Abstract:
A tube is made by first making a tubular billet with a central cavity by a primary shaping operation. Then the billet is fed to a radial forging machine where it is forged into the tube by decreasing an outer diameter of the billet and a radial thickness of a wall of the billet.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a tube-forging method. More particularly this invention concerns a method of making a tube from a billet. 
       BACKGROUND OF THE INVENTION 
       [0002]    U.S. Pat. No. 8,166,792 describes a method for making a seamless hot-finished steel tube, wherein a billet heated to molding temperature is formed in a first molding step by stamping into a hollow billet, and wherein subsequently a finished tube is produced in the same heat by radial forging. 
       OBJECTS OF THE INVENTION 
       [0003]    It is therefore an object of the present invention to provide an improved tube-forging method. 
         [0004]    Another object is the provision of such an improved tube-forging method that overcomes the above-given disadvantages, in particular that is more cost-efficient than the prior-art methods. 
       SUMMARY OF THE INVENTION 
       [0005]    A method of making a tube. The method has according to the invention the steps of making a tubular billet with a central cavity by a primary shaping operation, feeding the billet to a radial forging machine, and forging the billet into the tube by decreasing an outer diameter of the billet and a radial thickness of a wall of the billet. 
         [0006]    By producing the hollow billet with the central cavity with a primary-shaping method, the hollow billet is created in a particularly simple and effective manner. According to the principle of primary shaping, the billet and the central cavity required for forging the hollow billet into a pipe are generated in the same forming step so that the effort of preparing the hollow billet is reduced. 
         [0007]    Generally preferred, the invention relates to pipes from an iron-based alloy, in particular steel, or also from a nickel-based or a titanium alloy. 
         [0008]    In a first preferred embodiment of the invention, the primary shaping is electro-slag remelting. Through this, in particular for steels, an effective and universal method for the primary shaping of the hollow billet is provided. 
         [0009]    Alternatively, the primary-shaping method can also be centrifugal casting, which is particularly suitable for combination with a radial-forging method because it makes a hollow billet with a central cavity in one simple step. 
         [0010]    In a generally advantageous refinement of the invention, after being primary shaped, the billet is mechanically machined. Particularly advantageously, but not required, this can comprise removing a casting skin. However, this can also involve, for example equalizing dimensions of the cavity, deburring, or other suitable pretreatments of the primary shaped hollow billet prior to feeding it to a radial-forging apparatus. 
         [0011]    Generally preferred, the hollow billet is heated after being primary shaped and before being fed to the forging machine so as to achieve a defined forming temperature for the radial-forging process. This can be advantageous in particular in the case of alloys and microstructures that have a relatively narrow temperature range for forging. 
         [0012]    Alternatively, for saving energy and costs, in a particularly advantageous manner after primary shaping and before feeding to the forge, there is no intermediate heating of the hollow billet. Thus, the very high heat inherently available during the primary shaping process is used here to reach a temperature suitable for radial forging. With such an approach, if necessary, controlled cooling of the hollow billet can be performed prior to introducing it into the radial-forging apparatus. 
         [0013]    In a preferred detailed configuration of the invention, descaling the primary formed hollow billet is carried out in a immediately before forging preferably, but not required, by a high-pressure method. 
         [0014]    Furthermore, at least a region of the cavity of the hollow billet is lubricated prior to forging by a lubricant. Such a lubricant can preferably be formed based on glass and/or phosphate and/or graphite. 
         [0015]    Forging the hollow billet into a pipe while reducing its outer diameter and wall thickness is generally and advantageously carried out by a forging mandrel as an internal tool. Open die forging without using a forging mandrel is principally also conceivable; however, using a forging mandrel is particularly effective. In most cases, the forging process takes place such that the wall of the hollow billet is pressed by outer forging jaws against the forging mandrel arranged inside the cavity. In particular, the forging jaws can be driven hydraulically, whereby always a very controlled pressure flow is achieved on the workpiece. However, alternatively, it is also possible to provide different drive mechanisms for the forging jaws, for example drop weights or the like. 
