Unprecedented sharing of welding experience, you have certainly never seen such a complete one!

【Summary Description】 In the welding process, there are many things that need to be paid attention to. Once neglected, it may make a big mistake. So for SQE, if you audit the welding process, you must pay attention to these points. If you deal with welding quality accidents, you should still pay attention to these problems! 1, welding construction does not pay attention to choose the best voltage [Phenomenon] 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, and defects such as undercut, pores and splash 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, short arc operation should be adopted in order to obtain better penetration during bottoming welding, and arc voltage can be appropriately increased in order to obtain higher efficiency and melting width during filling welding or cover welding. 2. Welding does not control welding current [Phenomenon] 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. [Measures] 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. 3, do not pay attention to the welding speed and welding current, the diameter of the electrode is used in coordination [Phenomenon] When welding, do not pay attention to controlling the welding speed and welding current, and the diameter of the electrode and the welding position are used in coordination. For example, when backing welding the full penetration corner seam, due to the narrow root size, if the welding speed is too fast, 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 surplus 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. [Measures] Welding speed has a significant impact on welding quality and welding production efficiency. When selecting, appropriate welding speed is selected according to welding current, weld position (backing welding, filling welding, cover surface welding), weld thickness and groove size. On the premise of ensuring penetration, easy discharge of gas and welding slag, no burning through and good forming, larger welding speed is selected to improve productivity and efficiency. 4. Do not pay attention to control arc length during welding [Phenomenon] During welding, the arc length shall not be properly adjusted according to the groove form, welding layer number, welding form, electrode type, etc. Due to the improper use of welding arc length, it is difficult to obtain high-quality welds. [Measures] In order to ensure the weld quality, short arc operation is generally adopted during welding, but appropriate arc length can be selected according to different situations to obtain the optimal welding quality. For example, shorter arc should be used for the first layer of V-shaped groove butt joint and angle joint to ensure penetration and avoid undercut. The second layer can be slightly longer to fill the weld. Short arc should be used when the weld gap is small, and the arc can be slightly longer when the gap is large, 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 adopted, attention should be paid to keeping the arc length basically unchanged during the movement, so as to ensure that the melting width and penetration depth of the whole weld are consistent. 5, welding does not pay attention to control welding deformation [Phenomenon] During welding, no attention is paid to controlling deformation from the aspects of welding sequence, personnel arrangement, groove form, selection of welding specifications and operation methods, 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 laminar tearing with 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, if effective deformation control measures are not taken, the installation size of the workpiece will not meet the use requirements, and even cause rework or scrap. [Measures] Adopt reasonable welding sequence and select appropriate welding specifications and operation methods, and also adopt anti-deformation and rigid fixing measures. 6, multi-layer welding discontinuous welding, do not pay attention to control the inter-layer temperature. [Phenomenon] 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 if welding is not preheated again. If the interval is too short and the interlayer temperature is too high (over 900 ℃), it will also affect the performance of the weld and heat affected zone, causing coarse grains, resulting in decreased toughness and plasticity, and leaving potential hidden dangers to the joint. [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 so that the interlayer temperature can be consistent with the preheating temperature as much as possible, and the highest interlayer temperature 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. 7, multi-layer weld without removing welding slag and weld surface defects for lower welding [Phenomenon] 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. [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. 8. The joint butt or fillet butt combined weld required for penetration is not enough. [Phenomenon] For butt or fillet butt combined welds requiring penetration such as T-shaped joints, cross joints, fillet joints, etc., the size of the welding leg is not enough, or the size of the welding leg of the connection weld between the web plate of crane beam or similar components and the upper flange plate flange designed with fatigue checking requirements is not enough, which will make the welding strength and rigidity fail to meet the design requirements. [Measures] Butt combined welds requiring penetration such as T-shaped joints, cross joints and corner joints shall have sufficient weld leg requirements according to the design requirements. Generally, the size of weld leg shall not be less than 0.25t(t is the thinner plate thickness at the joint). 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. 9, welding in the joint gap plug welding rod head or iron block [Phenomenon] As it is difficult to fuse the welding rod 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 and iron block, it is difficult to ensure the material consistency with the base metal. If it is filled with electrode head and iron block with oil stains and impurities, defects such as pores, slag inclusion and cracks will occur in the weld. 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. [Measures] <1> When the assembly gap of the workpiece 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 surfacing 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. When using different thickness and width of the plate butt joint, not smooth transition [Phenomenon] When using plates with different thickness and width, do not 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 is not treated with smooth transition, welding defects such as stress concentration and incomplete fusion are easily caused at the position where the weld is higher than the thickness of the sheet, which will affect the welding quality. [Measures] When the relevant regulations are exceeded, the weld seam 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 slopes 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. 11. Do not pay attention to the welding sequence for components with cross welds [Phenomenon] For components with cross welds, the welding sequence is not reasonably arranged by analyzing the release of welding stress and the influence of welding stress on the deformation of components. Instead, the vertical and horizontal welds are randomly welded. As a result, the vertical and horizontal welds will be constrained to each other, resulting in large temperature shrinkage stress, plate deformation, uneven plate surface and possible cracks in welds. [Measures] For components with cross welds, a reasonable welding sequence should be established. When several kinds of vertical and horizontal cross welds are welded, the transverse seam with large shrinkage deformation should be welded first, and the longitudinal weld should be welded later, so that the transverse weld will not be constrained by the longitudinal weld, so that the shrinkage stress of the transverse seam will be released without constraint, which can reduce the welding deformation and ensure the quality of the weld, or weld the butt weld first and then weld the fillet weld. 12. When lap joint of section steel bar adopts surrounding welding, continuous welding shall be applied at the corner. [Phenomenon] When surrounding welding is adopted for the lap joint between the section steel bar and the continuous plate, the weld on both sides of the bar shall be welded first, and the weld at the end shall be welded later, and discontinuous welding shall be carried out. Although this is beneficial to reduce welding deformation, it is easy to produce stress concentration and welding defects at the corner of the rod, which affects the quality of the welded joint. [Measures] When the lap joint of section steel bar is welded around, it should be welded continuously at the corner. Do not weld to the corner and run to the other side to weld. 13, require equal strong docking, crane beam wing plate and web plate at both ends without arc plate and lead plate [Phenomenon] When welding butt welds, full penetration fillet welds, and welds between crane beam flange plates and web plates, no arc plate and lead-out plate are added to the arc striking and lead-out. In this way, when welding the starting and ending ends, due to the unstable current and voltage and the unstable temperature of the starting and ending points, defects such as incomplete fusion, incomplete penetration, cracks, slag inclusion, pores, etc. are easy to occur, reduce the weld strength, can not meet the design requirements. [Measures] When welding butt welds, full penetration fillet welds and welds between crane beam wing plates and web plates, arc lead plates and lead-out plates shall be set at both ends of the welds. Its function is to lead the defective parts at both ends to the outside of the workpiece, and then cut off the defective parts to ensure the quality of the welds.

Previous Page

[Material Class] Have you mastered these basic welding knowledge?

Next Page

Welding knowledge sharing, control and prevention in welding process