The ERW Tube Mill Machine unit sends the strip steel to the welded pipe unit. After rolling by multiple rollers, the strip is gradually rolled up to form a round tube blank with an open gap. The reduction of the squeeze roller is adjusted to control the weld gap. 1~3mm, and make both ends of the weld be flush. If the gap is too large, the proximity effect will be reduced, the eddy current will be insufficient, and the weld will not be fused or cracked due to poor intercrystalline joints. If the gap is too small, the proximity effect will increase, and the welding heat will be too large, causing the weld to burn; or the weld will form a deep pit after being squeezed and rolled, which will affect the surface quality of the weld.

The welding temperature is mainly affected by the high-frequency eddy current heat power. According to formula (2), the high-frequency eddy current heat power is mainly affected by the current frequency, and the eddy current heat power is proportional to the square of the current excitation frequency; and the current excitation frequency is also excited The influence of voltage, current and capacitance and inductance. The excitation frequency formula is:

f=1/[2π(CL)1/2]…(1)

f-excitation frequency (Hz); C-capacitance in excitation loop (F), capacitance=electricity/voltage; L-inductance in excitation loop, inductance=magnetic flux/current

The above formula of the longitudinal welded pipe unit shows that the excitation frequency is inversely proportional to the square root of the capacitance and inductance in the excitation circuit, or proportional to the square root of the voltage and current. As long as the capacitance, inductance or voltage and current in the circuit are changed, the excitation frequency can be changed. Size, so as to achieve the purpose of controlling the welding temperature. For low carbon steel, the welding temperature is controlled at 1250~1460℃, which can meet the requirements of penetration of pipe wall thickness of 3~5mm. In addition, the welding temperature can also be achieved by adjusting the welding speed.