Method for producing high-pressure pipe

Abstract

To provide a method for producing a high-pressure pipe (1).SOLUTION: An inner pipe (2) made of metal and an outer pipe (3) made of metal are drawn together through a first drawing die (4a). The outer diameter (D2) of the inner pipe is smaller than the inner diameter (D3) of the outer pipe. By drawing the outer tube and the inner tube extending therein together through the first drawing die, a very stable, force-locking connection between the inner tube and the outer tube is produced. This leads, on the one hand, to the production of tubes having a large wall thickness, which makes the produced tubes very robust and pressure-resistant. On the other hand, tubes produced by cold forming have a very high surface finish on the outside and in particular on the inside. Both properties allow a sufficiently high protection of the pipe to be produced against bursting when pressures in excess of 12,000 bar are applied to the pipe.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a tube. [Background technology]

[0002] In many technical fields, such as, for example, injection technology for internal combustion engines and the chemical industry, piping systems are used that are subject to high pressures and that must withstand these pressures for a long time. In this regard, a distinction is made between static, i.e. time- and space-invariant, and dynamic, i.e. time- and / or space-varying, pressure loads of piping systems. In many technical fields of application, pressure-carrying components are exposed to time- and space-varying, and in some cases also periodically pulsating, pressure loads, and therefore these components are subject to significantly higher demands in terms of pressure resistance depending on the load phase than components subject to static pressure loads. This results in components subject to dynamic pressure loads wearing out faster and having to be replaced more frequently than components subject to static pressure loads, given the same construction type. The degree of this wear depends on the respective load factors, such as, for example, the frequency of the pressure swings and the difference between the maximum and minimum pressures, i.e. the pressure delta.

[0003] While the static pressure resistance of a component depends primarily on the mechanical properties such as the elastic limit and tensile strength of the material used, the dynamic pressure resistance also depends on other important quantities such as the ductility (elongation at break) of the material, the depth of any existing cracks in the component wall, purity, microstructure and surface finish.

[0004] Failures of tubes and other components are generally caused by critical crack growth, for example in the form of a crack progressing from the inner tube surface to the outer tube surface. In this regard, local stress concentrations at material defects, for example in the form of lattice defects, and at rough surfaces can cause cracks to be initiated by peak stresses or existing cracks to be further expanded by pressure loads. Wear of components under dynamic pressure loads is therefore determined, among other things, by the crack growth per impact pressure and the material properties, and in this regard, good surface quality can significantly reduce the probability of crack initiation.

[0005] In order to achieve high pressure resistance of tubes and other components under dynamic pressure loads, the materials from which the tubes and components are manufactured are selected with a view to optimizing the above-mentioned decisive influencing factors, with subsequent improvement of the material properties, for example by autofrettage, often also being taken into account.

[0006] In order to withstand pressures of over 15,000 bar for a relatively long period of time, the wall thickness of the tube is a key factor in its manufacture, as well as the appropriate selection of the material. The wall thickness of the tube is calculated by subtracting the inner diameter from the outer diameter of the tube. Starting from the same material composition, the pressure resistance of the tube increases with the wall thickness of the tube, since the propagation of a crack from the inner wall of the tube to the outer wall of the tube will last longer with increasing wall thickness. Therefore, thick-walled tubes are often used in high-pressure technology. Summary of the Invention

[0007] Thus, there is a need for a method of manufacturing tubing that improves the dynamic pressure resistance or service life of the tubing in high pressure technology.

[0008] It is a further object of the present invention to provide a method for manufacturing a tube, in which the outer tube and the inner tube have a permanent and stable mechanical bond to one another.

[0009] Another object of the present invention is to provide a method for manufacturing a pipe having an inner surface with high surface quality.

[0010] At least one of the aforementioned problems is solved by a method for manufacturing a tube having the following steps: providing an inner metallic tube having a first inner diameter and a first outer diameter; providing an outer metallic tube having a second inner diameter and a second outer diameter, where the first outer diameter is smaller than the second inner diameter; inserting the inner tube into the outer tube such that the inner tube extends inside the outer tube; and drawing the inner and outer tubes together through a first drawing die, where the second inner diameter is reduced such that a force-coupled connection is caused between the outer tube and the inner tube.

