The weldability of metal materials refers to the ability of metal materials to obtain excellent welding joints using certain welding processes, including welding methods, welding materials, welding specifications and welding structural forms. If a metal can obtain excellent welding joints using more common and simple welding processes, it is considered to have good welding performance. The weldability of metal materials is generally divided into two aspects: process weldability and application 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 is evaluated based on a certain welding method and the specific process measures used. Therefore, the process weldability of metal materials is closely related to the welding process.
Service weldability: refers to the degree to which the welded joint or the entire structure meets the service performance specified by the product technical conditions. The performance depends on the working conditions of the welded structure and the technical requirements put forward in the design. Usually include mechanical properties, low temperature toughness resistance, brittle fracture resistance, high temperature creep, fatigue properties, lasting strength, corrosion resistance and wear resistance, etc. 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.
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 and other factors of the metal all have an impact on processes such as 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 stress, and large deformation during welding. Moreover, due to the long residence time at high temperature, the grains in the heat-affected zone grow, which is detrimental to the joint performance. Austenitic stainless steel has a large linear expansion coefficient and severe joint deformation and stress.
In terms of chemical composition, the most influential element is carbon, which means that the carbon content of the metal determines its weldability. Most of the other alloying elements in steel are not conducive to welding, but their impact 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 prone to occur. 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 to evaluate 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 joints after welding are also very good. Preheating and post-weld heat treatment are not 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, organizational state, etc. of steel all affect weldability to varying degrees. The weldability of steel can be improved by refining or refining grains and controlled rolling processes.
Welding materials directly participate in a series of chemical metallurgical reactions during the welding process, which determine the composition, structure, properties and defect formation of the weld metal. If the welding materials are improperly selected and do not match the base metal, not only will a joint that meets the usage requirements not be obtained, but defects such as cracks and changes in structural properties will also be introduced. Therefore, the correct selection of welding materials is an important factor in ensuring high-quality welded joints.
2. Process factors
Process factors include welding methods, welding process parameters, welding sequence, preheating, post-heating and post-weld heat treatment, etc. The welding method has a great influence on the weldability, mainly in two aspects: heat source characteristics and protection conditions.
Different welding methods have very different heat sources in terms of power, energy density, maximum heating temperature, etc. Metals welded under different heat sources will show different welding properties. For example, the power of electroslag welding is very high, but the energy density is very low, and the maximum heating temperature is not high. The heating is slow during welding, and the high temperature residence time is long, resulting in coarse grains in the heat-affected zone and a significant reduction in impact toughness, which must be normalized. To improve. In contrast, electron beam welding, laser welding and other methods 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 the welding process parameters and adopting other process measures such as preheating, postheating, 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 welded joints without crack defects that meet performance requirements.
3. Structural factors
It mainly refers to the design form of the welded structure and welded joints, such as the impact of factors such as structural shape, size, thickness, joint groove form, weld layout and its cross-sectional shape on weldability. Its influence is mainly reflected in the transfer of heat and the state of force. Different plate thicknesses, different joint forms or groove shapes have different heat transfer speed directions and rates, which will 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, which affects the stress state of the joint. Poor crystal morphology, severe stress concentration and excessive welding stress are the basic conditions for the formation of welding cracks. In the design, reducing joint stiffness, reducing cross welds, and reducing various factors causing stress concentration are all important measures to improve weldability.
4. Conditions of use
It refers to the operating temperature, load conditions and working medium during the service period of the welded structure. These working environments and operating conditions require welded structures 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; structures working in acid, alkali or salt media The welded container should have high corrosion resistance and so on. In short, the more severe the usage conditions, the higher the quality requirements for welded joints, and the harder it is to ensure the weldability of the material.
Identification and evaluation index of weldability of metal materials
During the welding process, the product undergoes welding thermal processes, metallurgical reactions, as well as welding stress and deformation, resulting in changes in chemical composition, metallographic structure, size and shape, making the performance of the welded joint often different from that of the base material, sometimes even Cannot meet usage requirements. For many reactive 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 less difficulty required to make a good welded joint from a material, 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 material The weldability is poor.
When manufacturing products, the weldability of the materials used must first be evaluated to determine whether the selected structural materials, welding materials, and welding methods are appropriate. There are many methods to evaluate the weldability of materials. Each method can only explain a certain aspect of the weldability. Therefore, tests are required to fully determine the weldability. Test methods can be divided into simulation type and experimental type. The former simulates the heating and cooling characteristics of welding; the latter tests according to actual welding conditions. The test content is mainly to detect the chemical composition, metallographic structure, mechanical properties, and presence or absence of welding defects of the base metal and weld metal, and to determine the low-temperature performance, high-temperature performance, corrosion resistance, and crack resistance of the welded joint.
Welding characteristics of commonly used metal materials
1. Welding of carbon steel
(1) Welding of low carbon steel
Low carbon steel has low carbon content, low manganese and silicon content. Under normal circumstances, it will not cause serious structural hardening or quenching structure due to welding. This kind of steel has excellent plasticity and impact toughness, and the plasticity and toughness of its welded joints are also extremely good. Preheating and postheating are generally not required during welding, and special process measures are not required to obtain welded joints with satisfactory quality. Therefore, low carbon steel has excellent welding performance and is the steel with the best 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 low carbon steel. When CE is close to the lower limit (0.25%), the weldability is good. As the carbon content increases, the hardening tendency increases, and a low-plasticity martensite structure is easily generated in the heat-affected zone. When the weldment is relatively rigid or the welding materials and process parameters are improperly selected, cold cracks are likely to occur. When welding the first layer of multi-layer welding, due to the large proportion of the base metal fused into the weld, the carbon content, sulfur and phosphorus content increase, making it easy to produce hot cracks. In addition, stomatal sensitivity also increases when the carbon content is high.
