Welding properties of metal materials
Concept of weldability of metallic materials
The weldability of metal materials refers to the ability of metal materials to obtain excellent welding joints under certain welding conditions, including welding methods, welding materials, welding specifications and welding structure forms. If a metal can obtain excellent welding joint by more common and simple welding processes, the weldability of the metal material with good welding performance can be generally divided into two aspects: process weldability and use weldability.
Process weldability: refers to the ability to obtain excellent and defect free welded joints under certain welding process conditions. It is not the inherent property of metal, but the evaluation based on a certain welding method and specific process measures. Therefore, the process weldability of metal materials is closely related to the welding process.
Use weldability: refers to the degree that the welded joint or the whole structure meets the use performance specified in the product technical conditions. The performance depends on the working conditions of the welding structure and the technical requirements put forward in the design. Generally, it includes mechanical property, low temperature toughness, brittle fracture resistance, high temperature creep, fatigue property, endurance strength, corrosion resistance and wear resistance. For example, the commonly used s30403 and s31603 stainless steels have excellent corrosion resistance, and the 16MnDR and 09MnNiDR low-temperature steels also have good low-temperature toughness resistance.
- 1. Influence factors of welding properties of metal materials
- 1.1 Material factor
- 1.2 Process factors
- 1.3 Structural factors
- 1.4 Service conditions
- 2. Identification and evaluation indexes of weldability of metal materials
- 3. Estimation and test method of weldability of metal materials
- 3.1 Indirect assessment method of process weldability
- 3.2 Direct assessment method of process weldability
- 4. Welding characteristics of common metal materials
- 4.1 Welding of carbon steel
- 4.2 Welding of low alloy high strength steel
- 4.3 Welding of stainless steel
1. Material factor
Materials include base metal and welding materials. Under the same welding conditions, the main factor determining the weldability of the base metal is its physical properties and chemical composition.
Physical properties: for example, the melting point, thermal conductivity, linear expansion coefficient, density, thermal capacity and other factors of metal all affect the thermal cycle, melting, crystallization, phase transformation and other processes, thus affecting the weldability. Stainless steel and other materials with low thermal conductivity have large temperature gradient, high residual stress and large deformation during welding,. Moreover, due to the long residence time at high temperature and the grain growth in the heat affected zone, the joint performance is not good. The austenitic stainless steel has large coefficient of linear expansion, serious deformation and stress of the joint.
In terms of chemical composition, the most influential element is carbon, that is to say, the amount of carbon content of metal determines its weldability. Most of the other alloy elements in steel are not conducive to welding, but their influence is generally much smaller than that of carbon. With the increase of carbon content in steel, the tendency of hardening increases, while the plasticity decreases, which is easy to produce welding cracks. In general, the sensitivity of metal materials to produce cracks and the change of mechanical properties in the welding joint area 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 excellent plasticity and impact toughness, and the plasticity and impact toughness of welded joint after welding are also very good. The welding process is easy to control, so it has good weldability.
In addition, the melting and rolling state, heat treatment state and microstructure state of steel all affect the weldability to varying degrees. The weldability of steel can be improved by means of refining or refining grains and controlled rolling process.
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 weld metal. If the welding material is chosen improperly and does not match with the base metal, it will not only fail to obtain the joint meeting the use requirements, but also introduce the generation of defects such as cracks and the change of structure and properties. Therefore, the correct selection of welding materials is an important factor to ensure the quality of welding joints.
2. Process factors
Process factors include welding method, welding process parameters, welding sequence, preheating, post heat treatment and post weld heat treatment. Welding method has a great influence on weldability, mainly in two aspects: heat source characteristics and protection conditions.
Different welding methods have different heat sources in power, energy density, maximum heating temperature and so on. When the metal is welded under different heat sources, it will show different welding properties. For example, the power of electroslag welding is very large, but the energy density is very low, the maximum heating temperature is not high, the heating is slow during welding, the high temperature retention time is long, which makes the grain in the heat affected zone coarse and the impact toughness significantly reduced, which can be improved only by normalizing treatment. On the contrary, electron beam welding, laser welding and other methods have low power, 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, adopting preheating, post heating, multi-layer welding and controlling inter layer temperature and other process measures can adjust and control welding thermal cycle, thus changing the weldability of metal. If preheat before welding or heat treatment after welding are adopted, it is possible to obtain welded joints without crack defects and meeting the requirements of service performance.
