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1. Characteristics and requirements of welding of boiler equipment 1. Characteristics of welding Boiler equipment requires that welded joints have sufficient strength, tightness, high temperature resistance, corrosion and other performance properties like the subject materials. The on-site welding position is horizontal fixed or inclined fixed butt joint of pipes, including flat welding, vertical welding, overhead welding, horizontal welding and other welding positions. Therefore, higher and stricter requirements are put forward for welders’ operating skills. 2. Requirements for welding management of boilers A. Boiler manufacturing, installation, and repair units must hold licenses issued by the quality and technical supervision administrative department. Manufacturing, installation, and repair units shall be responsible for the safety and quality of the special equipment they manufacture, install, and repair. The installation unit should establish and improve the quality management system and implement it effectively ； Managers, technical directors, professional engineers and other professional technical and management personnel are matched, have corresponding qualifications and are suitable for the work they undertake. B. Welders and non-destructive testing inspectors engaged in the manufacture, installation and repair of special equipment must obtain a special operator qualification certificate issued by the administrative department in accordance with relevant regulations. Welding personnel include welding technicians, welding quality inspectors, welding inspection personnel, welders and welding heat treaters. Welder management is currently implemented * * The "Examination and Management Rules for Welders of Boilers, Pressure Vessels and Pressure Pipes" promulgated by the General Administration of Quality Supervision, Inspection and Quarantine (National Quality Inspection Pot [2002] No. 109, hereinafter referred to as the 02 version of the "Examination Rules") will be implemented from October 1, 2002. Welders who have obtained certificates (qualified items) according to the original "Boiler and Pressure Vessel Welder Examination Rules" (hereinafter referred to as the 88th Edition "Examination Rules") can continue to use their qualified items until the original welder certificate (qualified items) expires. Welders should perform welding according to the welding procedure instruction book or welding procedure card. Low-stress welder code stamps should be stamped near the welds of boiler pressure components. C. There should be a sound welding management system, welding process evaluation, and welding operation instructions. When using welding methods to manufacture, install, repair and modify boiler pressure components, the welding unit should formulate a welding process instruction book and conduct welding process assessment. Only when the requirements are met can it be used for production. During the boiler manufacturing process, when the welding ambient temperature is below 0°C, no welding is allowed without preheating measures. When welding on-site during boiler installation and repair, if the ambient temperature is below 0°C, the provisions of the welding process documents should be complied with. Welding in the open air is not allowed when it rains or snows. It is strictly prohibited to ignite arcs, test current, or weld temporary supports on the surface of the workpiece to be welded. Welding of counterpart fixtures is not allowed on the surface of high-alloy steel materials. When spot welding, in addition to the welding materials, welding process, welder and preheating temperature should be the same as those during formal welding, the following requirements should also be met: (1) When spot welding the counterpart root, the quality of each solder joint should be checked after spot welding. If there are any defects, they should be removed immediately and spot welded again. (2) If thick-walled and large-diameter pipes are fixed using the filling method, when removing the temporary fixings, the base metal should not be damaged, and the remaining weld scars should be cleaned, polished and trimmed. The welding construction process includes four important processes: counterpart assembly, welding, heat treatment and inspection. Only when this process meets the requirements can the next process be carried out, otherwise the construction of the next process is prohibited. Before welding boiler products, the following welding joints shall be evaluated for welding process. a. Butt welded joints between pressure components. b. T-shaped joints or corner joints that require full penetration between pressure components or between pressure components and load-bearing non-pressure components. The currently implemented welding procedure qualification standards include: Appendix 1 of "Steam Boiler Safety Technical Supervision Regulations" (96 Edition): Welding procedure qualification. D. There should be complete welding operation logs or construction records, and welding inspection records. Records should be authentic and reliable with good traceability. 