METHOD FOR PRODUCING A HIGH-PRESSURE PIPE

Abstract

Method for producing a high-pressure tube with inner tube and outer tube made of metal are drawn together through a first drawing die. The outer diameter of the inner tube is smaller than the inner diameter and drawing forms a very stable frictional connection between the inner tube and the outer tube. The manufactured tube has a large wall thickness and is very robust and pressure-resistant while having a very high quality outer shell surface and in particular a very high quality inner shell surface by virtue of the cold forming process. These two properties allow a sufficiently high protection against bursting when pressures in excess of 12,000 bar are applied to the tube. The produced tube has improved dynamic pressure resistance against high pressures by combining a large wall thickness of the tube to be manufactured with a high quality inner shell surface of the tube to be manufactured.

Claims

1. A method for manufacturing a tube comprising the steps of: providing an inner tube made of metal having a first inner diameter and a first outer diameter; providing an outer tube made of metal with a second inner diameter and a second outer diameter, wherein the first outer diameter is smaller than the second inner diameter, and wherein inserting the inner tube into the outer tube such that the inner tube extends within the outer tube; and drawing of the inner tube and the outer tube together through a first drawing die, wherein the second inner diameter is reduced such that a frictional connection of the outer tube and the inner tube is established.

2. The method according to claim 1, wherein the tool diameter of the forming 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 first inner diameter of the inner tube during the drawing of the outer tube and inner tube extending therein together through the first drawing die is reduced by at most 5%.

3. The method according to claim 1 wherein the first outer diameter of the inner tube by drawing of the outer tube and the inner tube extending therein together through the first drawing die is reduced by at least 0.01 mm and by at most 0.3 mm.

4. Method according to claim 1, wherein the inner tube after drawing of the outer tube and the inner tube extending therein together through the first drawing die is strain-hardened and has a tensile strength of at least 900 N.

5. The method according to claim 1, wherein the tube to be manufactured after drawing of the inner tube and the outer tube through the first drawing die has a wall thickness, which is defined by half the difference between the second outer diameter of the outer tube and the first inner diameter of the inner tube, at least one third of the outer diameter of the outer tube, wherein the wall thickness is at least one third of the outer diameter of the outer tube.

6. The method according to claim 1, wherein the tool diameter of the forming inner surface of the first drawing die prior to drawing of the inner tube and the outer tube together through the first drawing die is by at least is 5% smaller than the second outer diameter of the outer tube prior to drawing.

7. The method according to claim 1, wherein the inner shell surface of the inner tube has a surface quality such that existing cracks on the surface do not exceed a depth of 50 m.

8. The method according to claim 1, wherein the inner tube is manufactured by drawing a hollow of metal through a second drawing die and over an inner die tool.

9. The method according to claim 1, wherein the inner tube is manufactured by in a pilger mill by rolling a hollow of metal over a mandrel.

10. The method according to claim 2, wherein the inner die tool or the mandrel is made of steel with a polished surface, such that an inner shell surface of the inner tube is burnished during drawing of the inner tube over the inner die tool or by rolling over the mandrel.

11. The method according to claim 1, wherein a material of at least the inner tube or the outer tube is selected from a group consisting of a carbon steel, a low alloy steel and a high-alloy steel or a combination thereof.

12. The method according to claim 1, wherein the first outer diameter of the inner tube prior to drawing of the outer tube and the inner tube extending therein together through the first drawing die is in a range from 6.25 mm to 6.45 mm, and after the common drawing is in a range from 6.08 mm to 6.28 mm, wherein the first outer diameter of the inner tube is reduced by the common drawing.

13. The method according to claim 1, wherein the inner tube and the outer tube are made of the same material.

14. The method according to claim 1, wherein the inner tube and the outer tube are made of different materials.

15. The method according to claim 2, wherein the first outer diameter of the inner tube by drawing of the outer tube and the inner tube extending therein together through the first drawing die is reduced by at least 0.01 mm and by at most 0.3 mm.

16. The method according to claim 3, wherein the inner die tool or the mandrel is made of steel with a polished surface, such that an inner shell surface of the inner tube is burnished during drawing of the inner tube over the inner die tool or by rolling over the mandrel.

Description

[0043] Further advantages, features and possible applications of the present invention will become apparent from the following description of an embodiment and the accompanying figures.

[0044] FIG. 1a shows a schematic cross-sectional view of an outer tube and an inner tube extending in the outer tube prior the common drawing through a first drawing die according to an embodiment of the method according to the present invention.

[0045] FIG. 1b shows a schematic cross-sectional view of a tube manufactured after common drawing through a first drawing die according to an embodiment of the method according to the present invention.

[0046] FIG. 2 illustrates a schematic sectional view in the longitudinal direction of a drawing bench for manufacturing a tube according to an embodiment of the method according to the present invention.

