Manufacturing process of a double-walled heat exchanger tube

Abstract

The invention relates to a method for manufacturing a double-walled heat exchanger tube comprising an outer tube and an inner tube, these tubes being metallic, cylindrical and coaxial. This method comprises the following steps: (i) providing: . a first tube having an internal diameter d1int and an external diameter d1ext, this first tube being intended to form the outer tube, . a second tube having an internal diameter d2int and an external diameter d2ext, this second tube being intended to form the inner tube, and .of a coaxial cylindrical tubular strip of Fe0 having an internal diameter dint and an external diameter dext, such that 0.15 mm ≤ (d1int - dext) ≤ 0.25 mm, 0.15 mm ≤ (dint – d2ext) ≤ 0.25 mm, and 10 µm ≤ (dext - dint) ≤ 200 µm; (ii) coaxially joining the second tube and the tubular strip inside the first tube, the tubular strip being positioned between the first and second tubes; (iii) brazing or bonding one end of the assembly; (iv) co-deforming the assembly; (v) cutting the ends of the assembly; (vi) welding the cut ends of the assembly; and (vii) heat treatment of the assembly by hot isostatic pressing, whereby the double-walled heat exchanger tube is obtained. No figure.

Description

Title of the invention: Method for manufacturing a double-walled heat exchanger tube TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a method of manufacturing a double-walled heat exchanger tube, this double-walled tube being intended in particular to equip devices of the heat exchanger type.

[0002] Such heat exchangers find application in particular in the chemical industry or in the energy field. STATE OF THE ART

[0003] Heat exchangers are devices for transferring thermal energy from a first fluid to a second fluid, without mixing them.

[0004] Different types of two-fluid heat exchangers exist, including those equipped with a double-walled tube, also known as a "double-walled tube", which comprises two cylindrical (tubular) and coaxial tubes threaded into each other, the internal surface of the smaller diameter tube (inner tube) being intended to come into contact with one of the fluids and the external surface of the larger diameter tube (outer tube) being intended to come into contact with the other fluid.

[0005] The particular structure of a double-walled heat exchanger tube has the advantage of providing increased safety to the heat exchanger equipped with it, insofar as each tube performs, in a redundant manner, a dual function, namely a sealing function to prevent the two fluids from coming into contact and a function of resistance to the pressure exerted by the circulation of these two fluids.

[0006] This dual function is particularly important, for example in cases where the heat exchanger is fitted to installations, such as chemical reactors, in which it is necessary to ensure, in a safe and efficient manner, the exchange of heat between a first fluid such as reactive molten metals or metal salts, for example based on sodium, lithium or potassium, and a second fluid such as water. Indeed, any failure which would occur at the level of a double-walled tube fitted to such installations would cause serious consequences on operation due to the chemical reactivity of the heat transfer fluid with the medium to be cooled. In this specific case, the stresses in service, in addition to the static pressure of the fluids and the corrosion of the surfaces, result from the stresses of thermal origin and the fatigue stresses imposed by the operating cycles or the vibrations induced by the thermohydraulics.

[0007] Most double-walled heat exchanger tubes consider leaving a mechanical clearance, or gap, between the inner and outer tubes.

[0008] By connecting this gap to a detection system, it is thus possible to detect the presence of fluid coming from the piercing of one of the two tubes before the integrity of the other tube is impaired.

[0009] However, the presence of this gap is particularly penalizing with regard to the thermal conductivity properties of the double-walled heat exchanger tube and, moreover, does not allow for an effective response to a hypothetical simultaneous piercing of the two tubes constituting it.

[0010] To improve the thermal conductivity of double-walled exchanger tubes, it has been proposed to at least partially fill the gap existing between the two tubes.

[0011] Document CN 203928838, referenced [1] in the present description, proposes a method for manufacturing a double-walled tube which comprises a step of filling the gap between the inner and outer tubes of such a tube with a metal powder. By ensuring a radial distribution of the temperature, this metal powder gives good thermal conductivity to the double-walled tube and also makes it possible to mitigate the risks of degradation linked to sudden thermal transitions.

[0012] Document CN 113458737, referenced [2], proposes a method for manufacturing a double-walled tube comprising an outer tube, an intermediate layer and an inner tube, the outer and inner tubes as well as the intermediate layer being metallic, cylindrical and coaxial. The method described in document [2] comprises the following successive steps:

[0013] (1) placing the inner tube coated with the intermediate layer in the tube external, the gap between the internal and external tubes being appropriate;

[0014] (2) sealing and welding the internal and external tubes at each of their ex junctions; and

[0015] (3) the installation of the tubes thus welded for an isostatic compression step hot under conditions suitable for diffusion welding.

