There are many things to pay attention to during welding. Once neglected, it may lead to big mistakes. If you review the welding process, you must pay attention to these points. If you deal with welding quality accidents, you still need to pay attention to these problems!
1. Not paying attention to the selection of the best voltage during welding construction
[Phenomenon] When welding, whether it is base, filling, or capping, regardless of the size of the groove, the same arc voltage is selected. This may not meet the required penetration depth and width, and defects such as undercut, pores, and spatter may occur.
[Measures] Generally, the corresponding long arc or short arc should be selected for different situations to obtain better welding quality and work efficiency. For example, in order to obtain better penetration depth during base welding, short arc operation should be used. In order to obtain higher efficiency and width during filling welding or capping welding, the arc voltage can be appropriately increased.
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2. Welding does not control the welding current
[Phenomenon] During welding, in order to rush the progress, the butt weld of medium and thick plates is not grooved. The strength index decreases, and even fails to meet the standard requirements. Cracks appear during the bending test, which will make the performance of the weld joint unguaranteed and pose a potential hazard to the safety of the structure.
[Measures] When welding, the welding current in the process assessment should be controlled, and a 10-15% fluctuation is allowed. The blunt edge size of the groove should not exceed 6mm. When butt welding, when the plate thickness exceeds 6mm, a groove should be opened for welding.
3. Not paying attention to the coordinated use of welding speed and welding current, and the electrode diameter
[Phenomenon] When welding, do not pay attention to the control of welding speed and welding current, and the electrode diameter and welding position should be coordinated. For example, when performing the base welding on the fully penetrated corner seam, due to the narrow root size, if the welding speed is too fast, the root gas and slag will not have enough time to be discharged, which may easily cause defects such as incomplete penetration, slag inclusion, and pores at the root; when performing the cover welding, if the welding speed is too fast, pores may also be easily generated; if the welding speed is too slow, the weld excess height will be too high and the shape will be uneven; when welding thin plates or welds with small blunt edges, the welding speed is too slow, which may easily cause burn-through and other situations.
[Measures] The welding speed has a significant impact on the welding quality and welding production efficiency. When selecting, the appropriate welding speed should be selected in combination with the welding current, weld position (base welding, filling welding, cover welding), weld thickness, and groove size. A larger welding speed should be selected under the premise of ensuring full penetration, easy discharge of gas and welding slag, no burn-through, and good forming, so as to improve productivity and efficiency.
4. Failure to control the arc length during welding
[Phenomenon] The arc length is not properly adjusted according to the groove form, number of welding layers, welding form, electrode model, etc. during welding. Due to improper use of welding arc length, it is difficult to obtain high-quality welds.
[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 situations to obtain the best welding quality. For example, the first layer of V-shaped groove butt joint and corner joint should use a shorter arc to ensure full penetration and no undercutting. The second layer can be slightly longer to fill the weld. When the gap between the welds is small, a short arc should be used. When the gap is large, the arc can be slightly longer, and the welding speed is faster. The arc of overhead welding should be the shortest to prevent molten iron from flowing down; in order to control the temperature of the molten pool during vertical welding and horizontal welding, low current and short arc welding should also be used. In addition, no matter what welding is adopted, it is important to keep the arc length basically unchanged during the movement to ensure that the molten width and depth of the entire weld are consistent.
5. Failure to control welding deformation during welding
[Phenomenon] Failure to control deformation during welding from aspects such as welding sequence, personnel arrangement, groove form, selection of welding specifications and operation methods, resulting in large deformation after welding, difficulty in correction and increased costs, especially for thick plates and large workpieces, which are difficult to correct and can easily cause cracks or layered tearing by mechanical correction. The cost of flame correction is high and poor operation can easily cause overheating of the workpiece. For workpieces with high precision requirements, failure to take effective measures to control deformation will result in the installation size of the workpiece not meeting the use requirements, and even cause rework or scrapping.
[Measures] Adopt a reasonable welding sequence and select appropriate welding specifications and operation methods, and also adopt anti-deformation and rigid fixing measures.
6. Multi-layer welding is not continuous, and the temperature between layers is not controlled.
[Phenomenon] When welding thick plates in multiple layers, the temperature between layers is not controlled. If the interval between layers is too long, cold cracks are easily generated between layers if the welding is not preheated again. If the interval is too short and the temperature between layers is too high (over 900°C), the performance of the weld and the heat-affected zone will also be affected, resulting in coarse grains, reduced toughness and plasticity, and potential hidden dangers for the joints.
[Measures] When welding thick plates in multiple layers, the temperature between layers should be controlled more strictly. During the continuous welding process, the temperature of the base material to be welded should be checked to keep the temperature between layers consistent with the preheating temperature as much as possible, and the maximum temperature between layers should also be controlled. The welding time should not be too long. If the welding is interrupted, appropriate post-heating and insulation measures should be taken. When welding again, the re-preheating temperature should be appropriately higher than the initial preheating temperature.
7. The welding of the lower layer is carried out without removing the welding slag and defects on the surface of the multi-layer weld
[Phenomenon] When welding thick plates in multiple layers, the welding of the lower layer is carried out directly without removing the welding slag and defects after each layer is completed, which is easy to cause defects such as slag inclusion, pores, cracks, etc. in the weld, reducing the connection strength, and at the same time causing spatter during the welding of the lower layer.
