Method for straightening of a FeCrAl alloy tube

Abstract

The present disclosure relates to a method for straightening of a tube comprising a ferritic FeCrAl-alloy. One reason for the challenges regarding the cold working of a hollow of a ferritic FeCrAl-alloy into a finished tube is that FeCrAl-alloys have a low ductility. Even if a tube of a FeCrAl-alloy is obtained by cold working a hollow into a tube, the tube can hardly be straightened. This is even more a problem if a tube obtained is annealed, wherein the annealing leads to a deformation of tube along the longitudinal direction of the tube. Therefore, there is a need for a method for straightening of a tube comprising a ferritic FeCrAl-alloy. Thus, according to the present disclosure a method for straightening of a tube is suggested, wherein the method comprises the steps of providing a tube comprising a ferritic FeCrAl-alloy, heating the tube, and straightening and forming the heated tube by stretching.

Claims

1. A method for manufacturing a tube, comprising the steps in the following order: providing a hollow comprising a ferritic FeCrAl-alloy; heating the hollow to a temperature below a recrystallization temperature of the ferritic FeCrAl-alloy; cold working the heated hollow into the tube; annealing the cold worked tube; and straightening the annealed tube using a straightening method including the steps of: heating the tube, and straightening and forming the heated tube by stretching, wherein the hollow, during cold working, has a temperature in a range from about 90° C. to about 600° C., wherein cold working the heated hollow into the tube is pilgering or drawing, and wherein the ferritic FeCrAl-alloy comprises, in wt-%: Cr 9 to 25, Al 3 to 7, balance Fe, and normally occurring impurities.

2. The method according to claim 1, wherein the heated tube is irreversibly stretched in a longitudinal direction of the tube.

3. The method according to claim 1, wherein during the step of straightening and forming, the tube is mounted at a first end of the tube and/or at a second end of the tube, and wherein at least the first end and/or the second end of the tube is pulled with a preset force.

4. The method according to claim 1, wherein during straightening and forming an electric voltage is applied to the tube, in order to heat the tube by an electric current flowing through the tube.

5. The method according to claim 1, wherein the tube is heated so that the tube during straightening and forming has a temperature in a range from about 100° C. to about 1400° C.

6. The method according to claim 1, wherein, during the step of straightening and forming, the tube is mounted at a first end of the tube and/or at a second end of the tube, and wherein at least the first end and/or the second end of the tube is pulled with a preset force, wherein, during the step of straightening and forming, the heated tube is irreversibly stretched in a longitudinal direction of the tube, wherein, during the step of straightening and forming, an electric voltage is applied to the tube, in order to heat the tube by an electric current flowing through the tube, and wherein heating the tube results in the tube having a temperature in a range from about 100° C. to about 1400° C. during the step of straightening and forming.

7. The method according to claim 1, wherein the temperature below the recrystallization temperature is 90° C. to 600° C.

8. The method according to claim 1, further comprising: before cold working the heated hollow into the tube, coating the hollow with a water-based polymer suspension.

9. The method according to claim 1, wherein, before heating the coated hollow to the temperature below the recrystallization temperature of the ferritic FeCrAl-alloy and cold working the heated hollow into the tube, the method further comprises: coating the hollow with a water-based polymer suspension, and drying the coating.

10. The method according to claim 9, wherein the temperature below the recrystallization temperature is 90° C. to 600° C.

11. The method according to claim 1, wherein annealing the cold worked tube is at a temperature of 700° C. to 1150° C.

12. The method according to claim 1, wherein the ferritic FeCrAl-alloy comprises, in wt-%: Cr 9 to 25, Al 3 to 7, Mo >0 to 5, C 0.01 to 0.05, N 0.01 to 0.06, O 0.02 to 0.10, Mn 0.05 to 0.50, P 0 to 0.80, S 0 to 0.005, Si 0 to 3.0, balance Fe, and normally occurring impurities.

13. The method according to claim 12, wherein the ferritic FeCrAl-alloy further comprises, in wt-%: Y 0.05 to 0.60, Z 0.01 to 0.30, Hf 0.05 to 0.50, Ta 0.05 to 0.50, and Ti 0 to 0.10.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Further advantages, features and applications of the present disclosure will become apparent from the following description of embodiments and the corresponding figures attached. The foregoing as well as the following detailed description of the embodiments will be better understood when read in conjunction with the appendant drawings. It should be understood that the embodiments depicted are not limited to the precise arrangements and instrumentalities shown.

(2) FIG. 1 is a schematic flow chart of a method for manufacturing a tube according to the present disclosure.

(3) FIG. 2 is a schematic side view of an apparatus with a tube for stretching this tube for straightening and forming of the tube.

