Welding knowledge sharing, control and prevention in welding process

【Summary Description】 1. Welding arc length control and prevention During welding, the arc length shall not be properly adjusted according to the groove form, welding layer number, welding form, welding rod type, etc. Due to the improper use of welding arc length, it is difficult to obtain high-quality welds. prevention and control measures In order to ensure the quality of the weld, short arc operation is generally used during welding, but the appropriate arc length can be selected according to different conditions to obtain the best welding quality. For example, the first layer of V-shaped groove butt joint and angle joint should use shorter arc to ensure penetration and no undercut. The second layer can be slightly longer to fill the weld. When the weld gap is small, short arc should be used. When the gap is large, the arc can be slightly longer, and the welding speed is accelerated. The overhead welding arc should be the shortest to prevent the molten iron from flowing down; in order to control the temperature of the molten pool during vertical welding and horizontal welding, small current and short arc welding should also be used. In addition, no matter what welding is taken during the movement, attention should be paid to keeping the arc length basically unchanged, so as to ensure that the melting width and penetration depth of the whole weld are consistent. 2. Weld leg size control and prevention For butt or fillet combined welds requiring penetration, such as T-joints, cross joints, fillet joints, etc., the size of the weld leg is not enough, or the size of the weld leg for the connection between the web plate and the upper wing plate edge of the crane beam or similar components designed with fatigue checking requirements is not enough, which will make the welding strength and rigidity fail to meet the design requirements. prevention and control measures Butt combined welds requiring penetration, such as T-joint, cross joint and fillet joint, shall be in accordance with the design requirements, and must have sufficient weld leg requirements. Generally, the size of weld leg shall not be less than 0.25t(t is the thinner plate thickness at the connection). The weld leg size of the connection weld between the crane beam or similar web and the upper flange designed with fatigue checking requirements is 0.5t and shall not be greater than 10mm. The allowable deviation of welding size is 0~4mm. 3. Common defects and prevention in multilayer welding When thick plates are welded in multiple layers, the lower layer is directly welded without removing welding slag and defects after each layer is welded, which is easy to cause defects such as slag inclusion, pores and cracks in the weld, reduce the connection strength, and cause spatter during lower layer welding. prevention and control measures When thick plates are welded in multiple layers, each layer shall be welded continuously. After each layer of weld is welded, welding slag, weld surface defects and spatter shall be removed in time. If slag inclusion, pores, cracks and other defects affecting the welding quality are found, welding shall be carried out after thorough removal. 4. Welding voltage and its selection basis During welding, the same arc voltage is selected regardless of the size of the groove, whether it is bottoming, filling or covering. In this way, the required penetration depth and width may not be reached, resulting in defects such as undercut, pores and splash. Selection basis Generally, according to different situations, the corresponding long arc or short arc should be selected respectively to obtain better welding quality and working efficiency. For example, short arc operation should be adopted in order to obtain better penetration during backing welding. The arc voltage can be appropriately increased in order to obtain higher efficiency and melting width during filling welding or cover welding. 5. Optimal welding current and its selection basis During welding, in order to catch up with the progress, the butt weld of medium and heavy plate shall not be beveled. The strength index decreases, even fails to meet the standard requirements, and cracks appear during bending test, which will make the performance of weld joints can not be guaranteed and pose potential hazards to structural safety. Selection basis The welding shall be controlled according to the welding current in the process qualification, and 10 ~ 15% floating is allowed. The size of the blunt edge of the groove should not exceed 6mm. When butt joint, when the plate thickness exceeds 6mm, the groove shall be opened for welding. 6. Welding speed, welding current and electrode diameter and selection basis Do not pay attention to control the welding speed and welding current during welding; the diameter of the electrode and the welding position are coordinated. For example, when backing welding the full penetration corner seam, due to the narrow root size, such as too fast welding speed, the root gas and slag inclusion do not have enough time to discharge, which is easy to cause defects such as incomplete penetration, slag inclusion and porosity at the root; when welding the cover surface, if the welding speed is too fast, it is also easy to produce porosity; if the welding speed is too slow, the weld excess height will be too high and the appearance is not neat; when welding thin plates or small welds with blunt edges, the welding speed is too slow and burn through is easy to occur. Selection basis The welding speed has a great influence on the welding quality and welding productivity. When selecting, the welding current, the welding seam position (backing welding, filling welding, cover surface welding), the thickness of the welding seam and the groove size shall be selected with appropriate welding speed. On the premise of ensuring penetration, easy discharge of gas and welding slag, no burning through and good forming, the larger welding speed shall be selected to improve the productivity. 