Patent Description:
Interest has grown in the development of light weight, high flux heat exchangers for various applications. For example, single stage reusable near space air breathing flight vehicles that include light weight, high flux heat exchangers have been proposed for use in low cost manned space flight operations, e.g., to cool hot (e.g., -<NUM>) inlet air at speeds of Mach <NUM> or higher. Proposed heat exchangers for such purposes are constructed using small diameter (e.g., <NUM> millimeter (mm) (~<NUM> inch) Outside Diameter (OD)), very thin wall (e.g., <NUM> micron (µm) (<NUM> inch)) superalloy tubes.

The dimensions and operating parameters required for such tubes present a variety of tube making challenges. For example, tube drawing of small diameter tubes generally requires highly specialized tube making skill and/or apparatus, even at relatively traditionally tube wall thicknesses (e.g., <NUM>-<NUM> (<NUM> to <NUM> inch). As the tube wall thickness decreases below <NUM> (e.g., towards <NUM>), tube drawing often results in undesirable through-the-wall defects. Forming small diameter tubes by tube drawing long lengths using conventional lubrication and degreasing practices can also result m carbon or nitrogen contamination of the material used to form the tube - which may be undesirable for some applications.

Several methods exist for producing small diameter, thin- wall tubing, but are not without challenges. For example, bench drawn ng may be used to form small diameter, thin-wall tubing meeting the above noted specifications. However, the cost of bench drawing such tubing may be high, the production capacity may be limited, tube lengths are limited, and use of that process may entail significant scrap loss.

Small diameter, thin-wall tubing may also be made by coil drawing using a coil welding and tube sinking (drawing without internal diameter support) process. The product produced by such a process may suffer from through the wall defects, however, both at and away from the weld. It may also be difficult to use some materials (e.g. , superalloys) in such a method, and the product may exhibit a rough surface with longitudinal defects.

<CIT> describes a well screen for separating particulated material from formation fluid comprising, in combination: an elongated, tubular mandrel having a longitudinal bore defining a production flow passage, said mandrel being radially intersected by a flow aperture; a fluid-porous, particulate-restricting member mounted on said mandrel and overlying said flow aperture; and, a sacrificial foil disposed intermediate said particulate-restricting member and said mandrel, said foil covering said flow aperture.

<CIT> describes a method for forming a layer of synthetic corrosion products on tubing surfaces, said method comprising the steps of: selecting an object tube and a sacrificial tube, said sacrificial tube having a thermal expansion coefficient which is not equal to a thermal expansion coefficient of said object tube; placing said object tube and said sacrificial tube so as to be disposed circumjacent, one inside the other, and thereby creating an annular region between said object tube and said sacrificial tube; filling said annular region with a sludge slurry; and heating at least said inside tube so as to expand said inside tube, said inside tube thereby exerting pressure on said sludge slurry and thereby adhering a layer of synthetic corrosion product from said sludge slurry onto said object tube.

<CIT> describes the manufacture of clad tubes using powder metallurgy and hot isostatic pressing is described. A tube of corrosion resistant material is placed inside another tube and the annular space filled with metal powder. The tubes are covered, evacuated, sealed, and consolidated by hot isostatic pressing creating a billet. The billet is then reduced in thickness into a clad tube. Alternatively, a low-alloy steel tube is surrounded by a thin tube of mild steel and the annular gap formed between them is filled with powder alloy with high corrosion resistance. The tubes are covered, evacuated, sealed and consolidated by hot isostatic process into a billet for subsequent reduction to result in a clad tube. The mild steel tube is sacrificial and is removed by pickling to leave a low alloy tube with a thin surface layer of corrosion-resistant alloy.

<CIT> describes a conduit for conveying hydrogen under conditions of high temperature and high pressure comprises a clad, seamless tubing having a substrate layer formed of a tough base material that includes iron, chromium, or nickel, or alloys thereof. The tubing further has a thin cladding or surface layer formed of an alloy including at least about <NUM>-<NUM>% aluminum, combined with other materials that include at least one of iron and nickel and chromium, in an amount sufficient to make the surface layer capable of resisting hydrogen permeation. The conduit is formed by draw bonding a cladding tube in the substrate tube.

<CIT> describes a low solvus, high refractory alloy having unusually versatile processing mechanical property capabilities for advanced disks and rotors in gas turbine engines.

<CIT> describes a microtube-strip counterflow heat exchanger consisting of a number of small modules connected in parallel. Each module typically contains eight rows of one hundred tubes, each of <NUM> outside diameter and <NUM> length. The tubes are metallurgically bonded via the diffusion welding technique to rectangular header tube strips at each end. Caps suitable for manifolding are welded over the ends. Cages are provided to cause the shell-side fluid to flow in counterflow fashion over substantially all of the tube length, and suitable manifolds are provided to connect the modules in parallel.

