Cold solder joint is one of the most common welding defects. Sometimes the steel strips seem to be joined after welding, yet they are not fully integrated. The bonding strength of the joint is extremely low. Since the welds go through various complex processes on the production line, especially high-temperature furnace zones and high-tension straightening zones, cold solder joints are highly likely to cause strip breakage, seriously disrupting normal production.

      Essentially, a cold solder joint occurs when the temperature at the weld interface is insufficient. The fusion zone is too small, or the materials only reach a plastic state rather than being fully melted. The strips are merely pressed together mechanically. Although the joint looks intact, complete fusion is not achieved.

      The causes of cold solder joints can be analyzed in the following steps:

      (1) First, check for rust, oil stains and other contaminants on the weld interface, as well as uneven surfaces and poor contact. These issues will increase contact resistance, reduce current and result in insufficient temperature at the joint.

      (2) Verify whether the weld lap length complies with standards. Check for reduced lap length or cracks on the driving side. Insufficient lap length cuts down the bonding area of steel strips, leaving the joint unable to bear high tension. In particular, cracks on the driving side will cause stress concentration, expand gradually and eventually lead to strip fracture.

      (3) Check if the current setting conforms to process requirements. When the strip thickness changes, failure to adjust the current accordingly will result in insufficient current and defective welding.

      (4) Inspect whether the welding wheel pressure is appropriate. Insufficient pressure leads to excessive contact resistance and decreased actual current. Although the welding controller is equipped with constant current control, once the resistance rises beyond a certain range (generally 15%), the current compensation function will reach its limit. The current can no longer be raised to the set value, and the system will trigger an alarm during normal operation.

      In actual operation, if the exact cause cannot be identified promptly, clean the strip ends properly, increase the weld lap length, and appropriately raise the welding current and wheel pressure for re-welding. Keep a close watch on weld formation during the process, which can resolve most cold solder joint issues temporarily. If the problem stems from control system faults or grid voltage fluctuations, other targeted solutions shall be adopted.