LAUSR

Piercing is the first step of seamless tube manufacturing. It is followed by different kinds of rolling processes (tube expansion, wall thickness reduction, diameter reduction).1) It can be performed by drilling or press forging or by the Mannesmann cross-roll piercing, schematically pictured in Fig. 1. In the latter, a full hot cylindrical billet is entrained by two counter-rotating conical rolls (it is thus a rolling process) with axes offset by the feed angle (a few degrees). It is thus given a helical movement which pushes it against the tip of a fixed piercing plug. Under the rolls, the diameter/contact length ratio is designed such that high triaxiality reigns on the billet axis, opening a cavity by ductile damage the so-called Mannesmann effect. The piercing plug then expands this cavity to deform the full billet into a hollow tube. The pressure and strain applied by the cross-rolls in their plane ovalize the preform. Hence, two lateral guiding tools are implemented to restrain or control ovalization of the shell so as to finish with an axisymmetric section. They are either static guiding shoes or rotating Diescher disks. A number of tool design parameters are critical to obtain both precise dimensions and good metal and surface quality. Cross-rolls can be either barrel-shaped or conical,1) with variable feed angle. The shape of the piercing plug Impact of the Different Friction Coefficients on the Tools on the Mechanics of the Mannesmann 2-roll Tube Piercing