3. Structural factors
It mainly refers to the design form of welding structure and welding joint, such as the influence of structural shape, size, thickness, joint groove form, weld layout and section shape on weldability. Its influence is mainly manifested in heat transfer and force state. The direction and speed of heat transfer are different for different plate thickness, different joint form or groove shape, which affects the crystal direction and grain growth of molten pool. The structural switch, plate thickness and weld arrangement determine the rigidity and restraint of the joint, and affect the stress state of the joint. Poor crystal morphology, serious stress concentration and excessive welding stress are the basic conditions for the formation of welding cracks. In the design, it is an important measure to reduce the joint stiffness, cross weld and stress concentration.
4. Service conditions
It refers to the working temperature, load condition and working medium of the welding structure in service. These working environment and operating conditions require the welding structure to have corresponding performance. For example, the welding structure working at low temperature must have the resistance to brittle fracture; the structure working at high temperature must have the resistance to creep; the structure working under alternating load has good resistance to fatigue; the welding container working in acid, alkali or salt medium should have high corrosion resistance, etc. In a word, the more severe the service conditions are, the higher the quality requirements for the welded joints are, and the less easy the weldability of the materials is to be guaranteed.
In the process of welding, the products undergo the action of welding heat process, metallurgical reaction, welding stress and deformation, which brings the changes of chemical composition, metallographic structure, size and shape, and makes the performance of welding joint often different from that of base metal, sometimes even unable to 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 less equipment and less difficulty are needed to make a good welding joint, the better the weldability of the material is; on the contrary, complex and expensive welding methods, special welding materials and process measures are needed, which indicates 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. Each method can only describe one aspect of weldability, so it needs to be tested before the weldability can be fully determined. The test method can be divided into simulation type and experimental type. The former simulates the heating and cooling characteristics of welding, while the latter is tested according to the actual welding conditions. The main contents of the test are to detect the chemical composition, metallographic structure, mechanical properties and 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.
1. Indirect assessment 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 weldability.
Carbon equivalent calculation formula of carbon steel and low alloy structural steel:
When CE < 0.4%, the plasticity of steel is good, the hardening tendency is not obvious, and the weldability is good. In general conditions of welding technology, the welded joint will not produce cracks, but preheating should be considered for thick and large pieces or welding at low temperature;
When CE is between 0.4% and 0.6%, the plasticity of steel decreases, the hardening tendency increases gradually, and the weldability is poor. The workpiece shall be preheated properly before welding, and slow cooling shall be paid attention to after welding to prevent cracks;
When CE > 0.6%, the plasticity of steel is poor. The harden tendency and cold crack tendency are large, 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. Proper heat treatment should be carried out after welding.
The greater the carbon equivalent value obtained from the calculation results, the greater the hardening tendency of the steel to be welded, and the cold cracks are easy to occur in the heat affected zone. Therefore, when CE > 0.5%, the steel is easy to harden, and the cracks can only be prevented by preheating during welding. With the increase of plate thickness and CE, the preheating temperature should also increase accordingly.
2. Direct assessment method of process weldability
The welding crack test method can be divided into hot crack, cold crack, reheat crack, stress corrosion, lamellar tear and so on.
- (1) T-joint welding crack test method, which is mainly used to evaluate the hot crack sensitivity of fillet weld of carbon steel and low alloy steel, as well as to determine the influence of welding rod and welding parameters on the hot crack sensitivity.
- (2) Butt welding crack test method of pressing plate, which is mainly used to evaluate the hot crack sensitivity of carbon steel, low alloy steel, austenitic stainless steel electrode and weld. It is through installing the test piece in the FISCO test device, adjusting the groove gap size has a great influence on the crack production. With the increase of the gap, the crack sensitivity increases.
