Weldability of typical welding materials

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Weldability refers to the ability of homogeneous or heterogeneous materials to be welded under manufacturing conditions to form a complete joint that meets the desired performance requirements. Material, design, process, and service environment are the four key factors that influence weldability. The principles for evaluating weldability mainly include: ① Assessing the tendency of welding joints to develop process defects, in order to provide a basis for formulating appropriate welding procedures; ② Evaluating whether the welding joints can meet the requirements regarding the structural performance. I. Weldability of alloy structural steels
1. High-strength steels: Steels with a yield strength σs ≥ 295 MPa can all be classified as high-strength steels. 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 and used in the hot-rolled or normalized state. 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). The reasons for this are as follows: ① If the strength of the welded joint is greater than that of the base material, its ductility and toughness decrease; ② When the strengths are equal, the service life of the joint is comparable; ③ If the strength of the welded joint is less, 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 normalized steel increase, so do its hardening tendency and susceptibility to cold cracks. Factors affecting it: (1) Carbon equivalent; (2) Hardening tendency; (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 a brittle mixed structure is formed. Problems to be addressed in welding low-carbon quenched and tempered steel: ① Preventing cracks; ② Improving the toughness of the weld metal and the heat-affected zone while ensuring compliance with high-strength requirements. 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 in medium-carbon quenched and tempered steel primarily serves to ensure hardenability and improve temper resistance, while the true strength properties mainly depend on the carbon content. Key features: 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, where alloying elements are added 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: trace elements can adsorb at the grain boundaries, slowing down the diffusion of alloying elements along these boundaries and thereby strengthening them. 16. The main problems in the welding of pearlitic heat-resistant steel 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. The temperature range from -10 to -196°C is referred to as “cryogenic temperatures”; temperatures below -196°C are termed “ultracryogenic temperatures”. II. Weldability of cast iron 1. 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 cast iron: matrix + worm-like graphite; Malleable cast iron: F matrix + nodular graphite. 4. Can low-carbon steel electrodes be used to weld cast iron? No. During welding, even with a low current, the proportion of base metal in the first weld seam is 25%-30%. Assuming a carbon content of 3% in the cast iron, the carbon content in the first weld seam is 0.75%-0.9%, which classifies it as a high-carbon steel. High-carbon martensite forms immediately after welding cooling, and a white cast structure appears in the welded HAZ, making mechanical processing difficult. 5. Arc heat welding: The cast part 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: Welding with a preheating temperature of 300–400°C is referred to as semi-hot welding. III. Weldability of stainless steel 1. Stainless steel: Stainless steel refers to an umbrella term for alloy steels that possess high chemical stability and are capable of withstanding 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 in which the entire metal surface in contact with the corrosive medium undergoes corrosion. Pitting corrosion occurs when most of the metal surface is not corroded or only slightly corroded, with localized corrosion occurring in scattered areas. Crevice corrosion takes place in electrolytes, such as environments with oxygen ions; when there are gaps between stainless steel surfaces or between these surfaces and foreign objects, the flow of solution within those gaps is hindered, leading to a concentration difference of Cl- ions. This results in the formation of concentration cells, causing the passivation layer on the stainless steel in those gaps to absorb Cl- ions and become locally damaged. Intergranular corrosion is a selective form of corrosion that occurs near the grain boundaries. Stress corrosion refers to the phenomenon of brittle cracking that occurs in stainless steel under the influence of specific corrosive media and tensile stress, resulting in a significant reduction in its strength. 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 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, primarily in ferritic steels with a Cr content of more 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 when the mass fraction of Cr is 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 of welds: 1) Through the use of welding materials, ensure that the weld metal is either ultra-low carbon or contains sufficient amounts of the stabilizing element Nb; 2) Adjust the weld composition to obtain a certain amount of δ phase. 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 welding to reduce the degree of overheating and the width of the coarse-grained areas formed during welding; ④ Weld the seams that come into contact with corrosive media last; ⑤ Use cross-welding; ⑥ Increase the contents 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 during welding. 10. Why is low-current welding used for stainless steel? It is to reduce the temperature in the heat-affected zone during welding, prevent intergranular corrosion in the weld, avoid overheating of the electrodes and wires, reduce welding deformation and welding stresses, and lower the heat input. 11. The three conditions that cause stress corrosion cracking: environment, a selectively 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; consideration is also needed regarding the compatibility between the composition and the medium; 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; 2) Use so-called \"super-alloyed\" welding materials with higher Cr and Mo contents than those of the base material. 14. Welding austenitic stainless steel can result in thermal cracks, stress corrosion cracks, welding deformation, and intergranular corrosion. 15. Causes of hot cracks in austenitic steel welding: 1) Austenitic steels have low thermal conductivity, a high coefficient of linear expansion, and high tensile stress; 2) Austenitic steels tend to undergo epitaxial crystallization, resulting in a weld microstructure characterized by highly directional columnar crystals, which facilitates the segregation of harmful impurities; 3) The alloy composition of austenitic steels is relatively complex, making them prone to forming soluble eutectics. 16. Measures to prevent hot cracking: ① Strictly limit the contents of P and S in the base metal and welding materials; ② Try to make the weld form a duplex structure; ③ Control the chemical composition of the weld; ④ Use low-current welding. 17. When selecting austenitic stainless steel materials, the following points should be noted: ① Adhere to the “principle of suitability”; ② Determine whether a particular welding material is suitable based on its specific composition; ③ Consider the potential effect of the welding method and process parameters used in the specific application on the fusion ratio; ④ Determine the degree of alloying according to the overall weldability requirements specified in the technical specifications; ⑤ Pay attention to the alloy system of the weld metal, the role of specific alloying elements within that system, as well as both the service performance requirements and process weldability requirements. 18. 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. IV. Weldability of Magnesium and Magnesium Alloys 1. Oxidation and Evaporation Due to magnesium’s high reactivity, an oxide film (MgO) is easily formed during welding. MgO has a high melting point (2500°C) and high density (3.2 g/cm3), which can lead to the formation of inclusions in the weld, thereby reducing the performance of the weld. At high temperatures, magnesium also readily reacts chemically with nitrogen in the air to form magnesium nitrides, which weakens the properties of the joint. Magnesium has a low boiling point, which means it evaporates easily at the high temperatures of an arc. 2. Coarse grains: Due to their high thermal conductivity, high-power heat sources and fast welding speeds are required when welding magnesium alloys; this can lead to overheating of the weld area and the metal in the vicinity of the weld, as well as to an increase in grain size. 3. Thermal stress: Magnesium alloys have a high coefficient of thermal expansion, approximately 1 to 2 times that of aluminum; this leads to significant welding deformation during the welding process, resulting in substantial residual stresses. 4. Deposition of weld metal: Since the surface tension of magnesium is lower than that of aluminum, it is easy for the weld metal to deposit during welding, which affects the quality of the weld shape. 5. Porosity: Similar to welded aluminum alloys, hydrogen porosity tends to occur during the welding of magnesium alloys. The solubility of hydrogen in magnesium decreases as the temperature drops. Moreover, magnesium has a lower density than aluminum, making it difficult for gases to escape; as a result, pores are formed during the solidification of the weld. 6. Thermal cracking: Magnesium alloys tend to form low-melting eutectic structures with other metals, which can lead to the formation of crystalline cracks in the welded joints. When the temperature at the joint is too high, the low-melting-point compounds in the joint structure melt at the grain boundaries, resulting in voids or grain boundary oxidation – a phenomenon known as \"overburning\".