[0011] The classical method for the profile deformation of individual metal tubes is to draw such a profile through a forming drawing die, which is generally carried out on a drawing table. The workpiece is gripped at its start end by a clamping mechanism and drawn through the drawing die. While the drawing die defines the outer periphery of the workpiece after deformation, in the deformation of hollow elongated workpieces, i.e. especially tubes and hollow profiles, not only the drawing die but also a second die can be additionally provided, which is arranged inside the workpiece to be deformed. Such a second die for cold deformation by drawing is either a cantilevered plug or a drawing core, or the plug or drawing core is attached to a rod inside the workpiece as a drawing inner die.

[0012] The principle of the drawing machine is based on drawing the workpiece to be deformed through a drawing die from the outlet side, in which the workpiece before deformation has a larger peripheral surface than the drawing die. It is therefore necessary to be able to apply a drawing force to the workpiece at the outlet side, i.e. behind the drawing die in the drawing or moving direction of the workpiece. For this purpose, the drawing machine is equipped with a drawing mechanism that has a clamping mechanism that is driven so as to be movable relative to the drawing die. This clamping mechanism grips the workpiece during the drawing process and applies the drawing force.

[0013] According to the invention, the inner tube is inserted into the outer tube, so that the inner tube extends inside the outer tube, and the inner tube and the outer tube are drawn together through the drawing die of the drawing table. This drawing die is called the first drawing die in the sense of the invention. In this case, in one embodiment, there is no inner die, such as a plug or drawing core, so that in this so-called blank drawing, only the outer diameter of the tube to be produced is reduced and smoothed. The wall thickness of the tube to be produced is thus reduced in absolute terms, without affecting the inner surface of the tube to be produced.

[0014] However, the drawing process through the die not only affects the second outer diameter, i.e. the outer diameter of the outer tube, but also affects the second inner diameter, i.e. the inner diameter of the outer tube, in the form of a reduction. In addition, the forces generated during the drawing through the die result in a force-coupled joining of the outer and inner tubes.

[0015] A force-locking connection arises essentially by the transmission of forces, for example in the form of compression or friction forces, the connection of which is ensured purely by application forces.

[0016] The method according to the invention has several advantages over the methods known from the state of the art.

[0017] (1) When the inner and outer tubes are drawn together through a drawing die, a force-locking connection is created between the two tubes, which prevents the inner and outer tubes from being displaced in the axial direction, while a positive connection exists in the radial direction. A permanently stable and intact connection between the inner and outer tubes plays an important role, especially when using such so-called double-walled tubes in high-pressure technology. If this permanently stable and intact connection does not occur, for example due to the presence of an air gap or a play between the inner and outer tubes or a lack of a force connection between the two tubes, this can make the inner tube significantly more susceptible to cracking due to crack propagation when high pressure is applied to the double-walled tube.

[0018] (2) The production of double-walled tubes, i.e. tubes consisting of an outer and an inner tube, according to the invention results in the tube having, on the one hand, a sufficiently large wall thickness and, on the other hand, a very high surface finish on the outer side and especially on the inner side. Only by realizing both requirements is it possible to provide a sufficiently high degree of protection against cracks on the tube when pressures of more than 12,000 bar are applied to the tube. The large wall thickness, which results from the sum of the wall thicknesses of the outer and inner tubes after drawing through the first drawing die, increases the pressure resistance of the tube, since cracks on the inner side have to travel a greater distance before progressing to the outer side. In addition, due to the force-coupled connection between the inner and outer tubes, these cracks do not extend along the contact surface between the inner and outer tubes, which would result in the inner tube cracking in the worst case. In addition, in one embodiment, the drawing of the inner and outer tubes together is carried out in the cold state of the starting material, i.e. the incoming tube. This improves the strength and the dimensional and surface quality of the tube to be produced compared to a method in which the tube to be processed is heated. The cold deformation of the tube does not result in temperature gradients due to the tube being heated inhomogeneously, which would lead to distortions in the metal structure of the tube, and in material defects or lattice defects such as elongation due to temperature rise. Cold deformation on the drawing table therefore makes it possible to produce extremely precise tubes with a high surface finish, but only up to a certain maximum wall thickness. If the wall thickness of the tube to be drawn exceeds this maximum wall thickness, the surface finish will deteriorate sharply. According to the invention, by producing the inner and outer tubes separately from each other and then drawing them together through the drawing die, the inner surface of the inner tube can be produced with a high surface finish, while still providing the tube with the wall thickness required for high compressive strength. It thus becomes clear that the method according to the invention is particularly suitable for producing tubes with a significantly improved high-pressure dynamic pressure resistance, due to the combination of the requirement, on the one hand, of a large wall thickness of the tube to be produced and, on the other hand, of a high surface finish of the inner surface of the tube to be produced.

