Welding problems and characteristics of austenitic stainless steel

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Welding problems and characteristics of austenitic stainless steel: The welding characteristics of austenitic stainless steel include very high elastic and plastic stresses and strain levels during welding, yet cold cracks occur very rarely. The welded joint lacks a hardened zone and grain coarsening, therefore the tensile strength of the weld is high. However, it also suffers from significant welding deformation; due to its grain boundary characteristics and sensitivity to certain trace impurities (S, P), it is prone to thermal cracking and other problems. The intergranular corrosion resistance of the welded joint is reduced. Intergranular corrosion: According to the chromium-depletion theory, when the weld and heat-affected zone are heated to the sensitization temperature range of 450–850°C, chromium carbides precipitate at the grain boundaries, resulting in chromium-depleted boundaries that are not strong enough to resist corrosion. To mitigate intergranular corrosion in welds and corrosion in the sensitization temperature range of the base material, the following measures can be taken: (1) Reduce the carbon content in both the base material and the weld; add stabilizing elements such as Ti and Nb to the base material to promote the formation of MC phases, thereby preventing the formation of Cr23C6. (2) To form a dual-phase structure in the weld consisting of austenite with a small amount of ferrite. When a certain amount of ferrite is present in the weld, it can refine the grains and increase their area, thereby reducing the amount of chromium carbide precipitated per unit area of the grain boundaries. Chromium has a high solubility in ferrite; Cr23C6 forms preferentially in ferrite, thereby preventing the austenite grain boundaries from becoming chromium-deficient. The ferrite dispersed among the austenite grains can prevent corrosion from spreading inward along the grain boundaries. (3) Control the residence time in the sensitization temperature range. Adjust the welding heat cycle to minimize the residence time at 600–1000°C; choose a welding method with high energy density (such as plasma TIG welding), use a lower welding wire energy, pass argon gas behind the weld seam or use copper pads to increase the cooling rate of the weld joint. Reduce the number of starts and stops during welding to avoid repeated heating, and when performing multi-layer welding, apply welding to the surfaces in contact with corrosive media as late as possible. (4) After welding, carry out solution treatment or stabilization annealing at 850–900°C, followed by air cooling, in order to promote the complete precipitation of carbides and accelerate the diffusion of chromium. Preventive measures against knife-edge corrosion in welded joints: Due to their high diffusion capacity, carbon tends to accumulate at the grain boundaries during cooling, creating a supersaturated state, while Ti and Nb remain within the grains because of their low diffusion capacity. When the welded joint is reheated within the sensitization temperature range, supersaturated carbon will precipitate intergranularly in the form of Cr23C6. (1) Reduce carbon content. For stainless steels containing stabilizing elements, the carbon content should not exceed 0.06%. (2) Adopt a reasonable welding process. Choose a lower welding wire energy to reduce the high-temperature dwell time in the overheated area, and be careful to avoid the occurrence of \"intermediate-temperature sensitization\" during welding. During double-sided welding, the welds that come into contact with corrosive media should be welded last (which is why the inner weld of large-diameter thick-walled welded pipes is carried out after the outer weld); if this cannot be done, the welding parameters and weld geometry should be adjusted to minimize any re-sensitization heating of the areas that come into contact with corrosive media. (3) Post-weld heat treatment. Solution treatment or stabilization treatment is carried out after welding. Images: Stress corrosion cracking, Measures to prevent stress corrosion cracking: (1) Proper selection of materials and appropriate adjustment of weld composition. High-purity chromium-nickel austenitic stainless steels, high-silicon chromium-nickel austenitic stainless steels, ferritic-austenitic stainless steels, and high-chromium ferritic stainless steels exhibit good resistance to stress corrosion; the weld metal, when of austenitic-ferritic duplex steel structure, also shows good resistance to stress corrosion. (2) Eliminate or reduce residual stress. Post-weld stress relief heat treatment is performed, and mechanical methods such as polishing, shot peening, and peening are used to reduce residual surface stresses. (3) Rational structural design. To avoid significant stress concentration. Image: Welding