How to weld two tubes together

Abstract

The present invention provides a method for coaxially welding two tubes, wherein the axial ends of the tube walls of each tube are machined to have a stepped profile across at least a first portion of the tube wall thickness, the stepped profiles being complementary to each other. The method includes the steps of: (A) coaxially positioning a first tube and a second tube relative to one another, where the machined axial ends contact each other at least at a first portion of the first tube wall thickness and a first portion of the second tube wall thickness, where the first stepped shape of the machined axial end of the first tube wall and the second stepped shape of the machined axial end of the second tube wall are fitted together, and where a seam exists between the first and second tube first portions of the thickness; (B) fastening the first and second tubes with attachment welds at a plurality of discrete locations around the seam; and (C) circumferentially welding the first and second tubes around the entire circumference of the first and second tubes, where the circumferential weld extends through the entire thickness of the first and second tube walls.

Description

[Technical Field]

[0001] The present invention relates to a method for coaxially welding two metal tubes. In this specification, "tube" refers not only to straight tubes but also to corners, T-pieces, reducers, and the like. The present invention can be implemented, for example, for the manufacture of piping for transporting liquids in petrochemical facilities, ships, or offshore structures. Such liquids are, for example, process liquids such as water (waste or potable) or oil. It is known to assemble such piping from so-called spools, which are formed by assembling tubes welded together and T-pieces, corners, reducers, flanges, and the like. Such spools typically have a length of up to about 4 meters and contain multiple steel tubes, for example, with diameters of 51 mm to 305 mm (2 inches to 12 inches) and wall thicknesses of 2 mm to 15 mm, welded together. The spools can be welded on-site. The piping is then fabricated by welding the spools together on-site. [Background technology]

[0002] To achieve good weld penetration, it is known to apply V- or U-shaped weld seams between coaxial tubes and / or the like. The tubes are secured by mounting welds at a small distance from each other, such that there is a 1.0 mm or 1.2 mm slit between the tubes at the bottom of the weld seam, before the tubes are circumferentially welded together. Circumferential welding of tubes is a very delicate process, and as a result, if this welding process is performed manually, it requires highly skilled welders, and if this welding process is automated or at least mechanized, there remains a relatively large risk of welding errors.

[0003] For example, UK Patent Publication No. 656,696A discloses a method of welding two tubes coaxially by first positioning the tubes opposite each other. The weld seams of the opposing tubes are located at the innermost radial positions of the tubes and have protrusions that protrude inward and forward relative to the weld seam itself. These protrusions have a stepped shape at their end faces. The weld seams are inclined away from each other. As a result, when the tubes are positioned opposite each other, the aforementioned U- or V-shaped slits are created. The slits are filled with welding material, and the tubes are welded coaxially. Once the welding is complete, the entire protrusions are removed.

[0004] AT12413 U1 also discloses a method for coaxially welding two tubes. The weld seams of the opposing tubes have a stepped shape at their radially innermost portion that protrudes forward relative to the weld seam of the tube. The weld seams are inclined, and when the tubes are placed facing each other, a U-shaped or V-shaped slit is formed. The tubes are welded coaxially by filling this slit with welding material. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] British Patent Application Publication No. 656,696 [Patent Document 2] Austrian Utility Model No. 12413 Summary of the Invention

[0006] The present invention aims to provide a method that reduces the possibility of welding errors, such as inadequate weld penetration and, conversely, breaking the weld pool through the weld seam. Accordingly, the present invention provides a method as set forth in claim 1. It has been discovered that by using tubes whose axial ends have tube walls with complementary stepped shapes, the tubes can be very easily aligned with each other and can be welded relatively reliably in an automated manner. Here, the complementary stepped shapes ensure accurate alignment of the tubes with each other before they are secured together in step B and subsequently circumferentially welded in step C.

[0007] In one embodiment, the first stepped shape extends within a length range between 0.01 mm and 1.50 mm, as viewed in the axial direction of the first tube. This width range allows for proper alignment of the first and second tubes with one another, while also providing adequate weld penetration. In a further embodiment, the length range is between 0.10 mm and 1.00 mm, more specifically, between 0.15 mm and 0.75 mm. At the lower end of this width range, there is an increased risk of the tubes being incorrectly positioned relative to one another before circumferential welding. At the other end of this width range, there is an increased risk of welding errors.

[0008] The quality of the weld joint appears to be ensured, particularly when the first portion of the first tube wall thickness extends within a length having a dimension between 2.0 mm and 12 mm when viewed radially of the first tube. In a further embodiment, said dimension is between 2.5 mm and 10 mm, more particularly between 3.0 mm and 8.0 mm.

