Looking forward to your favourable inquiry!

×

13 big problems that are easily overlooked in steel structure welding

Date:2019-02-26    View:515      Tag:13 big problems that are easily overlooked in steel structure welding
Steel structure refers to a structure mainly composed of steel materials and is one of the main types of building structures. The structure is mainly composed of steel beams, steel columns, steel trusses and other components made of steel and steel plates, and the joints, bolts or rivets are usually connected between the components or components. Because of its light weight and simple construction, it is widely used in large factories, stadiums, super high-rises and other fields.

In the welding process, there are many things that need to be paid attention to during the welding process. Once negligent, it is possible to make a big mistake.

1, welding construction does not pay attention to choose the best voltage

【phenomenon】

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.

[Measures]

Generally, the corresponding long arc or short arc should be selected for different situations to get better welding quality and working efficiency. For example, in the case of bottom welding, a short arc operation should be used in order to obtain a good penetration depth, and the arc voltage can be appropriately increased in order to obtain higher efficiency and melt width in the case of filler welding or cap welding.

2, welding does not control the welding current

【phenomenon】

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.

[Measures]

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 docking, when the thickness exceeds 6 mm, it is necessary to open the groove for welding.

3, do not pay attention to the welding speed and welding current, the diameter of the electrode is coordinated

【phenomenon】

When welding, care should be taken to control the welding speed and welding current, and the welding rod diameter and welding position are used in coordination. For the bottom penetration of the fully penetrated corner seam, due to the narrow root size, if the welding speed is too fast, the root gas and slag do not have enough time to discharge, which may cause defects such as impermeability, slag inclusion and pores in the root. When the surface is welded, if the welding speed is too fast, the air hole is easy to be generated; if the welding speed is too slow, the weld height will be too high and the shape will be irregular; when welding a thin plate or a weld with a small blunt edge, the welding speed is too high. Slow, prone to burn and so on.

[Measures]

The welding speed has a great influence on the welding quality and the welding production efficiency. When selecting, the welding current, the weld position (bottom welding, filler welding, cover welding), the thickness of the weld, and the groove size are selected. Ensure penetration, gas and welding slag are easy to discharge, do not burn through, and form a large welding speed on the premise of good forming to improve productivity.

4, do not pay attention to control arc length when welding

【phenomenon】

The arc length is not properly adjusted according to the groove form, the number of welded 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.

[Measures]

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. Short arcs should be used for weld gaps. When the gap is large, the arc can be slightly longer and the welding speed is faster. 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 taken, it is necessary to keep the arc length constant during the movement to ensure that the entire weld has the same width and penetration.

5, welding does not pay attention to control welding deformation

【phenomenon】

During welding, care should be taken not 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, which may result in the installation dimensions of the workpiece not meeting the requirements for use, or even causing rework or scrapping.

[Measures]

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

6, multi-layer welding is not continuous welding, do not pay attention to control the interlayer temperature

【phenomenon】

When thick plate multi-layer welding, do not 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 time is too short, the interlayer temperature is too high ( More than 900 ° C), will also affect the performance of the weld and heat affected zone, resulting in coarse grain, resulting in decreased toughness and plasticity, leaving potential hidden dangers to the joint.

[Measures]

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.

7. The multi-layer weld does not remove the welding slag and the surface of the weld is defective, and the lower layer is welded.


【phenomenon】

In the multi-layer welding of thick plates, the lower layer welding is directly carried out without removing the welding slag and defects after the completion of each layer welding, which may cause defects such as slag inclusion, pores and cracks in the weld seam, reduce the joint strength, and cause splashing during the lower layer welding. .

[Measures]

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

8, the required penetration of the joint butt or angle butt joint weld angle is not enough

【phenomenon】

T-joints, cross-joints, angle joints, etc., which require penetration butt or angle butt joint welds, whose weld feet are not sufficiently sized, or the webs of crane beams or similar members designed with fatigue check requirements are connected to the upper wing edges Insufficient solder fillet size can make the strength and stiffness of the solder meet the design requirements.

[Measures]

T-joints, cross-joints, angle joints, etc., which require penetration of the butt joint welds, shall have sufficient weld foot requirements according to the design requirements. The general weld foot size shall not be less than 0.25t (t is the thinner joint) Board thickness). The size of the soldering foot 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 be greater than 10mm. The tolerance of the welding size is 0 to 4 mm.

9. Soldering the electrode head or iron block in the joint gap

【phenomenon】

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 it is filled with a welding rod head or iron block with oil, 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.

[Measures]

(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.

10. When the plates of different thicknesses and widths are docked, the transition is not smooth.

【phenomenon】

When using plates of different thicknesses and widths, it is not necessary to pay attention to whether the difference in thickness of the plates is within the standard allowable range. If it is not within the allowable range and does not undergo a smooth transition treatment, the weld seam is likely to cause stress concentration and unwelded welding defects at a height above the thickness of the sheet, which affects the quality of the weld.

[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 a slope before welding, and the maximum allowable slope value is 1:2.5, when directly The slope of the structural slope that is subjected to dynamic loads and requires fatigue checking shall not be greater than 1:4. When the plates of different widths are butt jointed, they should be gently cut according to the factory and site conditions by means of hot cutting, machining or grinding. 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-weld joints, care should be taken to analyze the welding stress release and the influence of welding stress on the deformation of the components, and the welding sequence can be arranged reasonably, but the vertical and horizontal random welding can result in the vertical and horizontal seams constraining each other and causing large temperature shrinkage. The stress causes the plate to deform, the surface of the plate is uneven, and it is possible to cause cracks in the weld.

[Measures]

For components with cross welds, a reasonable welding sequence should be established. When there are several kinds of vertical and horizontal cross welds, the transverse joints with large shrinkage deformation should be welded first, and then the longitudinal welds should be welded, so that the transverse welds are not restrained by the longitudinal welds, so that the shrinkage stress of the transverse joints Release without restraint, reduce weld distortion, ensure weld quality, or weld fillet welds after butt welds.

12. When the steel lap joint is welded, it is continuously welded at the corner.

【phenomenon】

When the steel bar and the continuous plate lap joint are used for the surrounding welding, the welded joints on both sides of the first welding rod are used, and the welded ends of the welded ends are not continuously welded. Although this is advantageous for reducing the welding deformation, stress concentration and welding defects are easily generated at the corners of the rod, which affects the quality of the welded joint.

[Measures]

When the steel lap joint is welded, it should be welded once at the corner. Do not weld to the corner and run to the other side for welding.

13. The requirements are strong butt joints, and there are no arcing plates and lead plates at both ends of the crane beam and the web.

【phenomenon】

In the welding butt weld, the full penetration fillet weld, the weld flange of the crane beam flange and the web, no arc runner and lead plate are added at the arc start and exit, so that the current and voltage are at the start and end of the weld. It is not stable enough, and the temperature at the starting and ending points is not stable enough. It is easy to cause defects such as unmelted, unpenetrated, cracked, slag, and pores in the starting and ending end welds, which reduces the weld strength and fails to meet the design requirements.

[Measures]

In the welding butt weld, the full penetration fillet weld and the weld of the crane beam wing and the web, the arc runner and the lead plate shall be provided at both ends of the weld, the function of which is to make the defects at both ends easy to produce defects. After being introduced to the outside of the workpiece, the defect is divided to ensure the quality of the weld.

COPYRIGHT©2011~2018 DMH UNITED STEEL INDUSTRY CO.,LTD All Rights Reserved. www.united-steel.com    Links