The process known as the Mannesmann process is based on cross-roll piercing which, for decades, was used in combination with pilger rolling. Both rolling techniques were invented by the brothers Reinhard and Max Mannesmann towards the end of the 19th century.
The pierce-rolling process, which was patented in 1886, made it possible for the first time to roll a thick-walled seamless hollow body from a solid steel billet.
To do this, the billet is heated to between 1200 and 1300°C and then positioned between two rolls arranged at an angle to each other and operating in the same direction, thus locking the billet into a kind of gear mechanism. As the billet progresses lengthwise through the rolls in a helical motion, it is pierced by a plug which is supported at the run-out end of the piercing mill by a freely rotating bar. The alternating tension and compression generated by the helical rotating motion loosen the material's microstructure in the core, and this facilitates the internal "drilling” by the plug.
In the 1880s, while they were rolling starting material for the family's file factory, the Mannesmann brothers had noticed that rolls arranged at an angle to each other can loosen the core of an ingot and cause it to break open. Based on the bold idea of putting this phenomenon to systematic use, they managed to produce a seamless hollow body from a solid ingot – initially by rolling alone. Very soon, however, they optimized the rolling process by using a plug to ensure more uniform piercing and a smoother inside surface. Naturally, the process and the equipment have been continuously optimized over the more than 100 years that have passed since then, so the billets now no longer simply break open and the plug produces an almost perfectly smooth inside surface.
Additionally in the early 1890s, Max Mannesmann developed the pilger rolling process that rolls the pierced hollows into thinner-walled tubes of great length.
In the pilger rolling stand, a pair of conical-shaped rolls – one arranged above the other – operates in the opposite direction to the material flow. The thick-walled hollow body, with a cylindrical mandrel inside it, is guided towards the pilger rolls. As soon as it is gripped by the tapered portion of the work pass, a small material wave is sheared off the hollow. This wave is forged to the desired wall thickness on the pilger mandrel by the smoothing portion of the work pass, with the hollow body plus mandrel moving backwards in the same direction as the rolls are rotating – i.e. in the opposite direction to the rolling – until they reach the idler pass of the rolls and are released. As the hollow is rotated it is once again pushed forward between the rolls, and a new rolling cycle begins.