High-frequency welding process of submerged arc steel pipe

Here is an introduction to the high-frequency welding process of submerged arc steel pipe.

Firstly, Weld Gap Control: 

After undergoing multiple rolling processes, the strip is directed into the welded pipe unit. Gradual rolling of the strip steel forms a circular tube blank with an opening gap. Adjusting the reduction amount of the squeeze roller controls the weld gap to 1~3mm, ensuring both ends of the weld are flush. A too-large gap diminishes the proximity effect, leading to insufficient eddy current heat and poor weld crystal joint, causing non-fusion or cracking. Conversely, a too-small gap increases the proximity effect, elevating welding heat excessively, resulting in burnout or deep pits post-extrusion and rolling, affecting weld surface quality.

Secondly, Welding Temperature Control: 

Welding temperature is influenced by high-frequency eddy current thermal power, which in turn is affected by current frequency. The eddy current thermal power is proportional to the square of the current-encouraging frequency. The encouragement frequency depends on voltage, current, capacitance, and inductance in the loop. Adjusting capacitance, inductance, or voltage and current in the loop alters the excitation frequency, thereby controlling welding temperature. For low carbon steel, maintaining welding temperature at 12501460°C meets the 35mm pipe wall thickness penetration requirement. Welding speed adjustments compensate for insufficient input heat, preventing incomplete fusion or overburning.

Thirdly, Extrusion Force Control: 

Adequate extrusion force under extrusion roller action heats the tube blank edges to welding temperature. Metal grains form a joint, crystallizing to create a robust weld. Insufficient force leads to weakened weld strength and stress-induced cracking, while excessive force squeezes out molten metal, reducing weld strength and producing burrs and defects.

Fourthly, High-Frequency Induction Coil Position Control: 

Positioning the induction coil close to the extrusion roller ensures prolonged effective heating time. Distant coils widen the heat-affected zone, weakening weld strength, while insufficiently heated edges result in poor post-extrusion shaping. The resistor's cross-sectional area should be at least 70% of the pipe's inner diameter, forming an electromagnetic induction loop with the edge of the pipe blank weld and the magnetic rod, concentrating eddy current heat near the weld edge.

Lastly, Weld Scar Treatment: 

Weld scars are smoothed by rapid movement during pipe welding. Internal burrs are typically left unremoved inside the welded pipe.