[Measures] When welding thick plates in multiple layers, each layer should be welded continuously. After each layer of weld is completed, the welding slag, weld surface defects and spatter should be removed in time. If there are defects such as slag inclusion, pores, cracks, etc. that affect the welding quality, they should be thoroughly removed before welding.
8. The weld angle size of the joint butt or corner butt combination weld that requires full penetration is not enough
[Phenomenon] The weld leg size of the butt or corner butt combination weld that requires full penetration, such as T-joints, cross joints, corner joints, etc., is not enough, or the weld leg size of the web and upper wing edge connection weld of the crane beam or similar components with fatigue verification requirements is not enough, which will make the welding strength and stiffness fail to meet the design requirements.
【Measures】For butt joints such as T-joints, cross joints, and corner joints that require full penetration, sufficient weld leg requirements must be met in accordance with design requirements. Generally, the weld leg size should not be less than 0.25t (t is the thickness of the thinner plate at the joint). The weld leg size of the crane beam or similar web and upper flange connection welds designed with fatigue verification requirements is 0.5t, and should not be greater than 10mm. The allowable deviation of the welding size is 0-4 mm.
9. Welding the welding rod head or iron block in the joint gap
【Phenomenon】Since it is difficult to melt the welding rod head or iron block into one with the welded part during welding, welding defects such as incomplete fusion and incomplete penetration will occur, reducing the connection strength. If rusty welding rod heads or iron blocks are used for filling, it is difficult to ensure that the material is consistent with the parent material; if welding rod heads or iron blocks with oil stains, impurities, etc. are used for filling, defects such as pores, slag inclusions, and cracks will occur in the weld. These situations will greatly reduce the quality of the weld of the joint and fail to meet the quality requirements of the design and specifications for the weld.
【Measures】<1> When the assembly gap of the workpiece is large, but does not exceed the specified allowable range, and the assembly gap exceeds 2 times the thickness of the thin plate or is greater than 20mm, the surfacing method should be used to fill the concave part or reduce the assembly gap. It is strictly forbidden to use the method of filling the electrode head or iron block 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, and the size of the parts should be controlled. Do not increase the gap to ensure the external dimensions.
10. When using plates of different thicknesses and widths for docking, the transition is not smooth
【Phenomenon】When using plates of different thicknesses and widths for docking, do not pay attention to whether the thickness difference of the plate is within the standard allowable range. If it is not within the allowable range and no smooth transition is performed, the weld is prone to stress concentration and welding defects such as lack of fusion at the place where the thickness of the thin plate is higher, affecting the welding quality.
【Measures】When the relevant regulations are exceeded, the weld should be welded into a slope, and the maximum allowable slope should be 1:2.5; or one or both sides of the thickness should be processed into a slope before welding, and the maximum allowable slope should be 1:2.5. When the structure directly bears dynamic loads and needs fatigue verification, the slope should not be greater than 1:4. When plates of different widths are butt-jointed, thermal cutting, mechanical processing or grinding wheel grinding should be used according to factory and site conditions to make a smooth transition, and the maximum allowable slope value of the connection is 1:2.5.
11. Pay no attention to the welding sequence for components with cross welds
【Phenomenon】For components with cross welds, pay no attention to reasonably arranging the welding sequence by analyzing the influence of welding stress release and welding stress on component deformation, but weld randomly in the vertical and horizontal directions, resulting in mutual restraint between the vertical and horizontal seams, generating large temperature shrinkage stress, causing plate deformation, uneven plate surface, and possible cracks in the welds.
【Measures】For components with cross welds, a reasonable welding sequence should be formulated. When there are several cross welds, the transverse weld with larger shrinkage deformation should be welded first, and then the longitudinal weld. In this way, the transverse weld will not be constrained by the longitudinal weld when welding, so that the shrinkage stress of the transverse weld can be released without constraint, which can reduce welding deformation and ensure the quality of the weld. Alternatively, the butt weld should be welded first and then the fillet weld.
12. When the lap joint of the steel bar is welded by circumference welding, continuous welding should be performed at the corner.
[Phenomenon] When the lap joint of the steel bar and the continuous plate is welded by circumference welding, the welds on both sides of the bar are welded first, and then the end welds are welded, and the welding is not continuous. Although this is beneficial to reducing welding deformation, it is easy to produce stress concentration and welding defects at the corner of the bar, affecting the quality of the welded joint.
[Measures] When the lap joint of the steel bar is welded by circumference welding, it should be welded continuously at the corner at one time. Do not weld to the corner and then run to the other side to weld.
13. Equal strength butt joint is required, and no arc starter plate and lead-out plate are provided at both ends of the crane beam flange and web
[Phenomenon] When welding butt welds, full penetration fillet welds, and welds between crane beam flanges and webs, no arc starter plate and lead-out plate are provided at the arc starter and lead-out points. In this way, when welding the start and end ends, the current and voltage are not stable enough, and the temperature of the start and end points is not stable enough, which easily leads to defects such as incomplete fusion, incomplete penetration, cracks, slag inclusions, and pores at the start and end ends of the welds, reducing the weld strength and failing to meet the design requirements.
[Measures] When welding butt welds, full penetration fillet welds, and welds between crane beam flanges and webs, arc starter plates and lead-out plates should be provided at both ends of the welds. Their function is to lead the parts at both ends that are prone to defects to the outside of the workpiece, and then cut off the defective parts to ensure the quality of the welds.
Post time: Dec-02-2024
