Concept of welding properties of metallic materials
Weldability of metal materials refers to the ability of metal materials to obtain excellent welding joints under certain welding processes including welding methods, welding materials, welding specifications and welding structure forms.A kind of metal, if can use more common and simple welding process to obtain an excellent welding joint, the metal is considered to have good welding properties.
Weldability: it refers to the ability to obtain excellent and defect-free welded joints under certain welding conditions.It is not the inherent property of the metal, but the evaluation according to a certain welding method and the specific technological measures adopted.Therefore, the weldability of metal materials is closely related to the welding process.
Weldability: refers to the degree to which the welded joint or whole structure meets the service performance specified in the technical conditions of the product.The performance depends on the working conditions of the welded structure and the technical requirements of the design.It usually includes mechanical properties, low temperature toughness, brittle fracture resistance, high temperature creep, fatigue properties, durable strength, corrosion resistance and wear resistance.For example, commonly used S30403, S31603 stainless steel has excellent corrosion resistance, 16MnDR, 09MnNiDR low temperature steel also has a good low temperature toughness performance.
Influence factors of welding properties of metal materials
Materials include base material and welding material.Under the same welding conditions, the main factors determining the weldability of the base material are its physical properties and chemical composition.
Physical properties: such as melting point, thermal conductivity, linear expansion coefficient, density, heat capacity and other factors of the metal, all affect the thermal cycle, melting, crystallization, phase transformation and other processes, thus affecting the weldability.Materials with low thermal conductivity, such as stainless steel, have large temperature gradient, high residual stress and large deformation during welding.Moreover, because of the long residence time at high temperature, the grain in the heat-affected zone grows, which is bad for the joint performance.The linear expansion coefficient of austenitic stainless steel is large.
In terms of chemical composition, the most influential element is carbon, that is, the amount of carbon in a metal determines its weldability.Most of the other alloying elements in steel are also bad for welding, but their effect is generally much less than that of carbon.When the carbon content in steel increases, the hardening tendency increases, the plasticity decreases, and welding cracks are easy to occur.Generally, the sensitivity of cracks and the change of mechanical properties of welded joints are taken as the main indexes to evaluate the weldability of materials.So the higher the carbon content, the worse the weldability.Low carbon steel and low alloy steel with carbon content less than 0.25% have good plasticity and impact toughness.The welding process is easy to control, so it has good weldability.
In addition, the melting, rolling, heat treatment and microstructure of steel have different effects on weldability.The weldability of steel can be improved by refining or refining grain and controlled rolling process.
Welding materials directly participate in a series of chemical metallurgical reactions in the welding process, which determines the composition, structure, properties and defects of weld metal.If the welding material is not chosen properly and does not match the base material, not only can not meet the use requirements of the joint, but also introduce cracks and other defects and changes in microstructure and performance.Therefore, the correct selection of welding materials is an important factor to ensure the quality of welded joints.
Process factors include welding method, welding process parameters, welding sequence, preheating, post heat and post heat treatment.Welding method has a great influence on weldability, which is mainly reflected in the characteristics of heat source and protection conditions.
The heat sources of different welding methods vary greatly in power, energy density and maximum heating temperature.Metal welding under different heat sources will show different welding properties.For example, electroslag welding has high power but low energy density, and the highest heating temperature is not high. During welding, the heating is slow and the high-temperature residence time is long, which makes the grain in the heat-affected zone coarse and the impact toughness significantly reduced. Therefore, it must be improved by normalizing.On the contrary, electron beam welding, laser welding and other methods, the power is not large, but the energy density is high, heating quickly.The high temperature residence time is short, the heat-affected zone is very narrow, and there is no danger of grain growth.
