During the welding process of steel structure, there are many matters to be paid attention to. Once neglected, it may lead to a big mistake. The following content is the items that are often ignored in the process of steel structure welding, don't be careless

Welding construction does not pay attention to choose the best voltage

phenomenon, danger

When welding, regardless of the bottoming, filling, and covering, regardless of the size of the groove, the same arc voltage is selected. In this way, the required penetration depth and fusion width may not be met, and 

defects such as undercut, pores, and splashes may occur.

Prevention

Generally, according to different situations, the corresponding long arc or short arc should be selected respectively to obtain better welding quality and work efficiency. For example, short arc operation should be used in order to obtain better penetration during bottom welding; In order to obtain higher efficiency and melting width, the arc voltage can be appropriately increased

Do not pay attention to controlling the arc length during welding

phenomenon, danger

During welding, the arc length is not properly adjusted according to the groove form, number of welding layers, welding form, electrode type, etc. Due to improper use of welding arc length, it is difficult to obtain high-quality welds.

Prevention

In order to ensure the quality of the weld seam, 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 fillet joint should use The arc is shorter to ensure penetration without undercutting; the second layer can be slightly longer to fill the weld. When the weld seam gap is small, it is advisable to use a short arc. When the gap is large, the arc can be slightly longer, and the welding speed can be accelerated. The arc of overhead welding 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, low current and short arc welding should also be used. In addition, no matter what kind of welding is used during the movement, attention should be paid to keeping the arc length basically unchanged, so as to ensure that the fusion width and penetration depth of the entire weld are consistent.

Insufficient fillet size of joint butt or corner butt combination welds requiring penetration

phenomenon, danger

T-joints, cross joints, corner joints and other butt or corner butt combined welds that require penetration, the size of the weld leg is not enough, or the web of the crane girder or similar components that require fatigue checking is designed to connect with the edge of the upper flange If the size of the weld leg of the weld is not enough, the strength and stiffness of the weld will not meet the design requirements.

preventive measures

T-shaped joints, cross joints, fillet joints and other butt combined welds that require penetration shall have sufficient fillet requirements in accordance with the design requirements. Generally, the fillet size shall not be less than 0.25t (t is the thinner part of the joint). plate thickness). For crane girders or similar webs connected to the upper flange with fatigue checking requirements, the weld leg size is 0.5t and should not be greater than 10mm. The allowable deviation of welding size is 0~4mm.

Multi-layer welds do not remove welding slag and weld surface defects before performing lower-layer welding

phenomenon, danger

In multi-layer welding of thick plates, the lower layer is welded directly without removing the welding slag and defects after each layer is welded, which will easily cause defects such as slag inclusions, pores, cracks, etc. in the weld, reduce the connection strength, and cause spatter during the lower layer welding .

Prevention

When welding multiple layers of thick plates, each layer should be welded continuously. After each layer of weld is welded, the welding slag, weld surface defects and spatter should be removed in time, and the defects such as slag inclusions, pores and cracks that affect the welding quality should be completely removed before welding.

Welding does not control welding current

phenomenon, danger

During welding, in order to speed up the progress, the butt welds of medium and thick plates are not beveled. The strength index drops, or even fails to meet the standard requirements, and cracks appear during the bending test, which will make the performance of the welded joints unable to be guaranteed and pose a potential hazard to structural safety.

Prevention

When welding, it should be controlled according to the welding current in the process evaluation, and 10~15% fluctuation is allowed. The size of the blunt edge of the groove should not exceed 6mm. When docking, when the thickness of the plate exceeds 6mm, a bevel must be opened for welding.

Multi-layer welding discontinuous welding, do not pay attention to control the temperature between layers

phenomenon, danger

When welding thick plates with multiple layers, do not pay attention to interlayer temperature control. If the interval between layers is too long, cold cracks will easily occur between layers if welding is performed without reheating; if the interval is too short, the temperature between layers will be too high. (over 900 degrees Celsius), it will also affect the performance of the weld and the heat-affected zone, which will cause coarse grains, resulting in a decrease in toughness and plasticity, and will leave potential hidden dangers for the joints.

Prevention

When multi-layer welding of thick plates, the control of interlayer temperature should be strengthened. During the continuous welding process, the temperature of the base metal to be welded should be checked so that the interlayer temperature can be kept as consistent as possible with the preheating temperature. To be controlled. The welding time should not be too long. In case of interruption of welding, appropriate afterheating and heat preservation measures should be taken. When welding again, the reheating temperature should be appropriately higher than the initial preheating temperature.

Welding does not pay attention to control welding deformation

phenomenon, danger

When welding, do not pay attention to controlling the deformation from the aspects of welding sequence, personnel arrangement, groove form, welding specification selection and operation method, etc., resulting in large deformation after welding, difficult correction, and increased costs, especially for thick plates and large workpieces, which is difficult to correct , it is easy to cause cracks or lamellar tears with 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, if no effective deformation control measures are taken, the installation size will not meet the requirements for use, and even rework or scrap will result.

Prevention

Adopt a reasonable welding sequence and select appropriate welding specifications and operating methods, and also adopt anti-deformation and rigid fixing measures.