[0016] In this document [2], the intermediate layer, which is made of a material chosen from copper, nickel, chromium and vanadium, is made by chemical or electrolytic deposition. In step (1), the appropriate gap between the inner and outer tubes is between 0.2 mm and 2 mm.

[0017] Although they improve the thermal conductivity properties of double-walled tubes, the manufacturing processes described in documents [1] and [2] nevertheless prove to be relatively expensive and complicated to implement on an industrial level.

[0018] The aim of the present invention is, therefore, to overcome the drawbacks of the methods of the prior art and to propose a method for manufacturing a double-walled heat exchanger tube which has the dual function of sealing and pressure resistance, in particular by resisting the propagation of fatigue defects, as well as good thermal conductivity properties, this process making it possible to achieve advantageous productivity and industrial implementation costs compared to the processes described in documents [1] and [2]. Statement of the invention

[0019] The above-mentioned aims and others are achieved by a method of manufacturing a double-walled heat exchanger tube of the aforementioned type, i.e. comprising an outer tube and an inner tube, the outer and inner tubes being metallic, cylindrical and coaxial.

[0020] According to the invention, the manufacturing method comprises the following successive steps (i) to (vii):

[0021] (i) the supply:

[0022] . of a first tube having an internal diameter dHnt and an external diameter diext, this first tube being intended to form the external tube,

[0023] . of a second tube having an internal diameter d2int and an external diameter d2ext, this second tube being intended to form the internal tube, and

[0024] . of a cylindrical and coaxial tubular strip made of Fe° having an internal diameter dint and an external diameter dext,

[0025] such as

[0026] 0.15 mm < (dHnt - dext) < 0.25 mm,

[0027] 0.15 mm < (dint - d2ext) < 0.25 mm, and

[0028] 10 pm < (dext - dint) < 200 pm;

[0029] (ii) coaxially joining the second tube and the strip inside the first tube, the strip being positioned between the first and second tubes;

[0030] (iii) brazing or bonding one end of the assembly obtained at the end of step (ii);

[0031] (iv) the co-deformation of the assembly obtained at the end of step (iii);

[0032] (v) cutting the ends of the co-deformed assembly at the end of the step (iv);

[0033] (vi) welding the cut ends of the assembly obtained at the end of step (v); and

[0034] (vii) the heat treatment of the assembly obtained at the end of step (vi), this heat treatment being carried out by hot isostatic compression, whereby the double-walled heat exchanger tube is obtained.

[0035] The method according to the invention makes it possible to form, between the external and internal tubes, a dense metallic interphase of pure iron which fills the entire initial volume of the gap existing between the first and second tubes and thus makes it possible to ensure a junction uniform mechanics between the outer and inner tubes. The absence of mechanical play between the outer and inner tubes prevents the movement of these two tubes relative to each other: these two tubes, in a sense, no longer form a single tube, they then deform in the same way as a monolithic tube during mechanical stress. Thanks to the manufacturing method according to the invention, a double-walled heat exchanger tube is obtained with a particularly high-performance geometric quality.

[0036] This pure iron metallic interphase also makes it possible to give the double-walled heat exchanger tube excellent thermal conductivity properties by ensuring very good heat transfer between the external and internal tubes, which has the effect of increasing the efficiency of a heat exchanger equipped with such a double-walled heat exchanger. Unexpectedly and surprisingly, the Inventor found that this ductile and dense Fe° iron interphase ensures excellent thermal conductivity which tends to a value close to that of an equivalent solid wall.

[0037] This ductility of the iron metal interphase also makes it possible to deflect and / or stop the propagation of fatigue cracks, in particular high-cycle fatigue cracks, which would have formed and propagated within one of the two tubes, thus preserving the integrity of the other tube. In particular, when the fatigue cracks are deflected, they propagate in the ductile metal interphase, thus providing a 30% longer service life at room temperature.

[0038] This iron interphase is further characterized by a seal enabling it to avoid the capillary propagation of fluid coming from such a crack between the two tubes and, therefore, to drastically reduce the risks of contact between the first and second fluids of the heat exchanger, which is fundamental, in particular when the first and second fluids are reactive, when there is a risk of contamination or even when the tubes are not controllable in service.