<CIT> describes a method for producing a tube made of two sections joined together of two different metals having reciprocal metallurgical affinity, which method comprises the following steps; degassing and cleaning two tubular blocks of substantially the same length each made of one of said metals, and a casing made of a malleable metal, the hollow part of one of said blocks having transverse dimensions substantially equal to the outer cross-section of the other block so that the blocks can be fitted one into the other; fitting said blocks one into the other to form a billet; enclosing said billet in said casing, producing a vacuum therein and sealing it vacuum tight; bringing said billet and casing to a plastic state by heating; extruding said billet to obtain a composite tube of which the wall is of two layers of the different metals, which layers are bonded together by a metallurgical bond, the composite tube being enclosed in a metal envelope obtained from said casing as a consequence of the extrusion; forming the composite tube to obtain a progressive change in tube diameter over a portion of the tube where the joint between two adjacent sections is to be produced, which forming produces a formed tube section with inside and outside transverse dimensions different from the adjacent unformed section; finishing the extruded and formed composite tube produced as above in order to bring it to the final desired inner and outer transverse dimensions, which finishing is carried out over the entire length of the tube section by removing metal so that said final inner dimensions will correspond with or be in excess of the inside transverse dimensions of the unformed section of the composite tube, and the final outer transverse dimensions will correspond with or be less than the outside transverse dimensions of the formed section of the composite tube.

Features and advantages of embodiments of the claimed subject matter will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, wherein like numerals depict like parts, and in which:.

The present disclosure generally relates to composite tube assemblies, thin- walled tubing, and methods of forming the same. In general, the composite tube assemblies of the present disclosure include an inner tube and an outer tube, wherein one of the inner and outer tube is a functional tube, and the other of the inner and outer tube is a sacrificial tube. Put differently, the composite tube assemblies described herein include a functional tube and a sacrificial tube, wherein the functional tube is disposed inside the sacrificial tube, or the sacrificial tube is disposed inside the functional tube. The composite tube assemblies described herein are drawn into a drawn composite tube assembly that includes a drawn functional tube and a drawn sacrificial tube. Following drawing, the drawn sacrificial tube is removed (e.g., by exposure to a corrosive media), leaving the drawn functional tube. As will be described in detail below, the composite tube assemblies and methods of the present disclosure enable the formation of a drawn functional tube while reducing, minimizing, or even eliminating the formation of through the wall defects in the drawn functional tube. Alternatively or additionally, the composite tube assemblies and methods of the present disclosure may limit or even prevent carbon and/or nitrogen contamination of the drawn tube.

In particular, the invention is directed to a drawn composite tube assembly , comprising:
a drawn functional tube formed of a first material, the drawn functional tube having a first inner surface and a first outer surface; and.

The superalloy can comprise at least one of nickel, cobalt, and iron, and which: can have a yield strength (YS) of at least <NUM> megapascals (MPa); can have an ultimate tensile strength (UTS) of at least <NUM> MPa; and in which one or more of said YS and said UTS can be maintained at temperatures at or above <NUM>, or at temperatures greater than or equal to <NUM>% of the absolute melting temperature of the superalloy.

The second material can be selected from the group consisting of copper, a copper alloy, iron, an iron alloy, nickel, a nickel alloy, cobalt, a cobalt alloy, or a combination thereof.

The first material can be a nickel superalloy, and the second material is thoria dispersed nickel; or the first material can be a nickel superalloy, and the second material comprises greater than or equal to <NUM>% by weight of nickel.

The drawn functional tube can have a first outside diameter (OD1); the drawn sacrificial tube can have a second outside diameter (OD2); OD2 > OD1; and OD1 can be in the range of. <NUM> to <NUM> millimeters (mm).

The functional tube can have a first average wall thickness (WT1), wherein WT1 ranges from <NUM> to <NUM>, wherein optionally the drawn sacrificial tube has a second average wall thickness (WT2); and WT2 ranges from <NUM>(WT1) to <NUM>(WT1).

The functional tube can have a wall thickness variation that is less than or equal to <NUM>% of WT1.

The first outer surface and the second inner surface can be metallurgically bonded to each other.

The corrosive media can be selected from the group consisting of hydrochloric acid, hydrofluoric acid, nitric acid, phosphoric acid, sulphuric acid, or a combination thereof.

