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 use more common and simple welding processes to obtain excellent welded joints, 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 metal, but a qualification based on a certain welding method and 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 performance depends on the working conditions of the welded structure and the technical requirements put forward in the design.
It 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 sheets of steel also have good low-temperature toughness properties.
Influencing factors of welding performance of metal materials
1. Material factors
Materials include base materials and welding materials. Under the same welding conditions, the main factor that determines the weldability of the base metal is its physical properties and chemical composition.
Physical properties: factors such as metal melting point, thermal conductivity, linear expansion coefficient, density, heat capacity, etc., all affect processes such as thermal cycling, melting, crystallization, and phase change, thereby affecting weldability.
Materials with low thermal conductivity such as stainless steel have a large temperature gradient during welding, high residual stress, and large deformation.
Moreover, due to the long residence time at high temperatures, the grains in the heat-affected zone grow, which is detrimental to the joint performance. The 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 most influential element is carbon, which means that the amount of carbon in a metal determines its weldability.
Most of the other alloying elements in steel are also not conducive to welding, but their degree of influence is generally much smaller than that of carbon.
As the carbon content in steel increases, the hardening tendency increases, while the plasticity decreases and welding cracks are prone to occur.
Generally, the sensitivity of metal materials to cracks during welding and the changes in the mechanical properties of the welded joint area is 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 is also very good.
There is no need for preheating and post-weld heat treatment 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 structure state of the steel all affect the weldability to varying degrees. Improve the weldability of steel by means of refining, refining, or grain refinement and controlled rolling technology.
Welding materials directly participate in a series of chemical metallurgical reactions in the welding process, which determine the composition, structure, performance, and defect formation of the weld metal.
If the welding material is improperly selected and does not match the base metal, not only the joints that meet the requirements of the use cannot be obtained, but also defects such as cracks and changes in the structure and properties will be introduced.
Therefore, the correct selection of welding materials is an important factor to ensure high-quality welded joints.
2. Process factors
Process factors include welding method, welding process parameters, welding sequence, preheating, post-heating, and post-welding heat treatment. The welding method has a great influence on weldability, which is mainly manifested in the characteristics of the heat source and the protection conditions.
Different welding methods have very different heat sources in terms of power, energy density, and maximum heating temperature. Metals welded under different heat sources will show different welding performances.
For example, the power of electro slag welding is very large, but the energy density is very low, the maximum heating temperature is not high, the heating is slow during welding, and the high-temperature residence time is long, which makes the heat-affected zone grain coarse and the impact toughness is significantly reduced. It must be normalized. improve.
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.
Adjust the welding process parameters, take preheating, post-heating, multi-layer welding, and control interlayer temperature and other process measures to 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 and meet performance requirements.
3. Structural factors
Mainly refers to the design form of welded structure and welded joints, such as the influence of structural shape, size, thickness, joint groove form, weld layout, and cross-sectional shape, etc. on weldability. Its influence is mainly manifested in the heat transfer and the state of force.
Different plate thicknesses, different joint forms of groove shapes have different heat transfer speed directions and heat transfer speeds, which will affect the crystallization direction and grain growth of the molten pool.
The switch of the structure, the thickness of the plate, and the layout of the welding seam, etc., determine the stiffness and restraint of the joint and affect the stress state of the joint.
Poor crystalline 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 that cause stress concentration are important measures to improve weldability.
4. Conditions of use
Refers to the working temperature, load conditions, and working medium of the welded structure during service. These working environments and operating conditions require the welded structure to have corresponding performance.
If the welded structure works at low temperature, it must have the performance of brittle fracture resistance;
The structure working at high temperature must have creep resistance;
The structure working under alternating load has good fatigue resistance;
Welded containers that work in acid, alkali, or salt media should have high corrosion resistance and so on.
In short, the harsher the conditions of use, the higher the quality requirements for welded joints, and the less likely it is to ensure the weldability of the materials.