[0019] In one embodiment of the present invention, the mold diameter of the shaping inner surface of the first drawing die, the second outer diameter of the outer tube, the second inner diameter of the outer tube, the first outer diameter of the inner tube, and the first inner diameter of the inner tube are selected such that the reduction in the first inner diameter of the inner tube when the outer tube and the inner tube extending therethrough are drawn together through the first drawing die is at most 5%.

[0020] This upper limit of 5% is manifested by the fact that, above this upper limit, undesired changes in the sense of deformation of the inner surface of the inner tube occur, where the profile of the changes observed in the cross section of the inner tube at at least one position in the longitudinal direction of the inner tube differs from the average value of the first inner diameter by at most 5%. In one embodiment, the profile of the changes observed in the cross section of the inner tube at at least one position in the longitudinal direction of the inner tube differs from the average value of the first inner diameter by at most 3%. In a further embodiment, this difference is at most 1%.

[0021] Due to these requirements on the die diameter and on the first and second outer and inner diameters of the outer and inner tubes, the force coupling evoked when drawing the outer tube and the inner tube extending therein together through the first drawing die has almost or hopefully no effect on the surface quality of the inner surface of the inner tube. This is important in that the high surface quality of the inner surface of the inner tube is not adversely affected by drawing through the first drawing die. In this regard, the free selection of the individual above-mentioned diameters has the advantage that the force coupling evoked by drawing through the first drawing die can be selectively adjusted. The force coupling can therefore be optimally adapted to the corresponding material properties of the inner and outer tubes. This also makes it possible to obtain the desired properties of the double-walled tube to be produced, for example a permanently stable and intact connection between the inner and outer tubes by the force coupling and a high surface quality of the inner surface of the inner tube, and therefore an increase in the dynamic pressure resistance, which depends on the respective starting conditions of the inner and outer tubes used.

[0022] In one embodiment of the present invention, the first outer diameter of the inner tube is reduced by at least 0.01 mm and at most 0.3 mm by drawing the outer tube and the inner tube extending therethrough together through a first drawing die. The reduction in the first outer diameter of the inner tube is a measure for the force coupling that occurs. The greater the reduction in the first outer diameter of the inner tube, the stronger the force coupling coupling between the inner tube and the outer tube.

[0023] In a further embodiment of the method according to the invention, after drawing the outer tube and the inner tube extending therethrough together through the first drawing die, the inner tube is cold hardened and has a tensile strength of at least 900 N. In one embodiment, after drawing together, the inner tube has a tensile strength of at least 1050 N. The tensile strength is the maximum mechanical tensile stress that the material of the inner tube can withstand before it breaks or tears.

[0024] In the method according to the invention, the cold hardening of the inner tube can already take place during its production, i.e. before it is drawn together through the first drawing die. However, the cold hardening of the inner tube can also take place only after drawing the outer and inner tubes together through the first drawing die, or else by a combination of the production of the inner tube and the drawing together. Cold hardening generally results in much narrower dimensional tolerances and better surface properties than deformation under heat supply. The cold hardening of the inner tube present after drawing the outer and inner tubes together through the first drawing die has the advantage that this leads to an increase in the material strength. This can be explained by the fact that the dislocation density increases due to the plastic deformation, so that the dislocations interfere with each other during their movement. This increases the elastic limit and the strength. This means that a significantly higher stress is required to further deform the cold hardened inner tube. This is advantageous, in particular, when the tube to be produced according to the method according to the invention is exposed to high pressure, since the cold hardening of the inner tube significantly increases the pressure resistance. Thus, tubes to be manufactured according to the invention are suitable for use under high pressures.

[0025] In one embodiment of the present invention, the first outer diameter of the inner tube before the outer tube and the inner tube extending therethrough are drawn together through a first drawing die is in the range of 6.25 mm to 6.45 mm, and after drawing together is in the range of 6.08 mm to 6.28 mm, and this first outer diameter (D2) of the inner tube is reduced by the drawing together.

[0026] In a further embodiment of the invention, the tube to be produced, after drawing the inner and outer tubes together through the first drawing die, has a wall thickness of at least one third of the outer diameter of the outer tube, said wall thickness being determined by half the difference between the second outer diameter of the outer tube and the first inner diameter of the inner tube. Such a wall thickness provides the tube to be produced with great stability, thereby enabling it to be used as a high-pressure tube, so that it can withstand pressures of more than 10,000 bar without cracking.