7. Control interlayer temperature and prevention measures When welding thick plates in multiple layers, do not pay attention to the temperature control between layers. If the interval between layers is too long, cold cracks will easily occur between layers without preheating again. If the interval is too short and the interlayer temperature is too high (over 900 degrees Celsius), the performance of the weld and heat affected zone will also be affected, resulting in coarse grains, resulting in decreased toughness and plasticity, and potential hidden dangers to the joint. prevention and control measures During multi-layer welding of thick plates, the control of interlayer temperature should be strengthened. In the continuous welding process, the temperature of the welded base metal should be checked to make the interlayer temperature as consistent as possible with the preheating temperature, and the maximum temperature between the layers should also be controlled. The welding time should not be too long. In case of welding interruption, appropriate post-heating and heat preservation measures should be taken. When welding again, the re-preheating temperature should be appropriately higher than the initial preheating temperature. 8. Influence and prevention of welding deformation During welding, the deformation is not controlled from the aspects of welding sequence, personnel arrangement, groove form, welding specification selection and operation method, etc., which leads to large deformation after welding, difficulty in correction and increased cost, especially for thick plates and large workpieces, which are difficult to correct and easy to cause cracks or lamellar tearing by mechanical correction. The cost of flame correction is high and the operation is not good, which is easy to cause the workpiece to overheat. For workpieces with high precision requirements, effective deformation control measures are not taken, and the installation dimensions cannot meet the use requirements, and even cause rework or scrap. prevention and control measures Adopt a reasonable welding sequence and select the appropriate welding specifications and operating methods, but also the use of anti-deformation and rigid fixation measures. 9. Influence and prevention of plug electrode head or iron block in joint clearance Since it is difficult to fuse the electrode head or iron block with the welded part during welding, it will cause welding defects such as incomplete fusion and incomplete penetration, and reduce the connection strength. If it is filled with rusty electrode head or iron block, it is difficult to ensure the material consistency with the base metal. If there are oil stains and impurities on the electrode head and iron block, the weld will have defects such as pores, slag inclusion and cracks. These conditions will greatly reduce the weld quality of the joint and fail to meet the quality requirements of the design and specifications for the weld. prevention and control measures 1) When the workpiece assembly gap is large, but does not exceed the allowable range specified, and the assembly gap exceeds 2 times the thickness of the thin plate or is greater than 20mm, the welding method shall be used to fill the recessed part or reduce the assembly gap. It is forbidden to use the method of filling electrode head or iron block repair welding in the joint gap. 2) When processing and marking parts, attention should be paid to leaving enough cutting allowance and welding shrinkage allowance after cutting, controlling the size of parts, and not increasing the gap to ensure the external dimensions. 10. The influence and prevention of different thickness and width plate butt joint When butt joint of plates with different thickness and width is adopted, it is not necessary to pay attention to whether the thickness difference of the plates is within the allowable range of the standard. If it is not within the allowable range and no smooth transition treatment is done, such welding seam is easy to cause stress concentration and incomplete fusion and other welding defects when it is higher than the thickness of the plates, thus affecting the welding quality. prevention and control measures When exceeding the relevant regulations, the weld shall be welded into a slope shape, and the maximum allowable slope value shall be 1:2.5; Or one or both sides of the thickness shall be processed into a slope before welding, and the maximum allowable slope value is 1:2.5. When the structural slope directly bears dynamic load and needs fatigue checking calculation, the slope shall not be greater than 1:4. When the plates of different widths are jointed, thermal cutting, mechanical processing or grinding with grinding wheel shall be adopted according to the conditions of the factory and the construction site to make the transition smoothly, and the maximum allowable slope value of the joint is 1:2.5.

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