Analysis of the quality of barrel flange cracking (2)
Second, the welding edge pretreatment problem
Before seam welding, the welding of the barrel body is generally pre-treated. The traditional edging process adopted by most barrel factories also uses milling and other processes. However, whatever process is adopted, the purpose is to remove the corrosion of the steel plate surface. , dirt and other impurities, in order to facilitate the welding of the surface of the steel can have a good electrical conductivity, reduce the number of Weld and missed welding, and enhance the welding strength. However, in the actual production, the welding edge treatment is often unsatisfactory, sometimes the edge grinding temperature is too high, causing the surface to burn black; sometimes the surface impurities still exist; some galvanized steel sheet surface still remains part of the metal zinc. The most serious is that sometimes the margin is too large, resulting in corner corners, as shown in Figure 1. All of these disadvantages can result in poor strength during welding, especially in the case of missing corners, where the two ends of the barrel may be defective. The quality of the weld, of course, can not withstand the role of the flange when the flanging, there will be inevitable flanging cracks.
Therefore, to reduce the defects of flanging, the first step in the production process is to solve the problem of the quality of the edge pretreatment.
Third, the quality of seam welding process
The quality defect of the seam welding process itself is mainly due to the lack of welding strength, deep indentation, burn-through, etc. These defects are the main cause of flanging cracks. The production of these defects is mainly related to the irrational selection of welding process parameters. The main parameters affecting the welding quality include welding current, electrode pressure, welding time, rest time, welding speed, and welding wheel diameter.
1, welding current
The heat required to form a weld nugget by seam welding is generated by the current through the resistance of the weld zone. Under the conditions given by other conditions, the size of the welding current determines the penetration and overlap of the nugget. In seam welding of steel barrels, the average penetration rate of nuggets is 30 to 70% of the thickness of the steel plate, and 45 to 50% is the best. In order to obtain high strength weld nugget overlap should not be less than 15 ~ 20%. When the welding current exceeds a certain value, continuing to increase the current can only increase the penetration and overlap of the nugget without increasing the seam strength, which is not economical. If the current is too small, defects such as deep indentation and burn-through of the weld can occur.
2, electrode pressure
Electrode pressure during seam welding has a greater influence on the nugget size. Excessive electrode pressure will make the indentation too deep and will accelerate the deformation and wear of the welding wheel. If the pressure is insufficient, a shrinkage cavity will occur, and the life of the welding wheel will be shortened due to the excessive contact resistance.
3, welding time and rest time
During seam welding, the size of the nugget industry is mainly controlled by the welding time, and the overlap amount is controlled by the cooling time. At a lower welding speed, the ratio of welding to rest time is 1.25:1 to 2:1, and satisfactory results can be obtained. When the welding speed increases, the pitch of the solder joints increases. In order to obtain a weld with the same amount of overlap, the ratio must be increased. For this reason, at a higher welding speed, the ratio of welding to rest time should be 3:1. Or higher.
4, welding speed
The welding speed determines the contact area between the welding wheel and the steel plate, and the contact time between the welding wheel and the heating part, and thus affects the heating and heat dissipation of the welding seam. When the welding speed increases, the welding current must be increased in order to obtain sufficient heat. Excessive welding speed can cause burns on the surface of the drum and adhesion of the weld wheel. Therefore, even if external water cooling is used, the welding speed must be limited.
At present, the degree of automation of the domestic root boring machine is different, and three schemes of high speed, medium speed, and low speed can be adopted. When manually moving the barrel, medium speed is often used to facilitate alignment of the predetermined weld position. In automatic welding, if the capacity of the welder is sufficient, high speed or higher speed can be used. If the capacity of the welder is not enough, and if it is not possible to ensure a sufficiently large melt width and penetration without lowering the speed, only low speeds can be used.
When galvanized steel tanks are seam welded, care should be taken to prevent cracking. The reason for the crack is that the zinc that remains in the nugget and spreads to the heat affected zone makes the weld embrittlement and is caused by the stress. The method to prevent cracking is to choose the correct process parameters. Practice has proved that the smaller the penetration rate (10~26%), the fewer crack defects. When the seam welding speed is high, the heat dissipation conditions are poor, the surface is overheated, and the penetration depth is large, so cracks are easily generated. Generally, in the condition of ensuring the diameter of the nugget and the strength of the joint, a small current, a low welding speed, and strong external water cooling should be selected.
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