2. Introduction to common welding methods and processes for boiler welding based on the definitions of GB/T3375 "Welding Terminology": A method of joining workpieces together by applying heat or pressure, or both, with or without filling material. Advantages of welding Compared with screw connections, tension connections, castings and forgings, welding has the following advantages: (1) Save metal materials, reduce structural weight, and have good economic benefits. According to statistics, the welded structure can be 15%~20% lighter than the welded structure, 30%~40% lighter than the casting, and 30% lighter than the forging. (2) The processing and assembly process is simplified, the production cycle is short, and the production efficiency is high. (3) High structural strength and good joint sealing. The sealing performance of welded structural joints is much better than that of joints and castings. Therefore, welded containers can fully meet the requirements for strength and sealing under high temperature and high pressure conditions. (4) Provide greater flexibility for structural design. Materials can be optimally configured according to the stress of the structure, and materials with different strengths, wear resistance, corrosion resistance, high temperature resistance and other properties can be selected in different parts according to project needs. For example, using carbon steel as the base material and overlaying the stainless steel lining layer to make petrochemical pressure vessels, this not only ensures the corrosion resistance of the equipment, but also saves a lot of precious metal materials and funds. 5) Tailor welding can be used * * Breaking through the limitations of casting and forging capabilities, we can produce extra large forged and welded, cast and welded structures, and provide extra large and extra heavy equipment and blanks, which promotes the development of the national economy. (6) The welding process is easy to mechanize and automate. .Limitations of welding: (1) Structures processed by welding methods are prone to large welding deformations and welding residual stresses, which affect the load-bearing capacity, processing accuracy and dimensional stability of the structure. At the same time, stress concentration will occur at the interface between the weld and the weldment, which will have a greater impact on the fatigue fracture of the structure. (2) There are a certain number of defects in welded joints, such as cracks, pores, slag inclusions, unwelded seams, unfusion, etc. The existence of these defects will reduce the strength, cause stress concentration, and damage the density of the weld, which is one of the main reasons for the damage of the welded structure. (3) Welded joints have large performance unevenness. Since the composition and metallographic structure of the weld is different from that of the base metal, each part of the joint experiences different thermal cycles, resulting in different performance in different areas of the joint. (4) High temperature, strong light and some toxic gases are generated during the welding production process, which will cause certain damage to the human body. Therefore, the labor protection of welding operators must be strengthened. Welding method classification: Melting welding, gas welding, arc welding, manual arc welding, submerged arc welding, gas shielded welding, inert gas shielded welding, CO2 gas shielded welding, electroslag welding, electron beam welding, laser welding, pressure welding, forge welding, friction welding, resistance welding, spot welding, butt welding, explosion welding, diffusion welding, brazing, fusion welding: The method of locally heating and melting the joints of the connected components into a liquid, and then cooling and crystallizing them into one body is called fusion welding. Pressure welding: The use of physical effects such as friction, diffusion and pressure to overcome the unevenness of the two connecting surfaces, remove (squeeze away) the oxide film and other contaminants, and bring the atoms on the two connecting surfaces close to each other to the lattice distance (range of atomic gravity), so that the connection achieved under solid-state conditions is collectively called solid phase welding. Pressure is usually required during solid phase welding, so it is called pressure welding. There are two forms of pressure welding: 1) Heating the contact part of the metal to be welded to a plastic state or a partial melting state, and then applying a certain pressure to combine the metal atoms to form a welded joint, such as resistance welding, friction welding, etc. (2) No heating welding, only apply enough pressure on the contact surface of the metal to be welded, and use the plastic deformation caused by the pressure to bring the atoms close to each other, thereby obtaining a strong squeeze joint. Such as cold pressure welding, ultrasonic welding, explosion welding, etc. brazing: Brazing is a method that uses a metal material with a lower melting point than the base metal as the solder, heats the weldment and the solder to a temperature higher than the melting point of the solder, but lower than the melting point of the base metal, and uses capillary action to make the liquid solder wet the base metal, fill the joint gap, and mutually diffuse with the base metal to connect the weldment. This method is called brazing. Brazing is divided into the following two types 1) Soft soldering: Welding with fiber materials (mainly lead and tin alloys) with a melting point lower than 