         [0016]    In a preferred refinement, the forging mandrel has a coating that preferably, but not necessarily, comprises a scale coating, a ceramic coating, and/or a coating with an applied metal alloy. These coatings can be applied individually or in combination. Applied metal alloys and/or hard alloys are also to be understood as such coatings that comprise hard materials, in particular based on ceramics, such as for example tungsten carbide or the like, embedded in the metal alloy and/or hard alloy. Such coatings are often produced by a thermal process such as plasma deposition welding, arc deposition welding, or the like. Here, the metal alloy serves for providing a sufficiently ductile matrix that, on the one hand, forms a good, nonchipping bond with a substrate of the forging mandrel, in particular steel, and that, on the other, due to embedded hard material phases and/or hard material particles, achieves a suitably high hardness on the surface acting toward the outside. 
         [0017]    In a particularly preferred refinement of the forging mandrel a base body of the forging mandrel has a surface profiling, and the coating is applied to the surface profiling. This way, apart from material flow, additional positive locking is achieved that effectively prevents the coating from detaching from the base body. In terms of shape and orientation, the profiling can in particular be adapted to the respective mechanical loads, for example the forces acting through the respective forging jaws. In particular, in an axial direction of the forging mandrel, the surface profilings form at least one undercut. This ensures a good positive-locking fit that can also absorb particularly high forces that act toward detaching the coating. Particularly preferred, the surface profiling has a number of ridges and depressions on the surface of the base body. 
         [0018]    The base body of a forging mandrel for use in a method according to the invention preferably consists of steel. 
         [0019]    The coating of the forging mandrel protects advantageously against thermal as well as mechanical loads. For example, the coating can have a specific thermal conductivity so as to reduce a thermal effect on the base body. 
         [0020]    Generally advantageous, the coating can be applied by using a thermochemical coating method. 
         [0021]    In a generally advantageous refinement of a forging mandrel, in addition, internal cooling can be provided, wherein a coolant flow can be fed through the mandrel, if necessary. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0022]    The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which: 
           [0023]      FIG. 1  is a side view of a hot working in the form of a radial forging mandrel; 
           [0024]      FIG. 2  is a large-scale sectional view of the detail shown at Z in  FIG. 1  for the uncoated tool base body; 
           [0025]      FIG. 3  is a view like  FIG. 2  of the coated tool base body; 
           [0026]      FIG. 4  is a view like  FIG. 2  of an alternative embodiment of the coated tool base body; 
           [0027]      FIG. 5  is a first microphotograph of the detail Z according to  FIG. 1 ; 
           [0028]      FIG. 6  is a second microphotograph of the detail Z; and 
           [0029]      FIG. 7  is a schematic view of a radial forging apparatus according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    As seen in a pipe or tube is forged as shown in  FIG. 7  by steps of: 
         [0031]    a) Feeding a hollow billet  104  having a central cavity  104   a  to a radial-forging apparatus  101 , and 
         [0032]    b) Forging the hollow billet  104  into a pipe while reducing the outer diameter and the wall thickness of the hollow billet  104 . 
         [0033]    According to the invention, first, the hollow billet  104  together with its cavity  104   a  is produced in a step al using a primary-shaping method. This involves preferably a centrifugal casting method or a remelting method, for example, electro-slag remelting. 
         [0034]    After primary formation of the billet  104  with the cavity  104   a,  mechanical rework on the hollow billet is carried out, if necessary. This can involve, for example, descaling and/or reworking the cavity  104   a  for accurately adapting to a size and shape required for forging. 
         [0035]      FIG. 7  shows the apparatus  101  for radial forging according to the method of this invention. Here, the tubular billet  104  is held at one of its axial ends in a manipulator or holder  102  centered on an axis A. At the opposite axial end, a hot-working tool in the form of a forging mandrel  1  is driven axially by a schematically illustrated actuator  106  into the cavity  104   a.  Forging jaws  103  are pressed radially inward around the mandrel  1  from outside the hollow billet  104 . The forging jaws  103  are pressed with a defined pressure progression against the hollow billet  104 , preferably by actuators shown schematically at  107 , so as to achieve a radial forging of the hollow billet  104  into a pipe. In an alternative configuration, the forging jaws can be moved by a cam mechanism. 
         [0036]    The hollow billet  104  can be rotated and/or axially displaced by the manipulator  102  relative to the mandrel  2  and forging jaws  103  during the forging operation. Alternately in theory the workpiece  104  could be held stationary and the tools  2  and  103  rotated around it. 
         [0037]    It is to be understood that the method according to the invention can also be carried out on other radial-forging apparatuses. 