The welding heat cycle can be adjusted and controlled by adjusting the welding process parameters, taking other process measures such as preheating, post-heating, multi-layer welding and controlling the temperature between layers, so as to change the weldability of the metal.If measures such as preheating before welding or post-welding heat treatment are taken, it is completely possible to obtain welded joints without cracks and meet the requirements of performance
It mainly refers to the design form of welded structure and welded joint, such as the influence of the structural shape, size, thickness, groove form, weld layout and section shape on weldability.The influence is mainly reflected in the heat transfer and the state of force.The heat transfer velocity direction and the heat transfer velocity of different plate thickness, joint form or groove shape are different, which affect the crystal direction and grain growth of the molten pool.The structural switch, plate thickness and weld line layout determine the stiffness and constraint degree 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 forming welding cracks.It is an important measure to improve weldability to reduce joint stiffness, cross weld and stress concentration.
Conditions of use
It refers to the working temperature, load condition and working medium of welded structure during service.These working conditions and operating conditions require the welding structure to have the corresponding performance.For example, the welded structure working at low temperature must have brittleness fracture resistance.The structure working at high temperature should have anti-creep property.The structure working under alternating load has good fatigue resistance.Welding vessels working in acid, alkali or salt media should have high corrosion resistance and so on.In a word, the more severe the use conditions are, the higher the quality requirements are on the welded joint, and the more difficult it is to guarantee the weldability of the material.
Evaluation index of weldability of metal materials
In the welding process, the product through the welding thermal process, metallurgical reaction, and the role of welding stress and deformation, resulting in chemical composition, metallographic structure, size and shape change, so that the performance of the welded joint is often different from the base material, sometimes even can not meet the use requirements.For many active or refractory metals, special welding methods, such as electron beam welding or laser welding, should be used to obtain high-quality joints.The fewer the equipment conditions and the less difficult it is to make the excellent welding joint, the better the weldability of the material.On the contrary, complicated and expensive welding methods, special welding materials and technological measures are required, which indicates that the material is not good at weldability.
When making 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 ways to evaluate the weldability of materials. Each method can only explain one aspect of weldability.The test method can be divided into simulation type and experiment type.The former simulates the heating and cooling characteristics of welding.The latter is tested according to actual welding conditions.The test content is mainly to detect the chemical composition, metallographic structure, mechanical properties and whether there is any welding defect 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.
Evaluation and testing methods for weldability of metallic materials
Indirect evaluation method for weldability of process
Since the influence of carbon is the most obvious, and the influence of other elements can be converted into the influence of carbon, carbon equivalent is used to evaluate the excellent weldability.
Carbon equivalent calculation formula of carbon steel and low alloy structural steel:
When CE< 0.4%, the plasticity of steel is good, the tendency of hardening is not obvious, and the weldability is good.In general welding conditions, welding joint will not produce cracks, but for thick large pieces or welding at low temperature, should consider preheating;
When CE is 0.4 ~ 0.6%, the plasticity of steel decreases, the tendency of hardening increases, and the weldability is poor.The workpiece should be preheated properly before welding and cooled slowly after welding to prevent cracks.
At CE > 0.6%, the plasticity of steel becomes worse.The tendency of hardening and cold cracking is greater, and the weldability is worse.The workpiece must be preheated to a higher temperature, technical measures should be taken to reduce welding stress and prevent cracking, and appropriate heat treatment should be carried out after welding.
The higher the carbon equivalent value obtained by the calculation, the greater the hardening tendency of the welded steel, and the cold crack is easy to occur in the heat-affected zone. Therefore, when CE >0.5%, the steel is easy to harden, and the crack can only be prevented by preheating. With the increase of plate thickness and CE, the preheating temperature should be correspondingly increased.
Method for direct evaluation of weldability of processes
The welding crack test method can be divided into hot crack, cold crack, reheat crack, stress corrosion, laminar tear and so on.
1).t-joint welding crack test method, which is mainly used to evaluate the thermal crack sensitivity of carbon steel and low-alloy steel fillet welds, as well as to determine the influence of welding electrode and welding parameters on the thermal crack sensitivity.
2). pressure plate butt welding crack test method, which is mainly used to evaluate the thermal crack sensitivity of carbon steel, low alloy steel, austenitic stainless steel welding rods and welds.It is by installing the specimen in the FISCO test device that adjusting the size of groove clearance has a great influence on the generation of cracks. As the gap increases, the crack sensitivity increases.