[0039] Furthermore, the method according to the invention does not implement any step exposing the constituent elements of the double-walled tube to hydrogen, unlike the electrolytic deposition which is implemented in the method of document [2] and which, when the exposure to hydrogen is not controlled, can weaken the metallic material forming the constituent elements of the tube, in particular when this material is steel.

[0040] The method according to the invention makes it possible to manufacture a double-walled heat exchanger tube which has an internal diameter d1NT and an external diameter dEXT-

[0041] According to one embodiment, the internal diameter d1NT of the double-walled tube is between 6 mm and 30 mm and, advantageously, between 8 mm and 15 mm.

[0042] According to one embodiment, the external diameter d^y of the double-walled tube is between 6 mm and 30 mm and, advantageously, between 10 mm and 20 mm.

[0043] It is specified that the expression "between ... and..." which is used above but also in the remainder of this description to define an interval, must be understood as defining not only the values ​​of the interval, but also the values ​​of the limits of this interval.

[0044] The method according to the invention comprises steps (i) to (vii) mentioned above and detailed below.

[0045] During step (i) of the manufacturing method according to the invention, two tubes are provided, a first tube which is intended to form the outer tube of the double-walled heat exchanger tube, and a second tube which is intended to form the inner tube of this double-walled heat exchanger tube.

[0046] The first tube has an internal diameter dHnt and an external diameter diext, and the second tube has an internal diameter d2int and an external diameter d2ext.

[0047] A cylindrical and coaxial tubular strip is also provided which is made of iron with oxidation state 0, i.e. pure iron Fe°. This Fe° strip has an internal diameter dint and an external diameter dext.

[0048] The first and second tubes as well as the strip are such that:

[0049] 0.15 mm < (diint - dext) < 0.25 mm,

[0050] 0.15 mm < (dint - d2ext) < 0.25 mm, and

[0051] 10 pm < (deXt - dint) < 200 pm.

[0052] In other words, the strip has a thickness of between 80 μm and 200 μm.

[0053] In an advantageous embodiment, the strip has a thickness of between 80 μm and 150 μm.

[0054] In an advantageous variant of the method according to the invention, at least one of the elements chosen from the first tube, the second tube and the tubular strip is without welding.

[0055] In a preferred variant of the method according to the invention, the first tube, the second tube and the tubular strip are all without welding. In doing so, reliability in terms of sealing and safety is maximized.

[0056] The first and second tubes, whether or not they are seamless, can be produced by any conventional industrial process.

[0057] According to an advantageous embodiment, the first and second tubes are obtained by implementing, prior to step (i), the following successive steps (i0) and (ij):

[0058] (i0) hot extrusion or hot rolling of a pierced rod (also called connecting rod), whereby a tubular blank is obtained, and

[0059] (ii) cold drawing or pilgrim rolling of the tubular blank as obtained at the end of step (io).

[0060] The implementation of these steps (i0) and (ii) makes it possible to obtain seamless tubes which can reach lengths of up to 6 m.

[0061] According to an advantageous embodiment, the seamless tubular strip is obtained by implementing, prior to step (i), the following successive steps (i2) to (i4):

[0062] (i2) drilling an iron rod Fe°,

[0063] (i3) turning the drilled rod as obtained at the end of step (i2), by means of which gives us a tubular blank, and

[0064] (i4) pilgrim rolling of the tubular blank as obtained at the end of step (i3).

[0065] In a variant, which will be illustrated in the example described below, the pilgrim rolling step (i4) can be reproduced at least once. The method according to the invention can, in addition, comprise a heat treatment step (i4') preferably carried out between each step (i4) and, for example, at 850°C for 1 h.

[0066] According to an advantageous embodiment, the method according to the invention further comprises a step of cleaning at least the internal surface of the first tube, the internal and external surfaces of the tubular strip and the external surface of the second tube, this cleaning step being implemented prior to step (ii) of coaxial assembly.

[0067] This cleaning step makes it possible to optimize the subsequent contact between the tubular strip and the surfaces of the external and internal tubes with which this strip will be in contact and, in doing so, improve the sealing and thermal conductivity of the double-walled heat exchanger tube.

[0068] The manufacturing method according to the invention comprises, after step (i), a step (ii) of coaxially assembling the first and second tubes and the Fe° iron tubular strip. More particularly, in this step (ii), the second tube and the strip are introduced inside the first tube, the strip being disposed between the first and second tubes.

