In the welding process, there are many things that need to be paid attention to. Once neglected, it is possible to make a big mistake. If auditing the welding process, you must pay attention to these points. If you deal with welding quality accidents, then you still have to pay attention to these problems!
1. Welding construction does not pay attention to choose the best voltage
[Phenomena] During welding, the same arc voltage is selected regardless of the size of the groove, whether it is priming, filling, covering, or the size of the groove. In this way, the required penetration depth and width may not be achieved, and defects such as undercuts, pores, and splashes may appear.
[Measures] 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, and the arc voltage can be appropriately increased in order to obtain higher efficiency and penetration width during filler welding or cover welding.
2. Welding does not control welding current
[Phenomena] During welding, in order to speed up the progress, the butt weld of the medium and heavy plate is not grooved. The strength index decreases, 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 the structural safety.
[Measures] During welding, the welding current should be controlled according to the welding current in the process qualification, and a 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, bevel must be opened for welding.
3. Do not pay attention to the welding speed and welding current, and use the electrode diameter in coordination
[Phenomena] When welding, do not pay attention to control the welding speed and welding current, and use the electrode diameter and welding position in harmony. For example, when bottom welding a fully penetrated corner seam, due to the narrow size of the root, if the welding speed is too fast, the root gas and slag inclusions do not have enough time to discharge, which may easily cause defects such as incomplete penetration, slag inclusion, and pores at the root. ; During cover welding, if the welding speed is too fast, it is easy to generate pores; if the welding speed is too slow, the weld height will be too high and the shape will be irregular; when welding thin plates or welds with small blunt edges, the welding speed is too high. Slow, prone to burn through and so on.
[Measures] Welding speed has a significant impact on welding quality and welding production efficiency. The selection should be based on welding current, weld position (backing welding, filler welding, cover welding), weld thickness, and groove size to select appropriate welding On the premise of ensuring penetration, easy discharge of gas and welding slag, no burning through, and good forming, a larger welding speed is selected to improve productivity and efficiency.
4. Do not pay attention to controlling the arc length during welding
[Phenomena] During welding, the arc length is not properly adjusted according to the groove form, the number of welding layers, the welding form, and the electrode type. It is difficult to obtain high-quality welds due to improper use of the welding arc length.
[Measures] In order to ensure the quality of the welding 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, such as V-groove butt, fillet joint first The first layer should use a shorter arc to ensure penetration without undercutting, and the second layer can be slightly longer to fill the weld. When the weld gap is small, a short arc should be used. When the gap is large, the arc can be slightly longer and the welding speed can be accelerated. The overhead welding arc should be the shortest to prevent molten iron from flowing; 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 kind of welding is adopted, attention should be paid to keep the arc length basically unchanged during the movement process, so as to ensure that the fusion width and penetration depth of the entire weld are consistent.
5. Welding does not pay attention to control welding deformation
[Phenomena] When welding, we do not pay attention to controlling the deformation in terms of welding sequence, personnel arrangement, groove form, welding specification selection and operation methods, etc., resulting in large deformation after welding, difficulty in correction, and increased cost, especially for thick plates and large workpieces. Correction is difficult, and mechanical correction can easily cause cracks or laminar tearing. The cost of straightening with flame is high and the operation is not easy to cause the workpiece to overheat. For workpieces with high precision requirements, failure to take effective measures to control deformation will cause the installation size of the workpiece to fail to meet the requirements for use, and even cause rework or scrap.
[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 discontinuous and does not pay attention to controlling the temperature between layers
[Phenomena] When welding thick plates with multiple layers, do not pay attention to the temperature control between layers. If the interval between layers is too long, it is easy to cause cold cracks between layers if welding is performed without re-preheating; if the interval is too short, the temperature between layers is too short. If it is too high (over 900°C), it will also affect the performance of the weld and the heat-affected zone, resulting in coarse grains, resulting in a decrease in toughness and plasticity, leaving potential hidden dangers to the joint.
[Measures] When the thick plate is welded with multiple layers, the control of the 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 as consistent as the preheating temperature. The maximum 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 is performed again, the re-preheating temperature should be appropriately higher than the initial pre-heating temperature.
7. If the multi-layer welding seam does not remove the welding slag and the surface of the welding seam is defective, the lower layer welding shall be carried out.
[Phenomenon] In the case of multi-layer welding of thick plates, the lower layer is welded directly without removing the welding slag and defects after each layer of welding is completed. splash.
[Measures] When welding thick plates with multiple layers, each layer should be welded continuously. After each layer of welds is welded, the welding slag, weld surface defects and spatters should be removed in time, and slag inclusions, pores, cracks and other defects that affect the welding quality should be completely removed before welding.