Rigid butt crack test method, (3) this method is mainly used for determination of weld zone of hot crack and cold crack, also can determine cold crack of heat affected zone, near the specimen by positioning weld seam in stiffness very first floor, when the test according to the actual construction of the welding parameters test welding, it is mainly used for welding rod arc welding, after welding the specimens at room temperature for 24 h, check the weld surface, and then go to the sample cut in grinding, check the crack, general with crack and crack as the evaluation criteria, each of the conditions of welding two pieces of specimen.
Welding characteristics of common metal materials
Welding of carbon steel
(1) welding of low carbon steel
Low carbon steel contains low carbon content, manganese, silicon content, in general, will not be caused by welding serious tissue hardening or quenching structure.This kind of steel has good plasticity and impact toughness.Generally, there is no need to preheat or heat after welding, and no need to take special technological measures to obtain satisfactory welding joints. Therefore, low carbon steel steel has excellent welding performance, and it is the steel with the best welding performance among all steels.
(2) welding of medium carbon steel
Medium carbon steel has higher carbon content and worse weldability than low carbon steel.When CE is close to the lower limit (0.25%), the weldability is good. With the increase of carbon content, the hardening tendency increases, and low plastic martensite is easy to be produced in the heat-affected zone.When the rigidity of the welded parts is large or the welding materials and process parameters are not selected properly, cold cracks are easy to occur.When welding the first seam of multi-layer welding, due to the large proportion of the base material fused into the weld, the content of carbon, sulfur and phosphorus increases, and it is easy to produce hot cracks.In addition, stomatal sensitivity increases with high carbon content.
(3) welding of high carbon steel
The hard and brittle high carbon martensite can be easily produced in high carbon steel with CE greater than 0.6%.It is easy to crack and difficult to weld in weld seam and heat affected zone.Therefore generally need not this kind of steel manufacturing welding structure, and used in manufacturing high hardness or wear resistant parts or parts, most of their welding is the welding repair of broken parts.These parts should be annealed before welding, in order to reduce welding cracks, and then re-heat treatment after welding.
Welding of low alloy high strength steel
The carbon content of low alloy high strength steel is generally less than 0.20%, and the total alloying element is generally less than 5%.Due to the fact that low-alloy high-strength steel contains a certain amount of alloying elements, its welding performance is different from that of carbon steel. Its welding characteristics are as follows:
(1) welding cracks of welded joints
Cold crack low-alloy high-strength steel is easy to be hardened during welding due to the fact that it contains C, Mn, V, Nb and other elements that strengthen the steel. These hardened structures are very sensitive. Therefore, if the welding process is not appropriate, it is easy to produce cold cracks.Moreover, this kind of crack has a certain delay, which is very harmful.
Re - heat (SR) crack re - heat crack is an intergranular cracking of a welded joint near the coarse crystal zone of the fusion line during post-weld stress relief heat treatment or long term high temperature operation.It is generally believed that the production is due to the fact that the carbides such as V, Nb, Cr and Mo near HAZ are solid dissolved in austenite due to the high temperature of welding, and it is too late for them to be precipitated during cooling after welding, while they are dispersed and precipitated during PWHT, so as to strengthen the intra - crystal and make the creep deformation concentrated in the grain boundary during stress relaxation.
The welded joint of low-alloy high-strength steel is not easy to generate reheat crack, such as 16MnR, 15MnVR, etc.However, for mn-mo-nb and mn-mo-v low-alloy high-strength steel, such as 07MnCrMoVR, Nb, V and Mo are the elements that promote strong reheat crack sensitivity. Therefore, this kind of steel should avoid the sensitive temperature zone of reheat crack during post-weld heat treatment to prevent the occurrence of reheat crack.