[0047] FIG. 3 shows a schematic sectional view in the longitudinal direction of a drawing bench for manufacturing an inner tube according to an embodiment of the method according to the present invention.

[0048] In FIG. 1a schematic cross-sectional view of an outer tube 3 and an inner tube 2 extending in 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, wherein the first outer diameter D2 is smaller than the second inner diameter D3.

[0049] The inner tube 2 consists of the material HP 160, a high strength nitrogen-alloyed austenitic stainless steel of high corrosion resistance. In this case, the inner tube 2 has been manufactured in a pilger rolling mill by rolling a hollow over a mandrel of metal. To perform the rolling motion, there are two rolls rotatably mounted on shafts in a roll stand, which carries out a reciprocating motion. The rolls are driven by the reciprocating motion of the roll stand. The hollow arranged between the rotating rolls is rolled over a tapered mandrel and experiences a stepwise feeding after each rolling process.

[0050] The inner tube 2 prepared in this way is characterized by very precisely determinable dimensions of the first inner (D1) and outer diameter (D2) and in particular by a high surface quality on its inner shell surface 8. The surface quality of the inner shell surface 8 of the inner tube 2 has also been improved my the mandrel being made of high quality steel with a polished surface, and thus the inner shell surface 8 of the inner tube 2 is burnished during rolling. This has the consequence that cracks existing on the inner shell surface 8 have a maximum depth of only 7 m.

[0051] The inner tube 2 shown in FIG. 1a comprises a first inner diameter D1 of 1.6 mm and a first outer diameter D2 of 6.3 mm.

[0052] The outer tube 3 in FIG. 1a consists, like the inner tube 2 of the material HP 160 and thus is also characterized by high corrosion resistance and pressure resistance. The manufacturing of the outer tube 3 was also carried out in a cold pilger mill with a burnishing mandrel, such that also the outer tube 3 has a high surface quality and very precise dimensions with respect to the second inner (D3) and outer diameter (D4).

[0053] 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. Here, the second inner diameter D3 of the outer tube 3 is configured such that the inner tube 2 may extend longitudinally in the outer tube 3, as is apparent from the cross-sectional drawing of FIG. 1a.

[0054] FIG. 1b shows the same schematic cross-sectional view of an outer tube 3 and of an inner tube 2 extending in the outer tube 3 as FIG. 1a, but in contrast to FIG. 1a after the outer tube 3 has already been drawn together with the inner tube 2 through a first drawing die 4a of FIG. 2. By the common drawing through the first drawing die 4a, the inner tube 2 and the outer tube 3 have formed a very stable frictional connection 6 with each other.

[0055] This frictional connection occurs in particular due to a suitable choice of the dimensions of the first drawing die 4a as well as the outer tube 3 and inner tube 2. The tool diameter D5 of the forming inner surface of the first drawing die 4a for the drawing die 4a shown in FIG. 2 is 14.5 mm and has been selected such that by the drawing of the outer tube 3 and inner tube 2 together through the first drawing die 4a a frictional connection 6 of the outer tube 3 to the inner tube 2 is sufficiently large to provide a permanently stable connection between the inner tube 2 and outer tube 3. However, the frictional connection 6 formed should not exceed a predetermined upper value, in order for the first inner diameter D1 of the inner tube 2 not to be negatively affected in form of a change of the inner shell surface of the inner tube 2.

[0056] In the cross-sectional view illustrated in FIG. 1b the frictional connection between the inner tube 2 and the outer tube 3 established by drawing did not negatively affect the first inner diameter D1 of the inner tube 2 and thus the surface quality of the inner surface 8 of the inner tube 2, such that the first inner diameter D1 of the inner tube 2 after of cold forming in the drawing bench with the first drawing die has a value of still 1.6 mm. In contrast, the first outer diameter D2 was reduced from an initial value of 6.3 mm to a value of 6.15 mm by drawing of the outer tube 3 and inner tube 2 together. Due to the frictional connection 6 between the outer shell surface of the inner tube 2 and the inner shell surface of the outer tube 3 the second inner diameter D3 of the outer tube 3 also after the drawing of the outer tube 3 and inner tube 2 together is at least substantially 6.15 mm. The second outer diameter D4 of the outer tube 3 after the drawing of the outer tube 3 and inner tube 2 together has the same value as the tool diameter D5 of the first drawing die 4a, which in the drawing of FIG. 1b corresponds to a value of 14.5 mm. The wall thickness of the tube 1 manufactured, whose cross section is depicted in FIG. 1b, hence corresponds to 12.9 mm.

