Analysis on the quality problems of flanging crack

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Analysis on quality problems of barrel flanging crack

in the process of steel barrel production, there are various quality problems, but the biggest quality problem affecting the steel barrel is the leakage of the steel barrel. There are many reasons for the leakage problem, among which the flange crack of the barrel body is one of the most common reasons, which directly causes the leakage in the crimping and seam welding triangle area after sealing. In recent years, all domestic barrel factories have been constantly carrying out equipment transformation and process improvement, but due to the relatively unfamiliar production of steel barrels by equipment manufacturers, there are many problems with process equipment. Now, the common problems are analyzed, hoping to throw bricks and attract jade

I. quality problems of raw materials

there are generally two process methods for steel barrel flanging, one is rolling flanging, and the other is die flanging. The two processes are different, but the requirements for raw materials are the same. Due to the performance of raw materials, the quality of flanging can be directly affected. An important factor of flanging crack is the material problem

1. The raw material steel plate shall have good plasticity and surface quality

for flanging process, the raw material shall have good plasticity. In the flanging deformation area, the internal stress of the material is mainly tensile stress, and its deformation is mainly elongation and thickness thinning. When the main deformation part exceeds the forming limit, it will cause fracture. Therefore, the material is required to have good plasticity and stability of plastic deformation. For materials with good plasticity, the allowable forming limit range is large, which can reduce the flanging crack caused by poor material quality. The limit deformation degree during flanging can be analyzed by flanging coefficient, and its value is


where k - flanging coefficient

d0 - barrel diameter before flanging

d1 - maximum diameter of vertical edge after flanging

due to the elongation deformation of the metal in the deformation zone under the action of tangential tensile stress during flanging, the ultimate flanging coefficient mainly depends on the plasticity of the metal material. During flanging, the values of tangential elongation deformation of each point in the radial direction in the deformation area are different, and the maximum elongation deformation occurs at the edge of flanging, so the elongation deformation of the metal at the edge shall be ensured to be less than the allowable limit value of the material during flanging. The maximum elongation deformation on the inner edge of the deformation area during barrel flanging is:

ε= (d1-d2)/d0=1/K-1≤ δ

it can be seen from the above formula that the ultimate flanging coefficient of steel barrel flanging is related to the elongation of material δ In inverse proportion. But in fact, the elongation used in the above formula δ The reason is that the elongation deformation at each point in the diameter direction in the flanging deformation area is different: the elongation deformation at the edge is the largest, and the elongation deformation at each point decreases rapidly with the increase of the distance from the edge. Therefore, the most vulnerable part to fracture is the edge of flanging, which largely depends on the elongation of the material

it is the chemical composition, metallographic structure and mechanical properties that affect the plasticity of materials. Generally speaking, the increase of the content of carbon, silicon, phosphorus and sulfur in steel will eventually reduce the plasticity and increase the brittleness of the material. The carbon content has the greatest impact on the plasticity of the material. It is generally believed that low carbon steel with a carbon content of no more than 0.05~0.15% has good plasticity. Steel with silicon content less than 0.37% has little effect on plasticity, but steel with carbon content greater than this value will become hard and brittle. When sulfur is combined with manganese or iron in steel, it appears in the form of sulfide, which seriously affects the hot rolling properties of steel materials. Sulfide promotes the formation of strip structure and reduces the plasticity. Therefore, high-quality low-carbon steel plates shall be used for the production of steel drums

plates used to manufacture steel drums shall have good surface quality. If there are scratches, pitting, scratches, pores, shrinkage cavities and other defects on the surface, or there is delamination on the material section, stress concentration will occur at the defective part during flanging, resulting in fracture

2. Heat treatment of materials

materials for barrel making shall be subject to necessary heat treatment before the raw materials leave the factory to improve their processing performance. If the material is not well heat treated, it will have a greater hardness, resulting in brittle fracture during processing. Low temperature annealing is generally adopted for low carbon steel used for barrel making, with heating temperature of 600~650 ℃ and cooling in air. If the low-temperature annealing can not get good results, the high-temperature annealing can be adopted. The material can be heated to 700~780 ℃ for 20~40 minutes and then cooled in the air

II. Pre treatment of welding edge

before seam welding, the welding edge of barrel body generally needs to be pre treated. Most barrel factories adopt traditional edge grinding processes, and some adopt edge milling and other processes. However, no matter what process is adopted, the purpose is to check whether the online selection sensor is correctly selected; The rust, dirt and other impurities on the steel plate surface shall be removed to facilitate the steel plate surface to have good conductivity during welding, reduce false welding and missing welding, and enhance the welding strength. However, in actual production, the treatment of welding edge is often unsatisfactory, sometimes the grinding temperature is too high, resulting in surface blackening; Sometimes surface impurities still exist; There is still some metal zinc on the surface of some galvanized sheets. The most serious problem is that sometimes the grinding range is too large, resulting in corner missing, as shown in Figure 1. All these defects can lead to poor strength during welding, especially in case of lack of angle, the two ends of the barrel body may have defects of not being overlapped. Of course, such quality welds cannot bear the effect of flanging force during flanging, and flanging cracks are inevitable

therefore, in order to reduce the defects of flanging crack, the quality problem of weld edge pretreatment must be solved first in the production process

