Welding knowledge

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Weldability and its experimental evaluation 1. Welding: A process in which two objects are joined at the atomic level to form an inseparable whole, achieved through heating or pressing, with or without the use of filler material. 2. Weldability: Refers to the ability of homogeneous or heterogeneous materials to be welded under manufacturing conditions to form a complete joint that meets the required performance standards. 3. The four major factors affecting weldability are: material, design, process, and service environment. 4. The principles for evaluating weldability mainly include: ① Evaluating the tendency of welded joints to develop process defects, thereby providing a basis for formulating reasonable welding procedures ； ②Assess whether the welded joint meets the requirements for structural performance ； The design of new welding test methods follows the following principles: comparability, relevance, reproducibility, and cost-effectiveness. 5. Carbon equivalent: The content of alloying elements in steel is converted into an equivalent carbon content and summed up, serving as a parameter for roughly assessing the tendency of the steel to develop cold cracks. 6. Oblique Y-groove butt crack test: Its main purpose is to assess the tendency of cold cracking in the first layer of welds and the HAZ of low-alloy high-strength steels; it can also be used to develop welding procedures. 1) Specimen preparation: the thickness δ of the steel plate to be welded is 9–38 mm. The grooves of the butt joints are machined mechanically. Restraint welds are applied to both ends of the test plate within a range of 60 mm each, using double-sided welding. Be careful to prevent corner deformation and lack of weld penetration. Ensure there is a 2mm gap at the weld area of the sample to be welded in the middle. 2) Test conditions: The electrode used for the test weld should be compatible with the base material; the electrodes must be thoroughly dried. The electrode diameter is 4 mm, the welding current is (170±10) A, the welding voltage is (24±2) V, and the welding speed is (150±10) mm/min. The test weld can be fabricated at various temperatures; only one layer of weld is applied for the test weld, and the groove is not filled. After post-weld holding and natural cooling for 24 hours, samples were cut out and crack inspection was performed. 3) Crack detection and crack density calculation. Inspect the surface and cross-section of the weld and heat-affected zone for cracks using the naked eye or a hand-held 5-10x magnifying glass. It is generally believed that in low-alloy steels, cracks do not usually occur when the surface crack rate in the \"Koironen\" test is less than 20%. 7. Pin test: Purpose – mainly to assess the tendency of steel to hydrogen-induced delayed cracking; with additional equipment, it can also determine sensitivity to reheat cracking and layering sensitivity. 1) Specimen preparation: The steel to be welded is processed into cylindrical pin specimens, samples are taken along the rolling direction, and the position of the pin in the thickness direction is indicated. There is an annular or spiral notch near the upper end of the test rod. Insert the pin test rod into the corresponding hole in the base plate, ensuring that the notched end is level with the surface of the base plate. For plug test bars with annular notches, the distance a between the notch and the end face should be such that the weld penetration depth is tangent to or intersects the cross-section at the root of the notch, but the portion of the notch root’s circumference that is penetrated by the weld shall not exceed 20%. For low-alloy steel, the a value is 2 mm when the welding heat input is E=15 KJ/cm. 2) During the testing process, a layer of surfacing weld bead is melted on the base plate using the selected welding method and strictly controlled process parameters. The center line of the weld bead passes through the center of the specimen, and its penetration depth should be such that the tip of the notch lies in the coarse-grained region of the heat-affected zone. The length L of the weld bead is approximately 100–150 mm. During welding, the cooling time value t8/5 at 800–500°C should be determined; when welding without preheating, loading is carried out after the weld has cooled to 100–150°C ； During pre-welding heating, loading should be applied at a temperature 50-70°C higher than the preheating temperature. The load shall be applied within 1 minute, and before the temperature cools down to 100°C or 50–70°C above the preheating temperature. If post-heating is applied, loading should be done before the post-heating. When the test bar is loaded, the pin may break during the duration of the load; record the load-bearing time. Weldability of alloy structural steel 1. High-strength steel: Steels with a yield strength σs ≥ 295 MPa can all be referred to as high-strength steel. 