Welding Plug the electrode head or iron block in the joint gap

phenomenon, danger

Since it is difficult to fuse the electrode head or iron block with the workpiece to be welded 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 heads or iron blocks, it is difficult to ensure that it is consistent with the material of the base metal; if there are oil stains and impurities on the electrode heads or iron blocks, it will cause defects such as pores, slag inclusions, and cracks in the weld. These situations will greatly reduce the quality of the weld seam of the joint, which cannot meet the quality requirements of the design and specification for the weld seam.

Prevention

1) When the assembly gap of the workpiece is large, but does not exceed the allowed range of use, and the assembly gap exceeds 2 times the thickness of the thin plate or is greater than 20mm, the surfacing welding method should be used to fill the recessed part or reduce the assembly gap. It is strictly forbidden to use the method of filling the electrode head or iron block repair welding in the joint gap.

2) When processing and scribing parts, attention should be paid to leaving enough cutting allowance and welding shrinkage allowance after cutting to control the size of the parts, and not to increase the gap to ensure the overall size.

When plates of different thickness and width are butt-jointed, the transition is not smooth

phenomenon, hazard

When plates of different thicknesses and widths are used for butt joints, 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 does not make a gentle transition, such welds are likely to cause stress concentration and Welding defects such as lack of fusion occur, affecting welding quality.

Prevention

When it exceeds the relevant regulations, the weld should be welded into a slope, and the maximum allowable value of the 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 value of the slope should be 1:2.5. The slope of the structural slope that bears dynamic load and needs to be checked for fatigue should not be greater than 1:4. When plates of different widths are butt-connected, thermal cutting, machining or grinding wheel grinding should be used according to the factory and site conditions to make a smooth transition, and the maximum allowable slope at the joint is 1:2.5.

Pay no attention to the welding sequence for components with cross welds

phenomenon, danger

For components with intersecting welds, do not pay attention to rationally arrange the welding sequence by analyzing the welding stress release and the influence of welding stress on component deformation, but weld vertically and horizontally randomly, resulting in mutual restraint between vertical and horizontal seams, resulting in large temperature shrinkage Stress will deform the plate, the surface of the plate will be uneven, and it may cause cracks in the weld

Prevention

For components with cross welds, a reasonable welding sequence shall be established. When there are several kinds of vertical and horizontal cross welds to be welded, the transverse seams with larger shrinkage and deformation should be welded first, and then the longitudinal welds should be welded, so that the transverse welds will not be constrained by the longitudinal welds when welding the transverse welds, so that the shrinkage stress of the transverse seams Relieved without restraint to reduce welding distortion, maintain weld quality, or weld butt welds first and then fillet welds.

When the lap joints of section steel rods adopt surrounding welding, continuous welding is applied at the corners

phenomenon, hazard

When surrounding welding is used for lap joints between section steel rods and continuous plates, the welds on both sides of the rods are welded first, and the end welds are welded second, with discontinuous welding. Although this is beneficial to reducing welding deformation, it is prone to stress concentration and welding defects at the corners of the rods, which affects the quality of welded joints.

Prevention

When surrounding welding is used for lap joints of section steel members, it shall be completed by one continuous welding at the corner. Don't weld to a corner and run to the other side to weld.

Equal-strength docking is required, and no arc-starting plate and lead-out plate are provided at both ends of the flange plate of the crane beam and the web plate

phenomenon, danger

When welding butt welds, full-penetration fillet welds, and welds between crane girder flange plates and webs, arc strike plates and lead-out plates are not added at the arc strike and lead-out points, so that when welding starts and ends, due to current and voltage It is not stable enough, and the temperature at the start and end points is not stable enough, which will easily lead to defects such as incomplete fusion, incomplete penetration, cracks, slag inclusions, pores and other defects in the start and end welds, which will reduce the strength of the weld and fail to meet the design requirements.

Prevention

When welding butt welds, full-penetration fillet welds, and welds between crane girder flanges and webs, arc strike plates and lead-out plates should be installed at both ends of the welds. After being drawn out of the workpiece, the defective part is cut off to ensure the quality of the weld

Do not pay attention to the welding speed and welding current, and the diameter of the electrode should be coordinated

phenomenon, hazard

When welding, do not pay attention to controlling the welding speed and welding current; the diameter of the electrode and the welding position should be used in coordination. For example, when performing backing welding on fully penetrated corner joints, due to the narrow root size, if the welding speed is too fast, the root gas and slag inclusions will not have enough time to discharge, which will easily cause defects such as incomplete penetration, slag inclusions, and pores at the root ; During cover welding, if the welding speed is too fast, it is easy to produce pores; if the welding speed is too slow, the weld reinforcement will be too high and the shape will be irregular; Slow, easy to burn through and so on.

Prevention

Welding speed has a significant impact on welding quality and welding productivity. When selecting the welding current, welding seam position (bottom welding, filling welding, cover welding), welding seam thickness, and groove size, select the appropriate welding speed to ensure the welding Permeable, gas and welding slag are easy to discharge, no burn through, and a higher welding speed is selected under the premise of good forming to improve productivity.