III. quality problems in the seam welding process

the quality defects in the seam welding process are mainly insufficient welding strength, too deep indentation, burn through, etc. these defects are the main causes of flanging cracks. These defects are mainly related to the unreasonable selection of welding parameters. The main parameters affecting welding quality are welding current, electrode pressure, welding time, rest time, welding speed and welding wheel diameter

1. Welding current

the heat required for weld nugget formation in seam welding is generated by using the current through the resistance of the welding area. Under the given conditions of other conditions, the welding current determines the penetration and overlap of nuggets. During the seam welding of steel barrel, the average penetration rate of nugget is 30~70% of the thickness of steel plate, and 45~50% is the best. In order to obtain high strength weld nugget overlap shall not be less than 15~20%. When the welding current exceeds a certain value, increasing the current continuously can only increase the penetration rate and overlap of nuggets, but will not improve the joint strength, which is uneconomical. If the current is too small, there will be defects such as too deep indentation and weld burn through

2, electrode pressure

electrode pressure has great influence on nugget size during seam welding. Too high electrode pressure will make the indentation too deep and accelerate the deformation and loss of the welding roller. Insufficient pressure will cause shrinkage cavity and shorten the service life of the welding roller due to excessive contact resistance

3. Welding time and rest time

in seam welding, the size of nugget industry is mainly controlled by welding time, and the overlap is controlled by cooling time. At low welding speed, the ratio of welding to rest time is 1.25:1~2:1, which can obtain satisfactory results. When the welding speed increases, the spacing of welding points increases. At this time, in order to obtain welds with the same overlap, this proportion must be increased. Therefore, at 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, thus affecting the heating and heat dissipation of the weld. When the welding speed increases, the welding current must be increased in order to obtain sufficient heat. Excessive welding speed will cause burns on the surface of the steel barrel and adhesion of the welding wheel. Therefore, even if external water cooling is used, the welding speed will be limited

at present, three schemes of high speed, medium speed and low speed can be adopted according to the different degree of automation of welding machines in China. When moving the barrel body manually, medium speed is usually used to facilitate the alignment of the predetermined weld position. During automatic welding, if the capacity of the welding machine is sufficient, high speed or higher speed can be adopted. If the capacity of the welding machine is not enough, and the welding width and penetration cannot be guaranteed without reducing the speed, the low speed can only be used

during seam welding of galvanized steel barrel, attention shall be paid to prevent cracks. The reason for the crack is that the residual zinc in the nugget and the zinc diffused to the heat affected zone make the weld embrittled and caused by stress. The method to prevent crack is to select the process parameters correctly. Practice has proved that the smaller the penetration rate (10~26%), the less the crack defects. When the seam welding speed is high, the heat dissipation condition is poor, the surface is overheated, and the penetration is large, which is easy to produce cracks. Generally, under the condition of ensuring nugget diameter and joint strength, low current, low welding speed and strong external water cooling shall be selected as far as possible

IV. quality problems of flanging process

the last process that affects the flanging quality is the flanging process, which is also one of the most important processes. At present, the flanging process of the steel barrel manufacturing line in the domestic barrel industry has roughly two process methods: one is rolling flanging; One is flanging the die. Although the two flanging methods are different, the results are basically the same. For the materials at the flanging part, they are all the deformation caused by thinning and stretching. In addition to the other reasons analyzed above, the main problem affecting the quality is the process technology of flanging process. From the perspective of actual production, flanging crack is related to flanging angle, arc degree, flanging speed, etc. 3. Technical training: during equipment commissioning

according to the different steel drum production lines, some require the flanging angle (the included angle between the flanging part and the barrel body) to be 90 ° (i.e. vertical flanging), and some require it to be greater than 90 ° (such as 95 ° or 100 °). In comparison, the larger the flanging angle is, the smaller the flanging edge diameter is, the smaller the relative deformation is, and it is not easy to crack when flanging; For vertical flanging, the flanging edge has the largest diameter, the largest deformation and the most prone to fracture. In terms of the manufacturing process of steel barrels, many crimping and sealing machines need to press the barrel cover and barrel body closely before crimping. In order to make the barrel body flanging closer to the barrel cover, generally speaking, it is disadvantageous that the flanging is too straight (90 °). A larger flanging angle is better for the flanging edge to be close to the end

after flanging, the connecting part between the barrel body and the flanging part generally has a certain arc transition, and the arc radius is sometimes the main factor leading to flanging crack. When the arc radius is larger, the deformation force is relieved, the deformation time is relatively prolonged, which is conducive to the uniform transmission of flanging force, and the fracture is not easy to occur; If the arc radius is too small, the deformation comes suddenly, the deformation force is not transmitted quickly, and the deformation time is too short, the fracture is easy to occur. In production practice, there are many flanging cracks caused by this reason, which must be paid attention to

the time of flanging process is also one of the factors affecting flanging crack. The equipment of some manufacturers is die stamping flanging, which has a fast process, short deformation time, and easy transfer of deformation force, so the fracture is easier to occur; Some equipment

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