: The mechanical properties and corrosion resistance of duplex stainless steel welded joints depend on whether the welded joints can maintain proper proportions. Therefore, the welding is based on how to ensure the two-phase structure. When the amount of ferrite and austenite is close to 50%, the performance is better, close to the performance of the base metal. Changing this relationship will reduce the corrosion resistance and mechanical properties of duplex stainless steel welded joints. Duplex stainless steel 2205 ferrite content of the best 45%, too low ferrite content of less than 25% will lead to a decrease in strength and resistance to stress corrosion cracking; excessive ferrite content greater than 75% will also be detrimental to Corrosion resistance and reduced impact toughness.
9 Influencing factors: The equilibrium relationship between ferrite and austenite in welded joints is affected by the content of alloying elements in steel and by the filling metal, welding thermal cycle and shielding gas.
10 The influence of alloying elements: According to research and a large number of experiments, it is found that the nitrogen content of the base metal is very important. Nitrogen plays an important role in ensuring a sufficient amount of austenite in the weld metal and in the post-weld heat affected zone. Like nickel, nitrogen forms austenite and enlarges austenite. However, nitrogen has a greater capacity than nickel to prevent single-phase ferrite from occurring after welding and to prevent the precipitation of harmful metal phases. Due to the effect of the welding heat cycle, when the self-welding or filler metal composition is the same as that of the base material, the amount of ferrite of the weld metal increases sharply, and even pure ferrite structure appears. In order to suppress the excessive increase of ferrite in the weld, the weld metal which is dominated by austenite is the welding tendency of duplex stainless steel. Generally, it is adopted to increase nickel or nitrogen in the welding material. Usually, the content of nickel is 2%-4% higher than that of the base material. For example, the nickel content of the 2205 filler metal is as high as 8%-10%. The filler material containing nitrogen is more stable than the filler material which only improves nickel, but nitrogen is added. It not only delays the precipitation between metals, but also improves the strength and corrosion resistance of the weld metal. At present, the filler material is generally based on the increase of nickel, and then the nitrogen content of the base material is equivalent.
11 For duplex stainless steel 2205, sandvik 22.8.3L (ER2209) welding wire is used for tungsten argon arc welding, and Avesta 2205AC/DC welding rod is used for electrode arc welding to meet the requirements of welding materials. These characteristics of duplex stainless steel 2205 and welding materials on alloying elements provide a certain range for the selection of welding process parameters, ie welding line energy, which is very advantageous for welding.
12 Thermal cycling: The most important feature of duplex stainless steel welding is that the welding thermal cycle has an effect on the microstructure inside the welded joint. There is a phase change in both the weld and the thermal star zone, which has a great influence on the performance of the welded joint. Therefore, multi-layer multi-pass welding is beneficial, the subsequent bead has a heat treatment effect on the front bead, and the ferrite in the weld metal is further transformed into austenite, which becomes a two-phase structure dominated by austenite; The austenite phase in the heat-affected zone adjacent to the weld is correspondingly increased, and the ferrite grains can be refined to reduce the precipitation of carbides and nitrides from the grains and grain boundaries, thereby making the microstructure of the entire welded joint. Significant improvement. It is also because of the influence of the welding heat cycle that the weld bead which is required to contact the medium during the welding of the duplex stainless steel should be welded, which is exactly the opposite of the welding sequence requirement of the austenitic stainless steel.
13 Effect of process parameters: The number of welding processes, ie the welding line energy, also plays a key role in the balance of the two-phase structure. Since the duplex stainless steel is 100% ferrite at high temperature, if the line energy is too small, the heat affected zone cools quickly, and the austenite does not have enough precipitation of ferrite to be kept cold in the greenhouse. If the line energy is too large, the cooling rate is too slow. Although a sufficient amount of austenite can be obtained, it also causes the ferrite grain growth in the heat-affected zone and the precipitation of the harmful metal phase with the same σ, resulting in brittleness of the joint. In order to avoid this, the best measure is to control the weld line energy and the interlayer temperature and use a filler metal.
14 Effect of protective gas: In tungsten argon arc welding, 2% nitrogen can be added to argon to prevent the surface of the weld from losing nitrogen due to diffusion, which helps the balance between ferrite and austenite.
Source: China 2205 Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)