         [0038]      FIG. 1  schematically illustrates the mandrel or hot-working tool  1  for producing a seamless pipe. Depending on the requirements, the shape may differ and can in particular be cylindrical or slightly conical. The tool  1  has a base body  2  with a working section  3  extending over a given length in the direction of the a. In the working section  3 , the tool  1  is provided with a coating  4  that protects the tool  1  against thermal and/or mechanical load. 
         [0039]    The entire tool base body  2  shown in  FIG. 1  represents in the meaning of the invention an exchangeable mandrel tip that, for example, can be detachably mounted on a mandrel body, here of a shaft  105  (see  FIG. 7 ) of the radial forging mandrel  1 . Other configurations or partitions of an exchangeable mandrel tip  2  and mandrel body  105  are possible depending on requirements. 
         [0040]    The exact structure of the tool is seen in  FIGS. 2 and 3 . The radially outer surface of the tool base body  2  has a surface profiling  5  formed by a plurality of radially outwardly projecting ridges  6  that are axially flanked by radially onwardly open square-section grooves  7 . The ridges  6  have an axial dimension B that preferably ranges from about 250 μm to 4000 μm, a radial height D of the ridges  6  (or depth of the grooves  7 ) that ranges from about 500 μm to 5000 μm, and an axial spacing A is between the ridges  6  (or axial dimension of the grooves  7 ) that ranges preferably from about 200 μm to 2000 μm. 
         [0041]    The profiling  5  is formed on the outer surface of the base body  2  in such a manner that the base body  2  is first processed to be smooth and subsequently, the grooves  7 , which are web-shaped or rectangular in radial section, are machined, in particular by turning. 
         [0042]    After this premachining process, the surface of the tool base body  2  is provided with the coating  4  as shown in  FIG. 3 . Here, the total radial layer thickness C of the coating  4  fills the depressions  7  and exceeds beyond the height D of the ridges  6 . 
         [0043]    Thus, viewed parallel to the axis a, an undercut is created for the material of the coating  4  due to the surface profiling  5  so that the coating  4  very firmly adheres on the base body  2  when the tool  1  is in use. 
         [0044]      FIG. 4  shows a preferred embodiment or solution. Pre-machining the tool base body  2  is carried out as shown in FIGS.  FIGS. 2 and 3 , that is first the profiling  5  is machined into the smooth tool base body  2 . The run of the profiling corresponds to the one according to  FIG. 2 . 
         [0045]    Then, however, prior to applying the coating  4 , a portion of the material of the base body  2  is first converted into a protective layer by a thermochemical treatment. The converted material  8  runs in a uniform depth over the profiling  5  and is shown by dashed lines. Accordingly, the width of the ridges (webs)  6  and the depth of the grooves  7  that, again, are rectangular in cross-section, decrease, as shown in  FIG. 4 . 
         [0046]    Onto the material layer  8  converted in this manner, i.e. onto the primary or inner protective layer generated by the conversion, the coating  4  is applied as a second outer layer during or after the conversion, as shown in  FIG. 4  for the finished tool. This is carried out, again, by a thermochemical method or, for example, by flame spraying or plasma spraying. 
         [0047]    According to the solution illustrated in  FIG. 4 , thus, between the carrier material (base body)  2  and the layer  4 , a structure is created on the carrier material  2  prior to or during the application or generation of the layer  4 , which structure manifests itself in the converted material  8 . 
         [0048]      FIGS. 5 and 6  show examples of actual coatings. The inner, more porous layer  8  generated by converting the webs (ridges)  6  and filling the gaps (grooves)  7 , and the second outer layer  4  applied thereon are clearly visible. In the present case, the inner layer  8  (converted material) consists of iron oxides and grows from the surface of the base body or the profiling. The gaps between the webs (ridges) are filled by the outer coating  4 . 
         [0049]    In the illustrated embodiment according to  FIG. 4  and  FIG. 6 , the carrier material (tool base body) has been coated with iron oxides or material of the base body that has been converted into iron oxide. In the present case, the carrier material is steel. The maximum thickness of the coating on the base body  2  in this example is approximately 1000 μm. 
         [0050]    The structured transition between the carrier material and the coating can be optimized depending on the application so that complete detachment of the layer during use can be prevented. This way, in particular, the life of the tool  1  can be significantly increased. 
         [0051]    The surfaces of the coated tool can be smoothed prior to or during use by mechanical machining operations, for example grinding and polishing (prior to use), or rolling (during use). 
         [0052]    Smoothing the surface reduces the friction between the tool and the workpiece (rolled material).