(3) Welding of high carbon steel
High carbon steel with CE greater than 0.6% has high hardenability and is prone to produce hard and brittle high carbon martensite. Cracks are prone to occur in welds and heat-affected zones, making welding difficult. Therefore, this type of steel is generally not used to make welded structures, but is used to make components or parts with high hardness or wear resistance. Most of their welding is to repair damaged parts. These parts and components should be annealed before welding repair 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 alloying elements generally does not exceed 5%. It is precisely because low-alloy high-strength steel contains a certain amount of alloy elements that its welding performance is somewhat different from that of carbon steel. Its welding characteristics are as follows:
(1) Welding cracks in welded joints
Cold-cracked low-alloy high-strength steel contains C, Mn, V, Nb and other elements that strengthen the steel, so it is easy to be hardened during welding. These hardened structures are very sensitive. Therefore, when the rigidity is large or the restraining stress is high, if Improper welding process can easily cause cold cracks. Moreover, this type of crack has a certain delay and is extremely harmful.
Reheat (SR) cracks Reheat cracks are intergranular cracks that occur in the coarse-grained area near the fusion line during post-weld stress relief heat treatment or long-term high-temperature operation. It is generally believed that it occurs due to the high temperature of welding causing V, Nb, Cr, Mo and other carbides near the HAZ to be solid dissolved in the austenite. They do not have time to precipitate during cooling after welding, but disperse and precipitate during PWHT, thus strengthening the crystal structure. Within, the creep deformation during stress relaxation is concentrated at 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 series low-alloy high-strength steels, such as 07MnCrMoVR, since Nb, V, and Mo are elements that have strong sensitivity to reheat cracking, this type of steel needs to be treated during post-weld heat treatment. Care should be taken to avoid the sensitive temperature area of reheat cracks to prevent the occurrence of reheat cracks.
(2) Embrittlement and softening of welded joints
Strain aging embrittlement Welded joints need to undergo various cold processes (blank shearing, barrel rolling, etc.) before welding. The steel will produce plastic deformation. If the area is further heated to 200 to 450°C, strain aging will occur. . Strain aging embrittlement will reduce the plasticity of the steel and increase the brittle transition temperature, resulting in brittle fracture of the equipment. Post-weld heat treatment can eliminate such strain aging of the welded structure and restore toughness.
Embrittlement of welds and heat-affected zones Welding is an uneven heating and cooling process, resulting in an uneven structure. The brittle transition temperature of the weld (WM) and heat-affected zone (HAZ) is higher than that of the base metal and is the weak link in the joint. Welding line energy has an important impact on the properties 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 HAZ and cause cracks. If the line energy is too large, the grains of WM and HAZ will become coarse. Will cause the joint to become brittle. Compared with hot-rolled and normalized steel, low-carbon quenched and tempered steel has a more serious tendency to HAZ embrittlement caused by excessive linear energy. Therefore, when welding, the line energy should be limited to a certain range.
Softening of the heat-affected zone of welded joints Due to the action of welding heat, the outside of the heat-affected zone (HAZ) of low-carbon quenched and tempered steel is heated above the tempering temperature, especially the area near Ac1, which will produce a softening zone with reduced strength. The structural softening in the HAZ zone increases with the increase in welding line energy and preheating temperature, but generally the tensile strength in the softened zone is still higher than the lower limit of the standard value of the base metal, so the heat-affected zone of this type of steel softens As long as the workmanship is proper, the problem will not affect the performance of the joint.
3. Welding of stainless steel
Stainless steel can be divided into four categories according to its different steel structures, 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 bidirectional stainless steel.
(1) Welding of austenitic stainless steel
Austenitic stainless steels are easier to weld than other stainless steels. There will be no phase transformation at any temperature and it is not sensitive to hydrogen embrittlement. The austenitic stainless steel joint also has good plasticity and toughness in the welded state. The main problems of welding are: welding hot cracking, embrittlement, intergranular corrosion and stress corrosion, etc. In addition, due to poor thermal conductivity and large linear expansion coefficient, welding stress and deformation are large. When welding, the welding heat input should be as small as possible, and there should be no preheating, 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 appropriately reduced.
(2) Welding of austenitic-ferritic two-way stainless steel
Austenitic-ferritic duplex stainless steel is a duplex stainless steel composed of two phases: 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. Currently, there are three main types of duplex stainless steel: Cr18, Cr21, and Cr25. The main characteristics of this type of steel welding are: lower thermal tendency compared with austenitic stainless steel; lower embrittlement tendency after welding compared with pure ferritic stainless steel, and the degree of ferrite coarsening in the welding heat affected zone It is also lower, so the weldability is better.
Since this type of steel has good welding properties, preheating and postheating are not required during welding. Thin plates should be welded by TIG, and medium and thick plates can be welded by arc welding. When welding by arc welding, special welding rods with similar composition to the base metal or austenitic welding rods with low carbon content should be used. Nickel-based alloy electrodes can also be used for Cr25 type dual-phase steel.
Dual-phase steels have a larger proportion of ferrite, and the inherent embrittlement tendencies of ferritic steels, such as brittleness at 475°C, σ phase precipitation embrittlement and coarse grains, still exist, only because of the presence of austenite. Some relief can be obtained through the balancing effect, but you still need to pay attention when welding. When welding Ni-free or low-Ni duplex stainless steel, there is a tendency for 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 try to use small current, high welding speed, and narrow channel welding. And multi-pass welding to prevent grain coarsening and single-phase ferriteization in the heat-affected zone. The inter-layer temperature should not be too high. It is best to weld the next pass after cooling.
Post time: Sep-11-2023