[0009] A preferred way to implement each step profile is to implement a first step profile having an outer annular end face and an inner annular end face, which are spaced apart from each other when viewed in the axial direction of the first tube. The terms "inner" and "outer" relate to the axis of the respective tube. Such a step profile is advantageously obtained by milling the axial ends of the tube wall of each tube, for example using a three-axis milling machine.

[0010] For the purpose of creating the step profile and for the welding process itself, the outer and / or inner annular end surface preferably extends parallel to a radial plane oriented perpendicular to the axial direction of the first tube, thereby reducing the risk of the weld joint shearing.

[0011] In a further embodiment, the first portion of the first tube wall extends through the entire thickness of the first tube wall, and / or the first portion of the second tube wall extends through the entire thickness of the second tube wall. This allows the axial ends of the tube walls of each tube to contact each other across the entire thickness. Such an embodiment may be primarily advantageous when the thickness of each tube wall is up to 8 mm. With such wall thicknesses, a fully penetrated weld can be achieved with a single weld layer using appropriate welding techniques.

[0012] In a further embodiment, primarily for connecting tubes having wall thicknesses of 6.0 mm or greater, the machined end of the first tube wall is further machined radially outward of the first portion of the first tube wall thickness over a second portion of the first tube wall thickness that connects to the first portion of the first tube wall thickness, and similarly, the machined end of the second tube wall is machined radially outward of the first portion of the second tube wall thickness over a second portion of the second tube wall that connects to the first portion of the second tube wall thickness. After performing step A, an open seam is created between the first and second tubes. Such circumferential weld joints between tubes are typically constructed with multiple weld layers.

[0013] The open seam may be substantially V-shaped or U-shaped when viewed in longitudinal cross-section.

[0014] To achieve adequate weld penetration and a relatively narrow weld joint, in a further embodiment, a first portion of the thickness of the first and second tubes is located within the maximum axial dimension of the open seam when viewed axially. In a more specific embodiment, the first portion is located within the central 50% of the maximum axial dimension of the open seam, or even more specifically, within the central 10% of the maximum axial dimension of the open seam.

[0015] In a further embodiment, the method further comprises, prior to step A, milling the axial ends of the first and second tube walls to obtain machined ends of the first and second tube walls. The milling can be performed, for example, using a relatively simple three-axis milling machine.

[0016] An efficient method of welding two tubes together can be achieved, according to the present invention, when, during step C, a single weld layer is welded that extends across at least the entire first tube wall thickness and a first portion of the second tube wall thickness.

[0017] In a further embodiment, during step C, at least a portion of the circumferential weld extending through the first tube wall thickness and the first portion of the second tube wall thickness is welded by TIG welding. TIG welding provides a relatively deep penetration. Fronius offers the so-called ArcTIG process, which has proven suitable for implementing the present invention.

[0018] It is further beneficial to the welding process if the welding feed material is fed to the weld pool as wire, possibly in a preheated state, in an automated manner during TIG welding.

[0019] To perform the welding process in an automated manner, in a further embodiment of the method, while performing step C, a welding torch is placed in a fixed position relative to the seam between the first tube and the second tube that were fastened together during step B, and the first tube and the second tube that are fastened together are rotated about their coaxial axes. [Brief explanation of the drawings]

[0020] The invention will be further elucidated by the following description of some embodiments of the method according to the invention with reference to the figures. [Figure 1] FIG. 1 shows in axial section two tubes arranged coaxially relative to each other for the implementation of the method according to the invention. [Figure 2a] FIG. 2a shows the encircled area IIa of FIG. 1 in an unwelded state. [Figure 2b] FIG. 2b shows the area according to FIG. 2a in the welded state. [Figure 3] FIG. 3 shows in axial section two other tubes arranged coaxially relative to each other for the implementation of the method according to the invention. [Figure 4a] FIG. 4a shows the enclosed area IVa in FIG. 3 in an unwelded state. [Figure 4b]FIG. 4b shows the area according to FIG. 4a in the welded state. DETAILED DESCRIPTION OF THE INVENTION

[0021] FIG. 1 shows two steel tubes 1, 2, each having an outer diameter of 114.3 mm (4 inches). The tubes 1, 2 are arranged coaxially relative to each other and have cylindrical tube walls 3, 4, respectively. The thickness of the tube walls 3, 4 is equal to each other, as indicated by the letter d in FIG. 2a. In this embodiment, d is equal to 6.0 mm. The inner diameters of the tubes 1, 2 are also equal to each other, as are the outer diameters of the tubes 1, 2. A seam 5, more specifically a welded seam 5, is present between the opposing tubes 1, 2, more specifically between the axial end faces of the tube walls 3, 4 of the opposing tubes 1, 2.

