PROCEDURE FOR CONTINUOUS PULLING OF METAL TUBES.

TECHNICAL FIELD The invention relates to a process as defined in the precharacterising part of Claim 1. Such a process is known from German Patent (DE-PS) 37 39 730. BACKGROUND OF THE INVENTION In the manufacture of seamless metal pipes, one usually starts from cast blocks which are worked into rough tubes by pressing or hot rolling operations. In a subsequent production step, these rough tubes are reduced in both wall thickness and outer diameter in a cold pilger-rolling mill. The pilger-formed pipe is drawn to its final size in a plurality of drawing operations with the aid of female mould members and mandrels disposed inside the metal pipe. In order to prepare each drawing step, a mandrel and drawing oil must be introduced into the metal tube. Subsequently, the beginning of the pipe is pointed so that it can be guided through the female mould member of the drawing machine. In the case of a number of drawing operations, these preparatory steps must be executed several times wherein, in addition, the pointed pipe sections must be separated: from the preceding length of pipe. In order to avoid these shortcomings, attempts hijve been made to join individual lengths before the drawing Operation by, say, butt welding of the ends of the pipe lengths. However, it turned out that the junctions of the individual pipe lengths do not have the strength required to transmit the drawing forces arising in the drawing operation:. In order to relieve the junctions during the drawing operation, the known process contemplates hollow drawing over a certain length in the region of the junctions, i.e., drawing the pipe during this time without reducing the wall thickness. This implies that the wall thickness i«n a relatively long interval is greater than in the residual length of pipe. As an aggravating feature, the junctions must be recognized with certainty in subsequent drawing operations so that an other hollow drawing step without reduction of the wall thickness can be executed in that region. In addition to a corresponding tool outfit, considerable, expenses for automatic control are requiredi At the- end of the production process? the junctions must be ctit from the finished lengths of pipe and form waste. USz/jrb * 4 t « f • • * 9 » » f « « • SUMMARY OF THE INVENTION The «(oaT of the invention is to create a process for the continuous drawing of metal pipes, wherein the entire length of pipe produced can be drawn with uniform reduction of the wall thickness but without large expenses for automatic control means. According to the invention, the problem is solved in that the junctions between the lengths of pipe are by appropriate measures transferred into almost the same state of strength as the respective immediately adjacent pipe material and that thereafter the lengths of pipe, including the junctions, are drawn to a uniform wall thickness. The main advantage of the process according to the invention is that the output of a drawing plant can be substantially increased without need for large expenses for automatic control means and without the development of useless pipe material of increased wall thickness. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT According to a particular advantageous embodiment of the invention, the metal pipe is softened by annealing in the region of the junctions. Since the required drawing force depends, inter alia, upon the state of strength of the material to be deformed, one can assume that in the case of a soft-annealed material, the required drawing force will be smaller than in the case of hard-drawn material. By the special thermal treatment of the metal pipe lengths in the region of the junctions, a predetermined region acquires the strength of the junction, e.g., the tensile strength of a non-hardened welding seam. In the ensuing first drawing step, the junctions are subjected to the much lower converting force for soft-annealed material. It was unexpectedly observed that the junctions between the lengths of pipe have the required strength features and are not torn in drawing operations using a flying mandrel with the usual reduction of the cross section by about 30 - 40%. In order to reduce the drawing forces to be transmitted, the drawing rate can be reduced preferably in the region of the junctions. This safety measure in general will be required only in the firt drawing step after joining the lengths of pipe. As an alternative, an almost identical state of strength of the junction and the immediately neighbouring hard-drawn region can be advantageously obtained by subjecting the junction to a local cold deformation. 1452/jrtf The claims defining the Invention are as follows: 1. A process for the continuous drawing of seamless metal pipes, with a mandrel and a female mould member reducing the outer diameter of the pipe, both disposed Inside the metal pipe, wherein the wall thickness 1s reduced by the mandrel coactlng with the female mould member and a drawing force acts on the drawn pipe behind the female mould member, and wherein at least two lengths of pipe are joined at their undeformed ends prior to the drawing operation, characterized in that the junctions between the lengths of pipe are first brought to substantially the same strength as the pipe material immediately adjacent thereto and that thereafter the pipe lengths, including their junctions, are drawn to a uniform wall thickness. 2. The process according to Claim 1, characterised in that the pipe lengths are soft-annealed in the region of the junction. 3. The process according to Claim 2, characterised in that the junction of each pipe length and a portion of the pipe length immediately adjacent the junction are subjected to annealing, before the pipe lengths are drawn. 4. The process according to Claim 1, characterised in that the pipe lengths are subjected to local cold working in the region of the junction. 5. The process according to any one Of Claims 1 through 4, characterised in that the rate of drawing the pipe lengths is reduced in the region of the junctions. DATED this THIRTEENTH day of SEPTEMBER 1991 KM-kabe1 me ta1 Akt i engesells e haf t Patent Attorneys for the Applicant SPRUSON. & FERGUSON TMS/t439h Zt4n p >VT <&