The invention is also directed to a method of making a tube, comprising:
forming a first composite tube assembly, the first composite tube assembly comprising:.

Forming the first composite tube assembly can comprise:.

Forming the first composite tube can further comprise, after disposing said first sacrificial tube inside the first functional tube or disposing said first functional tube inside the first sacrificial tube, drawing the first functional tube and the first sacrificial tube through a drawing die, such that the first outer surface is brought into contact with the second inner surface, or the first inner surface is brought into contact with the second outer surface.

Metallurgically bonding can comprise annealing said first sacrificial tube and said first functional tube to cause diffusion bonding between the first material and the second material.

As used herein, the term "compatible," when used in reference to a material of a sacrificial tube and/or a functional tube, means that the materials of the sacrificial tube and <NUM> the functional tube do not form intermetallic compounds, voids, or other undesirable phases when they are heated while in contact with one another. The materials also respond in a similar fashion to the large plastic deformation (up to <NUM>% reduction in area) and annealing steps (to remove the cold work from drawing) utilized in tube drawing.

As used herein, the term "corrosive media" refers to a compound or composition that will remove the material of a sacrificial tube, e.g. , by corrosion, dissolution, or some other mechanism. Non-limiting examples of corrosive media include strong and weak acids, including but not limited to hydrochloric acid, hydrofluoric acid, nitric acid, phosphoric acid, sulphuric acid, metallic salts (e.g., ferric chloride), combinations thereof, and the like. In embodiments, the corrosive media is selected to selectively remove the material of a sacrificial tube, while not removing or not substantially removing a material of a functional tube. In that context, "not substantially removing" means that when a material of a functional tube is exposed to a corrosive media for a defined time and at a defined temperature (e.g., for <NUM> hour at room temperature), less than or equal to <NUM>% by weight of the material of the functional tube is removed.

As used herein, the term "tube" refers to an elongated hollow object having an interior volume that is defined by at least one wall. For the sake of convenience and ease of understanding the present disclosure will describe various embodiments m which the term tube is used to refer to an elongated object having interior volume having a circular cross section. However, the tubes described herein are not limited to elongated objects that have an interior volume with a circular cross section. Rather, the term "tube" encompasses elongated objects having an interior volume that is defined by at least one wall, wherein the interior volume has any geometric or irregular cross section. Thus, the term "tube" encompasses elongated hollow objects having an interior volume with a geometric (e.g., circular, elliptical, oval, trigonal (e.g., triangular), quadrilateral (square, rhomboid, rectangular, etc.), pentagonal, hexagonal, etc., or irregular (e.g., non-geometric) cross section.

As used herein, the term "superalloy" means an alloy that contains at least one of nickel, cobalt, and/or iron, and which: has a yield strength of at least <NUM> megapascals (MPa); has an ultimate tensile strength of at least <NUM> MPa; and in which one or more of such properties is maintained at temperatures at or above <NUM>, or at temperatures greater than or equal to <NUM>% of the absolute melting temperature of the alloy. Such alloys may also exhibit good creep resistance at such temperatures.

One aspect of the present disclosure relates to composite tube assemblies that include a functional tube and a sacrificial tube. Reference is therefore made to <FIG>, which is a cross sectional view of one example of a composite tube assembly <NUM> consistent with the present disclosure. As shown, the composite tube assembly <NUM> includes a first functional tube <NUM> and a first sacrificial tube <NUM>. For the sake of convenience and ease of understanding, <FIG> depicts an embodiment in which the first functional tube <NUM> is disposed inside the first sacrificial tube <NUM>. Such a configuration is not required, however, and the relative position of the first sacrificial tube and the first functional tube may be reversed (i.e., the first sacrificial tube <NUM> may be disposed inside the first functional tube <NUM>). Moreover while embodiments of the present disclosure include a single first functional tube and a single first sacrificial tube, multiple functional tubes (e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or more) and/or multiple sacrificial tubes (e.g., <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or more) may be used.

The first functional tube <NUM> is formed from a first material and the first sacrificial tube <NUM> is formed from a second material. In general, the first material is or includes a material that is selected to achieve desired physical properties in an end-product (e.g., a drawn functional tube), while remaining compatible with the second material and being resistant to a corrosive media that may be used to remove the second material. Non- limiting examples of suitable first materials that may be used include metals, metal alloys, combinations thereof, and the like. Suitable metal alloys include alloys that include nickel, iron, cobalt, or a combination thereof, either alone or in combination with other alloying elements. In embodiments, the first material is or includes a superalloy, such as a superalloy containing at least one of nickel, cobalt, and/or iron. Specific non-limiting examples of suitable alloys that may be used as or in the first material include one or a combination of two or more of the materials set out in Table <NUM> below:.