[0022] When viewed in axial cross section as shown in Figure 2a, the weld seam 5 has a stepped shape due to the stepped axial ends of the opposing tube walls 3, 4. These stepped shapes complement each other as the axial ends fit together when the tubes 1, 2 are coaxially positioned relative to each other as shown in Figure 1. The tubes 1, 2 lock together radially so that they cannot move radially relative to each other and therefore remain coaxial.

[0023] The stepped shape of the axial ends of the tube walls 3, 4 is obtained by milling. More specifically, the stepped shape of the axial end faces is the result of each axial end of the tube 3 having an inner annular end face 6 and an outer annular end face 7. The end faces 6, 7 of the tube wall 3 are spaced apart from each other by a distance t when viewed in the axial direction of the tube 1. In the selected example, t is equal to 0.25 mm. In the selected embodiment, the radial dimension d1 of the inner annular end face 6 is 4.5 mm, and the radial dimension d2 of the outer annular end face 7 is 1.5 mm. In the embodiment of FIGS. 1, 2a, and 2b, d is equal to d1 + d2. The inner annular end face 6 forms a protrusion at each axial end of the tube wall 3. In general, it is advantageous for d2 to be smaller than d1 so that the transition between d1 and d2 is relatively close to the outer diameter of the tube, which is advantageous for the reliability of the weld joints produced. On the other hand, if d2 is too small, it may be disadvantageous in that each protrusion becomes easily damaged. Generally, it is preferable to select d2 to be greater than 0.5 mm, or greater than 1.0 mm.

[0024] The wall tube 4 also has an inner annular end face 8 and an outer annular end face 9, which are axially spaced apart by a distance t. The radial dimensions of the end faces 8 and 9 are equal to d1 and d2, respectively. The outer annular end faces 9 are part of protrusions provided at the respective axial ends of the tube wall 4.

[0025] From the above description, the stepped shapes of the axial ends of the tube walls 3, 4 facing each other complement each other.

[0026] The tubes 3 and 4 can be welded together by, for example, a TIG welding process. To weld the tubes 3 and 4 together, the common axis 10 of the tubes 1 and 2 is oriented horizontally, and the tubes 1 and 2 are secured together by attachment welds at multiple, discrete, approximately equidistant locations outside the weld seam 5. In this secured state, one of the tubes 1 and 2 is clamped to a rotating device that can rotate the attached tubes 1 and 2 about their common axis 10. Looking parallel to the axis 10, a welding torch is then positioned directly above the weld seam 5 at a 12 o'clock position. The welding process is then initiated, and the welding torch remains in that position or substantially in that position while the weld seam 5 rotates 360° about the axis 10. The welding can be performed at a relatively high amperage, for example, 300 A or more, and at a relatively high speed, for example, 25-30 cm per minute. As a result of the welding, a weld layer 21 is formed, as shown in FIG. 2b. The weld layer 21 is a weld penetration and extends both inside as well as outside the tube walls 3, 4. While Fig. 2b shows a stepped shape at the axial ends of the tube walls 3, 4 for clarity and to illustrate the invention, it will be clear to those skilled in the art that all material of the tubes 1, 2 inside the weld layer 21 has melted. Advantageously, the same welding process is used for welding the weld layer 21 and the attachment weld.

[0027] FIG. 3 shows two coaxial steel tubes 31, 32 arranged opposite each other with a weld seam 35 between them. The tubes 31, 32 have cylindrical tube walls 33, 34 of equal thickness, but greater than the thickness d of the tubes 3, 4. In this embodiment, the thickness D of the tube walls 33, 34 is approximately 10 mm. The weld seam 35 has a stepped shape on the inside of the tube walls 33, 34, at least in the axial cross section, that is identical to the stepped shape of the weld seam 5. More specifically, D1 = d1, D2 = d2, and T = t. On the outside of the tube walls 33, 34, the weld seam 35 is open and, more specifically, is substantially U-shaped. The bottom of the U is rounded with a radius R, and the legs of the U diverge to enclose an angle α having a magnitude of 40 degrees. The step shape of the inner portion of the weld seam 35 is within the central 10% of the U-shape of the outer periphery of the tubes 33, 34 when viewed in the axial direction.