[0057] FIG. 2 shows a schematic cross-sectional view in the longitudinal direction of a drawing bench for manufacturing a tube 1 according to an embodiment of the method according to the present invention. During the drawing through the drawing bench shown in FIG. 2 an outer tube 3 and an inner tube 2 extending therein according to the cross-sectional drawing of FIG. 1a are drawn together through a first drawing die 4a, wherein the drawing direction 7 in FIG. 2 is denoted by the arrow. Due to the absence of an inner tool, in this so-called hollow drawing merely the outer diameter of the tube 1 is reduced and smoothed. The wall thickness of the manufactured tube 1 thus experiences a reduction in absolute terms, but without causing an influence on the inner shell surface of the tube to be manufactured.

[0058] During the drawing of the outer tube 3 and inner tube 2 together through the first drawing die 4a a combined force is applied consisting of a linearly acting tensile force and a radially acting compressive force on the outer tube 3 with the inner tube 2 extending therein. In this case, the tool diameter D5 of the first drawing die 4a is selected such that the frictional connection between the outer shell surface of the inner tube 2 and the inner shell surface of the outer tube 3 resulting from the applied combined force is sufficiently large, such that a lasting and stable connection between these two surfaces is provided. Therefore, after the common drawing the inner tube 2 and outer tube 3 form such a rigid connection, that a so-called double-walled tube is formed, as shown on the left side of FIG. 2 and denoted by reference number 1. In addition, the inner tube 2 is strain-hardened by the common drawing and has a tensile strength of 1100 N.

[0059] However, an upper limit of the frictional connection is not exceeded in order to avoid negative influences of the frictional connection to the surface texture of the inner surface 8 of the inner tube 2 so that the latter remains virtually unchanged or unaltered. Thus, the inner surface and the inner diameter of the tube 1 experience almost no or no change at all by the drawing process according to the present invention.

[0060] FIG. 2 also makes it clearly evident that the first inner diameter D1 of the inner tube 2 as a result of the drawing of the outer tube and the inner tube together through the first drawing die 4a does not change 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 each have undergone varying degrees of reduction by the common drawing. Due to the effected frictional connection 6 between the outer shell surface of the inner tube 2 and the inner shell 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 equal.

[0061] FIG. 3 shows a schematic cross-sectional view in a longitudinal direction through one embodiment of a drawing bench for manufacturing an inner tube 2 according to the method of the present invention. The drawing bench in FIG. 3 in addition to a second drawing die 4b has an inner drawing tool 5 fixed by means of a core rod not shown in FIG. 3, over which the inner tube 2 to be formed is drawn in the drawing direction 7 along the illustrated arrow. In this way, both the outer diameter D2 of the inner tube 2 and the inner diameter D1 of the inner tube 2 and the wall thickness of the inner tube 2 are reduced and brought into a narrow tolerance range.

[0062] In contrast to the drawing bench shown in FIG. 2 during manufacturing of the inner tube 2 during presence of an inner drawing tool 5, namely a drawing core, a more precise fitting of the inner diameter D1 of the inner tube 2 to the desired value occurs. Here, the value of the inner diameter D1 of the inner tube mainly results from the diameter of the inner drawing tool 5. Furthermore, the sliding of the blank over the inner drawing tool 5 results in a smoothing of the inner shell surface of the inner tube 2. The smoothing is further enhanced by use of a lubricant or drawing oil, which reduce the sliding friction between the tube to be drawn and the respective drawing tools and thus lead to a more homogeneous drawing speed.

[0063] 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. Accordingly, the inner tube 2 shown on the left hand side of FIG. 3 has a first inner diameter D1 of approximately 1.6 mm and a first outer diameter D2 of approximately 6.3 mm.

[0064] For the purpose of original disclosure it is noted that all features as they will become apparent to a person skilled in the art from the present description, the drawings and the claims, even if they have been specifically described only in combination with certain other features, both individually and in any combinations can be combined with other features or groups of features disclosed herein, as far as the combination had not expressly been excluded or technical conditions make such combinations impossible or meaningless. A comprehensive explicit description of all conceivable combinations of features is only omitted for brevity and readability.

[0065] While the invention has been shown and described in detail in the drawings and the foregoing description, this illustration and description is merely exemplary and is not intended as a limitation of the scope of protection as it is defined by the claims. The invention is not limited to the disclosed embodiments.

[0066] Modifications of the disclosed embodiments will be apparent to the person skilled in the art from the drawings, the specification and the appended claims. In the claims the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain features are claimed in different claims does not exclude their combination. Reference numbers in the claims are not intended to limit the scope of protection.

LIST OF REFERENCE NUMBERS

[0067] D1 first inner diameter [0068] D2 first outer diameter [0069] D3 second inner diameter [0070] D4 second outer diameter [0071] D5 diameter of the first drawing die [0072] D5 diameter of the second drawing die [0073] 1 tube [0074] 2,2 inner tube [0075] 3,3 outer tube [0076] 4a first drawing die [0077] 4b second drawing die [0078] 5 inner drawing tool [0079] 6 frictional connection [0080] 7 drawing direction [0081] 8, 8 inner shell surface of the inner tube