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Ductile iron pipe manufacturers have developed advanced techniques for controlling welding temperatures during the production process. These manufacturers feed a steel strip into a welding pipe unit where it passes through multiple rollers. As the strip moves forward, it is progressively shaped into a cylindrical billet with an open seam. Adjustments to the pressure applied by the squeezing rollers ensure that the gap between the edges of the strip stays within a range of 1 to 3 millimeters, ensuring that the edges align perfectly. If the gap is too wide, the proximity effect weakens, leading to insufficient eddy current heating. This results in poor intergranular bonding, potentially causing unfused areas or cracks. Conversely, if the gap is too narrow, the proximity effect intensifies, causing excessive heat generation. This can lead to overheated welds, burn-through, or even significant depressions after the weld is compressed and rolled, negatively affecting the surface quality.
The welding temperature is primarily influenced by the high-frequency eddy current heat power. Based on formula (2), ductile iron pipe manufacturers understand that the high-frequency eddy current heat power depends significantly on the current frequency. The eddy current heat power is proportional to the square of the current excitation frequency, which itself is impacted by factors such as the excitation voltage, current, capacitance, and inductance. The excitation frequency can be calculated using the following formula:
\[ f = \frac{1}{2\pi(CL)^{1/2}} \]
Here, \( f \) represents the excitation frequency (in Hz), \( C \) is the capacitance in the excitation circuit (measured in farads), and \( L \) is the inductance in the excitation circuit (measured in henries). It's evident that the excitation frequency is inversely proportional to the square root of the product of the capacitance and inductance. By altering these parameters—such as capacitance, inductance, voltage, and current—the excitation frequency can be adjusted to control the welding temperature effectively. For low-carbon steel pipes, the ideal welding temperature typically ranges from 1250 to 1460 degrees Celsius to ensure proper penetration of the pipe wall, usually around 3 to 5 millimeters.
Additionally, the welding temperature can be regulated by adjusting the welding speed. Insufficient input heat can result in the edges of the weld failing to reach the required temperature, leaving the metal structure solid and leading to issues like incomplete fusion or insufficient penetration. On the other hand, excessive input heat can cause the edges of the weld seam to exceed the desired temperature, leading to overheating or droplet formation, which can create holes in the weld.
In summary, ductile iron pipe manufacturers employ a combination of techniques to control the welding temperature, including precise adjustments to the gap width, modulation of the excitation frequency, and fine-tuning of the welding speed. These methods ensure consistent and high-quality welds that meet industry standards and requirements.
This source provides valuable insights into the methods used by ductile iron pipe manufacturers to regulate welding temperatures, offering a comprehensive understanding of the processes involved in creating durable and reliable pipes.