As noted above, the first sacrificial tube is formed from a second material. In general, the second material is or includes a material that is selected to facilitate drawing of the first functional tube into a drawn functional tube, while being compatible with the first material and being removable by exposure to a corrosive media. In an aspect of the disclosure suitable second materials include metals, metal alloys, and combinations thereof, such as but not limited to copper, copper alloys, nickel, nickel alloys that contain greater than <NUM>% by weight (e.g., greater than or equal to <NUM>% by weight) of nickel, thoria dispersed (TD) nickel, nickel molybdenum (NiMo) alloys, cobalt, cobalt alloys including one or more secondary elements (except chromium) that are present in the first material, iron, iron alloys (e.g., low' carbon steel, iron nickel alloys), combinations thereof, and the like. In embodiments, the second material is or includes nickel or a nickel alloy when the first material is or includes a nickel super alloy. In other embodiments, the second material is or includes cobalt or a cobalt alloy when the first material is or includes a cobalt superalloy. In specific non-limiting embodiments, the second material is or includes one or more of the materials set out in Table <NUM> below.

In embodiments, the first material is or includes a nickel superalloy (e.g., Haynes <NUM>) and the second material is or includes TD nickel. In other embodiments, the first material is or includes a nickel superalloy (e.g., Inconel <NUM>), and the second material is or includes nickel or a nickel alloy containing 〉 <NUM>% by weight of nickel (e.g., Nickel <NUM>).

As shown but not labeled in <FIG> , the functional tube <NUM> has a first inner surface and a first outer surface, and the sacrificial tube <NUM> has a second inner surface and a second outer surface. In the illustrated embodiment, composite tube assembly <NUM> is configured such that functional tube <NUM> is disposed within the sacrificial tube <NUM>. In such embodiments the first outer surface of the functional tube <NUM> contacts the first inner surface of the sacrificial tube <NUM>, e.g., along an interface <NUM> there between. Similarly - in embodiments where the sacrificial tube <NUM> is disposed within the functional tube <NUM>, the second outer surface of the sacrificial tube <NUM> may be in contact with the first inner surface of the functional tube, e.g., along a similar interface <NUM>.

The functional tube <NUM> and the sacrificial tube <NUM> may be joined to one another in any suitable manner. For example, the functional tube <NUM> and sacrificial tube <NUM> may be joined to one another along an interface <NUM>, e.g., by mechanical bonding, chemical (e.g., metallurgical) bonding, combinations thereof, and the like. In embodiments, functional tube <NUM> and sacrificial tube <NUM> are bonded to one another along an interface (e.g., interface <NUM>) by a metallurgical bond. For example, functional tube <NUM> and sacrificial tube <NUM> may be diffusion bonded to one another along their length and along an interface (e.g., interface <NUM>). In such instances, no voids, intermetallic compounds, or undesirable phases (e.g., brittle or corrosion resistant phases) are formed between the materials of functional tube <NUM> and sacrificial tube <NUM> along the interface <NUM>.

Functional tube <NUM> and sacrificial tube <NUM> may have any suitable physical dimensions. As shown in <FIG> , functional tube <NUM> may have a first outer diameter (OD1), a first inner diameter (ID <NUM>), and a first average wall thickness (WT1 ), and sacrificial tube <NUM> may have a second outer diameter (OD2), second inner diameter (ID2), and second average wall thickness (WT2). Such dimensions may be selected appropriately based at least in part on the configuration of the composite tube assembly <NUM>. For example, in the embodiment of <FIG> OD1 is selected to be less than ID2, such that functional tube <NUM> may be disposed within sacrificial tube <NUM>. Conversely in instances where a composite tube assembly is configured with a sacrificial tube disposed within a functional tube, ID1 is selected to be greater than OD2. In one aspect of the disclosure (and regardless of the configuration), OD1 is in the range of <NUM> to <NUM> millimeters (mm), such as from <NUM> to <NUM>, or even about <NUM><NUM> to <NUM>. In such instances, ID2 is greater than OD1 when sacrificial tube <NUM> is disposed around the functional tube <NUM> , or ID2 is less than OD1 when sacrificial tube is disposed within the functional tube <NUM>.

