Method for manufacturing a composite tube

1. A method for manufacturing a composite tube having at least one multi-layered portion with an annular outer layer of a first alloy and an annular inner layer of a second alloy, the method comprising the steps of:

providing an annular base component of the second alloy, wherein the annular base component has a central through-hole extending along a longitudinal axis of the annular base component, and wherein the annular base component has an externally threaded section;

providing an annular outer component of the first alloy, wherein the annular outer component has an internally threaded section configured to engage with the externally threaded section of the annular base component;

forming a tubular work piece by mounting the annular outer component around the annular base component such that the internally threaded section of the outer component is in engagement with the externally threaded section of the annular base component, forming a mechanical interlock between the threaded sections; and

hot working the tubular work piece, so that on one hand a metallic bond is formed between the threaded sections of the annular outer component and the annular base component while the mechanical interlock is maintained, and so that on the other hand the tubular work piece is elongated and an outer diameter of the tubular work piece is reduced, thus forming a composite tube,

wherein the step of hot working comprises hot extrusion by pushing the tubular work piece through an extrusion die with a leading end first.

2. The method according to claim 1 , wherein the externally threaded section has an axial extension corresponding to an axial extension of the internally threaded section.

3. The method according to claim 1 , wherein the externally threaded section corresponds to an end portion of the annular base component.

4. The method according to claim 3 , wherein the externally threaded section has a smaller outer diameter than an adjacent non-threaded section of the annular base component.

5. The method according to claim 1 , wherein the externally threaded section extends along an entire axial length of the annular base component.

6. The method according to claim 1 , wherein the leading end is an end portion at which the annular outer component is mounted around the annular base component.

7. The method according to claim 1 , wherein the method further comprises machining the leading end prior to hot extrusion, such that a rounded leading end is formed.

8. The method according to claim 1 , further comprising straightening the composite tube after hot extrusion.

9. The method according to claim 1 , further comprising, after hot extrusion, at least one of pickling the composite tube and blasting the composite tube.

10. The method according to claim 1 , further comprising pre-heating the tubular work piece prior to the step of hot working the tubular work piece.

11. The method according to claim 1 , wherein the first alloy is selected from a stainless steel alloy, a nickel-based alloy, an iron chromium aluminium alloy, a carbon steel alloy, a zirconium alloy, an aluminium alloy, a copper alloy, or a titanium alloy.

12. The method according to claim 1 , wherein the second alloy has a different composition than the first alloy and is selected from a stainless steel alloy, a nickel based alloy, an iron chromium aluminium alloy, a carbon steel alloy, a zirconium alloy, an aluminium alloy, a copper alloy, or a titanium alloy.

13. The method according to claim 1 , wherein the composite tube has a mono-layered portion of said second alloy.

14. The method according to claim 1 , wherein hot extrusion reduces the outer diameter of the tubular work piece.

15. The method according to claim 1 , wherein the outer diameter and an inner diameter of the composite tube are essentially-constant along the composite tube.

16. The method according to claim 1 , wherein in a longitudinal cross-section of the composite tube, a portion of a thread of the externally threaded section of the annular base component extends in a longitudinal direction.

17. The method according to claim 1 , wherein, in a longitudinal cross-section of the composite tube, a portion of a thread of the externally threaded section of the annular base component overlaps in a radial direction a portion of a thread of the internally threaded section of the annular outer component.