SAMAROV, Victor (6741 Lincoln Avenue, #54Buena Park, California, 90620, US)
| CLAIMS What is claimed is: 1. A method for cladding tubes comprising: providing a first tube and a second tube, wherein said first tube is configured to fit within said second tube; positioning said first tube within said second tube, thereby defining a gap therebetween; depositing a powdered metal in said gap; closing the gap using covers; evacuating contaminants from said gap; sealing at least one cover; hot isostatically pressing said first and second tube with said powdered metal sealed therein for a predetermined period of time to produce a billet; and reducing the billet into a clad tube. 2. The method of claim 1 wherein said clad tube is extruded to a predetermined configuration. 3. The method of claim 1 wherein said first tube is manufactured from a low alloy steel and said second tube is manufactured from one of an austenitic steel and a mild steel. 4. The method of claim 1 wherein said powdered metal has a composition about equal to that of an Inconel 625 alloy. 5. The method of claim 1 wherein said powdered metal has a composition of about fifty percent nickel and about fifty percent chromium. 6. The method of claim 1 wherein said hot isostatically pressing sinters said powdered metal and bonds said powdered metal to a portion of said first and second tubes. 7. The method of claim 1 wherein said powdered metal is fed into said gap using a pressurized air device. 8. A method for cladding tubes comprising: providing a first tube manufactured from a low alloy steel and a second tube manufactured from an austenitic steel, wherein said first tube is configured to fit within said second tube; positioning said first tube within said second tube, thereby defining a gap therebetween; depositing in said gap a mixture of powdered metals collectively having a heat expansion coefficient between a heat expansion of the first tube and the second tube; evacuating contaminants from said gap; sealing said gap; and hot isostatically pressing said first and second tube with said powdered metal sealed therein for a predetermined period of time to produce a billet; reducing the billet into a clad tube. 9. The method of claim 8 wherein said clad tube is reduced in thickness by a predetermined amount. 10. A method for cladding tubes comprising: providing a first tube and a second tube, wherein said first tube is configured to fit within said second tube; positioning said first tube within said second tube, thereby defining a gap therebetween; depositing a powdered metal, in said gap; sealing the gap; evacuating contaminants from the gap; hot isostatically pressing said first and second tube with said powdered metal sealed therein for a predetermined period of time to produce a billet; and reducing the billet to a clad tube. 1 1. The method of claim 10 wherein: the first tube is manufactured from a low alloy steel. 12. The method of claim 10 wherein: the second tube manufactured from a mild steel. 13. The method of claim 10 wherein: the powdered metal has a composition of about fifty percent nickel and about fifty percent chromium. 14. The method of claim 10 wherein said clad tube is extruded to a predetermined configuration. 15. The method of claim 1 wherein the step of depositing a powdered metal in said gap, comprises the steps of: positioning a sacrificial metal container in said gap, and depositing a powdered metal in the sacrificial container. 16. The method of claim 8 wherein the mixture of powdered metals has a composition about equal to that of an Inconel 625 alloy. |
TECHNICAL FIELD
[0001] The present disclosure relates generally to a method for cladding tubes, and more particularly, to a method using powder metallurgy and hot isostatic pressing to clad tubes.
BACKGROUND OF THE INVENTION
[0002] Steam generators, also referred to as boilers, are used in various systems. For example, boilers can be used to produce steam for use in a turbine for production of electricity and in chemical processes for providing energy to initiate a chemical reaction. Some boilers include one or more walls, each formed of a plurality of tubes, the walls being secured to one another thereby surrounding a combustion chamber within the boiler. Additional groups of tubes can be disposed within the combustion chamber.
[0003] Each of the tubes also has an inside surface defining a passage extending therethrough. One end of each of the plurality of tubes can be in fluid communication with a water supply header while an opposing end of each of the plurality of tubes can be in fluid communication with a steam header. During operation of the boiler, combustion generally occurs in the combustion chamber and heats water flowing through the passages, creating steam that is fed to the steam header. The outer surfaces of the tubes in the combustion chamber are exposed to fuel, combustion, heat and combustion byproducts that corrode the tubes. As a result, the useful life of the tubes is reduced.
SUMMARY OF THE INVENTION
[0004] According to aspects illustrated herein, there is provided a method for cladding tubes that includes positioning a first tube within a second tube. The inner diameter defined by the first tube and the outer diameter of the second tube is sized so that an annular gap is formed between the first and the second tubes. A powdered metal is deposited in the annular gap, thereby forming a tube assembly. Covers are placed over the gaps. Contaminants are evacuated from the annular gap and the gap is sealed. The tube assembly is then hot isostatically pressed for a predetermined period of time to produce a billet that can be processed into a clad tube. [0005] According to other aspects disclosed herein, the first tube can be manufactured from a low alloy steel and the second tube can be manufactured from an austenitic steel. The powdered metal deposited in the annular gap can have a composition of an Inconel 625 alloy.
[0006] According to other aspects disclosed herein, the first tube can be manufactured from a low alloy steel and the second tube can be manufactured from mild steel and acts as a sacrificial container to protect the powder layer and provide lubrication during further processing. In this case the powdered metal has a composition of about fifty percent nickel and about fifty percent chromium.
