What Is The Seamless Steel Pipe Manufacturing Process
2025-02-14
What Is The Seamless Steel Pipe Manufacturing Process
Pipes that are manufactured without welds are referred to as seamless steel pipes, typically made from materials such as carbon steel, stainless steel, or alloy steel. Seamless steel pipes offer superior strength, reliability, and resistance to corrosion, making them essential in critical industries such as oil and gas, chemicals, and power generation. Due to their excellent strength, durability, and resistance to corrosion, seamless carbon steel pipes are widely used across various sectors. This article will provide an in-depth overview of the different production methods used for seamless carbon steel pipes.
1. Mandrel Mill Process
In the Mandrel Mill Process, a solid round steel billet is used as the starting material. This billet is first placed in a rotary hearth furnace for heating. Once heated, a small hole is punched at the end of the billet, which serves as a starting point for the rotary piercing process.
Rotary piercing is a rapid and dynamic rolling technique where the preheated billet is cross-rolled at high speed between two barrel-shaped rolls. This cross-rolling action generates significant tensile stress at the center of the billet. The design of the piercer rolls guides the metal to flow smoothly over the piercer point, which is a high-temperature, water-cooled alloy tool. As the metal passes over this tool, it transforms into a pipe shell through the rotary process. After the pipe shell is formed, it is immediately moved to the floating mandrel mill.
The floating mandrel mill consists of eight rolling stands, 16 rolls, and a set of mandrel bars. These mandrel bars are inserted into the pierced pipe shell, which is then passed through the mandrel mill. The rolling process inside the mandrel mill, with the mandrel bar in place, ensures the formation of a pipe shell with precise outer diameter (OD) and controlled wall thickness, determined by the mandrel bar's dimensions.
The mandrel mill pipe shell is then reheated in a furnace to undergo the final rolling process, which ensures it reaches its final dimensions and microstructural properties. After exiting the reheat furnace, the pipe is subjected to a high-pressure water descaling process to remove any iron oxide scale formed during heating. This cleaning step is vital for ensuring a smooth, scale-free surface.
Finally, the pipe shell is reduced to the required dimensions in the stretch mill. The pipes are then cut to the specified length by the cutoff mechanism, after which they are bundled and sent for further finishing and inspection before shipment.
2. Mannesmann Plug Mill Process
The Mannesmann Plug Mill Process shares similarities with the Mandrel Mill Process, with the primary distinction being the use of a plug mill instead of a mandrel mill for the rolling operation.
In the plug mill, a pair of conical rolls—positioned one above the other—operates in the opposite direction of the material flow. The thick-walled hollow pipe shell, which has a cylindrical mandrel inside, is guided into the plug mill rolls. Once the pipe shell is gripped by the tapered section of the work pass, a small amount of material is sheared off the pipe shell. This material is then forged into the desired wall thickness by the smoothing section of the work pass. As the hollow pipe shell, along with the mandrel, moves backward in the same direction as the rotating rolls—opposite to the rolling direction—it reaches the idler pass of the rolls, where it is released. The pipe shell, now rotated, is then pushed forward between the rolls, starting a new rolling cycle.
This method is typically used for producing larger seamless pipes, usually in the range of 6 to 16 inches (150 to 400 mm) in diameter. A steel ingot weighing up to two tons is heated to approximately 2,370°F (1,300°C) before being pierced. The hole in the hollow shell is then enlarged using a rotary elongator, resulting in a short, thick-walled tube called a bloom.
3. Extrusion Process
Extrusion is a metal shaping process where a workpiece, typically a round billet, is forced through a die with a smaller cross-sectional area, transforming the material into a new shape. The length of the extruded part varies depending on the amount of material in the billet and the profile being formed. This method is capable of producing a wide variety of cross-sectional shapes.
Pipes can be directly extruded by attaching a mandrel to the dummy block. A hole is created along the axis of the workpiece, through which the ram applies force to initiate the extrusion. The mandrel is placed inside this hole. As the process begins, the ram is pushed forward, and the metal flows between the mandrel and the die surfaces, forming the pipe. The mandrel shapes the interior profile of the extrusion, while the die shapes the exterior profile.