- (3) Rigid butt crack test method, which is mainly used to determine the hot crack and cold crack in the weld area, and also to determine the cold crack in the heat affected area. The surrounding of the test piece shall be welded on the base plate with great rigidity first with the positioning weld. During the test, the weld shall be tested according to the actual construction welding parameters, mainly used for electrode arc welding. The test piece shall be placed for 24 hours at room temperature after welding, and the surface of the weld shall be inspected first And then cut off the grinding piece of the sample to check whether there is any crack. Generally, crack and non crack are the evaluation criteria, and two test pieces are welded under each condition.
1. Welding of carbon steel
(1) welding of low carbon steel
Low carbon steel has low content of carbon and low content of manganese and silicon, which will not cause serious structure hardening or quenching structure due to welding. The plasticity and impact toughness of the steel are excellent, and the plasticity and toughness of the welded joint are also extremely good. In general, no preheating and post heating are required during welding, and no special process measures are required to obtain the welding joints with satisfactory quality. Therefore, low carbon steel has excellent welding performance and is the best steel of all steels.
(2) welding of medium carbon steel
The carbon content of medium carbon steel is higher, 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. With the increase of carbon content, the hardening tendency increases, and the martensite structure with low plasticity is easy to be produced in the heat affected zone. When the rigidity of weldment is large or the choice of welding materials and process parameters is not right, cold cracks are easy to occur. When the first layer of weld is welded by multi-layer welding, due to the large proportion of base metal fusion into the weld, its carbon content, sulfur and phosphorus content increase, and it is easy to produce hot cracks. In addition, when carbon content is high, stomatal sensitivity is also increased.
(3) welding of high carbon steel
The high carbon steel with 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 weld and heat affected zone and difficult to weld. Therefore, this kind of steel is generally not used to make welding structures, but to make components or parts with high hardness or wear resistance. Most of their welding is repair welding of damaged parts. Annealing shall be carried out before repairing these parts to reduce welding cracks, and heat treatment shall be carried out again after welding.
2. 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 amount of alloy elements is generally less than 5%. It is because low alloy high strength steel contains a certain amount of alloy elements that its welding performance is different from that of carbon steel
(1) welding crack of welding joint
Because of the elements such as C, Mn, V and Nb, which strengthen the steel, the cold crack low alloy high strength steel is easy to harden during welding, and these hardening structures are very sensitive. Therefore, in the case of high rigidity or high restraint stress, if the welding process is improper, it is easy to produce cold crack. And this kind of crack has a certain delay, which is very harmful.
Reheat (SR) cracks reheat cracks are intergranular cracks near the coarse-grained zone of the fusion line in the process of post weld stress relief heat treatment or long-term operation at high temperature. It is generally believed that the formation of the carbides is due to the solid solution of V, Nb, Cr, Mo and other carbides in the vicinity of HAZ in austenite at high welding temperature, which are precipitated in time of cooling after welding, but dispersed in PWHT, thus strengthening the intragranular structure and concentrating the creep deformation at the stress relaxation on the grain boundary.
The welded joints of high strength low alloy steel are not easy to produce reheat cracks, such as 16MnR, 15MnVR, etc. However, for mn-mo-nb and mn-mo-v low-alloy high-strength steels, such as 07MnCrMoVR, Nb, V and Mo are the elements that promote the sensitivity of reheat cracks. Therefore, this kind of steel should avoid the sensitive temperature area of reheat cracks during post weld heat treatment to prevent the occurrence of reheat cracks.
(2) embrittlement and softening of welded joints
Before welding, the strain aging embrittlement welded joint needs to undergo various cold processes (blanking shear, cylinder rolling, etc.), and the steel will produce plastic deformation. If the area is heated at 200-450 ℃, strain aging will be caused. Strain aging embrittlement will reduce the plasticity of steel and increase the brittle transition temperature, which will lead to brittle fracture of equipment. Postweld heat treatment can eliminate the strain aging of welding structure and restore the toughness.
