Concept of welding performance of metal materials
The weldability of metal materials refers to the ability of metal materials to obtain excellent welded joints under certain welding processes including welding methods, welding materials, welding specifications and welding structure forms. If a metal can obtain excellent welded joints using more common and simple welding processes, it is considered that this metal has good welding performance. The weldability of metal materials is generally divided into two aspects: process weldability and use weldability.
Process weldability: refers to the ability to obtain excellent, defect-free welded joints under certain welding process conditions. It is not an inherent property of the metal, but an assessment based on a certain welding method and the specific process measures adopted. Therefore, the process weldability of metal materials is closely related to the welding process. Use weldability: refers to the degree to which the welded joint or the entire structure meets the use performance specified in the product technical conditions. The use performance depends on the working conditions of the welded structure and the technical requirements put forward in the design. Usually includes mechanical properties, low temperature toughness, brittle fracture resistance, high temperature creep, fatigue performance, endurance strength, corrosion resistance and wear resistance. For example, the commonly used S30403 and S31603 stainless steels have excellent corrosion resistance, and 16MnDR and 09MnNiDR low-temperature steels also have good low-temperature toughness resistance.
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Factors affecting the welding performance of metal materials
1 Material factors Materials include base metal and welding materials. Under the same welding conditions, the main factors that determine the weldability of the base metal are its physical properties and chemical composition. In terms of physical properties: factors such as the melting point, thermal conductivity, linear expansion coefficient, density, heat capacity, etc. of the metal all affect the processes of thermal cycle, melting, crystallization, phase change, etc., thereby affecting weldability. Materials with low thermal conductivity such as stainless steel have large temperature gradients, high residual stresses, and large deformations during welding. Moreover, due to the long high temperature residence time, the grains in the heat-affected zone grow, which is not conducive to the performance of the joint. Austenitic stainless steel has a large linear expansion coefficient, and the deformation and stress of the joint are more serious. In terms of chemical composition, the element with the greatest influence is carbon, that is, the amount of carbon content in the metal determines its weldability. Most of the other alloying elements in steel are also not conducive to welding, but their influence is generally much smaller than that of carbon. As the carbon content in steel increases, the hardening tendency increases, the plasticity decreases, and welding cracks are easily generated. Usually, the sensitivity of metal materials to cracks during welding and the changes in mechanical properties of the welded joint area are used as the main indicators for evaluating the weldability of materials. Therefore, the higher the carbon content, the worse the weldability. Low-carbon steel and low-alloy steel with a carbon content of less than 0.25% have excellent plasticity and impact toughness, and the plasticity and impact toughness of the welded joint after welding are also very good. No preheating and post-weld heat treatment are required during welding, and the welding process is easy to control, so it has good weldability. In addition, the smelting and rolling state, heat treatment state, and organizational state of steel affect weldability to varying degrees. The weldability of steel can be improved by refining and purifying or refining grains and controlled rolling processes. Welding materials directly participate in a series of chemical metallurgical reactions in the welding process, determining the composition, organization, properties, and defect formation of the weld metal. If the welding material is not selected properly and does not match the parent material, not only will it not be possible to obtain a joint that meets the use requirements, but it will also introduce defects such as cracks and changes in organizational properties. Therefore, the correct selection of welding materials is an important factor in ensuring the acquisition of high-quality welded joints. 2. Process factors Process factors include welding method, welding process parameters, welding sequence, preheating, post-heating and post-weld heat treatment. The welding method has a great influence on weldability, which is mainly manifested in two aspects: heat source characteristics and protection conditions. The heat sources of different welding methods are very different in terms of power, energy density, and maximum heating temperature. Metals welded under different heat sources will show different welding properties. For example, electroslag welding has a large power, but a very low energy density, and a low maximum heating temperature. The heating is slow during welding, and the high temperature residence time is long, which makes the grains in the heat-affected zone coarse and the impact toughness significantly reduced. Normalizing treatment is required to improve it. In contrast, methods such as electron beam welding and laser welding have low power, but high energy density and rapid heating. The high temperature residence time is short, the heat-affected zone is very narrow, and there is no danger of grain growth. Adjusting welding process parameters and taking other process measures such as preheating, post-heating, multi-layer welding and controlling interlayer temperature can adjust and control the welding thermal cycle, thereby changing the weldability of the metal. If measures such as preheating before welding or heat treatment after welding are taken, it is entirely possible to obtain a welded joint without crack defects that meets the performance requirements. 