Welding analysis of high strength materials
In modern industry, high-strength materials play an increasingly important role, but welding cracks, embrittlement, softening and other phenomena bring hidden dangers to safety production and product efficiency. Therefore, according to my own learning and practical experience, the author analyzes and expounds the welding characteristics of high-strength materials, especially high-strength steel.
Key words: high strength materials; Welding; characteristic
I. Overview of High Strength Materials
In the current pipelines and containers, high-strength materials play an increasingly important role. Among them, the most important thing is to add one or more alloy components (the proportion of alloy components is less than 5%) to the steel in order to strengthen the strength of the steel, improve the strength of the steel to 275MPa or higher and produce better comprehensive quality. This kind of steel is called high strength steel, and its basic advantages are high strength, high plasticity and high toughness. According to the yield strength and heat treatment characteristics of steel, there are generally two kinds of high-strength steel.
The yield strength of hot-rolled normalized steel is between 294 MPa and 490 MPa, and the service state is hot-rolled normalizing rolling. From the category, it is non-heat-treated strengthened steel, which is the most commonly used in reality.
Quenched and tempered steel, whose yield strength is between 490 MPa and 980 MPa, is usually used in quenched and tempered state and belongs to the category of heat treatment strengthened steel. This kind of steel is characterized by high strength, good plasticity and toughness, and can be directly welded when quenching and tempering. Therefore, this medium tempered steel is becoming more and more popular in use.
At present, the commonly used high-strength steels have the following grades: 16MnR,15mnr,13mnmonbr,18mnmonbr; There are several forging brands: 16Mn, 15MnV, 20MnMo, 20MnMoNb.
Second, the welding characteristics of high strength steel
The carbon content in high strength steel is usually not higher than 0.20%, and the total alloy composition is usually not higher than 5%. Because high-strength steel contains some alloy components, its weldability is somewhat different from other materials. The specific welding features are as follows:
1, welding cracks occurred during welding.
(1). High-strength steel uses elements such as carbon and manganese, which increase the strength of steel. It often hardens when welding, and the hardened part is often very sensitive. Therefore, when the stiffness is too large and the constraint stress is strong, if there is a problem with the welding method, cold cracks will occur. In addition, cracking has a long delay, which is easy to cause great harm.
(2) Reheat crack refers to intergranular crack caused by slowly eliminating stress heat after welding operation or being in the coarse grain position near the fusion line at high temperature for a long time. It is generally believed that the reason for this kind of crack is that the elements such as V, Nb, Cr and Mo near the heat-affected zone are dissolved in austenite at high welding temperature, and are not completely precipitated after welding, but dispersed at PWHT, which strengthens the grains and concentrates the creep deformation generated during stress relaxation at the grain boundary.
When welding high-strength steel, it usually does not cause reheat cracks, such as 16MnR and15mnr. However, for high strength steels such as Mn-Mo-Nb and Mn-Mo-V, Nb, V, Mo and other components are sensitive, which are common factors causing reheat cracks. Therefore, during heat treatment after welding, it is necessary to avoid the temperature range that is easy to cause reheat cracks, so as not to cause reheat cracks.
2. Brittleness and softening of welded parts
(1). Strain aging embrittlement. Before welding, all kinds of cold treatment (such as steel plate shearing, pipe and tank rounding, etc.) should be carried out on the welding part, and the material will be deformed. If the deformed parts are heated to 200-450℃, strain aging may occur, and then embrittlement may occur, which often leads to the weakening of plasticity of materials, thus causing brittle fracture of steel.
PWHT can weaken strain aging in welding process and restore toughness to some extent. The Steel Pressure Vessel made by 1998 clearly stipulates that the thickness of cylinder steel should meet the following standards: the thickness of carbon steel should not be less than 3% of the cylinder inner diameter; The thickness of other steels shall not be less than 2.5% of the inner diameter. In addition, the pressed products made in cold forming and medium temperature forming should be heat treated after forming.
