GB2057313A - Composite tube - Google Patents

Abstract

A composite tube comprises two or more coaxial seamless and/or seam welded tubes bonded together by means of a heat sensitive material, e.g. metal or adhesive. An inner tube is drawn down to a required internal diameter and is then introduced into the bore of an outer tube. A layer of heat sensitive bonding material is provided between the inner and outer tubes and the outer tube is drawn down onto the inner tube. The composite assembly thus formed, is then heated to effect bonding between the inner and outer tubes.

Description

SPECIFICATION Improvements in and relating to the production of metal tube This invention relates to improvements in and relating to the production of metal tube and in particular, but not exclusively, to the production of thick walled small bore metal tube. A particular example of such tube is steel diesel pipe, which is used for transferring liquid fuel from a metering pump to the injector of a diesel engine. Such pipe requires a thick wall to resist bursting or even elastic deformation when pressurised during use, however, for ease of bending the pipe must be of small outside diameter. These requirements are met by producing a tube with small bore and relatively thick wall, for example, the outside diameter of the tube may be from 5 to 6mm and the bore of the tube from 1.5 to 2mm.Because of the small bore dimension, the bore of the tube must be smooth and clean and must be uniform to within very close tolerances in order to achieve a low consistent flow resistance.

According to one aspect of the present invention a process for producing metal tube comprises the following stages: (a) forming a first seam welded or seamless tube; (b) drawing the first tube, using a floating plug and die, until the internal diameter of the tube is equal or substantially equal to the bore required for the final product; (c) introducing said first tube into the bore of a second seamless or seam welded metal tube, (d) providing a layer of a heat sensitive bonding agent between the outer surface of the first tube and the inner surface of the second tube; (e) reducing the second tube onto the first tube to form a composite assembly; and (f) heating the composite assembly to effect bonding between the first and second tubes.

According to a preferred aspect of the present invention the second metal tube is formed from strip metal which is wrapped about the first tube and the seam subsequently welded. By this means the first and second tubes may be assembled in a continuous process, using conventional tube rolling techniques.

Likewise where additional tubes are placed around the first and second tubes, these may be formed in a manner similar to the second tube.

Preferably the heat sensitive bonding agent is a metal of lower melting point than that used to form the first and second tubes, for example copper, brass, terne metal or silver solder, which may be fused and upon cooling will bond the tubes together, ie, by brazing, soldering or like process. Alternatively the bonding agent may be a hot melt adhesive or an adhesive which cures upon heating, to form the bond.

The bonding agent may be applied to the outer surface of the first tube and/or the inner surface of the second tube at any stage before the second tube is reduced down onto the first, to form the composite assembly. Preferably the bonding agent is applied to the metal strip before it is formed into the first and/or second tube.

The invention is now described by way of example only, with reference to a method of producing diesel pipe having a bore in the region of 1.5 to 2mm and an outside diameter in the region of 5 to 6mm.

Steel strip approximately 40mm wide and of the order of 0.9 to 1.0mum thick, coated on both sides with a layer of copper about 0.0025mm thick, is wound off a coil which is of the order of 1000 metres long and fed into a rolling mill. In the rolling mill the sheet is gradually formed into a tube of approximately 12mm diameter in which the edges of the sheet abut one another to form a longitudinal seam.

The longitudinal seam of the tube is then welded, inert gas being introduced into the bore of the tube in order to prevent oxidation of the weld bead and the bore of the tube. This welded tube is then gradually reduced in diameter by a stretch reducing mill, until its outside diameter is of the order of 6 to 7mm and its wall thickness from 1 to 0.9mm, and is annealed in line.

The welded tube is then cold drawn using a floating plug and die, to give an accurately dimensioned tube, having a smooth clean bore of 2mm diater and an outside diameter of 3.8mm.

The tube thus produced is fed in parallel with a further strip of copper coated steel of approximately 40mm wide and from 0.9 to 1 .Omm thick into a rolling mill, in which this second strip is wrapped around the welded tube until its longitudinal edges abut one another to form an outer tube of approximately 12mm diameter. The seam of this outer tube is again welded, with an inert gas in the bore of the tube to prevent oxidation of the weld bead and bore.

