New highlights of galvanized barrel seam welding process

The steel drums must be formed by a series of processes such as rolling and welding. Because spot welders and seam welders have large currents and low impedance, the secondary voltage is low, generally not more than 10 volts, and individual up to about 20 volts. Such oil on the steel barrel body plate, oxide skin, etc. are all bad conductors, in the role of electrode pressure, the oxide film and other parts of the broken, conductive changes in the steel barrel body plate current field distribution. The individual parts of the current line density, the heat is too concentrated, can easily cause the surface of the barrel welds to burn or burn along the outer edge of the weld. At the same time, it is also possible to make different sizes of contact surfaces between the plates at the same electrode pressure. The well-cleaned surface makes the welding area well contacted, the metal pressing range around the melting core will also expand, and the plastic ring will be wider when welding under the same specification, thereby improving the shear strength. Some materials have formed alloy wires because the surface oxide film has not been cleaned in time. In addition, when some soluble impurities enter the core and some alloying elements form a low-melting eutectic, it causes dangerous defects such as internal cracks in the core. As the surface oxides contaminate the electrodes, it is easy to overheat the electrodes and the surface quality of the solder joints is reduced. Increasing the number of electrode repairs shortens the life of the electrode, wasting a large amount of copper alloy, and reducing the production efficiency. It can be seen that the surface state affects spot welding, heat deposition during seam welding, core formation, defect generation, and electrode heat dissipation. It directly affects the joint strength and production efficiency.

For barrel makers, it is not difficult to make ordinary hot rolled steel sheets and ordinary cold rolled steel sheets. However, to make galvanized steel sheet barrels, it is easy to have problems. For galvanized steel drums, since the zinc melting point on the surface of the steel sheet is relatively low, when the edges are lapped, they are easily melted under the high temperature produced by the grinding, and the melted zinc is coated in the gap of the grinding wheel to cause the wheel to slip. In this case, it is not possible to use a plastic cutter for shaping. This is because: First, the time interval for each shaping is too short and needs to be reshaped; the second is that the amount of the plastic cutter is too large. The 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 pickling method to treat the zinc layer on the surface of the galvanized steel sheet.

To mill the edging process, the original grinding wheel is changed to an alloy milling cutter. Its working principle is similar to the planer. However, due to the large investment in equipment, the application is not extensive. The use of an acid washing method to treat the zinc layer on the surface of the galvanized steel sheet is widely used due to its simple process. However, the zinc layer on the surface of the galvanized steel sheet is removed by the pickling method, and the edging must be performed later. In terms of environmental protection, in addition to the dust brought by the original edging process, the dust and harmful gases (hydrogen chloride) brought about by pickling are increased, which greatly jeopardizes our production environment and human health. In addition, it also seriously eroded our production equipment.

After pickling the galvanized steel sheet, after grinding, during the spot welding and seam welding, the residual acid solution will decompose the toxic chlorine gas when the steel sheet is welded and melted. Therefore, environmental protection during the production of galvanized steel drums will become an important issue for steel drum manufacturers.

Based on this practical problem of galvanized steel barrel production, we successfully tackled this problem through a large number of experiments and technical breakthroughs, and we did not need to be edging and pickling. We just passed the galvanized steel plate through polishing wheel or polishing wheel. After polishing to remove oil from the welding surface, this problem was solved by changing the original welding specification during welding.

Because the melting point of zinc is very low, only 692K, during the 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. The coating layer in the welding zone melts and the extruded zinc and the surrounding area of ​​the welding zone melt more than 692K. The zinc is squeezed into the gap of the steel plate at the same time, forming a good conduction condition between the two steel plates, the current field is expanded, the current density on the contact surface is greatly reduced, when the core diameter is in a critical state, the current fluctuates slightly, then the incomplete penetration occurs. phenomenon. From this point of view, the spot welding of galvanized steel should increase the welding current IW. In general, the lower the melting point, the greater the IW should be. In order to break through the zinc layer so that the contact surface has good contact during spot welding, and the molten zinc layer is squeezed out of the contact surface, so the electrode pressure when spot welding galvanized steel sheet is lower than that of spot welding ordinary low carbon steel sheet When the electrode pressure should be increased by 20% to 25%. When the power of the welding machine is insufficient, a small current and low pressure can also be used for welding the galvanized steel sheet. However, the shunting of pressure and current must be avoided.

The problem of seam welding of galvanized steel sheet is similar to that of spot welding, because the galvanizing layer has a wider melting range, the diversion is more serious, and more current is required for welding. When the temperature of the welded joint exceeds 1173K, the zinc element diffuses into the heat-affected zone and the brittleness of the joint 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 welding zone of the barrel barrel forms a Cu-Zn alloy with the welding wheel, which increases the surface resistance of the welding wheel and makes the heat dissipation more intensified. Adhesion becomes more severe; overheating of the surface layer further enhances the diffusion of zinc into the heat affected zone, increasing the chance of crack formation. When the vapor of zinc in the core is precipitated from the solidified metal, pores are formed. Although the air hole has little effect on the weld tightness of the weld, it has a great influence on the formation of cracks. It is a key issue in seam welding of galvanized steel barrels. In order to reduce cracks, it is first necessary to improve the welding specification. Tests have shown that the smaller the penetration rate (10% to 20%), the fewer defects such as cracks. However, when the welding speed is high, the surface of the welding zone of the steel drum is overheated and the penetration depth is large, and cracks are apt to occur. Therefore, in the case of welding galvanized steel sheets, a small current should be selected as far as possible to satisfy the conditions for increasing the flow of the galvanized layer to be shunted; the welding speed should be lower, and the external cooling water should be used for cooling. The welding wheel is driven with an embossing roller (knurling knife) to correct and clean the roller surface at any time, maintaining the predetermined roller width.

Through experimental verification, I believe that in the production of galvanized steel drums, the galvanized steel sheet must be cleaned and spot-welded, and the following points must be achieved:

1, spot welding, seam welding oil must be polished;

2. The welding current during spot welding is 28% higher than that of ordinary low carbon steel plate spot welded.

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 when 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 mild steel steel plates;

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

The above are some of the experiences in our experiments for reference by peers.