FI97449B - Procedure for continuous drawing of metal piping - Google Patents

Abstract

The invention concerns a procedure for continuous drawing of seamless metal piping with a mandrel arranged inside the metal pipe and a die reducing the pipe external diameter, whereby the wall thickness is reduced by the mandrel and die working in conjunction and a drawing force acting on the drawn pipe behind the die, and whereby, at least two pipe lengths before the drawing, the pipes are joined at their unformed ends. In the case of continuous drawing of, for example, metal pipes joined to each other by butt welding, the joints do not normally have the strength necessary to enable the drawing forces arising during the drawing process to be transmitted. The procedure according to the invention will ensure that, first of all, the joints will be brought up to the same strength as the adjacent material in the pipe blank, and that then the total, resulting length can be drawn to a uniform wall thickness. The same strength value could preferably be obtained by heat treatment in the joint area.

Description

97449

Method for continuous drawing of metal pipes

The invention relates to a method according to the upper concept of claim 1. Such a method is known from DE-PS 3 739 730.

In the manufacture of seamless metal pipes, the starting point is usually cast ingots, which are then further processed by pressing or hot rolling into pipe blanks. In the next manufacturing step, the wall thickness and outer diameter of these tube blanks 10 are reduced in a cold stretch rolling mill. The step-stretched tube is then drawn in many stages by means of matrices and mandrels inside the metal tube to the final dimension. A mandrel 15 and traction oil must be placed in the metal tube in preparation for each draw. The beginning of the tube is then sharpened to fit through the matrix of the tractor. If there are several draws, these preparations must be made several times and, in addition, the end of the pipe sharpened for the previous draw must always be removed. To avoid these disadvantages, attempts have already been made to connect the individual pipe sections together, for example by means of an end weld, before the drawing is carried out. However, it has been shown that the joints of the pipe sections are not strong enough to withstand the tensile forces. Reducing the load on the joints. In order to make it 25 during the drawing, the joints are drawn in a known method from a certain distance as a hollow drawing, i.e. in such a way that the wall thickness of the pipe does not decrease during this time. This results in the wall always being thicker for a relatively long distance than elsewhere in the pipe. The inconvenience is that it must be possible to be sure in successive strokes: · to identify the joints so that their point can again be drawn as a hollow stroke without reducing the wall thickness.

In addition to the corresponding tool, considerably control technology is required for the manufacture 35 described above. At the end of the manufacturing process, the joints must be cut off and remain a residue.

The object of the invention is to provide a method for the continuous drawing of metal pipes, in which the entire manufacturing length 5 can be pulled equally thin in its wall without the need for large amounts of adjusting devices.

The object is solved in the invention in that the joints of the pipe sections are initially brought to almost the same strength state by suitable means as the pipe material in the immediate vicinity of the joint, and that the pipe sections, including the joints, are then drawn to the same wall thickness.

An essential advantage of the method according to the invention is that the power of the traction device can be significantly increased without having to use a large number of control devices at the same time, and without leaving a thicker wall of unusable pipe material.

According to a particularly preferred embodiment of the invention, the joint of the metal tube is soft annealed. Since the required attraction depends on e.g. the strength of the material to be molded, it can be assumed that the soft annealed material requires less traction than the hard drawn material. Thanks to the heat treatment of the joints, the predetermined area becomes strong at the joint, for example with tensile strength of 25 unreinforced welds. In the subsequent first drawing, the material of the joints is then loaded with only a considerably less modifying force of the soft-annealed material. Surprisingly, it has been found that the joints 30 between the pipe sections are sufficiently strong and do not tear when pulled with a movable mandrel in the usual 30-40% cross-sectional reductions. In order to reduce the transmitted tensile forces, it is primarily possible to pull the area of the joints at a lower tensile speed. However, this precaution is usually only required for the first pull after the pipe sections have been joined.

Alternatively, the almost identical strength state of the joint and the area in its immediate vicinity can advantageously be achieved also by subjecting the joint to local cold working.

Claims (5)

A method for continuous drawing of seamless metal tubes with a mandrel arranged within the metal tube and a die reducing the outer diameter of the tube, whereby the wall thickness of the tube is reduced by interaction between the mandrel and the die and a pulling force engages the drawn tube behind the die and wherein at least two pipe lengths prior to drawing are joined to each other at their non-shaped ends, characterized in that the joints of the lengths are first brought to the same state of strength with the appropriate abutment strength as the immediately adjacent pipe material, and that the pipe lengths including the joints are then pulled to one unit. 15 2. A method according to claim 1, characterized in that the joints of the pipe lengths are annealed gently. 3. A method according to claim 2, characterized in that the joint site and the area subsequently drawn in the drawing direction are subjected to annealing. Method according to claim 1, characterized in that the joint length of the pipe length is subjected to local cold working. 5. A method according to any one of claims 1-4, characterized in that the pulling speed is reduced at the joints.