Method for manufacturing a composite tube

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.