The functional tube <NUM> and sacrificial tube <NUM> may also have respective average wall thicknesses, WT1, WT2, as noted above. In this context, the term "average wall thickness" means the average of the thickness of the wall of a tube over length of the tube or around the circumference of the tube. WT1 may be selected to achieve desired physical and/or operational characteristics in a final product, and thus functional tube <NUM> may have any suitable WT1. Without limitation, in embodiments WT1 is in a range from greater than or equal to <NUM> to less than or equal to <NUM>, such as from greater than or equal to <NUM> to less than or equal to <NUM>, or even from greater than or equal to <NUM> to less than or equal to <NUM>. In embodiments, WT1 is in a range of <NUM> to <NUM>.

WT2 may be larger or smaller than WT1. In embodiments, WT2 ranges from one quarter of WTl to five times WTI (i.e., WT2 = <NUM>(WT1) to <NUM>(WT1), such as from one half of WT1 to three times WT1 (i.e., WT2 = <NUM>(WT1) to <NUM>(WT1). Without limitation, in embodiments WT1 ranges from <NUM> to <NUM>, and W2 ranges from <NUM> to <NUM> times WT1(i.e., from <NUM> to <NUM>) In any case, a variation in the average wall thickness WT1 over the length or around the circumference of the composite tube assembly <NUM> may be less than or equal to <NUM>(WT1), such as less than or equal to <NUM>(WT1), less than or equal to <NUM>(WT1), or even less than or equal to <NUM>(WT1).

The composite tube assemblies described herein (and hence, their constitute sacrificial and functional tubes) may have any suitable length. For example, the composite tube assemblies described herein have a length of greater than or equal to <NUM> meters, such as greater than or equal to <NUM> meters, greater than or equal to <NUM> meters, greater than or equal to <NUM> meters, or even greater than or equal to <NUM> meters, wherein the physical parameters of the sacrificial and functional tubes (i.e., WT1, WT2, variation of WT1, etc.) are within the above noted ranges. In embodiments, composite tube assembly <NUM> has a length of greater than or equal to <NUM> meters, wherein WT1 is in a range of <NUM> to <NUM>, and the variation in WT1 ranges is less than or equal to <NUM>(WT1) (e.g., less than or equal to <NUM>(WT1), less than or equal to <NUM> (WT1), or even less than or equal to <NUM>(WT1).

As will be described in detail later in connection with the method of <FIG> and <FIG>, the composite tube assembly <NUM> may be drawn until the functional tube <NUM> has achieved desired dimensions. Subsequently, the sacrificial tube <NUM> may be removed, e.g., by exposing it to a corrosive media. For example, the sacrificial tube <NUM> may be removed by immersing all or a portion of composite tube assembly <NUM> in a liquid corrosive media (e.g., <NUM>% nitric acid, or the like), and/or flowing the corrosive media through the sacrificial tube. The processing parameters (e.g., immersion time, temperature, type of corrosive media, etc.) may vary depending on the nature of thickness of the second material of the sacrificial tube <NUM>. In embodiments, removal of the sacrificial tube includes exposing (e.g., immersing) the composite tube assembly <NUM> in <NUM>% nitric acid for about <NUM> minutes to about <NUM> hours (e.g., about <NUM> to <NUM> hours), at a temperature ranging from <NUM>-<NUM> (e.g., about <NUM>). In any case, removal of the sacrificial tube <NUM> preferably does not (or does not substantially ) affect the functional tube <NUM>. Following removal of the sacrificial tube <NUM>, functional tube <NUM> may remain, and may exhibit no (zero) through the wall defects along the length of the tube.

Another aspect of the present disclosure relates to methods of making a tube. In that regard reference is made to <FIG> and <FIG>, which are flow diagrams depicting example operations of one example of a method <NUM> of making a tube consistent with the present disclosure. For the sake of clarity and ease of understanding, the operations of method <NUM> will be described in with <FIG>, which stepwise illustrate one example of a process for producing a tube consistent with the present disclosure.

As shown in <FIG>, method <NUM> begins at block <NUM>. The method may then proceed to optional block <NUM>, pursuant to which a first functional tube and a first sacrificial tube may be provided. In this context, the first functional tube and first sacrificial tube may be understood as precursors or raw materials that will be formed (or otherwise incorporated) into a first composite tube assembly.

The first functional tube and first sacrificial tube may be provided in any suitable manner. For example, such tubes may be formed by gun drilling bar stock formed of or including the relevant first or second material(s) to provide seamless tube, i.e. a seamless first functional tube and a seamless first sacrificial tube. Alternatively or additionally, the first functional tube and/or first sacrificial tube may be provided by extruding and/or pilgering a first material and/or second material, again to provide seamless tube. Still further, the first functional and/or sacrificial tubes may be provided by providing a continuous strip of the first and/or second material, forming the strip to bring opposing edges thereof into proximity and/or contact with one another along a seam, and welding the seam to provide welded tube.