(2) embrittlement and softening of welded joints
Strain aging embrittlement of welded joints before welding need to undergo a variety of cold processing (cutting material shear, barrel body winding, etc.), steel will produce plastic deformation, if the area again through 200 ~ 450℃ of thermal action will cause strain aging.The strain aging embrittlement will reduce the plasticity of steel and increase the brittle transition temperature, which will lead to the brittle fracture of the equipment.Post-weld heat treatment can eliminate such strain aging of welded structure and restore toughness.
Embrittlement welding of weld and heat affected zone is an uneven heating and cooling process, thus forming uneven structure.The brittle transition temperature of weld (WM) and heat affected zone (HAZ) is higher than that of base metal, which is the weak link in the joint.The welding line energy has an important effect on the properties of low alloy high strength steel WM and HAZ.When the wire energy is too large, the coarse grain of WM and HAZ results in embrittlement of the joint.Compared with hot rolled and normalized steel, low carbon tempered and tempered steel has a more serious tendency of HAZ embrittlement due to too much linear energy.Therefore, when welding, the line energy should be limited in a certain range.
Due to the action of welding heat, the heat affected zone (HAZ) outside the low-carbon tempered steel is heated above the tempering temperature, especially the area near Ac1, resulting in a softening zone with decreased strength.With the increase of welding line energy and preheating temperature, the microstructure softening of HAZ zone is aggravated. However, the tensile strength of the softening zone is generally higher than the lower limit of the standard value of the base material. Therefore, the softening of the heat-affected zone of this kind of steel will not affect the service performance of the joint as long as the technology is proper
Welding of stainless steel
Stainless steel according to the different structure of its steel can be divided into four categories, namely austenitic stainless steel, ferrite stainless steel, martensitic stainless steel, austenitic - ferrite duplex stainless steel.The following are the main analysis of austenitic stainless steel and two-way stainless steel welding characteristics.
(1) welding of austenitic stainless steel
Austenitic stainless steel is easier to weld than other stainless steels.It is not sensitive to hydrogen embrittlement, and austenitic stainless steel joints have good plasticity and toughness.The main problems of welding are: welding hot crack, embrittlement, intergranular corrosion and stress corrosion.In addition, due to poor thermal conductivity, linear expansion coefficient, welding stress and deformation.When welding, small welding heat input should be used as far as possible, and should not preheat, and reduce the interlayer temperature, the interlayer temperature is controlled below 60℃, welding joints staggered each other.To reduce the heat input, the welding speed should not be excessively increased, but the welding current should be reduced.
(2) welding of austenitic and ferrite two-way stainless steel
Austenitic ferrite duplex stainless steel is a duplex stainless steel composed of austenitic and ferrite phases.It has the advantages of austenitic steel and ferrite steel, so it has the characteristics of high strength, good corrosion resistance and easy welding.At present, there are three types of duplex stainless steel: CR18, CR21 and CR25.The main characteristics of this kind of steel welding are: compared with austenitic stainless steel, it has lower thermal tendency;Compared with pure ferrite stainless steel, it has a lower embrittlement tendency after welding, and the ferrite coarsening degree is also lower in the welding heat affected zone, so the weldability is better.
Due to the good welding performance of this kind of steel, there is no preheating and reheating during welding.TIG welding should be used for thin plate, medium and thick plate can be used for electrode arc welding, electrode arc welding should choose special electrode with similar composition and base material or austenite electrode with low carbon content.Nickel base alloy electrode can also be used for CR25 duplex steel.
Due to the large proportion of ferrite in the dual-phase steel, the inherent embrittlement tendency of ferrite steel, such as 475 ° c brittleness, the embrittlement and grain size of the transition phase, still exists. However, due to the balance effect of austenite, the embrittlement is alleviated to some extent.For no NI or low NI duplex stainless steel welding, the heat affected zone has a tendency to single-phase ferrite and grain coarsening, at this time should be paid attention to control the welding heat input, as far as possible with a small current, high welding speed, narrow welding and multichannel welding, in order to prevent the heat affected zone single-phase ferrite, grain coarsening and interpass temperature shoulds not be too high, cold welding again after the next best.