DETAILED DESCRIPTION

(4) FIG. 1 is a flow chart exemplarily describing a method for manufacturing a tube according to an implementation of the present disclosure. In a first step 100, a hollow of a FeCrAl-alloy is provided.

(5) In the example depicted in the flow chart of FIG. 1, the hollow provided in step 100 in step 101 is glass-blasted on its inner surface, only. By glass-blasting the inner surface, any corrosion on the inner surface is ablated enhancing the properties of the finished tube. Surprisingly, a blasting of the hollow on its outer surface does not further enhance the properties of the finished tube.

(6) After blasting, the hollow in step 102 is immersed into a water-based polymer suspension. By immersing the hollow into the polymer suspension, the polymer suspension coats the hollow. After drying of the hollow in warm air in step 103, the polymer contained in the polymer suspension coats the entire hollow as a film and serves as a lubricant for the hollow during the cold working thereof into a tube.

(7) After the coating has been dried, the coated hollow is fed into a drawing bench in order to cold work the hollow into a tube. The hollow in step 104 is heated to a temperature of 125° C., wherein the temperature is measured right before the tube enters the forming zone defined by the drawing die and the mandrel. Finally, the hollow is drawn in step 105 through the gap defined by the drawing die and the mandrel. Simultaneously with the drawing of the hollow into the tube, a lubricant is applied to the outer surface of the hollow.

(8) The cold working process, i.e. the drawing of the hollow through the gap defined by the drawing die and the mandrel, not only reduces and defines the dimensions of the tube, but the cold working below the recrystallization temperature of the FeCrAl-alloy leads to a strain hardening of the material of the tube. In order to enhance the ductility of the material the tube in step 106 is annealed at a temperature in a range from about 700° C. to about 1150° C., wherein the exact temperature will depend on the microstructure of the FeCrAl-alloy.

(9) After annealing and cooling to a temperature around room temperature the tube is no longer straight in a longitudinal direction of the tube. In order to straighten and form the tube after annealing, the tube is inserted into a stretching equipment as it is schematically depicted in FIG. 2. In step 107 the tube is then simultaneously heated and stretched as schematically depicted in FIG. 1. The stretching is denoted by reference number 108, wherein the heating is denoted by reference number 109. What is important is that before the stretching 108 can start the tube must have reached a temperature range from 100° C. to 1400° C., such that the tube is in a heated state during the stretching. In this particular implementation heating is carried during the stretching. However, generally it is sufficient to stretch the tube at the increased temperature. Thus, in an implementation of the present disclosure, the tube is heated prior to the stretching only.

(10) In order to enable heating and stretching simultaneously, the apparatus 1 for stretching the tube 2 has a first clamping means 3 at a first end 4 of the tube 2. This first clamping mechanism 3 is in a fixed position relative to a baseplate of the apparatus 1. A second clamping means 5 is provided at a second end 6 of the tube 2. In contrast to the fixed clamping means 3 the second clamping means 5 is movable in a longitudinal direction 7 of the tube 2, wherein a distance between the fixed clamping means 3 and the second clamping means 5 is enlarged. By applying a preset force during the pulling of the second clamping means 5 the tube 2 is stretched.

(11) In order to heat the tube 2 to a preset temperature in the given range which is then held during the actual stretching, the first end 4 and the second end 6 of the tube 2 are connected to a voltage source 8 applying a voltage across the tube such that a current will flow through the tube 2, wherein the resistance within the tube 2 leads to a heating of the tube 2.

(12) For purposes of the original disclosure, it is noted that all features become apparent to a person skilled in the art from the present description, the figures and the claims even if they have only been described with reference to particular further features and can be combined either on their own or in arbitrary combinations with other features or groups of features disclosed herein as far as such combinations are not explicitly excluded or technical facts exclude such combinations or make them useless. An extensive, explicit description of each possible combination of features has only been omitted in order to provide a short and readable description.

(13) While the disclosure has been shown in detail in the figures and the above description, this description is only an example and is not considered to restrict the scope of protection as it is defined by the claims. The disclosure is not restricted to the disclosed embodiments.

(14) Modifications to the disclosed embodiments are apparent for a person skilled in the art from the drawings, the description and the attached claims. In the claims, the word “comprising” does not exclude other elements or steps and the undefined article “a” does not exclude a plurality. The mere fact that some features have been claimed in different claims does not exclude their combination. Reference numbers in the claims are not considered to restrict the scope of protection.

REFERENCE NUMERALS

(15) 1 Apparatus 2 Tube 3 First clamping means (fixed) 4 First end of the tube 2 5 Second clamping means 6 Second end of the tube 2 7 Longitudinal direction 8 Current source 100 Providing the hollow 101 Glass blasting the hollow 102 Coating the hollow 103 Drying the coating 104 Heating 105 Drawing 106 Annealing 107 Straightening and forming 108 Stretching 109 Heating