3 Structural factors mainly refer to the design form of welding structures and welding joints, such as the influence of structural shape, size, thickness, joint groove form, weld arrangement and cross-sectional shape on weldability. Its influence is mainly manifested in the state of heat transfer and force. Different plate thicknesses, different joint forms or groove shapes have different heat transfer speed directions and heat transfer speeds, which affect the crystallization direction and grain growth of the molten pool. The structural switch, plate thickness and weld arrangement determine the stiffness and restraint of the joint and affect the stress state of the joint. Poor crystallization morphology, severe stress concentration and excessive welding stress are the basic conditions for the formation of welding cracks. Reducing the stiffness of the joint, reducing cross welds, and reducing various factors that cause stress concentration in the design are all important measures to improve weldability. 4 Use conditions refer to the working temperature, load conditions and working medium during the service of the welded structure. These working environments and operating conditions require the welded structure to have corresponding performance. For example, welded structures working at low temperatures must have brittle fracture resistance; structures working at high temperatures must have creep resistance; structures working under alternating loads must have good fatigue resistance; welded containers working in acid, alkali or salt media must have high corrosion resistance, etc. In short, the more stringent the use conditions, the higher the quality requirements for welded joints, and the more difficult it is to ensure the weldability of the material.
Identification and evaluation indicators of weldability of metal materials
During the welding process, the product undergoes welding heat process, metallurgical reaction, and welding stress and deformation, which brings about changes in chemical composition, metallographic structure, size and shape, so that the performance of the welded joint is often different from that of the parent material, and sometimes even cannot meet the use requirements. For many active metals or refractory metals, special welding methods such as electron beam welding or laser welding should be used to obtain high-quality joints. The fewer equipment conditions and the less difficulty required to make a material into an excellent welded joint, the better the weldability of the material; on the contrary, if complex and expensive welding methods, special welding materials and process measures are required, it means that the weldability of this material is poor. When manufacturing products, the weldability of the materials used must be evaluated first to determine whether the selected structural materials, welding materials and welding methods are appropriate. There are many methods to evaluate the weldability of materials, and each method can only explain one aspect of weldability. Therefore, it is necessary to conduct tests before the weldability can be fully determined. The test methods can be divided into simulation type and experimental type. The former simulates the characteristics of welding heating and cooling; the latter conducts tests according to actual welding conditions. The test content mainly detects the chemical composition, metallographic structure, mechanical properties, welding defects of the parent material and weld metal, and determines the low temperature performance, high temperature performance, corrosion resistance and crack resistance of the welded joint.
Estimation and detection methods for weldability of metal materials
1 Indirect evaluation method of process weldability Since the influence of carbon is the most obvious, the influence of other elements can be converted into the influence of carbon, so the carbon equivalent is used to evaluate the excellence of weldability. Carbon equivalent calculation formula for carbon steel and low alloy structural steel:
When CE<0.4%, the plasticity of the steel is good, the hardening tendency is not obvious, and the weldability is good. Under general welding technical conditions, the welded joint will not produce cracks, but preheating should be considered for thick and large pieces or welding at low temperatures; when CE is 0.4-0.6%, the plasticity of the steel decreases, the hardening tendency gradually increases, and the weldability is poor. The workpiece needs to be properly preheated before welding, and slow cooling should be paid attention to after welding to prevent cracks; when CE>0.6%, the plasticity of the steel deteriorates. The hardening tendency and cold cracking tendency are large, and the weldability is worse. The workpiece must be preheated to a higher temperature, and technical measures must be taken to reduce welding stress and prevent cracking. Appropriate heat treatment must be performed after welding. The larger the carbon equivalent value obtained by the calculation result, the greater the hardening tendency of the welded steel, and the heat-affected zone is prone to cold cracks. Therefore, when CE>0.5%, the steel is easy to harden, and preheating must be performed during welding to prevent cracks. With the increase of plate thickness and CE, the preheating temperature should also increase accordingly. 2 Direct evaluation method of process weldability Welding crack test method. The cracks generated in the welded joint can be divided into hot cracks, cold cracks, reheat cracks, stress corrosion, layered tearing, etc. (1) T-joint welding crack test method. This method is mainly used to evaluate the hot crack sensitivity of carbon steel and low alloy steel fillet welds. It can also be used to determine the influence of welding rods and welding parameters on hot crack sensitivity. (2) Platen butt welding crack test method. This method is mainly used to evaluate the hot crack sensitivity of carbon steel, low alloy steel, austenitic stainless steel welding rods and welds. It is achieved by installing the specimen in the FISCO test device. Adjusting the size of the groove gap has a great influence on the generation of cracks. As the gap increases, the crack sensitivity increases. (3) Rigid butt crack test method. This method is mainly used to determine hot cracks and cold cracks in the weld area. It can also determine cold cracks in the heat-affected zone. The test piece is first welded to a very rigid base plate with positioning welds around it. During the test, the test weld is welded according to the actual construction welding parameters. It is mainly used for arc welding. The test piece is placed at room temperature for 24 hours after welding. First, check the weld surface, then cut off the sample grinding piece and check for cracks. Generally, cracks or no cracks are used as the evaluation criteria. Two test pieces are welded under each condition.