In any case, the seamless and/or welded tube may be drawn to produce first functional tube and first sacrificial tube with desired physical dimensions (outside diameter, insider diameter, wall thickness, etc.). For example and as shown in <FIG>, first functional tube <NUM> may have a first inside diameter ID1 and first outside diameter OD1, and the first sacrificial tube <NUM> may have a second inside diameter ID2, and a second outside diameter OD2. Such drawing may be performed in any suitable manner, such as by mandrel drawing in straight lengths, tethered plug drawing in straight lengths, floating plug drawing in straight lengths, floating plug drawing in a coil (optionally followed by straightening and cutting), combinations thereof, and the like. Consistent with the above discussion, the physical dimensions of the first functional tube and the first sacrificial tube may be selected based on the configuration of the composite tube assembly that is to be formed, i.e., whether the first sacrificial tube is to be disposed inside or outside the first functional tube.

Of course, block <NUM> may be omitted m instances where the first functional tube and first sacrificial tube are obtained in some other manner (e.g., by purchase from a supplier). In any case, once the first functional tube and second functional tube have been provided (or if the operations of block <NUM> are omitted), method <NUM> may proceed to block <NUM>, pursuant to which a first composite tube assembly may be formed.

Attention is drawn to <FIG>, which depicts more detailed operations of block <NUM>. As shown, the formation of a first composite tube assembly may begin with block <NUM>, pursuant to which the first functional tube and first sacrificial tubes are assembled into a first composite tube assembly precursor. This may be accomplished in any suitable manner,
<NUM> e.g., by disposing the first sacrificial tube inside the first functional tube, or by disposing the first functional tube inside the first sacrificial tube.

That concept is shown in <FIG>, which depicts one example of a first composite tube assembly precursor <NUM>, in which a first functional tube <NUM> is disposed within the interior volume of a first sacrificial tube <NUM>. In such embodiments, the first sacrificial tube <NUM> has an inside diameter ID2 that is greater than the outside diameter OD <NUM> of the first functional tube <NUM>. Consequently, the first composite tube assembly precursor includes a gap <NUM> between the outer surface(s) of the first functional tube <NUM> and the inner surface(s) of the first sacrificial tube <NUM>. In such instances and where first functional tube <NUM> and first sacrificial tube <NUM> are both cylindrical, the size of the gap <NUM> may correspond to a difference between ID2 and OD1.

As noted above, in embodiments the first sacrificial tube <NUM> may be disposed within the interior volume of the first functional tube <NUM>. In such instances, the outer diameter OD2 of the first sacrificial layer will be less than the inner diameter ID1 of the first functional layer, and a gap <NUM> will be present between the outer surface of the first sacrificial tube and the inner surface of the first functional tube. Like the embodiment of <FIG>, when the first functional and sacrificial tubes are both cylindrical, the size of the gap <NUM> may correspond to a difference between OD2 and ID1.

Returning to <FIG>, once the operations of block <NUM> are complete the method <NUM> may proceed to optional block <NUM>, pursuant to which a surface of the first sacrificial tube <NUM> may be brought into contact with a surface of the first functional tube <NUM>, or vice versa, to produce a second composite tube precursor. Put differently, pursuant to block <NUM> operations may be performed to reduce or even eliminate the gap <NUM> between the first functional tube and the first sacrificial tube, such that the outside surface of whichever tube is on the inside of the first composite tube precursor is brought into contact with the inside surface of whichever tube is on the outside of the first composite tube precursor.

One example of that concept is shown by comparison of <FIG> More specifically, <FIG> depicts an embodiment in which gap <NUM> in first composite tube precursor <NUM> has been eliminated along its length, such that the outer surface of first functional tube <NUM> is in contact with the inner surface of the first sacrificial tube <NUM>, resulting in second composite tube precursor <NUM>' that includes an interface <NUM> between the first functional and sacrificial tubes <NUM>, <NUM>, wherein gap <NUM> is reduced or even eliminated. Put in other terms, in second composite tube precursor <NUM>' the outer surface of the inner tube (i.e., first functional tube <NUM> ) is in contact with the inner surface of the outer tube (i.e., first sacrificial tube <NUM>) along interface <NUM> and along the length of the second composite tube precursor <NUM>'.