[0069] In other words, at the end of step (ii), an assembly is obtained in which the Fe° iron tubular strip is positioned between the first and second tubes.

[0070] The manufacturing method according to the invention comprises, after the coaxial assembly step (ii), a step (iii) of brazing or bonding one end of the assembly obtained at the end of step (ii).

[0071] This brazing step (iii) makes it possible to maintain the assembly for the implementation of the following steps of the method according to the invention, in particular its co-deformation step (iv), but also to protect the tubular strip from any contamination, in particular that generated by the lubricants which are used during this subsequent co-deformation step (iv).

[0072] The brazing must advantageously have sufficient ductility to be able to be co-deformed with the tubes and the tubular strip during step (iv).

[0073] In a particular embodiment involving first and second stainless steel tubes, the brazing step (iii) is carried out with high silver content brazing.

[0074] The manufacturing method according to the invention comprises, after the brazing step (iii), a step (iv) of co-deformation of the assembly obtained at the end of step (iii).

[0075] This co-deformation step (iv) makes it possible to reduce the mechanical play volume existing between the first tube and the strip, on the one hand, and between the second tube and the strip, on the other hand, which promotes the homogeneous elongation of the tubes and the strip, the preservation of the geometry of the assembly of these elements during subsequent operations, by ensuring the absence of wrinkling of the strip during deformation.

[0076] This co-deformation step (iv) can, in particular, be implemented by co-winding or by co-laminating.

[0077] In an advantageous variant of the method according to the invention, this co-deformation step (iv) is carried out by co-laminating with a pitch of the assembly obtained at the end of step (iii).

[0078] The choice of such a pilgrim step co-rolling step (iv) has the advantage of reducing the volume of the mechanical clearance between the first and second tubes and the tubular strip. In doing so, the quantity of gas contained in the assembly becomes negligible and no degassing step is necessary before implementing the following steps, in particular the welding step (vi).

[0079] In one embodiment, the manufacturing method according to the invention does not comprise a degassing step between the co-deformation step (iv) and the welding step (vi).

[0080] The elimination of such a degassing step, which is particularly complex to implement on an industrial level in the context of the manufacture of double-walled tubes, contributes to a significant reduction in cycle times, costs and production lead times.

[0081] The manufacturing method according to the invention comprises, at the end of the co-deformation step (iv), a step (v) of cutting the ends of the co-deformed assembly, so as to eliminate the ends of the assembly affected, for one, by the brazing and, for the other, by the percolation of lubricating fluids.

[0082] These ends cut in step (v) are then subjected to a welding step (vi).

[0083] This step (vi) of welding the two ends of the assembly makes it possible to isolate the interfaces of the tubular strip with the first and second tubes from the environment.

[0084] In one embodiment, the welding step (vi) is carried out by an arc welding process with a non-fusible electrode, for example made of tungsten, and without filler of material. This welding can advantageously be carried out under neutral gas.

[0085] The manufacturing method according to the invention comprises, after the welding step (vi), a step (vii) of heat treatment of the assembly obtained at the end of step (vi).

[0086] This step (vii) of heat treatment of the assembly obtained at the end of step (vi), is carried out by hot isostatic compression (HIC), by means of which the double-walled heat exchanger tube is obtained.

[0087] This hot isostatic compression step (vii) makes it possible to weld, by diffusion, on the one hand, the internal surface of the first tube to the external surface of the tubular strip and, on the other hand, the external surface of the second tube to the internal surface of the tubular strip. At the end of the hot isostatic compression step (vii), a uniform mechanical junction is obtained between the first and second tubes, which is formed by an interphase of pure iron originating from the tubular strip. The presence of a small quantity of oxygen trapped in the interface roughnesses of the strip and the tubes (absence of degassing) contributes to the formation of mixed oxides of iron and chromium (in the case of steel tubes), these oxides playing a favorable role in delamination of the surfaces during the propagation of a fatigue crack.

[0088] This step (vii) of hot isostatic compression, which causes little or no deformation of the first and second tubes and of the tubular strip, makes it possible to optimize the conservation of the geometry of the assembly formed by these tubes and strip and, consequently, to respect the geometric tolerances of the finished tubes, which makes it possible to ensure reproducibility, and therefore reliability, of the double-walled heat exchanger tubes manufactured by the method according to the invention.

