Bimetallic tube and method for manufacturing a bimetallic tube

1. A bimetallic tube comprising a first metallic tube having an inner diameter (ID 1st ) and an outer diameter (OD 1st ), and a second metallic tube having an inner diameter (ID 2nd ) and an outer diameter (OD 2nd ),

wherein the first metallic tube is arranged within and force-fitted to the second metallic tube, wherein the first metallic tube comprises, in weight %,

Fe+Cr≤1.0;

C≤0.1;

O≤0.2;

Hf≤5.0;

balance Zr and normally occurring impurities,

wherein the second metallic tube is an austenitic stainless steel comprising, in weight,

C≤0.04;

Mn≤3.0;

P≤0.05;

S≤0.04;

Si≤1.0;

Cr 15.0-30.0;

Ni 7.0-25.0;

Mo≤1.0;

N≤0.10;

balance Fe and normally occurring impurities,

and wherein the force-fitting of the first metallic tube to the second metallic tube is at least 20 μm determined by the following procedure:

measuring the inner diameter (ID BM ) of the bimetallic tube;

removing the second metallic tube from the first metallic tube;

measuring the inner diameter (ID 1st ) of the first metallic tube after the second metallic tube has been removed; and

calculating the absolute value of the difference (ID BM −ID 1st ) between the inner diameter (ID BM ) of the bimetallic tube and the inner diameter (ID 1st ) of the first metallic tube after the second metallic tube has been removed.

2. The bimetallic tube according to claim 1 , wherein the content of Cr present in the second metallic tube is in a range of from 17 to 21 weight %.

3. The bimetallic tube according to claim 1 , wherein the content of Cr present in the second metallic tube is in a range of from 23 to 27 weight %.

4. A method for manufacturing a bimetallic tube comprising the steps of:

providing a first metallic tube having an inner diameter (ID 1st ) and an outer diameter (OD 1st ), and providing a second metallic tube having an inner diameter (ID 2nd ) and an outer diameter (OD 2nd ), wherein the outer diameter (OD 1st ) of the first metallic tube is smaller than the inner diameter (ID 2nd ) of the second tube;

inserting the first metallic tube into the second metallic tube; and

cold-drawing the first and second metallic tubes together, such that the first and second metallic tubes are force-fitted together,

wherein in the step of providing the first metallic tube, the first metallic tube comprises, in weight %;

Fe+Cr≤1.0;

C≤0.1;

O≤0.2;

Hf≤5.0;

balance Zr and normally occurring impurities,

wherein in the step of providing the second metallic tube, the second metallic tube is an austenitic stainless steel comprising, in weight %;

C≤0.04;

Mn≤3.0;

P≤0.05;

S≤0.04;

Si≤1.0;

Cr 15.0-30.0;

Ni 7.0-25.0;

Mo≤1.0;

N≤0.10;

balance Fe and normally occurring impurities,

and wherein after the step of cold-drawing the first metallic tube is force-fitted to the second metallic tube so that the spring back is least 20 μm determined by the following procedure:

measuring the inner diameter (ID BM ) of the bimetallic tube;

removing the second metallic tube from the first metallic tube;

measuring the inner diameter (ID 1st ) of the first metallic tube after the second metallic tube has been removed; and

calculating the absolute value of the difference (ID BM −ID 1st ) between the inner diameter (ID BM ) of the bimetallic tube and the inner diameter (ID 1st ) of the first metallic tube after the second metallic tube has been removed.

5. The bimetallic tube according to claim 4 , wherein the content of Cr present in the second metallic tube is in a range of from 17 to 21 weight %.

6. The bimetallic tube according to claim 4 , wherein the content of Cr present in the second metallic tube is in a range of from 23 to 27 weight %.

7. The method according to claim 4 , wherein in the step of cold-drawing, the outer diameter (OD 2nd ) of the second tube is reduced 10% or less compared to outer diameter (OD 2nd ) of the second metallic tube before cold-drawing.

8. The method according to claim 4 , wherein in the step of cold-drawing, the outer diameter (OD 2nd ) of the second tube is reduced 4% or less compared to outer diameter (OD 2nd ) of the second metallic tube before cold-drawing.

9. The method according to claim 4 , wherein after the step of cold-drawing the second metallic tube has an elongation of at least or equal to 35% and a hardness not more than or equal to 90 HRB.

10. The method according to claim 4 , wherein in the step of providing the first metallic tube and/or the second metallic tube is heat-treated before the step of inserting the first metallic tube into the second metallic tube.

11. The method according to claim 4 , wherein in the steps of providing first and second metallic tubes, the first metallic tube and/or the second metallic tube are cold worked tubes.

12. The method according to claim 4 , wherein in the step of providing the first metallic tube, the first metallic tube is a seamless tube.

13. The method according to claim 4 , wherein in the step of providing the first metallic tube, the average wall thickness of the first metallic tube is 0.3 mm to 2 mm.

14. The method according to claim 4 , wherein in the step of providing the second metallic tube, the second metallic tube is a seamless tube.

15. The method according to claim 4 , wherein in the step of providing the second metallic tube, the average wall thickness of the second metallic tube is 0.5 mm to 4 mm.

16. The bimetallic tube according to claim 2 , wherein the content of Cr present in the second metallic tube is in a range of from 18 to 20 weight %.

17. The bimetallic tube according to claim 3 , wherein the content of Cr present in the second metallic tube is in a range of from 24 to 26 weight %.

18. The bimetallic tube according to claim 5 , wherein the content of Cr present in the second metallic tube is in a range of from 18 to 20 weight %.

19. The bimetallic tube according to claim 6 , wherein the content of Cr present in the second metallic tube is in a range of from 24 to 26 weight %.