[0027] In one embodiment of the present invention, the wall thickness of the tube to be produced is at least as large as the inner diameter of the inner tube.

[0028] In one embodiment of the method according to the invention, the mold diameter of the shaping inner surface of the first drawing die before drawing the inner and outer tubes together through the first drawing die is at least 5% smaller than the second outer diameter of the outer tube. In one embodiment, the mold diameter of the shaping inner surface of the first drawing die before drawing the inner and outer tubes together through the first drawing die is at least 7% smaller than the second outer diameter of the outer tube. In a further embodiment, this mold diameter before drawing the inner and outer tubes together through the first drawing die is at least 10% smaller than the second outer diameter of the outer tube.

[0029] In order for a force-coupled connection between the inner and outer tubes to occur, it is a necessary prerequisite that the die diameter of the forming inner surface of the first drawing die is smaller than the second outer diameter of the outer tube before they are drawn together. In this respect, the percentage difference of the die diameter of the forming inner surface of the first drawing die from the second outer diameter of the outer tube represents a measure for the force coupling that occurs. Under the prerequisite of a constant drawing speed, the greater this percentage difference is, the stronger the force coupling that occurs. If the percentage difference of the die diameter of the forming inner surface of the first drawing die is too small, no force-coupled connection occurs and the second outer diameter of the outer tube is reduced less. However, in one embodiment, there is also an upper limit to the percentage difference, which is reached when the first inner diameter of the inner tube changes by more than 5% both in the longitudinal and circumferential direction of the inner tube. In a further embodiment, this limit is already reached with a corresponding change of more than 3%, while in a further embodiment, it is even reached with a corresponding change of more than 1%. This precludes the force coupling from having a detrimental effect on the high surface quality of the first inner diameter.

[0030] In one embodiment of the present invention, the inner surface of the inner tube has a surface finish such that the cracks present on the surface do not exceed a depth of 50 μm. In one embodiment of the method according to the present invention, the cracks present on the inner surface of the inner tube do not exceed a depth of 20 μm, and in a further embodiment, the cracks do not exceed a depth of 10 μm. On the contrary, a typical and achievable maximum depth of the existing cracks is 7 μm. Due to such high surface purity, the existing cracks are almost unlikely to propagate from the inner surface of the inner tube in the direction of the first outer diameter of the inner tube, and therefore the inner tube has a high pressure resistance.

[0031] In one embodiment of the present invention, the inner tube is produced by drawing a metal lupe through a second drawing die and along the surface of the inner drawing die. The inner drawing die can be either a fixed drawing core or a mandrel bar. In this regard, in one embodiment, the lupe is subjected to several drawing processes along the surface of a mandrel or a non-fixed drawing plug, where the lupe is the blank of the tube to be produced. The multiple drawing processes improve the size accuracy of the inner and outer diameters of the lupe, and therefore the wall thickness, which can be determined by the difference between the outer and inner diameters, as well as the surface quality of the inner and outer surfaces. The use of a lupe or drawing oil between the lupe and the inner drawing die can reduce the sliding friction occurring between the lupe to be drawn and the inner drawing die, which contributes to a uniform drawing speed. The use of drawing oil therefore further reduces the surface roughness of the tube to be produced.

[0032] In an alternative embodiment of the invention, the inner tube is produced by rolling a metallic tubing along the surface of a rolling mandrel in a pilger mill, preferably a cold pilger mill.

[0033] The most widespread method of rolling tubes is known as cold pilgering. During rolling, the roll is pushed along the surface of a tapered rolling mandrel with a calibrated, i.e. finished, inner diameter of the tube, surrounded from the outside by two rolls that define the calibrated, i.e. finished, outer diameter of the tube, and rolled longitudinally along the surface of the rolling mandrel. During cold pilgering, the roll is gradually fed with few revolutions towards and over the rolling mandrel, while the rolls rotate and reciprocate horizontally along the surface of the mandrel and thus the roll. The horizontal movement of the roll is determined by the roll stand, which rotatably supports the roll. In known cold pilger rolling installations, the roll stand reciprocates in a direction parallel to the rolling mandrel by means of a crank transmission, while the rolls themselves acquire their rotational movement by means of a rack gear that is immovable relative to the roll stand and which meshes with a gear that is fixedly connected to the roll shaft.