450°C will result in lower joint strength. (2) Brazing uses fiber materials (mainly copper, silver, and alloys) with a melting point higher than 450°C, and the joint strength is high. 1. Manual arc welding A. Principle This is a welding method that uses the arc heat between the welding rod and the weldment to melt the welding rod and part of the weldment to form a welded joint. During the welding process, the electrode coating melts and decomposes to generate gas and slag. Under the joint protection of gas and slag, the harmful influence of the surrounding air on the molten metal is effectively eliminated. Through the metallurgical reaction between the molten metal and the anode at high temperature, the weld metal is reduced and purified, thereby obtaining a high-quality weld. Welding specifications: (1) Welding current: The main factors that determine the welding current are the diameter of the electrode and the position of the weld. The larger the diameter of the welding rod, the more arc heat energy is required to melt the welding rod, so the welding current should be increased accordingly. When welding low carbon steel or low alloy steel in the flat welding position, the following formula is commonly used to calculate the welding current: I=Nd. In the formula, I - welding current (A); d - electrode diameter (mm); N - experience coefficient, usually 30~50. When welding flat welds, a larger current can be used. When welding in other positions, in order to prevent the molten metal from flowing out of the molten pool, the molten pool should be smaller and the welding current should be smaller accordingly. (2)Arc voltage: The arc voltage mainly affects the melting width of the weld. The higher the voltage, the greater the melting width. The arc voltage is determined by the arc length. A long arc means a high arc voltage, and a short arc means a low arc voltage. The arc should not be too long during manual arc welding, so the arc voltage is not high and the range of change is not large. It is generally 20~25Vo(3) electrode diameter: The diameter of the welding rod is mainly selected based on the thickness of the workpiece to be welded. The thinner the workpiece, the thinner the welding rod used; the thicker the workpiece, the thicker the welding rod used. Welding rods with a diameter of 3~5mm are the most widely used. When the workpiece thickness is greater than 12mm, the diameter of the welding rod can be 4~6mm. When flat welding, thicker electrodes can be used to increase productivity. However, for the first layer of multi-layer welding, a welding rod no larger than 3.2mm should be used to ensure root penetration. In the future, thicker electrodes can be used for each layer according to the thickness of the workpiece. (4)Welding speed: Welding speed refers to the speed at which the welding rod moves along the welding direction. The welding speed of manual arc welding generally does not have special regulations, but is controlled by the welder according to the size of the weld and the characteristics of the electrode. Generally, the thinner the workpiece, the higher the welding speed should be. (5)Number of welding layers: When manual arc welding of medium-thick steel plates, multi-layer welding should be used. For steel of the same thickness, when other conditions remain unchanged, increasing the number of welding layers will help improve the plastic toughness of the welded joint. The number of welding layers is determined based on practical experience, and is approximately the ratio of the thickness of the steel to the diameter of the welding rod (taken as an integer). B. Features: Simple equipment ； Easy to operate ； Suitable for various locations ； It can weld carbon steel, heat-resistant steel, stainless steel, non-ferrous metals and other materials. Low production efficiency ； High labor intensity ； High technical requirements for welders. C. Application: It is the most widely used and is used for welding almost all kinds of pipes. Welding position of manual arc welding: Manual arc welding can be performed in different positions. During welding, the spatial position of the welded joint is called the welding position. In GB/T3375-94 "Welding Terminology", two parameters, inclination angle and rotation angle, are used to divide different welding positions. Among them, flat welding position, vertical welding position, horizontal welding position and overhead welding position are the four basic welding positions. There are also four basic forms of welding positions for pipe girth welds, namely horizontal rotation, vertical fixation, horizontal fixation, and 45° position. 