Welding of austenitic stainless steel

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Welding characteristics of austenitic stainless steel: Elastic and plastic stresses and strains are very high during welding, yet cold cracks occur extremely rarely. The welded joint lacks quenched and hardened zones as well as grain coarsening, therefore the tensile strength of the weld is high. Main problems in welding austenitic stainless steel: significant welding deformation ； Due to its grain boundary properties and sensitivity to certain trace impurities (S, P), it is prone to thermal cracking. 5 Major Welding Problems of Austenitic Stainless Steel and Their Solutions 01 Formation of chromium carbide, which reduces the intergranular corrosion resistance of the welded joint. Intergranular corrosion: According to the chromium-depletion theory, when the welds and heat-affected zones are heated to the sensitization temperature range of 450–850°C, chromium carbide precipitates at the grain boundaries, resulting in chromium-depleted boundaries that are not strong enough to resist corrosion. (1) To mitigate intergranular corrosion in welds and corrosion in the sensitization temperature range of the base material, the following measures can be taken: a. 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. b. 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 preferentially forms in ferrite, thereby preventing the austenite grain boundaries from becoming chromium-deficient ； The ferrite dispersed among the austenite prevents corrosion from spreading inward along the grain boundaries. c. Control the residence time within 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 arc welding), use a lower welding wire energy, pass argon gas behind the weld or employ copper pads to increase the cooling rate of the weld joint. Reduce the number of start and stop operations to avoid repeated heating, and when performing multi-layer welding, apply welding to the surfaces in contact with corrosive media as late as possible. d. 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. (2) Knife-edge corrosion of welded joints; to prevent this, the following measures can be taken: Due to their strong diffusion capacity, carbon tends to accumulate at the grain boundaries during cooling, resulting in 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. a. Reduce carbon content. For stainless steels containing stabilizing elements, the carbon content should not exceed 0.06%. b. 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 further sensitizing heating of the areas that come into contact with corrosive media. c. Post-weld heat treatment. Solution treatment or stabilization treatment is carried out after welding. 02 Stress corrosion cracking: The following measures can be taken to prevent stress corrosion cracking: a. Proper selection of materials and reasonable 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; weld metals with an austenitic-ferritic duplex structure also show good stress corrosion resistance. b. Eliminate or reduce residual stresses. Post-weld stress-relief heat treatment is carried out, and mechanical methods such as polishing, shot blasting, and hammering are used to reduce surface residual stresses. c. Rational structural design. To avoid significant stress concentration. 03 Welding thermal cracks (weld crystallization cracks, liquidation cracks in the heat-affected zone). The sensitivity to thermal cracks depends mainly 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 grain structure with a strong directional pattern, which facilitates the segregation of harmful impurities and elements. This facilitates 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 welding hot cracks. Preventive measures: a. Strictly control the content of harmful impurities S and P. b. Adjust the microstructure of the weld metal. Welds with a biphase structure possess excellent crack resistance. The δ phase in the weld helps to refine the grain structure, eliminates the directional nature of single-phase austenite, reduces the segregation of harmful impurities at grain boundaries, and is capable of dissolving larger amounts of S and P; it also lowers the interfacial energy, thereby preventing the formation of liquid films between the grains. c. 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 susceptibility to thermal cracking can be reduced. d. Process measures. Minimize the molten pool overheating to prevent the formation of large columnar crystals, by using a low wire energy and a weld bead with a small cross-section. For example, austenitic steels of type 25-20 are prone to liquefaction cracking. This can be achieved by strictly controlling the impurity content and grain size of the base material, using welding methods with high energy density, reducing the wire energy input, and increasing the cooling rate of the joint. 04 Embrittlement of welded joints: Heat-resistant steels should ensure the plasticity of welded joints to prevent embrittlement at high temperatures ； Low-temperature steel requires good low-temperature toughness to prevent brittle fracture of welded joints at low temperatures. 05 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 usually selected. Submerged arc welding is used for welding medium-thickness 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 finer welding wire and a lower welding wire 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 greater than 20%, weld wires with high Mn content (6–8%) should be used, and alkaline or neutral fluxes should be chosen to prevent the addition of Si to the weld seam, thereby improving its crack resistance. The flux designed for austenitic stainless steel contains very little silicon, allowing it to supply alloying elements to the weld zone and compensate for the loss of these elements, thereby meeting the requirements regarding the properties and chemical composition of the weld.