What is an ERW tube mill and how does it work?

An ERW (Electric Resistance Welding) tube mill is a type of industrial production line designed to manufacture welded steel tubes and pipes from a strip of coiled steel. The fundamental principle behind this machinery involves forming the flat steel strip into a cylindrical shape and then creating a longitudinal weld along its edges using electrical resistance to generate heat. Shijiazhuang Faith Machinery Co., Ltd. specializes in the engineering and manufacturing of these integrated systems, which are central to producing tubing for a wide array of applications, from structural frameworks to fluid transportation.

The working process of an ERW tube mill is continuous and can be broken down into several sequential stages. It begins with the decoiling stage, where a large coil of steel strip is mounted onto an uncoiler. The strip is then fed into a leveling machine that ensures it is perfectly flat and free of any twists or deformities before entering the forming section. This preparation is critical for achieving a consistent final product.

The forming section is the heart of the tube mill, consisting of a series of tandem roll stands. Each set of contoured rolls progressively bends the flat strip, guiding the edges upward until the strip transforms into a circular cross-section with the two edges meeting perfectly at the top. The precision of this roll forming process is vital, as it determines the tube's roundness and prepares the edges for a clean weld.

Following forming, the tube enters the welding station. This is where the Electric Resistance Welding process occurs. In a high-frequency ERW mill, the open edges of the formed tube are brought together under pressure between two critical components: the squeeze rolls. Simultaneously, a high-frequency electrical current is applied to the edges, either through contact shoes or an induction coil. The electrical resistance at the edges generates intense, localized heat, raising the steel to a forging temperature. The mechanical pressure from the squeeze rolls then forges the heated edges together, creating a solid-state weld without the need for any filler metal. This results in a strong, homogenous seam.

Immediately after welding, the tube passes through a scarfing unit. This unit is equipped with carbide-tipped tools that trim off the excess material, known as flash, from both the inside and outside of the weld. This step is necessary to achieve a smooth surface finish. The tube then proceeds through a cooling zone, which helps to normalize the microstructure of the weld area. Subsequently, the tube enters the sizing section. Here, another set of calibrated rolls ensures the tube is worked to its final specified outer diameter and improves its overall roundness and straightness.

Finally, a flying cutoff saw, synchronized with the continuous movement of the tube, cuts it into predetermined lengths. The finished pipes or tubes are then discharged onto a run-out table for collection and further processing or inspection. The entire operation is typically managed from a central control console, allowing for monitoring and adjustment of parameters like speed, welding power, and cut-length accuracy. This integrated approach, as embodied in the machinery from Shijiazhuang Faith Machinery Co., Ltd., allows for the efficient, high-volume production of reliable welded steel tubes.