[0007] The above described and other features are illustrated by the following figures and detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Referring now to the Figures, which are exemplary embodiments, and wherein the like elements are numbered alike:
[0009] FIG. 1 is a cross sectional end view of a tube and a cladding tube, before pressing;
[0010] FIG. 2 is a cross sectional end view of a portion of a tube assembly before pressing;
[0011] FIG. 3 is a cross sectional end view of the portion of a tube assembly of FIG. 2 with a powdered metal disposed therebetween;
[0012] FIG. 4 is a cross sectional side view of the tube assembly of FIG. 3, with end covers disposed thereon; and
[0013] FIG. 5 is a cross sectional view of the tube assembly of FIG. 4, after pressing. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0014] Typically tubes operating in corrosive environments are coated, using techniques such as thermal spray or vapor deposition to provide a more protective surface layer. In the most aggressive environments clad tubing produced by co-extrusion has been used. However limitations in the integrity of the bond formed in this way can lead to debonding particularly during long exposures in thermal cycling conditions as a result of the stresses associated with the mismatch in thermal expansion coefficients between the austenitic and ferritic steels.
[0015] In the present invention the introduction of a layer of alloy powder will improve bond integrity and since the coefficient of thermal expansion is intermediate between those of the austenitic and ferritic steels, thermal stress effects will be reduced. [0016] FIG. 1 illustrates a tube 10 which is to be clad with a cladding tube 12. The tube 10 is manufactured from a low alloy steel and the cladding tube 12 is manufactured from a suitable corrosion resistant material, such as austenitic steel. The cladding tube 12 is sized to fit over the tube 10, as described below. Thus the cladding tube 12 has an inside surface 14 defined by a diameter Dl and the tube 10 has an outside surface 16 defined by an outside diameter D2 which is less than the inside diameter Dl . While the cladding tube 12 is described as being manufactured from austenitic steel, it is contemplated that the cladding tube can be manufactured from other materials also, depending upon its intended use.
[0017] As shown in FIG. 2, a tube assembly 18 includes the tube 10 positioned substantially coaxially inside the cladding tube 12 thereby defining an annular gap 20 therebetween of a substantially uniform width Wl . An interior surface 22 of the tube 10 and an exterior surface 24 of the cladding tube 12 are separated by a distance Tl . During assembly, the tube 10 is held in position inside the cladding tube 12 by a suitable fixture (not shown), to maintain the uniform width Wl . While the annular gap 20 is shown and described as having the substantially uniform width Wl , non-uniform cavities can also be employed without departing from the broader aspects disclosed herein.
[0018] As illustrated in FIG. 3, the annular gap 20 is filled with a powdered metal 26, for example one having a chemical composition approximately equivalent to that of an Inconel 625 alloy (UNS Designation N06625). The powdered metal 26 can be deposited in the annular gap 20 by any suitable method such as, but not limited to, a gravity feed and/or a pressurized air feed device. While the powdered metal 26 is described as having a chemical composition approximately equivalent to that of an Inconel 625 alloy, other compositions including but not limited to a powdered metal having a composition of about 50% nickel and about 50% chromium, a composition having about 40-60% chromium or a composition having about 40-60% nickel can also be employed without departing from the broader aspects disclosed herein.
[0019] Referring to FIGS. 3 and 4, cover 28 placed over the ends of the tubes 10, 12. Essentially all contaminants such as liquid, air and gas are evacuated from the annular gap 20, while the powdered metal 26 is retained therein. Such evacuation of the annular gap 20 can be accomplished by suitable means such as with a vacuum pump (not shown). In practice, the cover 28 is secured to the tube 10 and the cladding tube 12 adjacent the end 30 by welding; and another of the covers 28 is welded to the tube 10 and the cladding tube 12 adjacent the opposing end 32, thereby sealing the powdered metal 26 in the annular gap 20. Evacuation of the annular gap 20 is earned out by attaching a vacuum tube 36 into a port 38 that passes into gap 20. Vacuum tube 36 connects to an evacuation system. When evacuation is complete port 38 is sealed and the vacuum tube 36 is removed.
[0020] The tube assembly 18 illustrated in FIG. 4 is hot isostatically pressed for a predetermined period of time to sinter the powdered metal 26 and bond the powdered metal to the outside surface 16 of the tube 10 and to the inside surface 14 of the cladding tube 12. The hot isostatic pressing subjects the tube assembly 18 to a suitable elevated temperature and includes the application of a suitably elevated pressure, as indicted by the plurality of arrows designated "P", to the interior surface 22 of the tube 10, the exterior surface 24 of the cladding tube 12 and exposed surfaces 34 of the covers 28.
[0021] In FIG. 5, the tube assembly 18' is shown with port 38 sealed. After the hot isostatic pressing of tube assembly 18', the annular gap 20 is compressed to a uniform width W2 which is less than the width Wl before isostatic pressing. In addition, the distance Tl between the interior surface 22 and the exterior surface 24 has decreased to a distance T2, as a result of the hot isostatic pressing. The tube assembly 18' can be used as a billet which can be further processed by extrusion or some other reduction process to produce a length of clad tubing of the desired size and thickness.
[0022] Reduction and deformation of the billet into desired sizes, shapes and thicknesses actually improves the bond between the tubes 10, 12.
[0023] Application of the hot isostatic pressing to the tube assembly 18 using the powdered metal 26 as described herein, has utility in that bonds between the sintered powdered metal 26 and the inside surface 14 of the cladding tube 12 and the outside surface 16 of the tube 10 are improved, compared to clad tubes manufactured from other cladding processes, for example those manufactured from a process in which the tube and cladding are in contact with one another. The use of the powdered metal 26 having a composition equivalent to an Inconel 625 alloy is particularly useful in improving such bonding.
[0024] Application of the hot isostatic pressing to the tube assembly 18 wherein the powdered metal 26 has a composition of about 50% nickel and about 50% chromium, has further utility in providing a corrosion resistant layer between the tube 10 and the tube 12 which in this case is of mild steel and serves as a sacrificial component to enclose the powdered metal and which is removed by pickling from the finished tube.
[0025] While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
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