[0089] In a particular embodiment, step (vii) is carried out at a temperature of between 800°C and 1200°C, at a pressure of between 50 MPa and 200 MPa and for a duration of between 30 min and 4 h.

[0090] In an advantageous embodiment, step (vii) is carried out at a temperature of between 1000°C and 1200°C, at a pressure of between 100 MPa and 200 MPa and for a duration of between 30 min and 2 h.

[0091] The method according to the invention makes it possible to manufacture double-walled heat exchanger tubes of variable lengths and geometries.

[0092] The materials of the first tube and the second tube may be the same or different, and may in particular comprise iron.

[0093] In an advantageous variant, the first and second tubes are made of a material comprising iron, in particular stainless steel. This stainless steel may advantageously be a martensitic steel, advantageously Eurofer-97 or T-91 steel.

[0094] In a preferred variant, the materials of the first and second tubes are identical.

[0095] In one embodiment, the manufacturing method according to the invention further comprises, after the heat treatment step (vii), one or more of the following steps (viii) to (xii):

[0096] (viii) cooling the double-walled heat exchanger tube, for example at a rate of 50°C / h;

[0097] (ix) bending the double-walled heat exchanger tube obtained at the end of step (viii);

[0098] (x) the expansion of the double-walled heat exchanger tube obtained at the end of step (viii) or (ix);

[0099] (xi) heat treatment of the double-walled heat exchanger tube obtained at the end of step (ix) or (x), such as quenching or standardized tempering; and

[0100] (xii) straightening the double-walled heat exchanger tube obtained at the end of step (ix), (x) or (xi).

[0101] In particular, and specifically when the first and second tubes are made of stainless steel and, more particularly, of Eurofer-97, a cooling step (viii) makes it possible to maintain the double-walled tube in a state suitable for allowing the bending and / or expanding of the tube.

[0102] The double-walled tubes manufactured by the method according to the invention can be bent and assembled like conventional tubes.

[0103] It should be noted that the choice of pure iron for the tubular strip and, therefore, for the interphase of the double-walled tube does not modify the composition of the weld pool during assembly by welding of such tubes. It is therefore possible to envisage assembling the tubes in full thickness without significantly modifying the composition of the weld bead due to the presence of the iron interphase.

[0104] It would be completely different with an interphase of copper, chromium, nickel or vanadium as described in document [2] which requires a significant change in the composition of the weld bead, with the possible risk of modifying the mechanical properties of the weld bead of such a tube.

[0105] The method according to the invention therefore greatly promotes the industrial manufacture of double-walled heat exchanger tubes, by avoiding the creation of areas devoid of interphase, also called reserves, located at the level of future welds.

[0106] Generally speaking, the heat treatment step (xi) aims to confer the mechanical properties expected for the double-walled tube manufactured by the method according to the invention.

[0107] This heat treatment (xi) may, for example, be chosen from quenching, standardized tempering and annealing, or even be a combination of these (quenching followed by standardized tempering), this treatment being adapted to the specifications of the material(s). forming the first and second tube(s).

[0108] Furthermore, due to the change of phase(s) that may occur during the heat treatment step (xi), a geometric distortion of the double-walled heat exchanger tube is possible. A straightening step (xii) may then be envisaged to ensure the geometric conformity of this double-walled heat exchanger tube.

[0109] Other characteristics and advantages of the invention will appear on reading the following example which relates to the manufacture of a double-walled heat exchanger tube.

[0110] It is specified that this example is given only as an illustration of the object of the invention and does not constitute in any way a limitation of this object. [YES] DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT

[0112] A double-walled heat exchanger tube was manufactured by the method according to the invention.

[0113] Production of tubes and strips

[0114] The first and second tubes intended to form respectively the external tube and the internal tube of the double-walled heat exchanger tube were made of Eurofer-97 stainless steel, the composition of which (in mass percentage) is specified in Table 1 below. [Tables 1] V Cr Mn Ta W c N Fe 0.2 9 0.4 0.075 1.1 0.11 0.03 the remainder

[0115] These first and second tubes are seamless cylindrical tubes which have been produced by a conventional industrial process from a Eurofer-97 steel blank which has been drawn in such a way as to obtain tubes with a length of 2.5 m and:

[0116] - for the first tube, a thickness of 1.45 mm and external and internal diameters respective 14.00 mm (diext) and 11.12 mm (dHnt), and

[0117] - for the second tube, a thickness of 1.20 mm and external and internal diameters respective of 10.62 mm (d2ext) and 8.2 mm (d2int).

