Cold pilger rolling is a very well established process for efficiently and effectively controlling product quality and assisting in meeting stringent product manufacturing specifications.
Here we look at two of the most common milling techniques in VMR1 and HPTR2 which use compression rather than tension to achieve sometimes complex cross section dimensions.
One of the most important and abundant finish metallurgical products are seamless tubes. After hot rolling, a part of these tubes goes to the end-user, the other part serves as a billet to the cold pilger rolling process. The tubes, made by cold pilger rolling, are used by traditional and perspective consumers - machine-building enterprises.
As such, theses consumers make higher demand to the assortment and also the product quality. The following factors can be related with such demands: the usage of hardly-deformed materials, which have the high user’s quality and the manufacturing of the tubes with a complicated shape of cross-section – so called hollow profiles. The preciseness of the geometrical dimensions, the surface quality, the structure and mechanical characteristics of the ready tubes require under-recrystallisation temperatures for finish size deformation.
Cold pilgering is a longitudinal cold-rolling process that reduces the diameter and wall thickness of metal tubes in one process step. Depending on the material, the cold pilger process achieves cross-sectional reductions of more than 90 percent in a single working cycle.
Figure 1 shows the principle of the cold pilger process.
As mentioned above, the part of billet (hot-, cold rolled or extruded etc. tube) 1 that equals to feeding volume, gets into moving zone of working cage 2, where rolls 3 with variable dies radius are installed. The variation of dies radius is provided by specific tool calibration. In the rolling process summary deformation zone (working cone) 4 is formed at this stage. Its internal surface is formed by the mandrel 6, and the consequent deformation of the parts of billet with the low level of partial strains in working cone is the tube with finish size 5.
Considerable progress in the field of technology and pilger rolling equipment for manufacturing of precision tubes was indicated a long time ago. At that time the concepts such as cold and warm tubes rolling were formed, they had a new sense for technology and equipment improving.
Figure 1: The scheme of cold pilger rolling
The two most common ways to cold-reduce tubes by compression are VMR1 and HPTR2 cold pilgering mills. Although both VMR and HPTR dies reduce tubes via compression rather than tension, the complexity of tooling design and manufacturing varies greatly between these machine types.
The variable cross-sectional groove of a VMR die requires special expertise and equipment to be designed and built, as does the matching mandrel (conical mandrel). By contrast, the cams, cylindrical mandrels, and constant cross-sectional grooves of HPTR dies are relatively simpler to design and build. This allows the production on conventional machine tools, which gives HPTR die owners much more flexibility on sourcing tooling and modification of tool design.
Figure 2: VMR and HPTR pilgering process comparison
The cold pilgering process is suitable for all metals. Typical materials are mild steel, stainless steel, ferritic steel, low-alloy steel, copper and copper alloys, titanium alloys, zirconium alloys, and nickel alloys. Using cold pilgering to reduce precious metals is conceivable, because practically no material loss occurs. The deformation strengths of the cold pilgered tubes range from 400 N/mm2 for copper to more than 1,500 N/mm2 for special alloys.
Some applications, such as baseball bats and golf clubs, rely on cold pilgering to create the intermediate, tapered shape. Other applications are lightning poles, finned tubes, and nonround tubes with internal or external longitudinal ribs.
Table 1: Typical materials for cold pilgering