Patent Publication Number: US-2015068020-A1

Title: Method for producing multi-walled metallic tubes

Description:
The invention relates to a method for producing a tube from metal, which consists of an inner tube and at least one outer tube surrounding the same. To create the outer tube, a metal strip running in longitudinally is formed around the inner tube moved in its axial direction so as to form a slit tube in such a way that its edges running in the longitudinal direction lie against one another at a slit extending in the longitudinal direction. The slit is welded to complete an outer tube that is closed ail around. 
     Hereinafter, the term “metal” is understood as likewise including the term “alloy”. 
     Double-walled or multi-walled metallic tubes have diverse applications, in particular in the cable industry and for the transporting of gases and liquids. For example, a tube may be clad on the inside and/or on the outside for reasons of corrosion protection. Electrically conducting cables may consist of the inexpensive base material aluminum, with highly electrically conductive copper clad on it. In the case of such composite tubes, the desired properties of the metals used are advantageously exploited. 
     Known methods for producing double-walled metallic tubes are based on galvanic or mechanical bonding techniques. 
     A metallic layer may be galvanically applied to a metal tube, as happens for example in the case of the production of galvanized steel tubes. An inseparable bond of the two metal layers is obtained. Galvanizing has relatively low associated production rates. 
     In the case of the mechanical bonding techniques, the metal surfaces bond to one another by frictional contact. According to the nature of the surface and depending on the production process, variously strong bonds can be created. 
     Cladding is the term used for applying a metal layer to a second metal layer by pressure. In practice, the first layer is pressed or rolled onto the second. A clad metal strip can in this way be formed into the tube and welded. In the region of the weld, the two materials of the metal layers can mix and possibly fuse to become an alloy, so that the material properties in the weld differ considerably from the properties of the two connected metal layers. This difference in properties may be undesired, if for example the weld becomes considerably more brittle when welding a copper layer to an aluminum layer. 
     Longitudinal seam welding of one, two or more metal layers formed into a tube also has disadvantageous effects if the longitudinal seam welding of all the layers takes place together, there is a great overall wall thickness and the inner tube contains a core. This core may be damaged by the high heat input caused by the required high welding energy. 
     In the case of hydroforming, two metallic tubes are pushed one into the other and the inner tube is deformed by internal high pressure in such a way that it is pressed firmly against the outer tube. The internal high pressure is produced by a water-oil emulsion in the inner tube in a closed forming tool in which the tubes are located. This method requires relatively high costs and low production rates. 
     DE68916383T2 describes a technique of hot extrusion. This involves inserting two tubes one inside the other, heating them and subsequently drawing them through a die, whereby the diameters of the two tubes are adapted to one another and the metal layers are pressed together in such a way that a solid bond between the tubes is obtained. 
     JP 559 202117 A describes a method for producing a double-walled tube in which a metal strip is formed around an inner tube to form an outer tube and is welded at its edges. The outer tube, which encloses the inner tube, is fed to a heating device, in which it is heated to a specific temperature. After that, the two tubes are brought into bonding contact with one another in a reducing stage. JP S55 247786 A, 39 H08 290214 A and JP S58 221686 A describe similar methods, with the exception of the heating. 
     The invention is based on the object of providing a continuous method for producing encased tubes of metal that allows high production rates and in which the inner to and the outer tube are bonded by a high pressing force. 
     This object is achieved according to the invention 
     by the inner tube being cooled and 
     by the completed outer tube being reduced in its diameter until it is in contact with the cooled inner tube in at least one reducing stage. 
     Apart from the advantages arising directly from the way in which the object is achieved, the invention also provides the advantage that the double-walled or multi-walled tube produced by means of the continuous method according to the invention has a closed surface of the outer material, in the case of which the properties in the region of the weld differ only slightly from those of the rest of the tube. Likewise advantageously, the degree of pressing of the metal layers can be adapted to the respectively intended area of use of the encased tube. If the metal layers are made of the same material, the production process based on the method according to the invention is less expensive than the production of a thick-walled, single-layer tube with regard to the required capacity of the device that is used for the method. For example, the device has to be designed for lower flexural forces, and less welding power has to be installed. On the other hand, higher production rates are possible with the same welding power, even though more than one layer has to be welded. A further resultant advantage is the lower heat input into a possibly present core in the welding operations. Furthermore, tubes with lower flexural rigidity than the equivalent single-walled tubes can he produced. 
    
    
     
       The method according to the invention is explained on the basis of the drawing.  FIG. 1  shows in a schematic representation a device for producing a tube from metal according to one exemplary embodiment of the method according to the invention. 
     