(2) Brittleness of weld and heat affected zone. When welding materials, heating and cooling are often not very uniform, which will produce uneven structure. Weld and heat affected zone have certain brittleness, which is the weakest place in welded joint. The energy intensity of weld has a great influence on the performance of weld and heat affected zone of high strength steel. High strength steel hardens easily. If the linear energy is not high, HAZ will produce martensite and cause cracks. If the linear energy is too high, WM and HAZ will produce coarse grains, which will cause embrittlement of welded parts. If the linear energy is too high, the HAZ embrittlement caused by quenched and tempered steel is particularly obvious. Therefore, when welding, the linear energy should be controlled at an appropriate measured value.
(3) The heat affected zone of the welding part is softened. Due to the thermal effect during welding, the strength of some areas will be reduced, forming a certain softening zone. The microstructure softening in HAZ zone will deteriorate due to the increase of weld heat and preheating temperature, but the performance of ordinary softening zone can still reach the minimum standard of the specified standard value. Therefore, if the process is proper, the normal use of weldments will not be reduced.
Third, the welding characteristics of new high-strength materials
1, high strength pipeline steel
High strength pipeline steel refers to steel grade above X70. Up to now, X80 is the pipeline steel with the highest strength used in the completed pipeline steel. Ipsco Steel Company of Canada clearly pointed out in the annual report 1998 that the trial production of X90 and X 100SSAW steel pipes has been successfully carried out, with the ultimate goal of producing various specifications of X 100 steel pipes. NKK, Sumitomo Metal, Nippon Steel, Kawasaki Steel and European Steel Pipe Company have also successfully developed X90 and X 100UOE steel pipes, and are developing X 120 steel pipes.
In order to ensure the safety and reliability of the pipeline, the strength and toughness must be improved at the same time. In particular, steel pipes used for high-pressure gas transportation must have high CVN. Super bainite and super martensite are called pipeline steels in 2 1 century, and their steel grades are X80 ~ X 100 (bainite) and X 100 ~ X 120 (martensite). In terms of composition design, they all belong to (super) Mn-Nb-Ti system or Mn-Nb-V(Ti) system, and some elements such as Mo, Ni and Cu are added. Therefore, the toughness of the heat affected zone is not worse than that of low-strength pipeline steel, and its sensitivity to cold cracks is not great. For steel with strength higher than 600MPa, special attention should be paid to WM cold crack when welding, especially ultra-low hydrogen welding material must be used for spot welding butt girth weld.
2. Ultra-fine grained steel
In the 1990s, the world's major steel-producing countries successively carried out research on a new generation of steel materials, especially those from Japan. Super steel? Plan, China? Major basic research of new generation steel materials? And Korean? 2 1 century high performance structural steel? It has attracted the attention and enthusiastic participation of the world steel industry.
In the research of a new generation of steel materials, the most striking thing is the study of ultra-fine grains, and the goal of doubling the strength is achieved by ultra-fine grains (minimum 1mm). The biggest problem of ultra-fine grain steel welding is the trend of grain growth in heat affected zone. In order to solve this problem, laser welding, ultra-narrow gap MAG welding, pulse MAG welding and other low heat input welding methods must be adopted.
refer to
[1] Wang Jianli. Welding procedure qualification of high strength steel [J]. Yunnan Hydropower, 2007, (02).
Li Ming. Welding of high strength steel [J]. Modern welding, 2005, (03).
Li Zhuoxin, Liu Xiulong, Lee Hung, Li Guodong. Research progress of welding materials and weldability of high strength steel at home and abroad [J]. New Technology and New Process, 2007, (05).
Discussion on welding technology of materials
Abstract: Welding is the manufacturing or carving process of connecting metals or thermoplastics. In the welding process, the workpiece and solder melt to form a melting zone, and the molten pool cools and solidifies to form a connection between materials. In this process, pressure is usually needed. There are many sources of welding energy, including gas flame, arc, laser, electron beam, friction and ultrasound. Nowadays, welding robots are widely used in industrial applications, researchers are still deeply studying the essence of welding and continuing to develop new welding methods to further improve welding quality.
Keywords: welding; Metal; Energy; technology
1, Introduction to Welding Technology
1. 1 Physical essence of welding process
Welding is a process in which two or more materials of the same or different species are connected into a whole through the combination and diffusion between atoms or molecules. The method to promote the combination and diffusion between atoms and molecules is heating or pressurizing, or heating and pressurizing at the same time.
1.2 classification of welding
Metal welding can be divided into fusion welding, pressure welding and brazing according to the characteristics of its technological process.