Reduction and/or elimination of the gap <NUM> may be performed in any suitable manner. In embodiments, gap <NUM> is reduced and/or eliminated by subjecting the first composite lube precursor <NUM> to a gap elimination process. The gap elimination process may involve inserting a mandrel into the interior diameter of the inner tube of the first composite tube precursor <NUM> or providing a straight fixed plug for drawing, and drawing the first composite tube precursor <NUM> through a drawing die that reduces the inside diameter of the outer tube to the outside diameter of the inner tube. In the embodiment of <FIG>, for example, the first composite tube assembly precursor <NUM> may be drawn through a die to reduce ID2 of the first sacrificial tube <NUM> until ID2 equals or substantially equals OD <NUM> of the first functional tube <NUM>. Although not shown, it may be understood that such drawing may result in a corresponding extension in length and, thus, second composite tube precursor <NUM>' may be longer than first composite tube precursor <NUM>. Of course, block <NUM> is not limited to such operations and the gap between the first sacrificial tube and the first functional tube may be reduced or eliminated in any suitable manner.

Following the operations of block <NUM> (or if such operations are omitted) the method may proceed to block <NUM>, pursuant to which the first sacrificial tube and the first functional tube are joined, e.g., along an interface therebetween (e.g., along interface <NUM> in <FIG>). Such joining may be performed in any suitable manner. In embodiments, an annealing process is used to join the first sacrificial tube and the first functional tube. In general, the annealing process involves heating the second composite tube precursor for a sufficient time and at a sufficient temperature to produce a metallurgical bond (e.g., a diffusion bond) between the two tubes. For example, the annealing process may result in the formation of a diffusion bond between the first functional tube and the first sacrificial tube along an interface therebetween (e.g., interface <NUM>). In embodiments, the first functional tube and first sacrificial tube are joined by a vacuum annealing process, in which the second composite tube precursor is heated to a temperature in the range of <NUM> to <NUM> for <NUM> or more minutes, while under a vacuum (e.g., of less than or equal to <NUM> torr), so as to produce a diffusion bond between the first functional tube and first sacrificial tube along an interface therebetween. Notably, such annealing temperatures are lower than those typically used to anneal a superalloy, such as the example alloys noted above m Table <NUM>.

In addition to joining the first functional tube and first sacrificial tube, the annealing process may also be configured to impart desired physical properties to one or more of the first and second materials. For example, the annealing process may be configured such that the first and second materials of the first functional and sacrificial layers, respectively, are ductile enough (e.g., ductility ) <NUM>%) to be subsequently drawn to a desired length. The annealing process may also serve to regulate the variation in the wall thickness (WT1 ) of the first functional layer, such that the variation in the wall thickness is less than or equal to <NUM>% (e.g., less than or equal to <NUM>%, or even less than or equal to <NUM>%).

Following the operations of block <NUM> a first composite tube assembly may be formed. One example of that concept is shown in <FIG>, which depicts a first composite tube assembly <NUM>' that includes a first functional tube <NUM> joined to a first sacrificial tube <NUM>. The method may then proceed to block <NUM>, pursuant to which the first composite tube assembly is processed to form a drawn composite tube assembly. Formation of the drawn composite tube assembly may be accomplished in any suitable manner, such as via a drawing or a sinking process. For example, a drawn composite tube may be formed by drawing the first composite tube via mandrel drawing, fixed plug drawing, floating plug drawing, coil drawing, combinations thereof, and the like. In embodiments, a drawn composite tube assembly is formed by a plurality of drawing, degreasing, and annealing cycles until desired physical dimensions (e.g., inner diameter, outer diameter, etc.) of the functional tube are achieved. Each drawing, degreasing, and annealing cycle may include drawing the first composite tube (e.g., via coil drawing) using one or more lubricants (e.g., chlorinated parafins or olefins), degreasing the drawn composite tube to remove the lubricant(s), and annealing the drawn composite tube assembly, e.g., using a vacuum annealing process such as described above in conjunction with block <NUM>.

Degreasing of the drawn composite tube assembly may be performed in any suitable manner, and in some embodiments is performed with a two-phase mixture of a solvent and a non-reactive gas. Suitable solvents that may be used for that purpose includes solvents that have low viscosity (e.g., a viscosity low enough to flow through the small interior diameter of the drawn composite tube) and which are able to dissolve and transport the lubricant from the tube ID) for the lubricant(s) used in tube drawing. Non-limiting examples of such solvents include trichloroethylene, n-propyl bromide, acetone, chlorinated hydrocarbon solvents, chlorofluorohydrocarbon solvents, combinations thereof, and the like. Suitable non-reactive gases include inert gases (e.g., helium, neon, argon, krypton, xenon, radon, and combinations thereof), hydrogen, carbon dioxide, nitrogen, combinations thereof, and the like.