hot cracks (grain boundary cracks in the weld, liquefaction cracks in the heat-affected zone). The sensitivity to hot cracks depends primarily on the chemical composition, microstructure, and properties of the material. Ni readily forms low-melting-point compounds or eutectics with impurities such as S and P, while the segregation of elements like boron and silicon promotes the formation of thermal cracks. The welds tend to develop a coarse columnar crystal structure with a strong directional pattern, which facilitates the segregation of harmful impurities and elements. This promotes the formation of a continuous intergranular liquid film, increasing susceptibility to thermal cracking. If the welding does not result in uniform heating, large tensile stresses are likely to be generated, promoting the formation of weld heat cracks. Preventive measures: (1) Strictly control the content of harmful impurities S and P. (2) Adjust the microstructure of the weld metal. Welds with a biphase structure possess excellent crack resistance. The δ phase in such welds can refine the grain structure, eliminate the directionality of single-phase austenite, reduce the segregation of harmful impurities at grain boundaries, and it is also capable of dissolving larger amounts of S and P; furthermore, it can lower the interfacial energy and prevent the formation of liquid films between grains. (3) Adjust the alloy composition of the weld metal. By appropriately increasing the contents of Mn, C, and N in single-phase austenitic steel, and adding small amounts of trace elements such as cerium, hafnium, and tantalum (which can refine the weld microstructure and purify the grain boundaries), the sensitivity to thermal cracking can be reduced. (4) Process measures. Minimize the pool overheating to prevent the formation of coarse columnar crystals, by using a low wire energy and a narrow weld bead width. Image: Cracking of welded joints. Heat-resistant steels must maintain the plasticity of their welded joints to prevent cracking at high temperatures; steels intended for use at low temperatures require good low-temperature toughness to avoid brittle fracture of the welded joints. Image: Significant welding deformation occurs due to the low thermal conductivity and high expansion coefficient; fixtures can be used to prevent such deformation. Welding methods and selection of welding materials for austenitic stainless steel Austenitic stainless steel can be welded using methods such as Tungsten Inert Gas Welding (TIG), Metal Inert Gas Welding (MIG), Plasma Arc Welding (PAW), and Submerged Arc Welding (SAW). 　Austenitic stainless steel requires a lower welding current due to its low melting point, low thermal conductivity, and high electrical resistivity. Narrow welds and narrow bead patterns should be used to reduce the time spent at high temperatures, prevent the formation of carbides, minimize weld shrinkage stresses, and lower susceptibility to thermal cracking. The composition of the welding material, especially the alloying elements Cr and Ni, must be higher than that of the base metal. Welding materials containing a small amount (4–12%) of ferrite are used to ensure good crack resistance of the welds (cold cracking, hot cracking, stress corrosion cracking). When the presence of a ferrite phase is not allowed or impossible in the weld, welding materials containing alloying elements such as Mo and Mn should be used. C, S, P, Si, and Nb in welding materials should be kept as low as possible; Nb can cause solidification cracks in pure austenitic welds, but a small amount of ferrite in the weld can effectively prevent this. For welded structures that require stabilization or stress-relief treatment after welding, welding materials containing Nb are typically used. Submerged arc welding is used for welding medium plates, and the loss of Cr and Ni can be compensated for by the supply of alloying elements from the flux and wire. 　　Due to the large penetration depth, care must be taken to prevent the formation of thermal cracks in the central area of the weld and to avoid a decrease in the corrosion resistance of the heat-affected zone. Care should be taken to select a thinner welding wire and lower welding line energy; the wire should have low levels of Si, S, and P. 　　The ferrite content in the welds of heat-resistant stainless steel should not exceed 5%. For austenitic stainless steels with Cr and Ni contents exceeding 20%, welding wires with high Mn content (6–8%) should be used, and alkaline or neutral fluxes should be selected to prevent the addition of Si to the weld, thereby enhancing its crack resistance. 　　The flux designed for austenitic stainless steel contains very little Si, and it can supply alloying elements to the weld area, compensating for the loss of these elements due to welding processes, thereby meeting the requirements regarding the properties and chemical composition of the weld.