2. The solid solution strengthening effect of Mn is very significant; when ωMn ≤ 1.7%, it can improve toughness and reduce the brittle transition temperature. Si reduces plasticity and toughness, while Ni is an element that provides both solid solution strengthening and an increase in toughness, as well as a significant reduction in the brittle transition temperature – it is often used in low-temperature steels. 3. Hot-rolled steel (normalized steel): A low-alloy high-strength steel with a yield strength of 295–490 MPa, typically supplied in a hot-rolled or normalized state for use. 4. Design principles for welded joints of high-strength steel: High-strength steel is selected based on its strength; therefore, the principle for welded joints is that the strength of the welded joint should be equal to that of the base material (the equal-strength principle). Analysis: ① If the strength of the welded joint is greater than that of the base material, its ductility and toughness decrease; ② When they are equal, the service life is similar; ③ If the strength of the welded joint is lower, its strength is insufficient. 5. Weldability of hot-rolled and normalized steel: Hot-rolled steel contains a small amount of alloying elements, so it generally has a low tendency to develop cold cracks. Normalized steel, on the other hand, contains more alloying elements, which increases its tendency to harden; as the carbon equivalent and thickness of the normalized steel increase, so do its hardening tendency and susceptibility to cold cracks. Factors affecting hardness: (1) Carbon equivalent; (2) Hardening tendency: the hardening tendency of hot-rolled steel and that of normalized steel; (3) Maximum hardness in the heat-affected zone. The maximum hardness in the heat-affected zone is a simple method for assessing a steel’s hardening tendency and susceptibility to cold cracking. 6.SR cracks (stress-relief cracks, reheat cracks): In welded structures such as thick-walled pressure vessels made of Mo-containing normalized steel, another type of crack may occur during post-weld stress-relief heat treatment or subsequent high-temperature reheating. 7. Toughness is a property that characterizes a metal’s resistance to the initiation and propagation of brittle cracks. 8. When selecting welding materials for low-alloy steel, two aspects must be considered: ① there should be no welding defects such as cracks; ② the materials must meet the requirements regarding performance in service. When welding hot-rolled steel and normalized steel, the welding materials are generally selected based on their strength grades. The key points for making such selections are as follows: ① Choose welding materials of a grade that matches the mechanical properties of the base material; ② Take into account the effects of the fusion ratio and cooling rate; ③ Consider the impact of post-weld heat treatment on the mechanical properties of the weld. 9. Principles for determining the post-weld tempering temperature: ① Do not exceed the original tempering temperature of the base material to avoid affecting its properties; ② For materials that have been tempered, avoid temperature ranges in which temper brittleness may occur. 10. Quenched and tempered steel: quenching + tempering (at high temperature). 11. Using a \"low-strength match\" in the welding of high-strength steel can improve the crack resistance of the welded area. 12. When welding low-carbon quenched and tempered steel, two basic issues need to be considered: ① The cooling rate during the martensite transformation should not be too fast, so that martensite can undergo self-tempering to prevent the formation of cold cracks; ② The cooling rate between 800°C and 500°C must be higher than the critical rate at which brittle mixed structures form. Problems to be addressed in the welding of low-carbon quenched and tempered steel: ① Preventing cracks; ② Enhancing the toughness of the weld metal and heat-affected zone while ensuring that high strength requirements are met. 13. For low-alloy steels with low carbon content, increasing the cooling rate to form low-carbon martensite is beneficial for ensuring toughness. 14. The addition of alloying elements to medium-carbon quenched and tempered steel primarily serves to ensure hardenability and improve temper resistance, whereas the true strength properties mainly depend on the carbon content. Main characteristics: high specific strength and high hardness. 