The composite tube is then fed through a stretch reducing mill, where the outer tube is reduced down onto the inner tube. The composite tube is then passed through a furnace and heated to a temperature at which the copper layer between the inner and outer tube fuses, so that upon subsequent cooling of the tube the inner and outer tubes are brased together. The final tube has a clean smooth bore of 2mm diameter and an outside diameter of 6mm.

By the method described above diesel pipe of 5 to 6mm outside diameter and 1.5 to 2mm bore may be produced in lengths of 1000 metres or even greater, which may subsequently be cut to the required lengths.

Various modifications can be made without departing from the invention. For example, while in the process described above, a coating of the bonding agent copper, is provided on both surfaces of the first and second tubes, each tube may be coated with the bonding agent on only one surface or infact only one of the tubes need be coated, provided that a layer of bonding agent of sufficient thickness to produce an acceptable bond, is provided between the assembled tubes. When tubes of even greater wall thickness are required three or more tubes, with coatings of bonding agent between each tube, may be assembled in the manner described above and subsequently bonded. Where, for example, the final product is formed from three concentric tubes, the inner and outer tubes may be formed from plain steel strip, whilst the intermediate tube is formed from steel strip coated on both sides with bonding agent, thus providing the layers of bonding agent between each tube.

Claims (19)

1. A method of producing a metal tube including the following stages; (a) forming a seam welded or seamless tube; (b) drawing the first tube, using a floating plug and die, until the internal diameter of the tube is equal or substantially equal to the bore required for the final product; (c) introducing said first tube into the bore of a second seamless or seam welded metal tube; (d) providing a layer of heat sensitive bonding agent between the outer surface of the first tube and the inner surface of the second tube; (e) reducing the second tube onto the first tube to form a composite assembly; and (f) heating the composite assembly to effect bonding between the first and second tubes.

2. A method according to Claim 1 in which the second tube is formed from strip metal which is folded around the first tube and the seam is welded.

3. A method according to Claim 1 or 2 in which the heat sensitive bonding agent is a metal of lower melting point than that used for the tubes.

4. A method according to Claim 3 in which the heat sensitive bonding agent is copper, brass, terne metal or silver solder.

5. A method according to Claim 1 or 2 in which the heat sensitive bonding agent is a hot melt adhesive.

6. A method according to Claim 1 or 2 in which the heat sensitive bonding agent is an adhesive which cures upon heating.

7. A method according to any one of the preceding Claims in which the bonding agent is applied to the outer surface of the first tube and/or the inner surface of the second tube.

8. A method according to Claim 7 in which the first and/or second tube is made from a metal strip the seam of which is welded, the bonding agent is being applied to the metal strip before it is formed into the first and/or second tube.

9. A method according to any one of the preceding Claims in which the first and second tubes are located within one or more additional seamless or seam welded metal tubes, these additional tubes being reduced down onto the first and second tubes and bonded thereto.

10. A method according to Claim 9 in which an assembly of three or more metal tubes is formed and each tube is subsequently bonded to the adjacent tubes in a single operation, in which the composite assembly is heated.

11. A method of producing a tube substantially as described herein.

12. A tube comprising first and second co-axial seamless and/or seam welded metal tubes, the inner surface of the second tube being bonded to the outer surface of the first tube by means of a heat sensitive bonding agent.

13. Atube according to Claim 12 in which one or more additional seamless or seam welded metal tubes, co-axial with the first and second tubes are bonded to the outer surface of the second tube using a heat sensitive bonding agent.

14. A tube according to Claim 12 or 13 in which the heat sensitive bonding agent is a metal of lower melting point than that used to form the tubes,

15. A tube according to Claim 14 in which the heat sensitive bonding agent is copper, brass, terne metal or silver solder.

16. Atube according to Claim 12 or 13 in which the heat sensitive bonding agent is a hot melt adhesive.

17. A tube according to Claim 12 or 13 in which the heat sensitive bonding agent, is an adhesive which cures upon heating.

18. A tube formed in accordance with the method claimed in any one of Claims 1 to 11.

19. A tube substantially as described herein.