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Shijiazhuang Faith Machinery Co., Ltd has extensive experience in manufacturing ERW tube mills specifically engineered for processing carbon steel. These mills are designed to handle the specific characteristics of carbon steel, from low-carbon grades used in furniture to high-strength, low-alloy grades for structural applications.
The process starts with the payoff and feeding of carbon steel coils. The mill must be equipped with sufficient power to handle the tensile strength and thickness of the material. The strip is first leveled by a multi-roll leveler to eliminate any coil set or crossbow, ensuring a flat strip enters the forming section. The forming process is gradual, involving a sequence of breakdown passes, fin passes, and closing passes. Each set of rolls incrementally bends the flat strip into a cylindrical shape. The design of these rolls is critical to avoid overworking the material, which can lead to edge stretching or buckling.
For carbon steel, High-Frequency Induction Welding is commonly used. In this process, an induction coil located around the tube heats the edges of the strip to a forging temperature just before they make contact. The pressure from the squeeze rolls then forges the heated edges together to form a solid-state weld. The key advantage for carbon steel is the localized nature of the heat, which results in a narrow heat-affected zone and minimizes the metallurgical changes to the material. An impeder placed inside the tube at the weld point concentrates the magnetic field on the strip edges, improving welding efficiency.
Immediately after welding, the internal weld bead may be trimmed using a scarfing tool to create a smooth inner surface. The pipe then passes through a sizing section, which typically consists of several stands of rolls that calibrate the tube to its final, precise outer diameter and roundness. For some applications, especially thicker-walled pipes, an inline normalizing system may follow the welding process. This involves reheating the weld zone to a specific temperature and then allowing it to cool in a controlled manner. This normalizing process refines the grain structure of the weld and heat-affected zone, improving its mechanical properties and toughness, which is essential for pipes used in critical applications like energy transmission.
The final steps involve cooling the pipe, often with water sprays, and then cutting it to the required length using a flying cut-off saw. The entire process is monitored and controlled to ensure the final product meets dimensional standards such as ASTM A500 for structural tubing or API 5L for line pipe. The mill's design must be robust enough to maintain these tolerances consistently over long production runs.