15. There are three ways to improve the heat strength of pearlitic heat-resistant steels: ① Matrix solid solution strengthening, which involves adding alloying elements to strengthen the ferritic matrix; elements such as Cr, Mo, W, and Nb can significantly enhance heat strength. ② Second-phase precipitation strengthening: In heat-resistant steels with a ferritic matrix, the strengthening phases are mainly alloy carbides. ③ Grain boundary strengthening: The addition of trace elements allows them to adsorb at the grain boundaries, thereby slowing down the diffusion of alloying elements along these boundaries and strengthening them. 16. The main problems in welding pearlitic heat-resistant steels are cold cracks, hardening and softening in the heat-affected zone, as well as stress-relief cracks that occur during post-weld heat treatment or long-term use at high temperatures. 17.- The temperature range from -10 to -196°C is called \"low temperature,\" while temperatures below -196°C are referred to as \"ultra-low temperature.\" Stainless steel welding 1. Stainless steel: Stainless steel refers to an umbrella term for alloy steels that possess high chemical stability, enabling them to resist corrosion caused by air, water, acids, alkalis, salts and their solutions, as well as other corrosive agents. 2. The main forms of corrosion in stainless steel include uniform corrosion, pitting corrosion, crevice corrosion, and stress corrosion, among others. Uniform corrosion refers to the phenomenon where the entire surface of a metal in contact with a corrosive medium becomes corroded ； Pitting corrosion refers to localized corrosion that occurs sporadically, where most of the surface of the metal material remains uncorroded or shows only slight corrosion ； Crevice corrosion occurs in electrolytes, such as in an oxygen-ion environment, when there are gaps between stainless steel surfaces or between stainless steel and foreign objects. In such cases, the flow of solution within the crevices is impeded, leading to a local accumulation of Cl− ions. This creates a concentration cell, causing the passive film on the stainless steel within the crevices to adsorb Cl− ions and thus be locally damaged ； Intergranular corrosion, a selective form of corrosion that occurs near the grain boundaries ； Stress corrosion refers to the phenomenon of extremely brittle cracking in stainless steel under the action of specific corrosive environments and tensile stress, resulting in a strength that is much lower than normal. 3. Measures to prevent pitting corrosion: 1) Reduce the levels of chloride ions and oxygen ions. 2) Add alloying elements such as chromium, nickel, molybdenum, silicon, and copper to stainless steel. 3) Avoid cold working as much as possible to reduce the likelihood of pitting corrosion occurring at dislocation sites. 4) Lower the carbon content in the steel. 4. High-temperature properties of stainless steel and heat-resistant steel: Brittleness occurs at 475°C, mainly in ferritic steels with a Cr content greater than 13%. Prolonged heating between 430–480°C followed by slow cooling leads to an increase in strength and a decrease in toughness at room temperature or in low temperatures ； σ-phase embrittlement is typical at a mass fraction of Cr of 45%; it involves FeCr intermetallic compounds that are non-magnetic, hard, and brittle. 5. Corrosion resistance of austenitic stainless steel welded joints: 1) intergranular corrosion, 2) intergranular corrosion in the sensitized zone of the heat-affected area, 3) knife-edge corrosion. 6. Measures to prevent intergranular corrosion in welds: 1) By using welding materials, the weld metal can be made either ultra-low carbon or contain sufficient amounts of the stabilizing element Nb. 2) Adjust the weld composition to obtain a certain amount of δ phase. The intergranular corrosion theory is essentially the chromium-deficiency theory. 7. Intergranular corrosion in the sensitized zone of the heat-affected area: Refers to the intergranular corrosion that occurs in those parts of the welding heat-affected area where the peak heating temperature falls within the sensitization range. 8. Knife-edge corrosion: Intergranular corrosion that occurs in the weld zone, appearing in the form of knife-like cuts, hence it is called “knife-edge corrosion”. 9. Measures to prevent knife-edge corrosion: ① Use low-carbon base materials and welding materials. ② Employ stainless steels with a duplex microstructure. ③ Use low current for welding in order to reduce the degree of overheating and the width of the coarse-grained areas resulting from welding. ④ Weld the welds that come into contact with corrosive media last. ⑤ Perform cross-welding. ⑥ Increase the content of Ti and Tb in the steel, so that there is sufficient Ti, Tb available to combine with carbon at the grain boundaries of the coarse-grained areas formed by welding. 