Seamless pipes, processed starting from perforated and drawn steel bars, are employed in multiple applications, from the oil pipelines construction, petrochemical uses, up to hydraulic cylinders, mainly due to their capacity to withstand great pressures. These pipes are obtained from billets or blooms, or by heat rolling process.
Mannesmann, the creator of seamless pipes, implemented a process made of two separate phases, the production of the tube and the “pilger” milling phase, and it allows to obtain seamless pipes with a diameter up to 700mm and length up to 30m. Therefore, the process starts with the boring of round semifinished products (billets), obtained from the casting of molten steel, which are brought to a temperature of 1200°-1300°, placed in an oblique-cylinders rolling mill with double-conicity table, elongated and perforated thanks to thrusting and rotating operations in oblique axes by the cylinders themselves.
The tube thus formed is led into a hot rolling mill, and, through rotatory and compressive movements, the surface and tolerances are improved.
In the second phase, in order to make the diameter of the pipe a perfect circumference, the pipe is processed by an octagonal calibration spindle with a conical tip, which, by retracting its segments, is moved back and forth inside, smoothing the oval curves, and making it perfectly round.
The differences between seamless and welded pipes are the following:
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