    
    
     A metal strip  2  is drawn off from a strip supply  1  and fed to a forming device  3 , for example a rolling or sliding tool. Before entering the forming device  3 , the metal strip  2  may be cut to size by means of strip-edge shears  4 . To create the outer tube, the metal strip  2  is formed around a simultaneously fed inner tube  5  moved in its axial direction in the forming device so as to form a slit tube. The inner tube  5  may contain a cable core or a filling of some other kind. The edges running in the longitudinal direction of the slit tube and lying against one another are welded to one another in a welding device  6 , in order to obtain an outer tube  7  that is closed all around. The outer tube  7  is taken up by a drawing-off device  8  and drawn through a device  9  for reducing the diameter that is arranged upstream of the drawing-off device  8 . 
     When drawing down the outer tube  7 , the diameter of the same is reduced in such a way that the outer tube  7  is brought into contact with the inner tube, so that the two tubes are connected to one another. The diameter-reducing device  9  may for example be a drawing block, with which the two tubes are pressed together. Similarly, the diameter-reducing device  9  may consist of at least one roll stand (for example that known as a Turk&#39;s head), with which the outer tube is rolled onto the inner tube. The roll stand may be driven or not driven. 
     The drawing-off device  3  may for example consist of a strip draw-off or a collet draw-off. As represented in  FIG. 1 , a further drawing-off device  11  may be arranged upstream of the drawing block or roll stand  9 . The further drawing-off device  11  applies the force that is required to unwind the metal strip  2  from the unwinder  1  and draw it through the possibly present strip-edge shears  4  and the forming device  3 , if these do not have a drive of their own. The drawing-off device  8  then only has to apply the force that is required for deforming the outer tube in the drawing block or roll stand  9 . Furthermore, when drawing down the tube, minor vibrations and torsion of the outer tube may occur. These movements may have adverse effects on the welding result. The further drawing-off device  11  serves for isolating the influences of the reduction on the welding in the upstream welding device  6 . 
     The finished metal tube  10  can be subsequently fed to further processing or storing devices (not represented). In particular, the finished metal tube  10  can be fed as a new inner tube  5  to the forming device  3  in the described method according to the invention. In this way, two-layered and multi-layered tubes can be continuously produced from metal. Furthermore, the drawing down may be followed by an annealing process, for example recrystallization annealing, in order to reestablish the original microstructural state that was altered by the pressing or roll cladding. The finished double-walled or multi-walled metal tube  10  may be fed to a winder or a cutting unit. 
     The connection between the outer tube and the inner tube may be made with varying degrees of strength. The pressing pressure between the outer tube and the inner tube is determined in part by the diameter of the diameter-reducing device  9 , through which the outer tube is drawn down onto the inner tube. If the two tubes are pressed together so strongly that a virtually inseparable mechanical connection is produced between them, this is known as cladding. In this case, the inner tube may also be plastically deformed. 
     Furthermore, it is possible to influence the bonding between the two tubes, by the adhesion between the touching surfaces or contact surfaces) being appropriately adapted or changed. 
     For example, the inner surface of the outer tube and the outer surface of the inner tube (or only one of the two surfaces) may be cleaned and activated before the drawing down. This surface treatment may take place chemically, i.e. by means of a liquid or gas, or with plasma. Similarly, an adhesive may be applied to at least one of the two (possibly previously chemically activated) contact surfaces, before the outer tube is drawn down onto the inner tube. 
     The inner tube may be heated or cooled before the drawing down of the outer tube. If the inner tube is heated, its deformation resistance is reduced, so that the outer tube can be connected as solidly as possible to the inner tube, i.e. can be clad, lower forces being necessary than with a non-heated inner tube. If the inner tube is cooled, one of the effects is that its outer diameter shrinks slightly. If the material of the inner tube heats up after the drawing down of the outer tube, the inner tube tries to resume its initial diameter, so that a high surface pressure can be achieved between the two tubes. 
     Similarly, the outer tube may be heated before the drawing down. The shrinkage of the drawn-down outer tube during the cooling has the effect that the pressing pressure on the inner tube is increased in comparison with a previously non-heated outer tube. Furthermore, its deformation resistance is reduced, whereby the required shaping forces are lower than in the case of a non-heated outer tube. 
     Before feeding, the outer surface of the inner tube may be worked with a skiving tool, by brushing or by grinding. In particular, for example, an oxide film that hinders the cladding operation can be removed in this way. Similarly, the contact surface of the metal strip can be correspondingly pretreated. In order to prevent the formation of a new oxide film, the steps of feeding, welding and drawing down may take place under a shielding gas atmosphere. 
     The inner tube and the outer tube may consist of identical metals, for example of steel, high-grade steel or a nonferrous metal. For some applications it is advantageous to have a metal tube in which the inner tube and the outer tube consist of the same metal and the connection of the two starting tubes is less solid, so that the finished metal tube is less flexurally rigid than a single-layer metal tube with an equivalent wall thickness. If the inner tube encases a core, the usually undesired heat input into the core is reduced in comparison with that in the case of a single-layer thick-walled tube, since less energy is required for welding the comparatively thin inner tube than in the case of the thick-walled tube. When welding the outer tube, the inner tube has already cooled down. The heat input into the core is thus also less when welding the outer tube than in the case of a one-walled tube with a thick wall. The two-stage method makes the thermal loading of the core lower. 
     The inner tube and the metal strip may similarly consist of different metals. Such a tube, with diverse uses, is for example a CCA tube (copper-clad aluminum), which consists of an aluminum tube that has an cuter copper layer. 
     The finished metal tube  10  may run through a further reducing stage, in which the metal tube, which has a specific initial diameter, is drawn down to a final diameter that is reduced in comparison with the initial diameter. If it is necessary to obtain an outer tube that is as thin-wailed as possible, such as for example in the case of the CCA tube, the wall thicknesses of the inner tube and the outer tube may be reduced at the same time in this reducing stage, for example with the aid of a mandrel in the region of the drawing tool.