[0034] Cold pilger rolling is certainly more expensive than sliding drawing the tube through a drawing die, but it allows tubes to be obtained that are particularly accurate in size in terms of the outside and inside diameter of the tube to be produced. Cold pilger rolling can also use a lubricant between the tube and the rolling mandrel, a so-called mandrel bar lubricant, in order to reduce the sliding friction that occurs and to obtain a smoother surface on the inside of the inner tube to be produced.

[0035] In one embodiment of the invention, the inner drawing die or the rolling mandrel is made of surface-polished steel, so that the inner surface of the inner tube, defined by the first inner diameter and length of the inner tube, is burnished when the inner tube is drawn along the surface of the inner drawing die or rolled along the surface of the rolling mandrel. Within the scope of burnishing, also called smooth rolling, a highly hardened and polished inner drawing die or a highly hardened and polished rolling mandrel is pressed with high force against the surface of the inner tube to be processed and pressed there. This generates high compressive stresses at the roughness peaks, i.e. at the maximum of the surface roughness, which cause plastic deformation of the roughness peaks. The inner surface of the burnished inner tube is characterized by a low surface roughness, a relatively high dimensional accuracy and an increased hardness, i.e. a hardened surface. These properties are of great importance for the resistance to high pressures of more than 15,000 bar, since they clearly reduce the initiation and growth of cracks.

[0036] In one embodiment of the method according to the invention, the material of at least the inner or outer tube is selected from the group consisting of non-alloyed steel, low alloyed steel, and high alloyed steel, or a combination thereof. In one embodiment, the material is high alloyed steel. In a further embodiment, at least the inner or outer tube is made from HP160.

[0037] In high pressure technology, various metallic materials are used for the manufacture of tubes and other components. They are mainly unalloyed, low alloyed and high alloyed steels. Particularly high dynamic pressure resistance is achieved in the case of tubes or other components made of high alloy steels, which are cold hardened or tempered and subsequently burnished. HP160 is a high-strength, nitrogen-alloyed austenitic special steel, which is characterized by improved corrosion resistance, high purity, good deformability and the possibility of processing the material autofrettaged up to 12,000 bar compared to standard materials. Due to its chemical composition and high purity, HP160 has very good resistance to intercrystalline corrosion and hydrogen embrittlement. The high molybdenum content causes good resistance to pitting and contact corrosion as well as stress corrosion cracking. HP160 is therefore the preferred material for manufacturing tubes with high dynamic pressure resistance.

[0038] In one embodiment of the present invention, the inner tube is corrosion resistant, a feature that is advantageous for use in high pressure technology, since initiating corrosion, i.e. ongoing decomposition, of the inner tube would impair the pressure resistance of the inner tube.

[0039] In one embodiment of the invention, the inner and outer tubes are made of the same material, which results in a very stable connection between them due to the force-locking connection of the inner and outer tubes caused by drawing them together through the first drawing die. Under microscope observation, the same lattice structure is present in the metal structure in both the inner and outer tubes when they are made of the same material, so that the lattice structures on the outer surface of the inner tube and on the inner surface of the outer tube can be very well combined with each other.

[0040] In an alternative embodiment of the invention, the inner and outer tubes are made of different materials. This has the advantage that different material properties can be present on the inner and outer surfaces of the tube to be manufactured, depending on the demands of the technical application. In one embodiment, the inner tube is preferably made of a very high pressure- and corrosion-resistant material, for example HP160, in order to be able to withstand high pressures under dynamic pressure loads. If the inner tube is made of a very high pressure- and corrosion-resistant material, the outer tube can be made of a material with a lower pressure- and corrosion-resistant material compared to the inner tube. This allows a reduction in the manufacturing costs of the tube to be manufactured, nevertheless with a very high quality.

[0041] Further advantages, features and applicability of the invention will become apparent on the basis of the following description of an embodiment and the figures belonging thereto. [Brief description of the drawings]

[0042] [Figure 1a] 2 is a schematic cross-sectional view of an outer tube and an inner tube extending within the outer tube prior to drawing together through a first drawing die according to one embodiment of a method according to the present invention. FIG. [Figure 1b] 2 is a schematic cross-sectional view of the produced tubes after drawing together through a first drawing die according to one embodiment of the method according to the invention; FIG. [Diagram 2] 1 is a schematic longitudinal section of a drawing table for the production of tubes according to one embodiment of the method according to the invention; [Diagram 3] 1 is a schematic longitudinal section of a drawing table for the manufacture of an inner tube according to an embodiment of the method according to the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] 1a shows a schematic cross-sectional view of an outer tube 3 and an inner tube 2 extending within the outer tube 3. The inner tube 2 has a first inner diameter D1 and a first outer diameter D2, while the outer tube 3 has a second inner diameter D3 and a second outer diameter D4, where the first outer diameter D2 is smaller than the second inner diameter D3.

