Seamless steel pipes are produced from a hot round billet and take their main form through three processing stages: First, the hot billet is pierced (along its axis) and hot rolled or extruded into a hollow shell. Then, it is elongated, simultaneously reducing the diameter and thickness of the pipe wall. Finally, the ultimate form is obtained by hot rolling, cold rolling, and elongating.
The main methods of producing seamless pipes include:
- Mannesmann plug mill process
- Mandrel mill process
- Extrusion process
Mannesmann Plug Mill Process
The piercing of a solid hot billet with cross rolling, which was invented by the Mannesmann brothers in 1885, is commonly considered the start of the industrial manufacturing of seamless pipes. The Mannesmann plug mill process begins with a round steel billet heated in a rotary furnace. After heating, the billet is drilled using a conical mandrel and entered into a rolling mill where two conical rolls operate along the opposite direction of the flow of materials.
This cross drilling process creates a hollow shell with a thick wall. The second rolling operation using the Pilger mill elongates the hollow casing and reduces its wall thickness, producing a seamless steel pipe.
Mandrel Mill Process
This process also begins with a preheated cylindrical steel billet. The first stage is rotational piercing where the billet is rotated between two cross-sectional barrel rolls. The high tensile strength opens the billet’s axis. Meanwhile, the billet is drilled along its axis using a water-cooling tool, creating a hollow casing. Then, the pipe casing should be transferred to a floating mandrel mill comprised of multiple rolling bases, roll pairs, and a set of mandrel rods. The mandrel rods enter the drilled pipe casing, which is then transferred to the rolling station. Mandrel rods work as a control mechanism and create a standard outer diameter (OD) and a controlled thickness (THK).
After moving forward in the mandrel mill, the mandrel rods must be pulled from the pipe casing, cooled, and returned to the supply station to be ready for another casing. Then, the pipe is reheated and reduced to specific dimensions using a stretch-reducing mill. Then, the final pipe is cooled, cut, smoothed, and finished and inspected before shipping.
The pipe extrusion process is performed in extrusion presses.
The raw material, a hot billet, passes through the extrusion mold with a lower cross-section using a mandrel. The extruded metal is pressed between the mandrel and the mold. The external form of the pipe is made by the mold and the internal form is made by the mandrel. This method can produce numerous profiles.