8. The joint butt or corner butt combination weld that requires penetration is insufficient in size
[Phenomena] T-joints, cross joints, fillet joints and other butt joints or corner butt combination welds that require penetration, the size of the weld foot is insufficient, or the web and upper wings of crane beams or similar components are designed with fatigue checking requirements. The welding leg size of the plate edge connection weld is not enough, so that the strength and stiffness of the welding will not meet the design requirements.
[Measures] Butt joint welds that require penetration, such as T-joints, cross joints, fillet joints, etc., should have sufficient welding leg requirements according to the design requirements. Generally, the size of the welding leg should not be less than 0.25t (t is the connection point) thinner plate thickness). The weld foot size of the connection weld between the crane girder or similar web and the upper flange designed with fatigue checking requirements is 0.5t, and should not be larger than 10mm. The allowable deviation of welding dimensions is 0 to 4 mm.
9. Weld the electrode head or iron block in the joint gap
[Phenomenon] Because 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. Such as filling with rusted electrode tips and iron blocks, it is difficult to ensure that the material of the base metal is consistent; such as filling with electrode tips and iron blocks with oil stains, impurities, etc., it will cause defects such as pores, slag inclusions, and cracks in the weld. All of these conditions will greatly reduce the quality of the weld seam of the joint, failing to meet the quality requirements of the design and specification for the weld seam.
[Measures] <1> When the assembly gap of the workpiece is large, but does not exceed the specified allowable range, and the assembly gap exceeds the thickness of the thin plate by 2 times or 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 for repair welding in the joint gap. <2> When scribing the parts, pay attention to leave enough cutting allowance and welding shrinkage allowance after cutting, control the size of the parts, and do not increase the clearance to ensure the overall size.
10. When using plates of different thicknesses and widths, the transition is not smooth
[Phenomena] When using plates of different thicknesses and widths to butt, 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 smooth transition treatment is not performed, the welding seam will easily cause stress concentration and welding defects such as non-fusion when the thickness of 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 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 slopes before welding, and the maximum allowable slope should be 1:2.5 , the slope of the structural slope that is directly subjected to dynamic load and needs to be checked for fatigue should not be greater than 1:4. When the plates of different widths are butted, thermal cutting, machining or grinding methods should be used to make the transition smoothly according to the factory and site conditions, and the maximum allowable slope value of the connection is 1:2.5.
11. Do not pay attention to the welding sequence of components with cross welds
[Phenomena] For components with crossed welds, the welding sequence is not properly arranged by analyzing the effect of welding stress release and welding stress on the deformation of the components, but the vertical and horizontal welding is randomly performed, resulting in mutual restraint between the vertical and horizontal seams, resulting in large The temperature shrinkage stress will deform the plate, the surface of the plate will be uneven, and it may cause cracks in the weld.
[Measures] For components with cross welds, a reasonable welding sequence should be formulated. When there are several vertical and horizontal cross welds for welding, the horizontal seam with larger shrinkage deformation should be welded first, and then the longitudinal weld is welded, so that the transverse weld will not be constrained by the longitudinal weld, so that the shrinkage stress of the horizontal seam will be reduced. Released without restraint to reduce welding deformation and ensure weld quality, or weld butt welds first and then fillet welds
12. When surrounding welding is adopted for the lap joint of the profiled steel member, continuous welding shall be performed at the corner.
[Phenomena] When the lap joint between the section steel rod and the continuous plate is welded, the welding seam on both sides of the rod member is first welded, and the end welding seam is welded afterwards, and the welding is discontinuous. Although this is beneficial to reduce welding deformation, it is easy to produce stress concentration and welding defects at the corners of the rod, which affects the quality of welded joints.
[Measures] When surrounding welding is used for the lap joint of the section steel member, it should be completed by one continuous welding at the corner, and it should not be welded to the corner and then run to the other side for welding.
13. Equal-strength butt joints are required, and there are no arc-starting plates and lead-out plates at both ends of the crane beam wing plate and the web plate
[Phenomena] When welding butt welds, full penetration fillet welds, and welds between the crane girder flange plate and the web, the arc ignition plate and the lead plate are not added at the arc ignition and lead-out, so that when welding the start and end ends, the Because the current and voltage are not stable enough, and the temperature at the start and end points is not stable enough, it is easy to cause defects such as incomplete fusion, incomplete penetration, cracks, slag inclusions, and pores in the start and end welds, which reduces the strength of the weld and fails to meet the design requirements.
[Measures] When welding butt welds, full penetration fillet welds, and welds between crane girder flanges and webs, arc-starting plates and lead-out plates should be installed at both ends of the weld to prevent After the defective part is drawn out of the workpiece, the defective part is divided to ensure the quality of the weld.
