How to explain the causes of defects and how to prevent them
It is the basic skill of NDT personnel to understand and explain the image defects on the negative film, but how to explain to peers or welder friends how the welding defects are produced? How to prevent during welding? NDT personnel are required to learn relevant welding knowledge to help them understand and explain the formation and prevention of defects. Now the welfare has come. The editor has collected and summarized some causes of welding defects and preventive measures, which can be used as reference for the national NDT friends.
1. Do not pay attention to choose the best voltage for welding construction
[phenomenon]
During welding, the same arc voltage shall be selected regardless of the backing, filling, covering and groove size. In this way, the required penetration and width may not be achieved, and defects such as undercut, porosity and spatter may occur.
[measures]
Generally, according to different situations, the long arc or the short arc should be selected respectively to get better welding quality and work efficiency. For example, in order to get better penetration, short arc operation should be used in backing welding, and in order to get higher efficiency and width of fusion, arc voltage can be increased properly in filling welding or cover welding.
2 welding does not control welding current
[phenomenon]
During welding, in order to speed up the progress, the butt weld of medium and thick plates shall not be beveled. The strength index declines, even fails to meet the requirements of the standard, and cracks appear during bending test, which will make the performance of the weld joint unable to be guaranteed and pose a potential hazard to the structural safety.
[measures]
During welding, the welding current 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 shall not exceed 6mm. When butt joint, when the plate thickness is more than 6mm, groove shall be made for welding.
3. Do not pay attention to the coordinated use of welding speed, welding current and electrode diameter
[phenomenon]
During welding, the welding speed, welding current, electrode diameter and welding position shall not be controlled. For example, when backing welding the full penetration fillet weld, due to the narrow root size, such as too fast welding speed, insufficient time to discharge the gas and slag at the root, it is easy to cause defects such as incomplete penetration, slag inclusion and air hole at the root; when covering welding, such as too fast welding speed, it is also easy to produce gas hole; if the welding speed is too slow, the weld reinforcement will be too high and the shape will be irregular; when welding thin plate or blunt edge When the size of the weld is small, the welding speed is too slow, and it is easy to burn through.
[measures]
The welding speed has a significant impact on the welding quality and welding production efficiency. When selecting, the welding current, weld position (backing welding, filling welding, cover welding), weld thickness, groove size shall be selected with appropriate welding speed. On the premise of ensuring the penetration, gas and welding slag are easy to discharge, not burn through, and good forming, the larger welding speed shall be selected to improve the productivity efficiency.
4. Do not pay attention to control the arc length during welding
[phenomenon]
During welding, the arc length shall not be adjusted properly according to groove form, number of welding layers, welding form, electrode model, 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, the short arc operation is usually adopted during welding, but the appropriate arc length can be selected according to different situations to obtain the optimal welding quality. For example, the short arc shall be used for the first layer of V-groove butt joint and corner joint to ensure the penetration and avoid undercut. The second layer can be slightly longer to fill the weld. The short arc should be used when the gap is small. When the gap is large, the arc can be slightly longer and the welding speed can be accelerated. The arc of overhead welding shall be the shortest to prevent molten iron from flowing down; in order to control the temperature of weld pool, small current and short arc welding shall also be used in vertical welding and horizontal welding. In addition, no matter what kind of welding is adopted, the arc length shall be kept basically unchanged during the movement, so as to ensure that the weld width and penetration are consistent.
5. Welding deformation is not controlled
[phenomenon]
During welding, the deformation is not controlled from the aspects of welding sequence, personnel arrangement, groove form, selection of welding specification and operation method, resulting in large deformation after welding, difficulty in correction and increased cost, especially for thick plates and large workpieces, which are difficult to be corrected, and it is easy to cause cracks or lamellar tears by mechanical correction. The cost of flame correction is high and the operation is not good, which is easy to cause overheating of workpiece. If we do not take effective deformation control measures for workpieces with high precision requirements, the installation dimension of workpieces will not meet the use requirements, or even cause rework or scrap.
[measures]
Reasonable welding sequence, proper welding specification and operation method, anti deformation and rigid fixing measures shall be adopted.
6. The multi-layer welding is discontinuous, and the interlayer temperature is not controlled
[phenomenon]
When welding thick plates in multiple layers, pay no attention to the temperature control between layers. If the interval between layers is too long, cold cracks will be produced easily when welding without re preheating; if the interval is too short and the temperature between layers is too high (over 900 ℃), the performance of the weld and the heat affected zone will also be affected, which will cause the grain coarseness, reduce the toughness and plasticity, and leave potential hazards to the joint.
[measures]
When the thick plate is welded in multiple layers, the control of the interlayer temperature shall be strengthened. During the continuous welding process, the temperature of the welding base metal shall be inspected to make the interlayer temperature as consistent as possible with the preheating temperature, and the maximum interlayer temperature shall also be controlled. The welding time shall not be too long. In case of welding interruption, appropriate post heating and heat preservation measures shall be taken. When welding again, the re preheating temperature shall be appropriately higher than the initial preheating temperature.
7. For multi-layer welds, the lower layer welding shall be carried out without removing welding slag and defects on the weld surface
[phenomenon]
When the thick plate is welded in multiple layers, the lower layer is directly welded without removing the welding slag and defects after each layer is welded, which is easy to cause slag inclusion, air hole, crack and other defects in the weld, reduce the connection strength, and at the same time cause splashing during the lower layer welding.