[0044] The inner tube 2 consists of material HP160, i.e. austenitic special steel with high corrosion resistance, high strength and nitrogen alloying. It was produced by rolling a metallic roll along the surface of a rolling mandrel in a pilger rolling mill. To perform the rolling movement, two work rolls are present in a reciprocating roll stand, which are rotatably supported on a shaft. These work rolls are driven by the reciprocating movement of the roll stand. The roll, which is placed between the rotating work rolls, is rolled along the surface of a tapered rolling mandrel and is fed in small increments after each individual rolling process.

[0045] The inner tube 2 produced in this manner features very precisely determinable dimensions of the first inner diameter (D1) and the outer diameter (D2) as well as, inter alia, a high surface finish of the inner surface 8 of the inner tube 2. The surface finish of the inner surface 8 of the inner tube 2 was further improved in that the rolling mandrel consisted of a high-value steel with a polished surface, whereby the inner surface 8 of the inner tube 2 was burnished during rolling. As a result, the maximum depth of cracks present on the inner surface 8 is only 7 μm.

[0046] The inner tube 2 shown in FIG. 1a has a first inner diameter D1 of 1.6 mm and a first outer diameter D2 of 6.3 mm.

[0047] The outer tube 3 of Fig. 1a, like the inner tube 2, is also made of material HP160 and is therefore likewise characterized by high corrosion and pressure resistance. The manufacture of the outer tube 3 is also carried out in a cold pilger rolling plant with burnished rolling mandrels, so that the outer tube 3 also has a high surface quality and very precisely manufactured dimensions with respect to the second inner diameter (D3) and the outer diameter (D4).

[0048] The second inner diameter D3 of the outer tube 3 shown in Fig. 1a is 6.6 mm and the second outer diameter D4 of the outer tube is 15.9 mm. The second inner diameter D3 of the outer tube 3 is configured such that the inner tube 2 can extend longitudinally within the outer tube 3 as can be seen in the cross-sectional view of Fig. 1a.

[0049] Figure 1b shows the same schematic cross-sectional view as Figure 1a of an outer tube 3' and an inner tube 2' extending within the outer tube 3', but in contrast to Figure 1a after the outer tube 3 together with the inner tube 2 has already been drawn through the first drawing die 4a of Figure 2. By drawing together through the first drawing die 4a, the inner tube 2' and the outer tube 3' have a very stable force-locking connection 6 with each other.

[0050] This force-coupled connection is achieved, inter alia, by suitable selection of the size of the first drawing die 4a and the outer and inner tubes 3 and 2. The inner forming die diameter D5 of the first drawing die 4a in the drawing die shown in FIG. 2 is 14.5 mm, which is selected in this respect so that the force-coupled connection 6 between the outer and inner tubes 3 and 2, which is generated by drawing the outer and inner tubes 3 and 2 together through the first drawing die 4a, is large enough to provide a permanently stable connection between the inner and outer tubes 2' and 3'. However, it is desirable that the force-coupled connection 6 thus generated does not exceed a certain upper value, so as not to have a detrimental effect on the first inner diameter D1 of the inner tube 2' in the form of a change in the inner surface of the inner tube 2'.

[0051] In the cross-section shown in FIG. 1b, the first inner diameter D1 of the inner tube 2' still has a value of 1.6 mm after the cold deformation performed on the drawing table with the first drawing die, since the force coupling between the inner tube 2' and the outer tube 3' performed by the drawing process had no effect on the first inner diameter D1 of the inner tube 2 and thus on the surface quality of the inner surface 8' of the inner tube 2'. In contrast, the first outer diameter D2' has been reduced from an initial value of 6.3 mm to a new value of 6.15 mm by drawing the outer tube 3 and the inner tube 2 together. Due to the force coupling type coupling 6 between the outer surface of the inner tube 2' and the inner surface of the outer tube 3', the second inner diameter D3' of the outer tube 3' is also at least approximately 6.15 mm after drawing the outer tube 3 and the inner tube 2 together. The second outer diameter D4' of the outer tube 3', after the outer tube 3 and the inner tube 2 have been drawn together, has the same value as the die diameter D5 of the first drawing die 4a, which in Fig. 1b corresponds to a value of 14.5 mm. The wall thickness of the produced tube 1, the cross-section of which is shown in Fig. 1b, therefore corresponds to 12.9 mm.