Welding characteristics of common metal materials
1 Welding of carbon steel (1) Welding of low carbon steel Low carbon steel has low carbon content and low manganese and silicon content. Under normal circumstances, it will not cause serious microstructure hardening or quenching microstructure due to welding. This steel has excellent plasticity and impact toughness, and its welded joints also have extremely good plasticity and toughness. Generally, no preheating or postheating is required during welding, and no special process measures are required to obtain a weld joint with satisfactory quality. Therefore, low carbon steel has excellent welding performance and is the best steel in terms of welding performance among all steels.
(2) Welding of medium carbon steel Medium carbon steel has a higher carbon content and its weldability is worse than that of low carbon steel. When CE is close to the lower limit (0.25%), the weldability is good. As the carbon content increases, its hardening tendency increases, and low-plasticity martensite structure is easily generated in the heat-affected zone. When the weldment is rigid or the welding materials and process parameters are not properly selected, cold cracks are easily generated. When welding the first layer of weld seam with multi-layer welding, due to the large proportion of the base material fused into the weld seam, its carbon content and sulfur and phosphorus content increase, and hot cracks are easily generated. In addition, when the carbon content is high, the porosity sensitivity also increases.
(3) Welding of high carbon steel High carbon steel with CE greater than 0.6% has high hardenability and is easy to produce hard and brittle high carbon martensite. Cracks are easily generated in the weld seam and heat-affected zone, making it difficult to weld. Therefore, this type of steel is generally not used to manufacture welded structures, but is used to manufacture high-hardness or wear-resistant components or parts. Most of their welding is to repair damaged parts. Before welding these parts and components, they should be annealed to reduce welding cracks, and then heat treated again after welding.
2 Welding of low-alloy high-strength steel The carbon content of low-alloy high-strength steel generally does not exceed 0.20%, and the total amount of alloying elements generally does not exceed 5%. It is precisely because low-alloy high-strength steel contains a certain amount of alloying elements that its welding performance is somewhat different from that of carbon steel. Its welding characteristics are manifested in:
(1) Welding cracks of welded joints Cold cracks Low-alloy high-strength steel contains elements such as C, Mn, V, and Nb that strengthen the steel, and is easy to harden during welding. These hardened structures are very sensitive. Therefore, in the case of high rigidity or high restraint stress, if the welding process is improper, cold cracks are easily generated. Moreover, this type of crack has a certain delay, and its harm is extremely great. Reheat (SR) cracks Reheat cracks are intergranular cracks that occur in the coarse grain area near the fusion line during the stress relief heat treatment process after welding or long-term high-temperature operation of the welded joint. It is generally believed that the high welding temperature causes carbides such as V, Nb, Cr, and Mo near the HAZ to dissolve in austenite, which do not have time to precipitate during cooling after welding, but dispersedly precipitate during PWHT, thereby strengthening the grains and concentrating the creep deformation during stress relaxation on the grain boundaries. Low-alloy high-strength steel welded joints are generally not prone to reheat cracks, such as 16MnR, 15MnVR, etc. However, for Mn-Mo-Nb and Mn-Mo-V low-alloy high-strength steels, such as 07MnCrMoVR, since Nb, V, and Mo are elements that promote strong reheat crack sensitivity, this type of steel should be carefully avoided during post-weld heat treatment to avoid the sensitive temperature zone of reheat cracks and prevent the occurrence of reheat cracks.
