Welding gap for welded steel pipe

In the manufacturing process of welded steel pipes, the welding gap is a critical parameter that significantly influences the quality of the final product. The strip steel is processed through a series of rolls to form a circular tube blank with an open gap. This gap is carefully controlled to ensure optimal welding conditions.

Adjusting the Welding Gap 

The welding gap, typically maintained between 1 to 3 mm, is adjusted by modifying the pressing amount of the pressing roll. This ensures that the ends of the solder joints are aligned. If the gap is too large, it can lead to reduced proximity effect, insufficient eddy current, and poor weld intergranular joint, potentially causing unmelting or cracking. Conversely, if the gap is too small, the proximity effect increases, leading to excessive welding heat, which can result in weld burn or the formation of deep pits that affect the weld's surface quality.

1. Control of Welding Temperature

Welding temperature is primarily influenced by the thermal power generated by high-frequency eddy currents. The thermal power is proportional to the square of the current excitation frequency, which in turn is affected by the excitation voltage, current, and the values of capacitance and inductance. Precise control of these factors is essential to achieve the desired welding temperature.

2. Control of Pressing Force

Once the tube edges reach the soldering temperature, they are pressed by the pressing roller, allowing common metal crystal grains to infiltrate and crystallize, forming a robust weld seam. Insufficient pressing force can lead to fewer common crystals, reduced weld metal strength, and potential cracking under stress. Excessive pressing force, on the other hand, can extrude molten metal from the weld, reducing weld strength and causing internal and external burrs, as well as defects like welding seams.

3. Regulation of High-Frequency Induction

Coil Position The high-frequency induction coil should be positioned as close as possible to the squeeze roller. A coil that is too far from the roller can extend the effective heating time, widen the heat-affected zone, and decrease weld strength. Conversely, if the coil is too close, the weld edge may not be adequately heated, leading to poor extrusion formation.

4. Role of the Resistor in Welded Pipes

The resistor, typically a special magnetic rod or a set of rods, plays a crucial role in forming an electromagnetic induction circuit between the induction ring and the edge of the tube blank. This circuit generates a proximity effect, concentrating the eddy current in the tube blank weld. The resistor's cross-sectional area should not be less than 70% of the steel pipe's inner diameter. It is dragged through the tube blank with a wire, and its center position should be relatively fixed near the center of the squeeze roller. Due to the rapid movement of the tube blank during startup, the resistor is subject to significant friction damage and needs frequent replacement.

5. Post-Welding Extrusion and Cleaning

After the weld is formed and extruded, a weld bead is produced, which needs to be removed. The cleaning process involves fixing a tool on a frame and smoothing the welding bead through the rapid movement of the welded pipe. Generally, burrs inside the welded pipe are not removed, focusing on the exterior quality of the weld.

In summary, managing the welding gap and related parameters in welded steel pipe production is crucial for ensuring the quality and integrity of the final product. Each step, from adjusting the gap to controlling the welding temperature and force, plays a vital role in achieving a strong and reliable weld.