[measures]
When thick plates are welded in multiple layers, each layer shall be continuously welded. After the completion of each layer of welding, the welding slag, weld surface defects and splashes shall be removed in time. If any slag, air hole, crack and other defects affecting the welding quality are found, the welding shall be carried out after thorough removal.
8. The weld fillet size of butt joint or combination of fillet butt weld requiring penetration is not enough
[phenomenon]
The size of welding foot of butt joint or corner butt joint combination weld requiring penetration, such as T-joint, cross joint and corner joint, is not enough, or the size of welding foot of connecting weld between web and upper flange edge of crane beam or similar components designed with fatigue checking calculation requirements is not enough, which will make the strength and rigidity of welding not meet the design requirements.
[measures]
For butt combination welds requiring penetration, such as T-joints, cross joints, fillet joints, etc., there must be enough weld legs according to the design requirements. Generally, the weld leg size shall not be less than 0.25t (t is the thinner plate thickness at the joint). The weld leg size of crane beam or similar web and upper flange connection weld designed with fatigue checking calculation requirements is 0.5T, and shall not be greater than 10 mm. The allowable deviation of welding dimension is 0-4 mm.
9. Plug the electrode head or iron block in the joint gap
[phenomenon]
Because it is difficult to melt the electrode head or iron block into a whole with the weldment during welding, it will cause welding defects such as lack of fusion, lack of penetration, etc., and reduce the connection strength. For example, it is difficult to ensure the same material with the base metal if the rusty electrode head and iron block are used for filling; for example, if the electrode head and iron block with oil and impurities are used for filling, defects such as porosity, slag inclusion and crack will occur in the weld. These conditions will greatly reduce the weld quality of the joint, which can not meet the design and specification requirements for the weld quality.
[measures]
(1) When the assembly clearance of the workpiece is large, but it does not exceed the allowable range, and the assembly clearance is more than 2 times of the thickness of the thin plate or more than 20 mm, the concave part shall be filled or the assembly clearance shall be reduced by the method of surfacing. It is strictly prohibited to fill the electrode head or iron block in the joint gap for repair welding.
(2) When marking out parts, pay attention to leaving enough cutting allowance and welding shrinkage allowance after cutting, control the size of parts, and do not increase the gap to ensure the overall size.
10. When plates of different thickness and width are used for butt joint, uneven transition
[phenomenon]
When the plates of different thickness and width are used for butt joint, it is not noticed 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 carried out, the weld seam is easy to cause stress concentration and lack of fusion and other welding defects at the position higher than the thickness of the sheet, which will affect the welding quality.
[measures]
In case of exceeding the relevant provisions, the weld shall be welded into a slope, and the maximum allowable slope 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 shall be 1:2.5. When the structural slope directly bears the dynamic load and needs to be checked for fatigue, the slope shall not be greater than 1:4. For butt joint of plates with different width, the method of thermal cutting, machining or grinding wheel shall be adopted according to the conditions of the factory and the construction site to make the transition smooth, and the maximum allowable slope value of the joint is 1:2.5.
11 do not pay attention to the welding sequence for members with cross weld
[phenomenon]
For members with cross weld, the welding sequence shall be arranged reasonably by analyzing the influence of welding stress release and welding stress on the deformation of members, but welding shall be carried out at will vertically and horizontally, which will result in mutual restraint of longitudinal and transverse joints, large temperature shrinkage stress, deformation of plate, uneven surface of plate and possible crack of weld.
[measures]
For members with cross welds, a reasonable welding sequence shall be established. When there are several kinds of vertical and horizontal cross welds, the transverse weld with large shrinkage deformation shall be welded first, and then the longitudinal weld shall be welded, so that the transverse weld will not be constrained by the longitudinal weld, so that the shrinkage stress of the transverse weld can be released without constraint, which can reduce the welding deformation, ensure the weld quality, or weld the butt weld first and then weld the fillet weld.
When the lap joint of 12 type steel bar adopts girth welding, it shall be welded continuously at the corner
[phenomenon]
When the lap joint of section steel member and continuous plate adopts girth welding, the weld on both sides of the member shall be welded first, then the end weld shall be welded, and the welding shall not be continuous. Although it is beneficial to reduce the welding deformation, it is easy to produce stress concentration and welding defects at the corner of the member, which will affect the quality of the welding joint.
[measures]
When the lap joint of section steel member adopts girth welding, it shall be welded continuously at the corner. Do not weld to the corner and run to the other side for welding.
13 equal strength butt joint is required, and no run on plate and run-off plate are set at both ends of the flange plate and web plate of the crane beam
[phenomenon]
When welding the butt weld, full penetration fillet weld, and the weld between the flange plate and web plate of crane beam, no run on plate and run-off plate are added at the run on and run-off points, so that when welding the start and stop ends, the temperature of the start and stop points is not stable enough due to the unstable current and voltage, which may lead to the lack of fusion, incomplete penetration, crack, slag inclusion, air hole and other defects of the start and stop welds, so as to reduce the weld seam The strength can not meet the design requirements.
[measures]
When welding the butt weld, full penetration fillet weld and the weld between the wing plate and web plate of the crane beam, arc striking plate and leading out plate shall be set at both ends of the weld, whose function is to lead the parts easy to produce defects at both ends out of the workpiece, and then cut off the defective parts to ensure the quality of the weld.