In the welding process, there are many things to be aware of in the welding process. Once negligent, it may cause a big mistake. Today, Xiaobian organizes the items that are often overlooked during the welding process of steel structures. Don't care too much.

 

Welding construction does not pay attention to choose the best voltage

Phenomenon, harm

When welding, whether it is bottoming, filling, or covering, regardless of the size of the groove, the same arc voltage is selected. This may not achieve the required penetration, melt width, and defects such as undercuts, pores, and splashes.

Prevention

Generally, for different situations, the corresponding long arc or short arc should be selected to get better welding quality and working efficiency. For example, short arc operation should be used in order to obtain better penetration depth during bottom welding; when filling or cap welding In order to obtain higher efficiency and widening, the arc voltage can be appropriately increased.

 

Inadvertent control of arc length during welding

Phenomenon, harm

The arc length is not properly adjusted according to the groove form, the number of welding layers, the welding form, the welding rod type, etc. during welding. Due to the improper use of the welding arc length, it is difficult to obtain a high quality weld.

Prevention

In order to ensure the quality of the weld, short arc operation is usually used during welding, but the appropriate arc length can be selected according to different conditions to obtain the best welding quality. For example, the V-groove butt joint and the first layer of the corner joint should be used. Shorter arcs to ensure penetration and no undercutting; 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 is increased. The arc welding should be the shortest to prevent the underwater flow of iron; in order to control the temperature of the molten pool during vertical welding and horizontal welding, small current and short arc welding are also required. In addition, no matter what welding is used during the movement, it is necessary to keep the arc length constant, so as to ensure that the entire weld has the same width and penetration.

 

Insufficient joint butt joint or angle butt joint weld angle is not enough

Phenomenon, harm

T-joints, cross-joints, angle joints, etc., which require penetration or angle butt joint welds, which are not sufficiently welded, or the webs of crane beams or similar components designed to meet fatigue calculation requirements are connected to the upper wing flanges. Insufficient solder fillet size can make the strength and stiffness of the solder meet the design requirements.

Preventive measures

T-joints, cross-joints, corner joints, etc., which require penetration of the butt joint welds, shall have sufficient weld foot requirements according to the design requirements. Generally, the weld foot size shall not be less than 0.25t (t is a thinner joint) Board thickness). The size of the welding feet of the crane beam or similar web and the upper flange connecting the welded joint with the fatigue check calculation requirement is 0.5t, and should not exceed 10mm. The tolerance of the welding size is 0~4mm.

 

Multi-layer welds do not remove the weld slag and the weld surface is defective.

Phenomenon, harm

When multi-layer welding of thick plates, the welding of the slag and defects is not carried out after each layer is welded, and the lower layer welding is directly carried out, which may cause defects such as slag inclusion, pores and cracks in the weld, reduce the joint strength, and cause splashing during the lower layer welding. .

Prevention

When thick plate multi-layer welding, each layer should be continuously welded. After each weld is welded, the weld slag, weld surface defects and spatter should be removed in time. It is found that the slag, pores, cracks and other defects that affect the welding quality should be thoroughly removed before welding.

 

Welding does not control the welding current

Phenomenon, harm

When welding, in order to grab the progress, the groove is not opened for the butt weld of the plate. The strength index drops, even if it does not meet the standard requirements, and cracks appear during the bending test, which will make the weld joint performance unguaranteed and pose a potential hazard to the structural safety.

Prevention

Welding should be controlled according to the welding current in the process evaluation, allowing 10 to 15% floating. The blunt edge of the groove should not exceed 6mm. When the butt joint exceeds 6 mm, the groove is to be welded.

 

Multi-layer welding is not continuously applied, and care is not taken to control the interlayer temperature.

Phenomenon, harm

When thick plate multi-layer welding, pay attention to the interlayer temperature control, such as the interval between layers is too long, it is easy to produce cold cracks between layers without reheating; if the interval is too short, the interlayer temperature is too high (More than 900 degrees Celsius), it will also affect the performance of the weld and heat affected zone, which will cause the grain to be coarse, resulting in a decrease in toughness and plasticity, leaving potential hidden dangers to the joint.

Prevention

When multi-layer welding of thick plates, the temperature control between the layers should be strengthened. During the continuous welding process, the temperature of the base metal of the welding should be checked so that the interlayer temperature can be consistent with the preheating temperature as much as possible, and the maximum temperature between the layers is also To 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 re-welding, the reheating temperature should be appropriately higher than the initial preheating temperature.

 

Welding does not pay attention to control welding deformation

Phenomenon, harm

During welding, care is not taken to control the deformation from the welding sequence, personnel layout, groove form, welding specification selection and operation method, resulting in large deformation after welding, difficulty in correction, and increased cost, especially for thick plates and large workpieces. It is easy to cause cracks or lamellar tears by mechanical correction. The use of flame correction for high cost and poor operation is likely to cause overheating of the workpiece. For workpieces with high precision requirements, effective control deformation measures are not taken, the installation dimensions are not up to the requirements of use, and even rework or scrap is caused.

Prevention

Use a reasonable welding sequence and select the appropriate welding specifications and operating methods, as well as anti-deformation and rigid fixing measures.

 

Soldering the electrode head or iron block in the joint gap

Phenomenon, harm

Since it is difficult to fuse the electrode head or the iron block with the workpiece to be welded during welding, welding defects such as unfusion and penetration are caused, and the joint strength is lowered. If it is filled with a rusted electrode head or iron block, it is difficult to ensure that it is consistent with the material of the base material; if there is oil on the electrode head, oil on the iron block, impurities, etc., the weld will cause defects such as pores, slag inclusions and cracks. In all cases, the weld quality of the joint is greatly reduced, failing to meet the quality requirements of the design and specification for the weld.

Prevention

1) When the assembly clearance of the workpiece is large, but does not exceed the specified allowable range, when the assembly gap exceeds the thickness of the thin plate by 2 times or more than 20 mm, the surfacing method is used to fill the recessed portion or reduce the assembly gap. It is strictly forbidden to use the method of filling the welding head or iron block in the joint gap.

2) When scribing parts, pay attention to the remaining allowance and the welding shrinkage after cutting, and control the size of the parts. Do not increase the gap to ensure the size.