The embrittlement welding of weld and HAZ is a non-uniform heating and cooling process, thus forming non-uniform 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 influence on the properties of WM and HAZ of low alloy high strength steel. The low alloy high strength steel is easy to harden, and if the line energy is too small, the HAZ will appear martensite to cause cracks; if the line energy is too large, the coarse grains of WM and HAZ will cause joint embrittlement. The HAZ embrittlement tendency of low carbon quenched and tempered steel is more serious than that of hot rolled and normalized steel. Therefore, when welding, the line energy should be limited to a certain range.
The HAZ softening of the welded joint is due to the heat effect of welding. When the HAZ of the low carbon quenched and tempered steel is heated to the temperature above the tempering temperature, especially the area near AC1, there will be a softening band with reduced strength. The softening of HAZ zone is aggravated with the increase of welding line energy and preheating temperature, but generally the tensile strength of HAZ zone is still higher than the lower limit requirement of base metal standard value, so as long as the process is proper, the softening of HAZ zone of this kind of steel will not affect the use performance of its joint.
3. Welding of stainless steel
Stainless steel can be divided into four categories according to its different 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 bidirectional stainless steel.
(1) welding of austenitic stainless steel
Austenitic stainless steel is easier to weld than other stainless steels. It is insensitive to hydrogen embrittlement and has good plasticity and toughness in as welded austenitic stainless steel joints. The main problems of welding are: welding hot crack, embrittlement, intergranular corrosion and stress corrosion. In addition, due to poor thermal conductivity, large coefficient of linear expansion, large welding stress and deformation. During welding, small welding heat input shall be used as much as possible, and preheating shall not be allowed, and the interlayer temperature shall be reduced. The interlayer temperature shall be controlled below 60 ℃, and the weld joints shall be staggered with each other. In order to reduce the heat input, the welding speed should not be increased excessively, but the welding current should be reduced.
(2) welding of AUSTENITIC FERRITIC duplex stainless steel
Austenite ferrite 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 kind of steel welding are: it has a lower thermal tendency than austenitic stainless steel; it has a lower embrittlement tendency than pure ferritic stainless steel after welding, and the degree of ferritic coarsening in the heat affected zone of welding 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 required during welding. TIG welding should be used for thin plates, and electrode arc welding can be used for medium and thick plates. Special electrode with similar composition to base metal or austenitic electrode with low carbon content should be used for electrode arc welding. Nickel base alloy electrode can also be used for Cr25 dual phase steel.
There is a large proportion of ferrite in the dual phase steel, but the inherent embrittlement tendency of the ferritic steel, such as 475 ℃ embrittlement, σ phase precipitation embrittlement and coarse grain, still exist. Only because of the balance effect of austenite, it can be relieved to a certain extent. During welding, attention should be paid. When welding non Ni 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, it is necessary to control the heat input of welding, and try to use small current, high welding speed, narrow pass welding and multi pass welding to prevent grain coarsening and single-phase ferrite in the heat affected zone. The interlayer temperature should not be too high. It is better to weld the next pass after cooling.
Source: China Fittings Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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Which metal is used in welding?
Welding can be used to weld steel, iron, aluminum, copper, and nickel.
How does welding affect material properties?
The weld metal is comparatively stronger, and the joint properties are controlled by weld metal chemical composition and microstructure. The strong carbide/nitride forming elements, like Nb, Ti, V, etc., have very limited solubility in ferrite and austenite, and normally the precipitates act as fine dispersion of carbides, nitrides and/or carbonitrides and contribute to strength due to precipitation hardening. This could be the reason that the yield strengths of all the joints increase compared to the yield strength of the parent metal. The yield point elongation is attributed to the interaction of solute atoms and moving dislocations. This could be the reason that the yield strengths of all the joints increase compared to the yield strength of the parent metal.
What materials can be welded together?
In terms of weldability, commonly used materials can be divided into the following types:
Aluminium and its alloys.
Nickel and its alloys.
Copper and its alloys.
Titanium and its alloys.
What is the hardest metal to weld?
While the hardest known mineral in the universe is diamond, the honor of the hardest metal goes to chromium. Chromium is used in the well-known alloy stainless steel to make it harder.