Estimation and Testing Method of Weldability of Metallic Materials
1. Indirect assessment method of process weldability
Because 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 excellent weldability.
Carbon equivalent calculation formula for carbon steel and low-alloy structural steel:
When the CE is 0.4 to 0.6%, the plasticity of the steel decreases, the hardening tendency gradually increases, and the weldability is poor. The workpiece must be preheated properly before welding, and slowly cooled after welding to prevent cracks;
When CE> 0.6%, the plasticity of the steel becomes worse. The hardening tendency and cold cracking tendency are large, and the weldability is worse. The workpiece must be preheated to a higher temperature, technical measures to reduce welding stress and prevent cracking must be taken, and proper heat treatment must be carried out 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 the welding must be preheated to prevent cracks. , As the plate thickness and CE increase, the preheating temperature should also increase accordingly.
2. Direct evaluation method of process weldability
In the welding crack test method, the cracks produced in the welded joint can be divided into hot cracks, cold cracks, reheat cracks, stress corrosion, laminar tears, etc.
(1) T-joint welding crack test method. This method is mainly used to assess 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) Pressure plate butt welding crack test method. This method is mainly used to assess the hot crack sensitivity of carbon steel, low alloy steel, austenitic stainless steel electrodes, and welds. It is through the installation of the test piece in the FISCO test device, adjusting the size of the groove gap has a great impact on the generation of cracks. With the increase of the gap, the greater the crack sensitivity.
(2) Rigid butt joint crack test method. This method is mainly used to measure hot cracks and cold cracks in the weld zone. It can also measure cold cracks in the heat-affected zone. On the bottom plate, the test weld is applied according to the actual construction welding parameters during the test. It is mainly used for electrode arc welding. After the test piece is welded, it is placed at room temperature for 24 hours. For cracks, cracks and non-cracks are generally evaluated, and 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, low manganese, and silicon content, and will not cause severe structural hardening or quenched structure due to welding under normal circumstances.
This steel has excellent plasticity and impact toughness, and the plasticity and toughness of its welded joints are also extremely good. Generally, there is no need for preheating and post-heating during welding, and no special process measures are required to obtain a satisfactory welded joint. Therefore, low carbon steel has excellent welding performance and is the steel grade 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 that of low-carbon steel. When the CE is close to the lower limit (0.25%), the weldability is good. As the carbon content increases, the hardening tendency increases, and it is easy to produce a low-plastic martensite structure in the heat-affected zone.
When the rigidity of the weldment is large or the welding material and process parameters are not selected properly, cold cracks are prone to occur. When multi-layer welding is used to weld the first layer of welds, due to the large proportion of the base metal fused into the weld, the carbon content, sulfur, and phosphorus content are increased, and thermal cracking is easy to produce.
In addition, when the carbon content is high, the pore sensitivity also increases.
(3) Welding of high carbon steel
High-carbon steel with a CE greater than 0.6% has high hardenability and is easy to produce hard and brittle high-carbon martensite. It is easy to produce cracks in the weld and heat-affected zone, and it is difficult to weld.
Therefore, this type of steel is generally not used to manufacture welded structures but used to manufacture high-hardness or wear-resistant parts or parts, and most of their welding is repair welding of damaged parts.
These parts and components should be annealed before welding to reduce welding cracks, and then heat treatment should be performed 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 precise because low-alloy high-strength steel contains a certain amount of alloying elements that its welding performance is different from that of carbon steel. Its welding characteristics are shown in:
(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. It is easy to harden during welding. These hardened structures are very sensitive. Therefore, if the rigidity is high or the restraint stress is high if Improper welding process can easily produce cold cracks. Moreover, this type of crack has a certain degree of delay, which is extremely harmful.
Reheating (SR) cracks Reheating cracks are intergranular cracks that occur in the coarse-grained region near the fusion line during the stress relief heat treatment process or long-term high-temperature operation of the welded joint. It is generally believed that the occurrence is due to the high welding temperature that causes the carbides such as V, Nb, Cr, Mo, etc., near the HAZ to dissolve in the austenite, which is too late to precipitate during cooling after welding but is dispersed and precipitated during PWHT, thereby strengthening the crystal. Inside, the creep deformation during stress relaxation is concentrated on the grain boundaries.