[0118] The cylindrical tubular strip of pure iron was prepared from a pure iron marketed by the company Liffs under the name ARMCO® Pure Iron grade 4.

[0119] Tubular blanks 500 mm long and with respective outer and inner diameters of 16.22 mm and 12.57 mm were first machined by drilling and then by turning before being rolled.

[0120] The tubular blanks were then subjected to a first pilgrim rolling, followed by a first heat treatment at 850°C for 1 h, then to a second pilgrim rolling followed by a second heat treatment at 850°C for 1 h, then to a third pilgrim rolling followed by a third heat treatment at 850°C for 1 hour and, finally, a fourth and final pilgrim rolling.

[0121] At the end of these rolling steps interspersed with heat treatment steps, the seamless tubular strip was obtained having a thickness of 100 μm and respective external and internal diameters of 10.97 mm (dext) and 10.77 mm (dint).

[0122] Surface preparation

[0123] The internal surface of the first tube intended to form the external tube was cleaned by means of Corundum sandblasting so as not to affect the internal diameter dHnt by more than 50 pm.

[0124] The external surface of the second tube intended to form the internal tube was brushed using an abrasive brush of the Scotch-Brite™ type so as not to affect the external diameter d2ext by more than 50 pm.

[0125] The internal and external surfaces of the first and second tubes as well as the internal and external surfaces of the tubular strip of pure iron were carefully cleaned with a cloth then with acetone.

[0126] Assembly of tubes and strips

[0127] The second tube and the tubular strip were then coaxially assembled inside the first tube, the tubular strip being positioned between the first and second tubes.

[0128] Taking into account the respective external and internal diameters of the first and second tubes and of the tubular strip as obtained after the preparation of the different surfaces, a technical clearance of at least 150 μm exists between the strip and the first tube, on the one hand, and between the strip and the second tube on the other hand, which obviously facilitates the assembly of these three elements.

[0129] Brazing the assembly

[0130] The second inner tube was positioned so as to protrude from the first tube by several centimeters. The first and second tubes were then heated with an oxyacetylene torch before depositing a silver braze between the two tubes. It is specified that the silver braze has a content that is compatible with the steel of the first and second tubes which comprises 9% by mass of chromium. The braze percolates into the assembly clearance between the tubes over the extent of the area heated by the torch. It should be noted that this braze is characterized by sufficient ductility to be co-deformed with the tubes and the strip during the following co-deformation step.

[0131] Step cooling was then carried out in order not to generate a fragile martensitic structure.

[0132] Co-deformation of the assembly

[0133] The assembly was then subjected to pilgrim step rolling.

[0134] Table 2 below shows the external and internal diameters (denoted dextern, dintern and expressed in mm) of the first and second tubes and of the tubular strip before and after this step of co-deformation by pilgrim step rolling of the assembly.

[0135] [Tables2] Before co-deformation After co-deformation ^external ^internal ^external ^internal First tube 14.00 11.12 13.50 10.60 Strip 10.97 10.77 10.60 10.40 Second tube 10.62 8.20 10.40 8.00

[0136] Cutting and welding

[0137] The ends of the assembly as obtained at the end of the co-deformation step were cut to eliminate the end thermally affected by the brazing as well as the opposite end possibly affected by the percolation of the lubricating fluid.

[0138] The assembly was then cut into sections having a length compatible with the enclosure in which the hot isostatic compression (HIC) step was then carried out.

[0139] The sections were then welded using a tungsten electrode welding process without adding material and under neutral gas. This welding step is carried out without prior degassing, the technical clearance between the tubes and the strip being very small after the co-deformation step.

[0140] Heat treatment by hot isostatic pressing (HIP)

[0141] The sections were then placed in an enclosure to undergo a hot isostatic compression cycle conducted at a temperature of 1100°C, at a pressure of 1200 bar (120 MPa), for 1 h.

[0142] Complementary treatments

[0143] After the CIC step, the double-walled tube sections as obtained were cooled at a rate of 50°C / h, giving them a ferritic metallurgical state (no martensite) allowing them to be bent like conventional tubes.

[0144] These tubes thus shaped and welded were then heat treated by quenching and tempering.

[0145] The double-walled tube sections thus manufactured by the method according to the invention had an external diameter of 13.50 mm (± 0.25 mm) and a thickness of 2.75 mm (± 0.35 mm).