10. Why is low-current welding used for stainless steel? This helps to reduce the temperature in the heat-affected zone of welding, prevents intergranular corrosion in the weld, avoids overheating of the electrodes and wires, reduces welding deformation and welding stress, and can also lower the heat input. 11. The three conditions that cause stress corrosion cracking: environment, selective corrosive medium, and tensile stress. 12. Measures to prevent stress corrosion cracking: 1) Adjusting the chemical composition – ultra-low carbon levels help improve resistance to stress corrosion; compatibility between the composition and the medium is also important. 2) Eliminating residual welding stresses. 3) Addressing electrochemical corrosion by conducting regular inspections and making timely repairs. 13. To improve pitting resistance: 1) It is necessary to reduce the segregation of Cr and Mo on one hand; 2) On the other hand, so-called \"super-alloyed\" welding materials with higher Cr and Mo contents than those of the base material should be used. 14. Welding austenitic stainless steel can cause hot cracks, stress corrosion cracks, welding deformation, and intergranular corrosion. 15. Causes of welding thermal cracks in austenitic steel: 1) Austenitic steel has low thermal conductivity and a high linear expansion coefficient, as well as high tensile stresses; 2) It tends to form weld microstructures with strongly directional columnar grains through co-precipitation, which facilitates the segregation of harmful impurities; 3) Its alloy composition is complex, making eutectic formation likely. 16. Measures to prevent thermal cracking: ① Strictly limit the P and S contents in the base material and welding materials. ② Try to achieve a duplex microstructure in the weld. ③ Control the chemical composition of the weld. ④ Use low current for welding. 17. What are the differences in the weld microstructure of type 17.18-8 and type 25-20 in terms of preventing hot cracks? The welds of 18-8 steel form an A+δ microstructure; the δ phase can dissolve large amounts of P and S, with its proportion typically ranging from 3% to 7%. The welds of 25-20 steel form an A+primary carbide microstructure. 18. When selecting austenitic stainless steel, the following points should be taken into account: ① Adhere to the \"principle of suitability\"; ② Determine suitability based on the specific composition of the welding materials chosen; ③ Consider the extent of fusion that may result from the welding methods and process parameters used in the specific application; ④ Decide on the degree of alloying in accordance with the comprehensive weldability requirements specified in the technical conditions; ⑤ Pay attention to the alloy system of the weld metal, the role of specific alloy components within that system, as well as the requirements regarding performance and process weldability. 19. Analysis of weldability of ferritic stainless steels: 1) Intergranular corrosion in welded joints 2) Embrittlement of welded joints, high-temperature embrittlement, σ-phase embrittlement, and embrittlement at 475°C. Welding of cast iron 1. The three main characteristics of cast iron: vibration damping, oil absorption, and wear resistance. 2. The properties of cast iron mainly depend on the shape, size, quantity, and distribution of graphite, while the matrix structure also has a certain influence. 3. Ductile iron: F matrix + spherical graphite ； Gray cast iron: F matrix + flake graphite ； Ceramic iron: matrix + worm-like graphite ； Malleable cast iron: F matrix + flocculent graphite. 4. Can low-carbon steel electrodes be used to weld cast iron? No, during welding, even with a low current, the base material accounts for 25%-30% of the first weld layer. If we consider a carbon content of 3% in cast iron, the carbon content in the first weld layer will be 0.75%-0.9%, which classifies it as a high-carbon steel. High-carbon martensite forms immediately after welding cooling, and the weld heat-affected zone develops a white cast structure, making mechanical processing difficult. 5. Arc heat welding: The cast piece is preheated to 600–700°C and then welded while it is in a plastic state; the welding temperature must be at least 400°C. To prevent cracking during welding, stress-relief treatment and slow cooling are carried out immediately after welding. This casting repair method is known as arc heat welding. 6. Semi-hot welding: When the preheating temperature is between 300–400°C, it is referred to as semi-hot welding.