[0052] Figure 2 shows a schematic longitudinal section of a drawing table for the production of a tube 1 according to an embodiment of the method according to the invention. When drawing through the drawing table shown in Figure 2, the outer tube 3 according to the cross section of Figure 1a and the inner tube 2 extending therein are drawn together through a first drawing die 4a, the drawing direction 7 being set by the direction of the arrow in Figure 2. Since there is no inner die, in this so-called blank drawing, only the outer diameter of the tube 1 to be produced is reduced and smoothed. The wall thickness of the tube 1 to be produced is thus reduced in absolute terms, without the inner surface of the tube to be produced being affected.

[0053] When the outer tube 3 and the inner tube 2 are drawn together through the first drawing die 4a, a combined force consisting of a linearly acting drawing force and a radially acting compression force is applied to the outer tube 3 and the inner tube 2 extending therein. The die diameter D5 of the first drawing die 4a is selected in such a way that the combined force creates a sufficiently large force connection 6 between the outer surface of the inner tube 2 and the inner surface of the outer tube 3, which allows a permanently stable connection of these two surfaces. Thus, after drawing together, the inner tube 2 and the outer tube 3 are firmly connected, so that a so-called double-walled tube is obtained, as shown by reference number 1 on the left side of FIG. 2. In addition, after drawing together, the inner tube 2' is cold hardened and has a tensile strength of 1100 N.

[0054] In this case, however, the upper limit of the force coupling is not exceeded in order to prevent the undesirable effects of the force coupling on the surface quality of the inner surface 8' of the inner tube 2', so that the surface quality remains almost or not changed at all, and therefore the inner surface and the inner diameter of the tube 1 to be produced are also almost or not changed by the drawing process according to the invention.

[0055] 2 that the first inner diameter D1 of the inner tube 2 does not change by drawing the outer tube and the inner tube together through the first drawing die 4a, while the first outer diameter D2 of the inner tube 2, the second inner diameter D3 of the outer tube 3, and the second outer diameter D4 of the outer tube 3 are each reduced by different intensities by drawing together. Due to the force-coupled coupling 6 between the outer surface of the inner tube 2 and the inner surface of the outer tube 3, the first outer diameter D2 of the inner tube and the second inner diameter D3 of the outer tube are approximately the same size.

[0056] Fig. 3 shows a schematic longitudinal section of an embodiment of a drawing table for the production of an inner tube 2 according to the method according to the invention. The drawing table in Fig. 3 additionally has not only a second drawing die 4b but also an inner drawing die 5, which is fixed by means of a core rod not depicted in Fig. 3, along the surface of which the inner tube 2 to be deformed is drawn in a drawing direction 7 along the depicted arrow. In this way, both the outer diameter D2 of the inner tube 2 and the inner diameter D1 of the inner tube 2 as well as the wall thickness of the inner tube 2 are reduced to within narrow tolerances.

[0057] In contrast to the drawing table shown in FIG. 2, the presence of the inner drawing die 5, specifically the drawing core, allows the inner diameter D1 of the inner tube 2 to be precisely adapted to the desired value during production. The value of the inner diameter D1 of the inner tube 2 is then essentially determined by the diameter of the inner drawing die 5. In addition to this, smoothing of the inner surface of the inner tube 2 is achieved by sliding the blank along the surface of the inner drawing die 5. The adaptation of the outer diameter D2 of the inner tube 2 to the desired value is determined by the diameter D5' of the second drawing die 4b, which also causes a smoothing of the outer surface of the inner tube 2. The smoothing is further improved by the use of a lubricant or drawing oil, which reduces the sliding friction occurring between the tube to be drawn and the respective drawing die, and thus the drawing speed becomes uniform.

[0058] The drawing core 5 shown in Fig. 3 has a maximum diameter of 1.6 mm, while the diameter of the second drawing die 4b is 6.3 mm. The inner tube 2 shown on the left in Fig. 3 thus has a first inner diameter D1 of approximately 1.6 mm and a first outer diameter D2 of approximately 6.3 mm.

