Connection tube and its method of manufacturing

1. A method for manufacturing a connection tube for connecting two tubes of different alloys, comprising the steps of:

forming a composite tube having an annular outer layer of a first alloy and an annular inner layer of a second alloy;

working a first end portion of the composite tube such that a diameter of the composite tube gradually decreases from a middle portion of the composite tube toward the first end portion;

removing the annular inner layer from the first end portion; and

removing the annular outer layer from a second end portion of the composite tube, thus forming a connection tube having a first end portion of said first alloy and a second end portion of said second alloy,

wherein the step of forming the composite tube comprises 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, wherein the annular base component has a threaded section with a helically extending external thread and a non-threaded section with a transition surface including a first plurality of surface portions, wherein one of the first plurality of surface portions is concave or convex,

providing an annular outer component of the first alloy, wherein the annular outer component has a threaded section with a helically extending internal thread configured to engage with the helically extending external thread of the threaded section of the annular base component and a non-threaded section with an end surface including a second plurality of surface portions, wherein one of the second plurality of surface portions is concave or convex and the end surface is configured to conformally engage with the transition surface of the non-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 (i) the helically extending internal thread of the threaded section of the annular outer component is in engagement with the helically extending external thread of the threaded section of the annular base component, forming a mechanical interlock between the threaded sections, and (ii) the end surface of the non-threaded section of the annular outer component conformally engages with the transition surface of the non-threaded section of the annular base component, and

hot working the tubular work piece, wherein hot working (a) forms a metallic bond between the helically extending internal thread of the threaded section of the annular outer component and the helically extending external thread of the threaded section of the annular base component while the mechanical interlock and conformal engagement between the end surface of the non-threaded section of the annular outer component and the transition surface of the non-threaded section of the annular base component are maintained, and (b) elongating the tubular work piece and reducing an outer diameter of the tubular work piece.

2. The method according to claim 1 , wherein the composite tube has a dimension defined by an outer diameter (φ1), an inner diameter (φ2) and an interfacial diameter (φ3), and wherein the step of working the first end portion of the composite tube is carried out such that the interfacial diameter (φ3) of the first end portion corresponds to the inner diameter (φ2) of the middle portion.

3. The method according to claim 1 , wherein at least one of the step of removing the annular inner layer and the step of removing the annular outer layer comprises cutting machining and/or abrasive machining.

4. The method according to claim 1 , wherein the threaded section extends along a major part of an axial length of the annular base component.

5. The method according to claim 1 , wherein the internally threaded section extends along a major part of an axial length of the annular outer component.

6. The method according to claim 1 , wherein the step of hot working comprises hot extrusion.

7. 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.

8. 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.

9. The method according to claim 1 , wherein a first one of the first plurality of surface portions is concave and a second one of the first plurality of surface portions is convex.

10. The method according to claim 9 , wherein the first one of the first plurality of surface portions is at a first end of the transition surface and the second one of the first plurality of surface portions is at a second end of the transition surface.

11. The method according to claim 10 , wherein the second end of the transition surface is radially outward of the first end of the transition surface.

12. The method according to claim 9 , wherein the transition surface is S-shaped and the first one of the first plurality of surface portions is at a first end of the transition surface and the second one of the first plurality of surface portions is at a second end of the transition surface.

13. The method according to claim 1 , wherein a first one of the first plurality of surface portions is convex and a second one of the first plurality of surface portions is convex.