Welding processes for austenitic stainless steel

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1. Welding characteristics of austenitic stainless steel: (1) It is prone to hot cracks. Preventive measures: (a) Strive to achieve a duplex microstructure in the weld metal, keeping the ferrite content below 3-5%. Because ferrite can dissolve large amounts of harmful S and P impurities. (b) Prefer high-quality electrodes with alkaline fluxes to limit the contents of elements such as S, P, and C in the weld metal. (2) 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 carbide precipitates at the grain boundaries, resulting in chromium-depleted boundaries that are not strong enough to resist corrosion. Preventive measures: (a) Use low-carbon or ultra-low-carbon welding materials, such as A002, etc ； Welding electrodes containing stabilizing elements such as titanium and niobium, such as A137, A132, etc., are used. (b) A certain amount of ferrite-forming elements are introduced into the weld from the welding wire or electrode, resulting in a dual-phase structure of austenite + ferrite in the weld metal (the ferrite content is generally kept between 4% and 12%). (c) Reduce overheating of the weld pool by using a lower welding current and a faster welding speed, thereby accelerating the cooling rate. (d) Perform post-weld stabilization annealing on weldments with high requirements for intergranular corrosion resistance. (3) Stress corrosion cracking: Stress corrosion cracking is a phenomenon of delayed cracking that occurs in welded joints under specific corrosive environments when subjected to tensile stress. Stress corrosion cracking of austenitic stainless steel welded joints is a severe form of failure in such joints, manifesting as brittle fracture without plastic deformation. Preventive measures: (a) Reasonably design the forming and assembly processes to minimize cold working deformation as much as possible, avoid forced assembly, and prevent various types of scratches from occurring during assembly (all types of assembly scratches as well as arc burns can serve as sources of cracking for SCC, leading to corrosion pits). (b) Proper selection of welding materials: The weld metal should match the base metal well, without the formation of any undesirable microstructures such as grain coarsening or hard and brittle martensite. (c) Adopt appropriate welding processes: ensure good weld formation, without any defects such as stress concentration or pitting, like undercutting ； Adopt a reasonable welding sequence to reduce the level of welding residual stress. (d) Stress relief treatment: post-weld heat treatment, such as full annealing or annealing after welding ； When heat treatment is difficult to carry out, post-weld hammering or shot blasting is used. (e) Production management measures: Control of impurities in the medium, such as O2, N2, H2O, etc., in liquid ammonia media ； H2S in liquefied petroleum gas ； O2, Fe3+, Cr6+ in chloride solutions, etc ； Corrosion protection: such as coating, lining, or cathodic protection, etc ； Add a corrosion inhibitor. (4) Low-temperature embrittlement of weld metal: For austenitic stainless steel welded joints, the plasticity and toughness of the weld metal are critical issues when used at low temperatures. At this point, the presence of ferrite in the weld microstructure always deteriorates low-temperature toughness. Preventive measures: Achieving a single austenitic weld by using pure austenitic welding materials and adjusting the welding process. (5) σ-phase embrittlement of welded joints: After being exposed to high temperatures for a certain period of time, a brittle σ-phase precipitates in the weld zone of welded components, resulting in overall embrittlement of the joint and a significant decrease in plasticity and toughness. The precipitation temperature range of the σ phase is 650–850°C. During high-temperature heating, the σ phase is mainly formed from the transformation of ferrite. The longer the heating time, the more σ-phase precipitates. Preventive measures: (a) Limit the ferrite content in the weld metal (to less than 15%) ； Superalloyed welding materials, namely high-nickel welding electrodes, are used. (b) Use a small pass size to minimize the residence time of the weld metal at high temperatures. (c) If conditions permit, carry out a solution treatment on the precipitated σ phase to dissolve it into austenite. 2. Key points for selecting electrodes for austenitic stainless steel: Stainless steel is primarily used as corrosion-resistant steel, but it is also employed as heat-resistant and low-temperature steel. Therefore, when welding stainless steel, the properties of the welding rod must be compatible with the intended use of the stainless steel. Stainless steel welding electrodes must be selected based on the base material and working conditions (including working temperature and contact media, etc.). (1) Generally speaking, when selecting a welding electrode, one can refer to the material of the base metal; an electrode with the same or similar composition as the base metal should be chosen. For example: A102 corresponds to 0Cr19Ni9 ； A137 corresponds to 1Cr18Ni9Ti. (2) Since the carbon content has a significant impact on the corrosion resistance of stainless steel, stainless steel electrodes with a carbon content that is not higher than that of the base material are generally chosen. For 316L, A022 welding electrodes must be used. (3) The weld metal of austenitic stainless steel shall ensure mechanical properties, which can be verified through welding procedure qualification. (4) For heat-resistant stainless steels (austenitic heat-resistant steels) used in high-temperature environments, the welding electrodes selected should primarily ensure the heat-cracking resistance of the weld metal as well as the high-temperature performance of the welded joint. a) For austenitic heat-resistant steels with a Cr/Ni ratio of ≥1, such as 1Cr18Ni9Ti, austenitic-ferritic stainless steel electrodes are generally used, with a ferrite content of 2% to 5% in the weld metal being desirable. When the ferrite content is too low, the crack resistance of the weld metal is poor ； If it is too high, the σ embrittlement phase is likely to form during long-term use at high temperatures or during heat treatment, leading to cracks. Such as A002, A102, A137. In certain special applications where the use of fully austenitic weld metal is required, electrodes such as A402 and A407 can be employed. b) For stable austenitic heat-resistant steels with a Cr/Ni ratio of less than 1, such as Cr16Ni25Mo6, it is generally necessary to ensure that the weld metal has a chemical composition similar to that of the base material, while increasing the contents of elements such as Mo, W, and Mn in the weld metal. This approach helps to enhance the crack resistance of the weld while also maintaining its thermal strength. Such as A502, A507. (5) For corrosion-resistant stainless steels used in various corrosive media, welding electrodes should be selected based on the medium and operating temperature, to ensure their corrosion resistance (by conducting corrosion resistance tests on the welded joints). a) For media with a working temperature above 300°C and high corrosivity, welding electrodes containing Ti or Nb stabilizing elements or ultra-low carbon stainless steel must be used. Such as A137 or A002, etc. b) For media containing dilute sulfuric acid or hydrochloric acid, stainless steel electrodes containing Mo or those containing both Mo and Cu are commonly used. Such as A032, A052, etc. c) Stainless steel electrodes without Ti or Nb can be used only for equipment with low corrosivity under operating conditions or where rust contamination needs to be avoided. To ensure the stress corrosion resistance of the weld metal, super-alloyed welding materials are used, meaning that the content of corrosion-resistant alloying elements (such as Cr, Mo, Ni, etc.) in the weld metal is higher than that in the base material. If welding material of the 00Cr18Ni12Mo2 type (such as A022) is used to weld 00Cr19Ni10 weldments. (6) For austenitic stainless steels operating under low-temperature conditions, it is necessary to ensure the low-temperature impact toughness of the welded joint at the operating temperature; therefore, pure austenitic welding electrodes are used. Such as A402, A407. (7) Nickel-based alloy electrodes can also be used. When welding Mo6-type super austenitic stainless steel using nickel-based weld materials containing up to 9% Mo. (8) Selection of electrode coating type: a) Since the weld metal of duplex austenitic steel contains a certain amount of ferrite, it possesses good plasticity and toughness. From the perspective of the crack resistance of the weld metal, the difference between electrodes with alkaline coatings and those with titanium-calcium coatings is not as significant as that seen with carbon steel electrodes. Therefore, in practical applications, more emphasis is placed on the welding process properties, and electrodes with coating codes of 17 or 16 are generally used (such as A102A, A102, A132, etc.). b) Alkaline electrode coatings with code 15, such as A107 and A407, are considered only when the structure has high rigidity or when the crack resistance of the weld metal is poor (such as in certain martensitic chromium stainless steels and chromium-nickel stainless steels with a pure austenitic structure). 3. Key points for welding austenitic stainless steel: Generally speaking, austenitic stainless steel has excellent weldability. Almost all fusion welding methods can be used to weld austenitic stainless steels, and the thermophysical properties and microstructural characteristics of these steels determine the key aspects of their welding processes. ① Due to the low thermal conductivity and high thermal expansion coefficient of austenitic stainless steel, large deformations and welding stresses tend to occur during welding; therefore, welding methods that concentrate welding energy should be used as much as possible. ② Due to the low thermal conductivity of austenitic stainless steel, a greater penetration depth can be achieved compared to low-alloy steel under the same current. At the same time, due to its high resistivity, in shielded metal arc welding, a lower welding current is required compared to carbon steel or low-alloy steel electrodes of the same diameter, in order to prevent the electrode from turning red. ③Welding specifications. Large welding energy is generally not used. In shielded metal arc welding, it is advisable to use electrodes of small diameter and carry out rapid multiple passes of welding. For welds with high quality requirements, cold water can even be applied to accelerate cooling. For pure austenitic stainless steels and super-austenitic stainless steels, due to their high susceptibility to thermal cracking, the welding heat input must be strictly controlled in order to prevent excessive growth of weld grains and the occurrence of welding thermal cracks. ④ To improve the heat-cracking resistance and corrosion resistance of the weld, special attention must be paid to keeping the welding area clean during welding, in order to prevent harmful elements from penetrating into the weld. ⑤ Austenitic stainless steel generally does not require preheating during welding. To prevent grain growth and carbide precipitation in the weld and heat-affected zone, and to ensure the plasticity, toughness, and corrosion resistance of the welded joint, a lower interpass temperature should be maintained, generally not exceeding 150°C. 4. Welding processes and welding material selection for austenitic stainless steel: Austenitic stainless steel has good weldability, and no special welding procedures are generally required. However, if the welding materials are not selected appropriately or the welding process is incorrect, defects such as grain boundary corrosion and hot cracks can occur. Furthermore, austenitic stainless steel has a high linear expansion coefficient, which can easily cause significant warping deformation during welding. Welding process: Shielded metal arc welding is the most commonly used method for welding austenitic stainless steel. However, this method has a low transition coefficient for alloying elements; when the welding rod is changed, repeated heating at the weld joints has an adverse effect on the quality of the joint.