(2) Brittleness and softening of welded joints Strain aging embrittlement Before welding, welded joints need to undergo various cold processing (cutting, barrel rolling, etc.), which will cause plastic deformation of the steel. If the zone is subjected to heat at 200-450°C, strain aging will occur. Strain aging embrittlement will reduce the plasticity of the steel and increase the brittle transition temperature, which will lead to brittle fracture of the equipment. Post-weld heat treatment can eliminate this type of strain aging in welded structures and restore toughness. Weld and heat-affected zone embrittlement Welding is an uneven heating and cooling process, which forms an uneven structure. The brittle transition temperature of the weld (WM) and heat-affected zone (HAZ) is higher than that of the parent material, and is the weak link in the joint. The welding line energy has an important influence on the performance of low-alloy high-strength steel WM and HAZ. Low-alloy high-strength steel is easy to harden. If the line energy is too small, martensite will appear in the HAZ to cause cracks; if the line energy is too large, the grains of WM and HAZ will be coarse, which will cause joint embrittlement. Compared with hot-rolled and normalized steel, low-carbon quenched and tempered steel has a more serious tendency to HAZ embrittlement caused by excessive line energy. Therefore, when welding, the line energy should be limited to a certain range. Softening of the heat-affected zone of the welded joint Due to the heat of welding, the outer side of the heat-affected zone (HAZ) of low-carbon quenched and tempered steel is heated to above the tempering temperature, especially the area near Ac1, which will produce a softening zone with reduced strength. The structural softening of the HAZ zone increases with the increase of welding line energy and the increase of preheating temperature, but generally the tensile strength of the softening zone is still higher than the lower limit requirement of the standard value of the parent material. Therefore, as long as the process is proper, the softening problem of the heat-affected zone of this type of steel will not affect the performance of its joint.
3 Welding of stainless steel Stainless steel can be divided into four categories according to its steel structure, namely austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and austenitic-ferritic duplex stainless steel. The following mainly analyzes the welding characteristics of austenitic stainless steel and duplex stainless steel.
(1) Welding of austenitic stainless steel Austenitic stainless steel is easier to weld than other stainless steels. Phase change will not occur at any temperature, it is not sensitive to hydrogen embrittlement, and austenitic stainless steel joints also have good plasticity and toughness in the welded state. The main problems of welding are: welding hot cracks, embrittlement, intergranular corrosion and stress corrosion. In addition, due to poor thermal conductivity, large linear expansion coefficient, welding stress and deformation are large. When welding, the welding heat input should be as small as possible, and preheating should not be performed, and the interlayer temperature should be reduced. The interlayer temperature should be controlled below 60°C, and the weld joints should be staggered. To reduce heat input, the welding speed should not be increased excessively, but the welding current should be reduced accordingly.
(2) Welding of austenitic-ferritic duplex stainless steel Austenitic-ferritic duplex stainless steel is a duplex stainless steel composed of austenite and ferrite. It combines the advantages of austenitic steel and ferritic steel, so it has the characteristics of high strength, good corrosion resistance and easy welding. At present, there are mainly three types of duplex stainless steel: Cr18, Cr21, and Cr25. The main characteristics of this type of steel welding are: it has a lower thermal tendency than austenitic stainless steel; it has a lower embrittlement tendency after welding than pure ferritic stainless steel, and the degree of ferrite coarsening in the heat affected zone of welding is also low, so the weldability is better. Due to the good welding performance of this type of steel, preheating and post-heating are not required during welding. TIG welding is suitable for thin plates, and arc welding can be used for medium and thick plates. When arc welding, special welding rods with a composition similar to that of the parent material or austenitic welding rods with low carbon content should be used. Nickel-based alloy welding rods can also be used for Cr25 duplex steel. Because there is a large proportion of ferrite in duplex steel, the inherent embrittlement tendency of ferritic steel, such as 475℃ brittleness, σ phase precipitation embrittlement and coarse grains, still exists. It is only alleviated to a certain extent due to the balancing effect of austenite. Attention should still be paid during welding. When welding Ni-free or low-Ni duplex stainless steel, there is a tendency of single-phase ferrite and grain coarsening in the heat-affected zone. At this time, attention should be paid to controlling the welding heat input, and small current, high welding speed, narrow pass welding and multi-pass welding should be used as much as possible to prevent grain coarsening and single-phase ferrite in the heat-affected zone.
Post time: Jan-06-2025