Regardless of the type of sol vent and non-reactive gas, the degreasing process may involve flushing the solvent through the inner diameter of the first composite tube assembly. Flushing of the solvent may occur under at a pressure ranging from <NUM> to <NUM>,<NUM> pounds per square inch (<NUM>,<NUM> to <NUM>,<NUM> Bar) (PSI: e.g., <NUM>-<NUM>,<NUM> PSI (<NUM>,<NUM>-<NUM>,<NUM> Bar)), and may be performed for a time ranging from <NUM> to <NUM> minutes (e.g., <NUM> to <NUM> minutes). Subsequently, the non-reactive gas may be blown through the interior volume of the first composite tube assembly to remove residual solvent. In embodiments, the non-reactive gas is blown under a pressure ranging from <NUM> to <NUM>,<NUM> PSI (<NUM>,<NUM> to <NUM>,<NUM> Bar) (e.g., <NUM>-<NUM>,<NUM> PSI (<NUM>,<NUM>-<NUM>,<NUM> Bar)) for a time ranging from <NUM> minute to <NUM> hours, such as from <NUM> minutes to <NUM> hours.

The drawn composite tube assembly may have physical dimensions that differ from the first composite tube assembly discussed above. For example, the length of the drawn composite tube assembly may be substantially longer than the length of the first composite tube assembly. Moreover, the inner and outer diameters of the drawn composite tube and/or drawn functional tube m the drawn composite assembly may differ from the corresponding dimensions of the first composite tube assembly. That concept is shown in <FIG>, which is a cross sectional view of one example of a drawn composite tube assembly <NUM>" that includes a drawn functional tube <NUM> and a drawn sacrificial tube <NUM>'. As shown, the drawn functional tube <NUM> has an inner diameter ID3, wherein ID3 may be the same as or different from ID1. In embodiments, ID3 is less than ID1. For example, ID <NUM> may range from <NUM> to <NUM> of ID1. The outer diameter of drawn functional tube <NUM> may differ similarly from the outer diameter of the functional tube <NUM> in the first composite tube assembly <NUM>'.

The method may then proceed to block <NUM>, pursuant to which the sacrificial tube is removed. Removal of the sacrificial tube may be accomplished in any suitable manner, but is preferably accomplished by exposing the drawn composite tube assembly to a corrosive media, such as a corrosive liquid. For example, the sacrificial tube may be removed by immersing the composite tube assembly in a corrosive media and/or flowing a corrosive media through an interior diameter of the sacrificial tube. In such instances the corrosive media may be a strong or weak acid that attacks the material of the sacrificial tube, but does not or does not substantially affect the functional tube. In specific non-limiting embodiments, a drawn composite tube assembly may be exposed to nitric and/or phosphoric acid for enough time (e.g., <NUM> to <NUM> hours) and temperature (e.g., about <NUM> to <NUM>) to remove the sacrificial tube, while leaving the functional tube unaffected or substantially unaffected. This concept is shown in <FIG>, which depicts drawn functional tube <NUM> after drawn sacrificial tube <NUM>' is removed with a corrosive media.

Following the operations of block <NUM> the method may proceed to optional block <NUM>, pursuant to which one or more surfaces of the drawn functional tube may be passivated. In this context, passivation means that one or more surfaces of the drawn functional tube is chemically treated with a mild oxidant (e.g., a nitric acid solution), e.g., to removing free iron or other active metals from the surface thereof. In embodiments, the operations of block <NUM> may include treating the drawn functional tube with a nitric acid solution. In some instances the nitric acid solution may contain <NUM> to <NUM> volume % of nitric acid and <NUM> +/- <NUM> weight % of sodium dichromate, and the drawn functional tube is exposed to the solution for a minimum of <NUM> minutes at a temperature in the range of <NUM> to <NUM>. In other instances the nitric acid solution contains <NUM> to <NUM> volume % of nitric acid, and the drawn functional tube is exposed to the solution for a minimum of <NUM> minutes at a temperature in the range of <NUM> to <NUM>.

Claim 1:
A drawn composite tube assembly (<NUM>), comprising:
a drawn functional tube (<NUM>) formed of a first material, the drawn functional tube (<NUM>) having a first inner surface and a first outer surface; and
a drawn sacrificial tube (<NUM>) formed of a second material, the drawn sacrificial tube (<NUM>) having a second inner surface and a second outer surface;
wherein:
the drawn functional tube (<NUM>) is disposed inside or outside the drawn sacrificial tube (<NUM>);
wherein the first material comprises a superalloy;
wherein the second material is removable from the assembly (<NUM>) by exposure to a corrosive media,
wherein the first inner surface and the second outer surface are metallurgically bonded to each other.