1.2. 1 fusion welding is a method of heating the workpiece interface to a molten state without adding pressure during welding. When welding, the heat source quickly heats and melts the interface between two workpieces to be welded, forming a molten pool. The molten pool moves forward with the heat source, and after cooling, it forms a continuous weld to connect the two workpieces into a whole. In the welding process, if the atmosphere is in direct contact with the high-temperature molten pool, the oxygen in the atmosphere will oxidize metals and various alloy elements. Nitrogen and water vapor in the atmosphere enter the molten pool, and defects such as porosity, slag inclusion and cracks will also be formed in the weld during the subsequent cooling process, which will worsen the quality and performance of the weld. In order to improve the welding quality, people have developed various protection methods. For example, gas shielded arc welding is to isolate the atmosphere with argon, carbon dioxide and other gases to protect the arc and molten pool rate during welding; For example, when welding steel, adding titanium iron powder with high affinity for oxygen to covered electrode coating for deoxidation can protect beneficial elements manganese and silicon in covered electrode from being oxidized and entering the molten pool, and obtain high-quality weld after cooling.
Pressure welding (1.2.2) is to combine two workpieces in solid state under pressure, which is also called solid state welding. The commonly used pressure welding process is resistance butt welding. When the current passes through the connection end of two workpieces, the temperature there rises due to the large resistance, and when it is heated to a plastic state, it is connected into a whole under the action of axial pressure. The common feature of various pressure welding methods is that pressure is applied during welding without filling materials. Most pressure welding methods, such as diffusion welding, high frequency welding and cold pressure welding, have no melting process, so there is no problem of beneficial alloy elements burning and harmful elements invading the weld, which simplifies the welding process and improves the welding safety and hygiene conditions. At the same time, because the heating temperature is lower than that of fusion welding, the heating time is short and the heat affected zone is small. Many materials that are difficult to weld by fusion welding can often be welded by pressure welding into high-quality joints with the same strength as the parent metal.
1.2.3 Brazing is a welding method that uses a metal material with a melting point lower than that of the workpiece as the brazing filler metal, heats the workpiece and the brazing filler metal to a temperature higher than or lower than the melting point of the workpiece, wets the workpiece with liquid brazing filler metal, and fills the interface gap to realize mutual diffusion between atoms and the workpiece.
1.2.4 when welding, the seam formed to connect two connected objects is called weld. When welding, both sides of the weld will be affected by welding heat, and the structure and properties will change. This area is called the heat affected zone. When welding, due to the difference of workpiece material and welding current, etc. After welding, the weld seam and heat affected zone may be overheated, brittle, hardened or softened, which will also reduce the performance of the weldment and worsen the weldability. Therefore, it is necessary to adjust the welding conditions. Preheating the interface of weldment before welding, keeping temperature during welding and heat treatment after welding can improve the welding quality of weldment. In addition, welding is a local rapid heating and cooling process. Due to the constraints of the surrounding workpiece body, the welding zone cannot expand and contract freely, and the cooled weldment will produce welding stress and deformation. Important products need to eliminate welding stress and correct welding deformation after welding.
1.2.5 modern welding technology has been able to weld welds with mechanical properties equivalent to or even higher than those of the joints, and there are no internal and external defects. The mutual position of welded bodies in space is called welded joint, and the strength of the joint is not only affected by the quality of weld, but also related to its geometry, size, stress and working conditions. The basic forms of joints are butt joint, lap joint, T-joint (positive joint) and angle joint. The cross-sectional shape of butt weld depends on the thickness of the welded body before welding and the groove form of the two edges. When welding thick steel plates, grooves of various shapes are made at the joints for penetration, so that covered electrode or welding wire can be easily fed. Groove forms include single-sided welding groove and double-sided welding groove. When choosing the groove form, in addition to ensuring the penetration depth, factors such as convenient welding, less metal filling, small welding deformation and low groove processing cost should also be considered. When two steel plates with different thicknesses are butted, in order to avoid serious stress concentration caused by the sharp change of cross section, the thicker plate edges are often gradually thinned to reach the same thickness at the two butted edges. The static strength and fatigue strength of butt joint are higher than those of other joints. Butt welding is usually the first choice for connections working under alternating and impact loads or in low temperature and high pressure vessels.
