New highlights in the galvanized barrel seam welding process

Xin Qiaojuan

With the further improvement of environmental protection awareness, steel drum enterprises must minimize the environmental pollution and serious harm to operators in the production process of steel drums while ensuring the quality of steel drums. Therefore, in the international arena, in addition to the ISO9000 standard quality certification, steel drum enterprises must also pass the ISO14000 environmental standard certification. If the ISO14000 environmental standard is not certified as soon as possible, the steel barrel export will be greatly affected under the severe international environmental protection situation. Therefore, steel drum enterprises must strengthen the innovation of quality awareness and position the product quality standards at the international advanced level. Only in this way can we win by quality. To this end, in order to reduce environmental pollution, we must first edging and seam welding from the previous process of barrel making.

Steel drums must be formed by a series of processes such as coiling and welding. Because the spot welding machine and the seam welding machine have large current and low impedance, the secondary voltage is low, generally not more than 10 volts, and some are up to 20 volts. In this way, the oil stain and scale on the steel barrel body are all poor conductors. Under the action of the electrode pressure, the oxide film and the like are partially broken, and the current field distribution on the steel barrel body plate is changed during the conduction. The current lines in individual parts are dense, and the heat is too concentrated, which may cause burns on the surface of the barrel weld or burn along the outer edge of the weld. At the same time, it is also possible to make the contact faces of the plates different in size under the same electrode pressure. The well-cleaned surface is in good contact with the weld zone, and the metal compression range around the molten core is also expanded. The plastic ring is wider when welded under the same specification, thereby improving the shear strength. Some materials form an alloy wire because the surface oxide film is not cleaned in time. In addition, when some readily soluble impurities enter the core and form some low-melting eutectic with some alloying elements, they cause dangerous defects such as cracks in the core. Since the surface oxide contaminates the electrode, the electrode is easily overheated, and the surface quality of the solder joint is lowered. When the number of electrode repairs is increased, the electrode life is shortened, a large amount of copper alloy is wasted, and the production efficiency is lowered. It can be seen that the surface state has an influence on spot welding, heat deposition during seam welding, core formation, defect generation and electrode heat dissipation, etc. It is directly related to joint strength and production efficiency.

For barrel companies, it is not difficult to make ordinary hot-rolled steel sheets and ordinary cold-rolled steel sheets. However, if a galvanized steel sheet drum is to be produced, it is easy to cause problems. For galvanized steel drums, due to the low melting point of zinc on the surface of the steel sheet, it is easy to melt at the high temperature generated by grinding when edging, and the melted zinc is coated in the gap of the grinding wheel, causing the grinding wheel to slip. In this case, it is impossible to use a grinding wheel shaping knife. This is because: First, the time interval between each shaping is too short, and it needs to be shaped from time to time; the second is that the amount of plastic knives is too large. Production efficiency is extremely low. At this time, some barrel manufacturers use the "milling edge" process, and most manufacturers use the chemical cleaning method, that is, the acid washing method to treat the zinc layer on the surface of the galvanized steel sheet.

In the milling and edging process, the original grinding wheel is changed to an alloy milling cutter, and its working principle is similar to a planer. However, due to the large investment in equipment, the application is not extensive. The zinc layer on the surface of the galvanized steel sheet treated by the pickling method is widely used because of its simple process. However, the zinc layer on the surface of the galvanized steel sheet is removed by pickling, and must be edging afterwards. In this way, in addition to the dust caused by the original edging process, the dust and harmful gases (hydrogen chloride) brought by pickling are added to the environmental protection, which greatly jeopardizes our production environment and personal health. In addition, it has seriously eroded our production equipment.

After pickling, the galvanized steel sheet is ground, and during spot welding and seam welding, the residual acid liquid will decompose the highly toxic chlorine gas in the welded state of the steel sheet. Therefore, environmental protection during the production of galvanized steel drums will become an important issue for steel drum manufacturers.

According to the practical problem of galvanized steel drum production, we have successfully overcome this problem through a large number of experiments and technical breakthroughs, and without edging and pickling, we only pass the galvanized steel sheet through the polishing cloth wheel or polishing wheel. After polishing to remove the oil stain on the soldered surface, the original soldering specification is used to solve the problem during soldering.

Because the melting point of zinc is very low, only 692K, when spot welding, the melting range of the zinc layer on the contact surface between the steel plates is wider than that of the ordinary steel plate, and the coating of the weld zone melts and the extruded zinc and the weld zone are more than 692K. The zinc is simultaneously squeezed into the gap of the steel plate to form a good conductive condition between the two steel plates. The current field is enlarged, and the current density on the contact surface is greatly reduced. When the core diameter is in a critical state, the current slightly fluctuates, and the incomplete penetration occurs. phenomenon. From this point of view, when spot welding a galvanized steel sheet, the welding current IW should be increased. In general, the lower the melting point, the larger the IW should be. In order to break through the zinc layer to make good contact between the contact surfaces during spot welding and to squeeze the molten zinc layer out of the contact surface, the electrode pressure when spot welding galvanized steel sheet is better than spot welding ordinary low carbon steel thin steel sheet The electrode pressure should be increased by 20% to 25%. When the power of the welding machine is insufficient, the welding of the galvanized steel sheet can also be performed with a small current and a low pressure. However, the shunting of pressure and current must be avoided.

The problem of seam welding of galvanized sheet steel is similar to that of spot welding, because the galvanized layer has a wider melting range, the shunt is more serious, and a larger current is required to weld. When the temperature of the welded joint exceeds 1173K, the zinc element diffuses to the heat-affected zone, and the brittleness of the joint structure increases. Under certain conditions, cracks extending from the core to the heat-affected zone are formed. The molten zinc layer on the surface of the barrel of the steel barrel body forms a Cu-Zn alloy with the brazing wheel, which increases the surface resistance of the welding wheel and further increases the heat dissipation. The adhesion is more serious; and the superheating of the surface layer further strengthens the diffusion of zinc into the heat-affected zone, expanding the chance of crack formation. When the vapor of zinc in the core is precipitated from the solidified metal, pores are formed. Although the pores have little effect on the airtightness of the weld, they have a great influence on the formation of cracks, which is a key problem in the seam welding of galvanized steel barrels. In order to reduce cracks, it is first necessary to improve the welding specifications. Tests have shown that the smaller the penetration rate (10% to 20%), the fewer defects such as cracks. When the welding speed is high, the surface of the steel barrel welding zone is overheated, the penetration depth is large, and cracks are prone to occur. Therefore, when welding galvanized steel sheets, in order to meet the conditions that the galvanizing layer is melted to increase the shunt, a small current should be selected as much as possible; the welding speed should be lower, and the cooling is performed using strong external cooling water. The welding wheel is driven by an embossing roller (knurling knife) to correct and clean the surface of the roller at any time to maintain the predetermined roller width.

Through the test and verification, I believe that in the production of galvanized steel drums, the galvanized steel sheets should be subjected to spot cleaning and seam welding without direct pickling and seam welding. The following points must be achieved:

1. The oil stains in spot welding and seam welding must be polished;

2. The welding current during spot welding is increased by 28% compared to the welding current when spot welding ordinary low carbon steel plates;

3. The electrode pressure during spot welding is 22% higher than that of spot welding ordinary low carbon steel plate;

4. The welding current and welding speed during seam welding are the same as those for welding ordinary low carbon steel plates;

5. The electrode pressure during seam welding is increased by 7% compared with the electrode pressure when welding ordinary low carbon steel plates;

6. During seam welding, in addition to cooling inside the equipment, external cooling water must be forced to cool the welded barrel.

The above is some of the experience in our experiments for peer reference.