2. Submerged arc automatic welding. During submerged arc welding, the arc burns under a layer of granular meltable flux, and the arc is not exposed. A. Working principle: The welding arc burns between the welding wire and the workpiece, and the arc melts the welding wire and the base metal near the arc. The molten metal forms a molten pool, and the molten flux forms slag. The molten pool is protected by the slag, and the arc moves forward, forming a weld. B. Features: The welding current used is large, the current density is large, and the penetration depth is large. Thick plates can be welded and formed in one go, with high production efficiency. ； High welding speed ； The welded joint has beautiful shape and high welding quality. ； No splashing, saving materials and electricity ； No arc radiation, little welding smoke, little harm to welders ； Conducive to automation. The equipment is complex, not as flexible as hand arc welding, and is not suitable for all-position welding. C. Application: It is widely used in the production of spiral welded steel pipes. The welding materials for submerged arc automatic welding are welding wire and flux. The welding wire is a bare metal wire, similar to the electrode core of manual arc welding. It is continuously melted and filled in the weld during welding. Flux is similar to the coating of manual arc welding electrodes. Specifications for welding of automatic buried welding: One of the main advantages of automatic buried welding is that the weld seam is well formed. In arc welding, weld formation can usually be expressed by two indicators: weld formation coefficient (shape coefficient) and fusion ratio. The weld forming coefficient refers to the ratio of the weld melting width to the melting depth (referred to as the ratio of melting width to melting depth). A small forming coefficient means that the weld is deep and narrow, and the welding heat affected zone is small. This is advantageous in terms of making full use of arc heat energy, reducing the size of the heat affected zone and reducing welding deformation. However, if the forming coefficient is too small, low-melting-point impurities and gases in the weld crystallization will not easily emerge from the molten pool, and the weld will easily produce cracks, pores, and slag inclusions. Generally, it is more appropriate to control the weld forming coefficient between 1.3 and 2.0. The cross-section percentage of the base metal in the weld is called the fusion ratio. The fusion ratio can affect the chemical composition, metallographic structure and mechanical properties of the weld. Especially when the chemical composition of the filler metal and the base metal is different, the chemical composition of the weld close to the base metal will change greatly, and the amplitude of the change is related to the difference in the chemical composition of the two metals and the fusion ratio. The fusion ratio of buried welding is between 60% and 70%. The main welding specification parameters of automatic welding include welding current, arc voltage, welding speed, wire diameter and extension length, etc. (1)Welding current and arc voltage: When the welding current increases, the weld penetration depth increases but the weld width changes little. This is because when the welding current increases, the heat generated by the arc and the heat transferred to the weldment will increase, and the arc blowing force will increase, which will push the liquid metal in the welding pool' away from the lower part of the welding wire, directly heating the unmelted metal at the bottom of the molten pool, thereby increasing the penetration depth. At the same time, as the arc penetrates deep into the molten pool, the exposed part of the arc is reduced and its mobility is reduced, so the melting width remains basically unchanged. In summary, as the welding current increases, the weld forming coefficient decreases and the fusion ratio increases. When the welding current is too large, the penetration is too deep and the width of the melt changes little, making it difficult for the gas and inclusions in the molten pool to rise, easily forming pores, slag inclusions and cracks, and may also cause burn-through. In order to avoid these defects, when increasing the welding current, the arc voltage should be increased accordingly to make the forming coefficient appropriate. When the arc voltage increases, the welding width increases significantly, while the penetration depth decreases. (2)Welding speed: When other conditions remain unchanged, as the welding speed increases, the arc heat obtained within the unit length of the weld is reduced, and the melting amount of the welding wire on the unit length of the weld is also reduced, so the melting width and reinforcement height of the weld are reduced. The change trend of penetration depth with welding speed is more complicated: when the welding speed is small and increases, the penetration depth increases; when the welding speed reaches a certain value and continues to increase, the penetration depth becomes smaller. Excessively increasing the welding speed will cause defects such as incomplete penetration, pores, and undercuts. When the welding speed is too low and the arc voltage is high, a "mushroom-shaped" weld will be formed, and cracks will easily form inside the weld. (3) Welding wire diameter and extension length: When other parameters remain unchanged, the diameter of the welding wire increases, the diameter of the arc column increases, and the range of arc heating expands, causing the welding width to increase and the penetration depth to decrease. On the contrary, the diameter of the welding wire decreases, the current density increases relatively, the penetration depth increases and the penetration width decreases. When the extension length of the welding wire increases, due to the increase in resistance, the preheating effect of the extended part of the welding wire is enhanced, and the melting speed of the welding wire is accelerated, resulting in shallow penetration and increased weld