Lap joint is easy to prepare before welding, easy to assemble, with small welding deformation and residual stress, and is often used for on-site installation of joints and unimportant structures. Generally speaking, lap joints are not suitable for working under alternating load, corrosive medium, high temperature or low temperature. T-joints and corner joints are usually used for structural needs. The working characteristics of incomplete fillet weld on T-joint are similar to those of lap joint. When the weld is perpendicular to the direction of external force, it becomes a frontal fillet weld, and the surface shape of the weld will cause stress concentration to varying degrees. The stress of penetration fillet weld is similar to that of butt joint. The bearing capacity of fillet joint is low, so it is generally not used alone. It can only be improved by full welding, or when there are fillet welds inside and outside, and it is mostly used at the corner of closed structure. Welded products are lighter than riveted parts and cast forgings, which can reduce their own weight and save energy for transportation tools. The welding has good sealing performance and is suitable for manufacturing various containers. With the development of combined machining technology, welding, forging and casting can be combined to manufacture large-scale, economical and reasonable cast-welded structures and forged-welded structures, which has high economic benefits. The welding process can effectively use materials, and the welding structure can use materials with different properties in different parts, giving full play to the advantages of various materials and achieving economic and high quality. Welding has become an indispensable and increasingly important processing method in modern industry.
1.2.6 In the future welding process, on the one hand, we should develop new welding methods, welding equipment and welding materials to further improve the welding quality and safety and reliability, such as improving the existing welding energy sources such as arc, plasma arc, electron beam and laser; Using electronic technology and control technology, the technological performance of arc is improved and a reliable and portable arc tracking method is developed. On the other hand, it is necessary to improve the level of welding mechanization and automation, such as program control and digital control of welding machine; Develop a special welding machine to realize the whole process automation from preparation, welding to quality control; In the automatic welding production line, the promotion and expansion of numerical control welding manipulator and welding robot can improve the welding production level and improve the welding hygiene and safety conditions.
2. Welding-Industrial Art
The appearance of welding caters to the demand of new technology and new means for the development of metal art. On the other hand, the unique and wonderful changes of metal under the action of welding heat also meet the demand of metal art for new artistic expression language. In today's metal art creation, welding can and is being shown as a unique artistic expression language. This paper analyzes the emergence and application of this technology.
2. 1 artistic creation and technical methods are always inseparable. As an industrial technology, the emergence of welding caters to the needs of the development of metal art for new technical means. On the other hand, the unique and wonderful changes of metal under the action of welding heat also meet the demand of metal art for new artistic expression language. In today's metal art creation, welding can and is being shown as a unique artistic expression language. Metal welding art can be separated from traditional metal art and become a relatively independent art form, because welding is artistic.
2.2 Welding can produce expressive artistic language.
Welding is usually carried out at high temperature, and metal will produce many wonderful and rich changes at high temperature. The metal substrate will change color and be thermally deformed (i.e. welding heat affected zone); The welding wire will form some beautiful textures after melting; Welding defects are often used in welding art, and welding defects refer to defects that do not meet the design or process requirements in the welding process. Its main manifestations are welding cracks, blowholes, undercut, incomplete penetration, incomplete fusion, slag inclusion, flash, collapse, pits, burn-through, inclusions and so on. This is a very interesting phenomenon: the artistry of welding is usually reflected in some failed operations of industrial welding or hidden in some welding defects that industrial welding tries to avoid. Secondly, the language of welding art is unique. Choosing different metal materials and adopting different welding processes can give full play to the artistry of welding in different metal art forms.
