Sharing China-Made with Global Customers
Sharing China-Made with Global Customers
In the process of feeding strip steel into the welded steel pipe unit, the strip steel is gradually rolled into circular billets with open gaps through multiple passes of rolling. By adjusting the amount of pressure applied by the squeezing rollers, the gap of the weld seam is controlled within 1~3mm, ensuring both ends of the weld are level. If the gap is too large, it reduces the proximity effect, leading to insufficient eddy current heat and poor intergranular bonding of the weld seam, resulting in incomplete fusion or cracking. Conversely, if the gap is too small, it increases the proximity effect, causing excessive welding heat, resulting in weld burn-through or the formation of deep pits on the weld seam surface after squeezing and rolling, affecting the surface quality of the weld seam.
Welding temperature is primarily influenced by the high-frequency eddy current heat power. According to relevant formulas, the high-frequency eddy current heat power is mainly affected by the frequency of the current. The eddy current heat power is proportional to the square of the excitation frequency; and the excitation frequency is affected by excitation voltage, current, and the capacitance and inductance of the circuit. The formula for excitation frequency is:
f=2π(CL)1/21
Where:f - excitation frequency (Hz); C - capacitance in the excitation circuit (F), capacitance = charge/voltage; L - inductance in the excitation circuit, inductance = magnetic flux/current.
From the above formula, it can be seen that the excitation frequency is inversely proportional to the square root of the capacitance and inductance in the excitation circuit, or directly proportional to the square root of the voltage and current. By changing the capacitance, inductance, voltage, or current in the circuit, the excitation frequency can be adjusted to control the welding temperature. For low carbon steel, welding temperature is controlled at 1250-1460℃to meet the requirements of full penetration welding for wall thicknesses of 35mm in welded steel pipes. Additionally, welding temperature can also be adjusted by regulating welding speed.
Insufficient input heat results in the heated edges of the weld not reaching the welding temperature, with the metal structure remaining in a solid state, resulting in incomplete fusion or penetration. Conversely, excessive input heat leads to the overheating of the heated edges of the weld, causing over-burning or droplet formation, resulting in weld craters.
After the edges of the billet are heated to the welding temperature, under the squeezing of the squeezing rollers, they form mutual penetration and crystallization of common metal grains, ultimately forming a strong weld. If the squeezing pressure is too low, the number of common crystals formed is small, resulting in a decrease in the strength of the weld metal, leading to cracking under stress; if the squeezing pressure is too high, it will squeeze the molten metal out of the weld, not only reducing the strength of the weld but also producing a large number of internal and external burrs, and even causing defects such as lap welding.
The high-frequency induction coil should be placed as close as possible to the position of the squeezing roller. If the induction coil is far from the squeezing roller, the effective heating time is longer, the heat-affected zone is wider, and the strength of the weld decreases. Conversely, if the edges of the weld are insufficiently heated, the forming after squeezing is poor.
An impedance regulator is a special magnetic rod or group of magnetic rods for welded steel pipes. The cross-sectional area of the impedance regulator should generally not be less than 70% of the cross-sectional area of the steel welded pipe. Its function is to form an electromagnetic loop with the induction coil and the edge of the weld of the billet, generating a proximity effect, concentrating the eddy current heat near the edge of the weld of the billet, and heating the edge of the billet to the welding temperature. The impedance regulator is dragged by a steel wire inside the billet, and its central position should be relatively fixed near the center position of the squeezing roller. When starting the machine, due to the rapid movement of the billet, the impedance regulator experiences significant wear due to friction against the inner wall of the billet and needs to be replaced frequently.
After welding and squeezing, weld scars are formed and need to be removed. The removal method is to fix a cutting tool on the frame and rely on the rapid movement of the welded steel pipe to scrape off the weld scars. Generally, the burrs inside the welded steel pipe are not removed.
Taking the production of straight seam welded pipes with a diameter ofφ32×2mm as an example, the process parameters are briefly described:
Strip steel specifications: 2×98mm with a small amount of forming allowance after unwinding according to the median diameter
Material: Q235A steel
Input excitation voltage: 150V Excitation current: 1.5A Frequency: 50Hz
Output DC voltage: 11.5kV DC current: 4A Frequency: 120000Hz
Welding speed: 50 meters/minute
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