reinforcement. The extension length of the submerged arc automatic welding wire is usually 30~40mm. 3. Tungsten arc welding A. Principle: An arc welding method that uses external argon gas as the protective medium and tungsten electrode as the conductive electrode. The arc occurs between the electrode and the weldment, and oxygen is passed around the arc to form a continuous closed air flow to protect the arc and molten pool from air. B. Features: advantage: a: Argon gas can effectively isolate the surrounding air ； It itself is insoluble in metals and does not react with metals ； During the tungsten arc welding process, the arc also has the function of automatically removing the oxide film on the surface of the workpiece. Therefore, non-ferrous metal stainless steel and various alloys that are easily oxidized, nitrided and chemically active can be successfully welded. b: The tungsten arc is stable and can burn stably even under small welding current (less than 10A). It is especially suitable for welding of thin and ultra-thin plate materials. c: The heat source and filling wire can be controlled separately, so the heat input is easy to adjust, and welding can be performed in various positions, and the ideal method of single-sided welding and double-sided forming can also be achieved. d: Since the filler wire does not pass through the arc, there will be no spatter and the weld will be beautifully formed. The shortcomings are: a.: The melting depth is shallow, the deposition speed is small, and the production efficiency is low. b: The ability of the tungsten electrode to carry current is poor. Excessive current will cause the tungsten electrode to melt and evaporate, and its particles may enter the molten pool, causing pollution (tungsten inclusion). c: Inert gases (argon, helium) are more expensive, and compared with other arc welding methods (such as manual arc welding, submerged arc welding, CO2 gas shielded welding, etc.), the production cost is higher. C. Application: Tungsten arc welding can be used for welding almost all metals and alloys, but due to its high cost, it is usually used for welding aluminum, magnesium, titanium, copper and other non-ferrous metals, as well as stainless steel, heat-resistant steel, etc. For metals with low melting points and easy evaporation (such as lead, tin, zinc, etc.), welding is more difficult. It is often used for welding of small diameter pipes and primer welding of large diameter pipes. The welding specification parameters of argon arc welding mainly include welding current, arc voltage, welding speed, welding wire diameter, argon gas flow rate, nozzle diameter, etc. 4. Carbon dioxide gas shielded welding A. Principle: An arc welding method that uses external CO2 gas as the protective medium and automatically fed welding wire as the conductive electrode. In CO2 gas shielded welding, carbon dioxide gas surrounds the arc and molten pool, which can effectively prevent the harmful effects of air on the molten metal. However, carbon dioxide is different from inert gases. It is an oxidizing gas itself and can oxidize metal elements at high temperatures. Moreover, at high arc temperatures, carbon dioxide will decompose into carbon monoxide and atomic oxygen. These atomic oxygens are more likely to oxidize and burn iron and other alloy elements, thus reducing the alloy content and mechanical properties of the weld. The generated iron oxide, silicon dioxide, etc. form scum floating on the surface of the molten pool. The large amount of carbon monoxide produced by the reaction did not have time to precipitate completely during the cooling process of the molten pool, forming many pores. Since elements such as manganese and silicon are more likely to combine with oxygen than iron, manganese and silicon are commonly used as deoxidizers in steelmaking. In carbon dioxide gas shielded welding, these elements can also be used for deoxidation, thereby solving the problem of oxidation of iron by carbon dioxide and compensating for the burning loss of alloy elements. Therefore, when selecting carbon dioxide gas shielded welding wire, it must be ensured that the welding wire contains a sufficient amount of deoxidizing elements. The welding wire commonly used for carbon dioxide gas shielded welding is H08Mn2SiA, etc. The carbon dioxide gas used for carbon dioxide gas shielded welding is generally supplied in bottles and transported to the welding area through pipelines and nozzles. Gas purity should be no less than 99.5%. B. Features: advantage: a: low cost b: good quality c: high production efficiency d: good operating performance Disadvantages: a When a larger current is used for welding, there will be larger spatter; b there will be more smoke and arc intensity; c the surface of the weld will not be beautiful enough; d, when the control or operation is improper, it is easy to produce pores; e, the welding equipment is more complicated. C. Application: Mostly used for welding structural parts. 5. Principle of argon arc welding: An arc welding method that uses external argon gas as the protective medium and automatically fed welding wire as the conductive electrode. 6. Gas welding principle: The heat generated by the oxygen-acetylene flame is used to heat the welding wire and base metal to form a weld. application: Welding of non-ferrous metals. 7. Electroslag welding