In welding sculpture works, welds and incisions do not exist passively as traces of technical treatment, but are embodied in a wonderful and indispensable expression language. In a welding sculpture, thick welds are exposed on the surface of the sculpture, and all kinds of irregular cutting marks have become the artist's beautiful artistic language. In many cases, due to the rough and simple style pursued by welding sculptures, according to the needs of works, most of them retain the corrosion and defects of metals. Therefore, a kind of uncut primitive beauty can often be felt in welding sculptures. The welding seam of the steel plate splicing at the lower part of the sculpture is very thick. Judging from the firmness of the welding process, this is obviously not only due to the consideration of the firmness of the sculpture. In this sculpture, the lower twisted welds, as an important factor of the overall aesthetics of the sculpture, have become an indispensable part. On the whole, there are distorted weld marks everywhere, whether it is the text modeling in the upper part or the texture processing in the lower part, and the whole work has achieved the unity of the overall visual language. Manual plasma cutting method, using the heat of current during cutting, makes the cutting edge produce heat affected zone, thus obtaining bright white stainless steel? Dyeing? A circle of slightly gradient colors is applied. At the same time, by adjusting the welding specification, the powerful air jet from the cutting gun will be at the cutting edge at the moment when the cutting steel plate melts. Blow? A circle of randomly formed textures solidifies into beautiful cuts after metal cutting and cooling, which is in contrast with the smooth and bright stainless steel plate in the middle. The formation process of this random effect is accidental, but it is an inevitable phenomenon under certain welding specifications. In terms of size, semi-automatic CO2 gas shielded welding can be used to weld larger artistic wall decorations, and manual TIG welding can be used to weld smaller ones.
If a mural is regarded as a painting, the treatment of points, lines, surfaces, black, white, gray and even colors in the picture can be realized by welding. Metal wires of various models and materials will appear on the screen in different forms with different welding processes. The colors of different metals are different, such as bright silver in stainless steel, silver in aluminum, jet black in carbon steel, titanium steel, bronze, copper and brass, and as far as steel is concerned, different steels will have different color changes when heated at high temperature, that is, the welding heat affected zone is different. In addition, cutting is also one of the creative methods of welding art wall decoration, which can be used in combination with welding or alone, depending entirely on the creator's creative intention and mastery of technology and effect. Taken together, these methods mentioned above can be imagined to be varied.
3. Causes of fire and explosion accidents in welding operations
3. 1 During welding and cutting operations, especially during gas cutting, Mars, molten beads and iron slag are splashed everywhere due to the jet of compressed air or oxygen flow (larger molten beads and iron slag can splash to a place 5 meters away from the operation point). When inflammable and explosive substances or gases exist in the working environment, fire and explosion accidents may occur.
3.2 During high-altitude welding and cutting operations, inflammable and explosive articles within the reach of Mars are not cleaned up, and workers throw covered covered electrode heads during the operation, and do not carefully check whether there is fire after the operation.
3.3 acetylene generator was not placed during gas welding and gas cutting, and the safety devices of welding (cutting) torch, rubber pipeline and acetylene generator were not checked as required before work.
4. Preventive measures for fire and explosion accidents in welding operations.
4. 1 During welding and cutting operations, clean all inflammable and explosive articles within the lOm scope of the working environment, and pay attention to whether there are flammable liquids and flammable gases in the trenches and sewers of the working environment, and whether it is possible to leak flammable and explosive materials into the trenches and sewers, so as to avoid disasters and accidents caused by welding slag and metal sparks.
4.2 When welding and cutting at high altitude, it is forbidden to throw covered electrode's head, and the lower part of welding and cutting operations should be isolated. After the operation is completed, you should carefully check and confirm that there is no fire hazard before leaving the site.
4.3 Gas cylinders that meet the requirements of relevant national standards and regulations shall be used, and the storage, transportation and use of gas cylinders shall strictly abide by the safety operation procedures.
4.4 Pipelines conveying combustible gas and combustion-supporting gas shall be installed, used and managed according to regulations, and operators and inspectors shall receive special safety technical training.
4.5 When welding and repairing fuel containers and pipelines, the welding and repairing methods shall be determined according to the actual situation. When implementing the replacement method, the replacement should be thorough, and the shadow of combustible substances should be strictly controlled in the work. When the pressureless replacement method is implemented, a certain voltage should be maintained as required. Oxygen content should be strictly controlled in the work. It is necessary to strengthen detection, pay attention to monitoring, and have safety organization measures.
As an industrial technology, the appearance of welding caters to the demand of new technology and new means for the development of metal art. On the other hand, the unique and wonderful changes of metal under the action of welding heat also meet the demand of metal art for new artistic expression language. In today's metal art creation, welding can and is being shown as a unique artistic expression language.
The manifestations of the above welding defects and welding heat affected zone are formed by welding operation under certain specifications, and these artistic languages can only be produced through welding. The surface effect of welding art works is impossible or difficult to achieve by other metal processing technologies, so welding art has unique artistry.