Abstract: A method of preparing bimetallic tubes or pipes for roll-forming ribs therefrom, in which the external tube is subjected to initial compressive stress is introduced, is characterized by roughening the ends of the internal tube, and after placing the internal tube in the external tube the ends of the external tube are clamped on the roughened segments of the internal tube with simultaneous axial displacement of a part of the material of the external tube corresponding to said compressive stresses of a required quantity. From the tube assembly thus prepared ribs are drawn. It is preferable that at least one end of the external tube is formed conically during clamping.

Abstract: A method and apparatus for rolling transversely ribbed pipe consists in that the rib after forming is deflected in the direction of rolling parabolically, the external pipe is pressed down to the core pipe, and deflecting the rib and pressing down the external pipe onto the core pipe is carried out during one revolution of the pipe, and afterwards the rib is straightened.The apparatus has three tools provided with roll-formed disks, for supporting and straightening, whereby the second - in the direction of rolling - part of the profile of supporting disks is concave, preferably parabolically, and the last - in the direction of rolling - tool is provided with a supporting disk the outer diameter of which is from 0.1 to 0.8 mm larger than the outer diameter of the other supporting disks.

Abstract: A method of forming ribs on tubes or pipes is characterized by applying to the material of the tube, from which the rib is formed, initial compressive stresses preferably ##EQU1## where Fo denotes the area of the cross-section of the tube before drawing, Fr denotes the area of the cross-section of the core of the tube after drawing the rib, and Re is the plasticity limit. Next, ribs are drawn by driven tools with roll-formed disks while maintaining said compressive stress.