Low-alloy high-strength steel welded joints are generally not prone to reheat cracks, such as 16MnR, 15MnVR, etc. But 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 promote reheat cracking sensitivity, this type of steel should be heat treated after welding. Care should be taken to avoid the sensitive temperature zone 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 working (cutting, shearing, cylinder rolling, etc.) before welding, and the steel will undergo plastic deformation. If the area is subjected to heat at 200～450℃, it will cause strain aging. . Strain aging embrittlement will reduce the plasticity of steel and increase the brittle transition temperature, which will result in brittle fracture of the equipment. Post-weld heat treatment can eliminate the strain aging of the welded structure and restore the 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, which is the weak link in the joint.
Welding heat input has an important influence on the properties of low-alloy high-strength steel WM and HAZ. Low-alloy high-strength steel is easy to harden. If the heat input is too small, the HAZ will appear martensite to cause cracks;
If the heat input is too large, the coarse grains of WM and HAZ will cause joint embrittlement. Compared with hot-rolled and normalized steel, low-carbon quenched and tempered steel has a more serious tendency to embrittle HAZ caused by excessive heat input. Therefore, when welding, the heat input should be limited to a certain range.
Softening of the heat-affected zone of welded joints Due to welding heat, 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 softened zone with reduced strength. The softening of the structure in the HAZ zone increases with the increase of the welding heat input and the increase of the preheating temperature, but generally, the tensile strength of the softening zone is still higher than the lower limit of the standard value of the base material, so the heat-affected zone of this type of steel is softened As long as the process is proper, the performance of the joint will not be affected.
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 two-way stainless steel
(1) Welding of austenitic stainless steel
Austenitic stainless steel is easier to weld than other stainless steel. No phase change occurs at any temperature, and it is not sensitive to hydrogen embrittlement. Austenitic stainless steel joints also have good ductility and toughness in the welded state.
The main problems of welding are welding hot cracking, embrittlement, intergranular corrosion, and stress corrosion.
In addition, due to poor thermal conductivity, large linear expansion coefficient, large welding stress, and deformation. When welding, the welding heat input should be as small as possible, and should not be preheated, and the interlayer temperature should be lowered. The interlayer temperature should be controlled below 60℃, and the weld joints should be staggered. To reduce the heat input, the welding speed should not be excessively increased but should be adapted to reduce the welding current.
(2) Welding of austenitic-ferritic two-way stainless steel
Austenitic-ferritic bidirectional stainless steel is duplex stainless steel composed of two phases 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: compared with austenitic stainless steel, it has a lower thermal tendency; compared with pure ferritic stainless steel, it has a lower tendency to embrittlement after welding, and the degree of ferrite coarsening in the welding heat-affected zone It is also lower, so the weldability is better.
Due to the good welding performance of this kind of steel, preheating and post-heating are not necessary during welding. Thin plates should be welded with TIG, and medium and thick plates can be welded with electrode arc welding. Special electrodes with a similar composition to the base metal or austenitic electrodes with low carbon content should be selected for electrode arc welding. Nickel-based alloy electrodes can also be used for CR25 dual-phase steel.
Due to the existence of a large proportion of ferrite in dual-phase steels, the inherent embrittlement tendency of ferritic steels, such as brittleness at 475°C, σ phase precipitation embrittlement, and coarse grains, still exist because of the presence of austenite The balancing effect of the welding machine can be relieved to a certain extent, and it is still necessary to pay attention when welding.
When welding duplex stainless steels without or with low NI, 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 trying to use low current, high welding speed, and narrow pass welding. And multi-pass welding to prevent grain coarsening and single-phase ferrite in the heat-affected zone, the temperature between layers should not be too high, and it is best to weld the next pass after the cold.