Thread Content
1 The second-generation duplex stainless steels are generally referred to as standard duplex stainless steels. Their composition is characterized by ultra-low carbon content and nitrogen inclusion; their typical composition is 22% Cr + 5% Ni + 0.17% N. Compared to the first-generation duplex stainless steels, 2205 has a higher nitrogen content, which enhances its resistance to stress corrosion and pitting corrosion in acidic media with high chloride ion concentrations. Nitrogen is a strong austenite-forming element; when added to duplex stainless steel, it increases the strength of the steel without significantly compromising its ductility and toughness, and it also suppresses the precipitation and retards the formation of carbides. 2 Structural characteristics: At room temperature, in the solid solution of duplex stainless steel, austenite and ferrite each account for approximately half, thus exhibiting the characteristics of a two-phase structure. It retains the characteristics of ferritic stainless steels, such as low pitting susceptibility, resistance to crevice corrosion and chloride stress corrosion. At the same same time, it also possesses the advantages of austenitic stainless steels, including good toughness, a relatively low brittle transition temperature, resistance to intergranular corrosion, as well as excellent mechanical and weldability properties. 3. Outstanding performance: In terms of yield strength and resistance to stress corrosion, the yield strength of duplex stainless steels is nearly twice that of austenitic stainless steels. Under the same pressure rating conditions, this allows for material savings. Its linear coefficient of thermal expansion is lower than that of austenitic stainless steel, and is close to that of low-carbon steel. This makes the joining of duplex stainless steel and carbon steel more feasible, which has great engineering significance. Forging and cold stamping are not as good as with austenitic stainless steel. 4 Weldability: Duplex stainless steel 2205 has good weldability; it exhibits low sensitivity to both cold and hot cracking during welding. Typically, no preheating is done before welding, and no heat treatment is performed after welding. Due to its relatively high nitrogen content, the tendency toward single-phase ferritization in the heat-affected zone is reduced. When the welding material is appropriately selected and the welding heat input is well controlled, the weld joint exhibits good overall properties. 5 Thermal cracking: Thermal cracking is much less sensitive than austenitic stainless steels. This is because the nickel content is not high; there are very few impurities prone to forming low-melting-point eutectics, so a low-melting-point liquid film is unlikely to form. Additionally, there is no risk of grains growing rapidly at high temperatures. 6 Embrittlement of the heat-affected zone: The main problem in welding duplex stainless steels lies not in the weld itself, but in the heat-affected zone. Because under the welding heat cycle, the heat-affected zone is in a rapidly cooled non-equilibrium state, and more ferrite remains after cooling, thereby increasing the tendency to corrosion and sensitivity to hydrogen-induced cracking (brittleness). 7 Welding metallurgy: During the welding of duplex stainless steels, under the effect of thermal cycles, a series of changes occur in the microstructure of the weld metal and the heat-affected zone. At high temperatures, the microstructure of all duplex stainless steels consists entirely of ferrite; austenite precipitates during the cooling process. The amount of austenite precipitation is influenced by many factors. 8 Phase ratio requirements: The mechanical and corrosion resistance properties of welded joints in duplex stainless steels depend on whether the joints can maintain an appropriate phase ratio. Therefore, welding is carried out with the aim of preserving their duplex microstructure. When the amounts of ferrite and austenite are each close to 50%, the properties are relatively good, approaching those of the base material. Altering this relationship will degrade the corrosion resistance and mechanical properties of welded joints in duplex stainless steels. For duplex stainless steel 2205, the optimal ferrite content is 45%. A ferrite content of less than 25% will result in a decrease in strength and resistance to stress corrosion cracking; conversely, a ferrite content exceeding 75% will also impair corrosion resistance and reduce impact toughness. Factors influencing the phase ratio: The balance between ferrite and austenite in welded joints is affected both by the content of alloying elements in the steel, as well as by the filler metal, welding thermal cycle, and shielding gas. 10 Effects of alloying elements: Based on research and numerous experiments, it has been found that it is very important for the base metal to contain nitrogen. Nitrogen plays an important role in ensuring the formation of a sufficient amount of austenite in the weld metal and the post-weld heat-affected zone. Like nickel, nitrogen is an element that forms and expands the austenitic phase; however, its effectiveness exceeds that of nickel. It can prevent the formation of a single-phase ferrite after welding and inhibit the precipitation of harmful metallic phases. Due to the effect of the welding thermal cycle, when the composition of the self-fluxing weld or filler metal is the same as that of the base metal, the amount of ferrite in the weld metal increases sharply, and a pure ferrite structure may even form. To suppress the excessive increase of ferrite in the weld, the use of weld metal with a predominance of austenite is a trend in the welding of duplex stainless steels. Generally, two approaches are taken: increasing nickel content or adding nitrogen to the welding material. Typically, the nickel content is 2%–4% higher than that of the base material; for example, the nickel content in 2205 filler metal is as high as 8%–10%. The use of nitrogen-containing filler materials yields more stable results than using filler materials that only increase nickel content; however, adding nitrogen not only delays intermetallic precipitation, but also enhances the strength and corrosion resistance of the weld metal. Currently, filler materials generally involve adding nitrogen in an amount equivalent to that of the base material, on top of increasing the nickel content. 11 For duplex stainless steel 2205, using Sandvik 22.8.3L (ER2209) wire for TIG welding and Avesta 2205AC/DC electrodes for shielded metal arc welding meets the requirements for welding materials. These characteristics of duplex stainless steel 2205 and its welding materials regarding alloying elements provide a certain range for the selection of welding process parameters, namely the welding line energy; this is highly beneficial for welding. 12 Thermal cycling: The most significant characteristic of welding duplex stainless steels is that the welding thermal cycle affects the microstructure within the weld joint. Phase transformations occur both in the weld and in the heat-affected zone, which has a considerable impact on the properties of the weld joint. Therefore, multi-layer and multi-pass welding is beneficial; the subsequent passes exert a heat treatment effect on the previous layers. This causes the ferrite in the weld metal to further transform into austenite, resulting in a two-phase structure dominated by austenite ； The austenite phase in the heat-affected zone adjacent to the weld also increases accordingly, and it can refine the ferrite grains as well as reduce the precipitation of carbides and nitrides from within the grains and at the grain boundaries, thereby significantly improving the microstructural properties of the entire welded joint. It is also due to the effect of the welding thermal cycle that, when welding duplex stainless steel, the weld beads in contact with the medium must be welded; this is exactly the opposite of the requirements regarding the welding sequence for austenitic stainless steel. 13 Influence of process parameters: The number of welding processes, i.e., the welding line energy, also plays a crucial role in maintaining the balance of the duplex structure. Since duplex stainless steel is 100% ferritic at high temperatures, if the heat input is too low, the cooling rate in the heat-affected zone becomes rapid; as a result, austenite has no time to precipitate, and the excess ferrite remains in a supercooled state at room temperature. If the linear energy is too high and the cooling rate is too slow, although sufficient austenite can be obtained, it may also lead to the growth of ferrite grains in the heat-affected zone and the precipitation of harmful metallic phases such as σ phase, resulting in joint embrittlement. To avoid the above situations, the best measures are to control the welding wire energy and interpass temperature, and to use filler metal. 14 Effect of shielding gas: In tungsten inert gas welding, 2% nitrogen can be added to argon to prevent the loss of nitrogen from the weld surface due to diffusion; this helps maintain a balance between ferrite and austenite.