[0028] Figure 4b shows the cross section according to Figure 4a in a welded state. The weld seam 35 is welded across the entire thickness D of the tube walls 33, 34 by a base weld layer 51, an upper weld layer 52 and a third weld layer 53 extending on both sides of the tube walls 33, 34. The weld layers 52, 53 arranged on the base weld layer 51 may advantageously be welded with a different welding process, such as a MIG / MAG process, to allow for faster filling of the weld seam.

Claims

1. 1. A method of coaxially welding a first metal tube having a first tube wall and a second metal tube having a second tube wall, comprising: the inner diameter of the first tube is equal to the inner diameter of the second tube; an axial end of the first tube wall is machined such that, when viewed in longitudinal cross section, the axial end of the first tube wall has a first stepped shape across at least a first portion of the first tube wall thickness; the stepped shape extends within a length between 0.10 mm and 1.00 mm in an axial direction of the first tube, the first portion extending from an interior side of the first tube wall and having an inner annular end surface and an outer annular end surface, the outer annular end surface being smaller than the inner annular end surface and having a thickness of at least 1.0 mm; an axial end of the second tube wall is machined so that, when viewed in longitudinal cross section, the axial end of the second tube wall has a second stepped shape across at least a first portion of the second tube wall thickness, the first portion extending from an interior side of the second tube wall; the first step shape and the second step shape complement each other; The method comprises: (A) coaxially positioning the first tube and the second tube relative to one another, the respective machined axial ends contact each other at least at a first portion of the first tube wall thickness and a first portion of the second tube wall thickness; the first step profile machined on the axial end of the first tube wall and the second step profile machined on the axial end of the second tube wall are fitted together such that the fitting of the first step profile and the second step profile prevents radial movement of the first tube and the second tube relative to each other; a positioning step; (B) securing the first tube and the second tube by attachment welding at a plurality of discrete locations around the circumference of the seam; (C) circumferentially welding the first tube and the second tube around a full circumference of the first tube and the second tube, the circumferential weld extends through the entire thickness of the first tube wall and the entire thickness of the second tube wall. welding; A method comprising:

2. The step shape extends within a length having a size between 0.15 mm and 0.75 mm when viewed in the axial direction of the first tube. The method of claim 1.

3. the first portion of the first tube wall thickness extends within a length, as viewed in the radial direction of the first tube, having a dimension between 2.0 mm and 12 mm, preferably between 2.5 mm and 10 mm, and more preferably between 3.0 mm and 8.0 mm; 3. The method according to claim 1 or 2.

4. The outer annular end surface and the inner annular end surface are arranged at a distance from each other when viewed in the axial direction of the first tube. The method according to any one of claims 1 to 3.

5. the outer annular end surface and / or the inner annular end surface extend parallel to a radial plane oriented perpendicular to the axial direction of the first tube; The method of claim 4.

6. the first portion of the first tube wall extends through the entire thickness of the first tube wall, and / or the first portion of the second tube wall extends through the entire thickness of the second tube wall; 6. The method according to any one of claims 1 to 5.

7. the machined end of the first tube wall is further machined radially outwardly of the first portion of the first tube wall thickness over a second portion of the first tube wall thickness that joins the first portion of the first tube wall; the machined end of the second tube wall is further machined outside the first portion of the second tube wall thickness over a second portion of the second tube wall thickness that joins the first portion of the second tube wall; after performing step A, an open seam exists between the first tube and the second tube at the second portion of the thickness; 6. The method according to any one of claims 1 to 5.

8. the open seam is substantially V-shaped or U-shaped when viewed in longitudinal cross-section; The method of claim 7.

9. a first portion of the thickness of the first tube and the second tube is located within a maximum axial dimension of the open seam, preferably within a central 50% of the maximum axial dimension of the open seam, more preferably within a central 10% of the maximum axial dimension of the open seam, when viewed in the axial direction; 9. The method according to claim 7 or 8.

10. before step A, milling axial ends of the first and second tube walls to obtain machined ends of the first and second tube walls; 10. The method according to any one of claims 1 to 9.

11. When performing step C, a single weld layer is welded to extend across at least the first tube wall thickness and the first portion of the second tube wall thickness.

11. The method according to any one of claims 1 to 10.

12. When performing step C, at least a portion of the circumferential weld extending through the first tube wall thickness and a first portion of the second tube wall thickness is welded by TIG welding.

12. The method according to any one of claims 1 to 11.

13. welding feed material is fed as wire, preferably preheated, into the weld pool during said TIG welding in an automated manner; The method of claim 12.

14. During step C, a welding torch is positioned in a fixed position relative to the seam between the first tube and the second tube secured together in step B; the fixed first tube and second tube rotate about their coaxial axes; 14. The method of any one of claims 1 to 13.

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