[0146] The double-walled tube thus manufactured makes it possible to contain pressurized water at an average temperature of 325°C, it being specified that each of the first and second tubes can, on its own, withstand the forces linked to water pressure. BIBLIOGRAPHY

[0147] [1] CN 203928838

[0148] [2] CN 113458737

Claims

Claims

1. A method of manufacturing a double-walled heat exchanger tube comprising an outer tube and an inner tube, the outer and inner tubes being metallic, cylindrical and coaxial, said manufacturing method being characterized in that it comprises the following successive steps (i) to (vii): (i) providing: . a first tube having an internal diameter diint and an external diameter diext, this first tube being intended to form the outer tube, . a second tube having an internal diameter d2int and an external diameter d2ext, this second tube being intended to form the inner tube, and .of a cylindrical and coaxial tubular strip made of Fe° having an internal diameter dint and an external diameter dext, such that 0.15 mm < (diint - dext) < 0.25 mm, 0.15 mm < (dint - d2ext) < 0.25 mm, and 10 pm < (dext - dint) < 200 pm; (ii) coaxial assembly of the second tube and the strip inside the first tube, the strip being positioned between the first and second tubes; (iii) brazing or bonding one end of the assembly obtained at the end of step (ii); (iv) co-deformation of the assembly obtained at the end of step (iii); (v) cutting the ends of the co-deformed assembly at the end of step (iv); (vi) welding the cut ends of the assembly obtained at the end of step (v); and (vii) heat treatment of the assembly obtained at the end of step (vi), this heat treatment being carried out by hot isostatic pressing, whereby the double-walled heat exchanger tube is obtained.

2. A method according to claim 1, wherein the first and second tubes provided in step (i) are seamless, each of these first and second tubes having been advantageously obtained by carrying out, prior to step (i), the following successive steps (i0) and (ij): (i0) hot extrusion or hot rolling of a pierced rod, whereby a tubular blank is obtained, and (ii) cold drawing or pilgrim rolling of the tubular blank as obtained at the end of step (i0).

3. Method according to claim 1 or 2, in which the tubular strip provided in step (i) is weld-free and is advantageously obtained by carrying out, prior to step (i), the following successive steps (i2) to (i4): (i2) drilling an Fe° iron rod, (i3) turning the drilled rod as obtained at the end of step (i2), whereby a tubular blank is obtained, and (i4) pilgrim rolling the tubular blank as obtained at the end of step (i3).

4. The method of claim 3, wherein the pilgrim rolling step (i4) is repeated at least once, the method being further capable of comprising a heat treatment step (i4') preferably conducted between each step (i4) and, for example, at 850°C for 1 h.

5. Method according to any one of claims 1 to 4, further comprising a step of cleaning at least the internal surface of the first tube, the internal and external surfaces of the tubular strip and the external surface of the second tube, this cleaning step being carried out prior to step (ii) of coaxial assembly.

6. Method according to any one of claims 1 to 5, in which the co-deformation step (iv) is carried out by co-stretching or by co-rolling and, preferably, by pilgrim step co-rolling, of the assembly obtained at the end of step (iii).

7. A method according to any one of claims 1 to 6, which does not comprise a degassing step between the co-deformation step (iv) and the welding step (vi).

8. Method according to any one of claims 1 to 7, in which the welding step (vi) is carried out by an arc welding process with a non-consumable electrode, for example made of tungsten, and without adding material.

9. A method according to any one of claims 1 to 8, wherein step (vii) is carried out at a temperature of between 800°C and 1200°C, at a pressure of between 50 MPa and 200 MPa and for a time of between 30 min and 4 h.

10. A method according to any one of claims 1 to 9, further comprising, after step (vii), one or more of the following steps (viii) to (xii): (viii) cooling the double-walled heat exchanger tube, for example at a rate of 50°C / h; (ix) bending the double-walled heat exchanger tube obtained at the end of step (viii); (x) expanding the double-walled heat exchanger tube obtained at the end of step (viii) or (ix); (xi) heat treating the double-walled heat exchanger tube obtained at the end of step (ix) or (x), such as quenching or normalized tempering; and (xii) straightening the double-walled heat exchanger tube obtained at the end of step (ix), (x) or (xi).

11. A method according to any one of claims 1 to 10, wherein the first and second tubes are made of martensitic stainless steel, advantageously Eurofer-97 or T-91 steel.

12. A method according to any one of claims 1 to 11, wherein the materials of the first and second tubes are the same.