[0059] For the purposes of the original disclosure, it is pointed out that all features that are apparent to a person skilled in the art from the specification, drawings and claims, even if specifically described only in connection with other specific features, can be combined with other features or features disclosed herein, either alone or in any configuration, unless this has been expressly excluded or the technical reality makes such combination impossible or meaningless. It is only for the sake of brevity and readability of the specification that all possible combinations of features are not comprehensively and explicitly set forth herein.

[0060] While the invention has been illustrated and described in detail in the drawings and the foregoing specification, this illustration and description is made by way of example only and is not to be considered as a limitation on the scope of protection as defined by the claims. The invention is not limited to the disclosed embodiments.

[0061] Variations of the disclosed embodiments will be obvious to one skilled in the art from the drawings, the description and the appended claims. In the claims, the word "comprise" does not exclude other elements or steps, and the indefinite article "eine" or "ein" does not exclude a plurality. The mere fact that certain features are claimed in different claims does not exclude a combination of those features. Signs in the claims are not to be considered as limiting the scope of protection. [Explanation of symbols]

[0062] 1 tube 2, 2' inner tube 3, 3' outer tube 4a First drawing die 4b Second drawing die 5. Inner drawing die 6 Force-coupled coupling 7 Pulling direction 8, 8' Inner surface of inner tube D1 First inner diameter D2 First outer diameter D3 Second inner diameter D4 Second outer diameter D5 Diameter of first drawing die D5' Diameter of the second drawing die

Claims

1. The following steps, namely Providing a metallic inner tube (2) having a first inner diameter (D1) and a first outer diameter (D2); providing an outer metallic tube (3) having a second inner diameter (D3) and a second outer diameter (D4), providing said first outer diameter (D2) smaller than said second inner diameter (D3); inserting the inner tube (2) into the outer tube (3) so that the inner tube (2) extends inside the outer tube (3); drawing the inner tube (2) and the outer tube (3) together through a first drawing die (4a), the second inner diameter (D3) being reduced so as to cause a frictional bond (6) between the outer tube (3) and the inner tube (2); A method for producing a tube (1), comprising: the die diameter (D5) of the inner shaping surface of the first drawing die (4a), the second outer diameter (D4) of the outer tube (3), the second inner diameter (D3) of the outer tube (3), the first outer diameter (D2) of the inner tube (2) and the first inner diameter (D1) of the inner tube (2) are selected such that when the outer tube (3) and the inner tube (2) extending inside the outer tube (3) are drawn together through the first drawing die (4a), the reduction in the first inner diameter (D1) of the inner tube (2) is at most 5%; The inner tube (2) is produced by drawing a hollow metal through a second drawing die (4b) and over an inner die (5) or by rolling a hollow metal over a mandrel; wherein the inner die (5) or the mandrel is made from surface-polished steel such that the inner shell surface of the inner tube (2) is burnished when the inner tube (2) is drawn over the inner die (5) or rolled over the mandrel.

2. The method according to claim 1, characterized in that the tube (1) to be produced, after the inner tube (2) and the outer tube (3) are drawn together through the first drawing die (4a), has a wall thickness that is at least one-third of the second outer diameter (D4) of the outer tube (3), and that the wall thickness is half the difference between the second outer diameter (D4) of the outer tube (3) and the first inner diameter (D1) of the inner tube (2).

3. A method according to claim 1 or 2, characterized in that the mold diameter (D5) of the inner molding surface of the first drawing die (4a) before the inner tube (2) and the outer tube (3) are passed through the first drawing die (4a) and drawn together is at least 5% smaller than the second outer diameter (D4) of the outer tube (3) before drawing.

4. A method according to any one of claims 1 to 3, characterized in that the material of at least the inner tube (2) or the outer tube (3) is selected from the group consisting of non-alloy steel, low-alloy steel, and high-alloy steel, or a combination thereof.

5. A method according to any one of claims 1 to 4, characterized in that the first outer diameter (D2) of the inner tube (2) before the outer tube (3) and the inner tube (2) extending into the outer tube (3) are passed through the first drawing die (4a) and drawn together is in the range of 6.25 mm to 6.45 mm, and after the outer tube (3) and the inner tube (2) extending into the outer tube (3) are drawn together, the first outer diameter (D2) of the inner tube (2) is reduced by the drawing together.

6. A method according to any one of claims 1 to 5, characterized in that the inner tube (2) and the outer tube (3) are manufactured from the same material.

7. A method according to any one of claims 1 to 6, characterized in that the inner tube (2) and the outer tube (3) are manufactured from different materials.

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