Process of welding two pipes together, corresponding piping

Abstract

Method for welding two pipes to each other, corresponding piping The welding method comprises the following successive steps: - removing a layer from an inner surface (9) of the end (11, 13) of at least one of the pipes, thereby creating a hollow area (15) in the inner surface (9) extending to the free edge (14); - filling the hollow area (15) by depositing a stainless steel (17); - butt welding the pipe ends (11, 13) to each other. Figure for abstract: 8

Description

Title of the invention: Method of welding two conduits to each other, corresponding piping

[0001] The present invention relates generally to the welding of two conduits to each other.

[0002] The circuits of the safety injection system of a pressurized water nuclear reactor (circuits designated by the acronyms RIS and RRA) comprise stainless steel pipes of complex shapes. These pipes consist of several conduits welded to each other. The presence of cracks resulting from a stress corrosion phenomenon has been observed in certain nuclear reactors. These cracks are present on the internal surface of the pipes, near the welds.

[0003] This situation is illustrated in [Fig. 1]. The piping shown in this figure comprises two conduits 1, 3 welded to each other by a peripheral weld 5. The free edges of the ends of the two conduits 1 and 3 delimit between them a chamfer filled by the filler metal. Cracks 7 have been shown on the internal surface 9 of the conduits 1 and 3, near the weld 5.

[0004] One possibility to repair this piping and eliminate the cracks would be to completely replace conduits 1 and 3 with new conduits, made of a material resistant to stress corrosion.

[0005] Such a solution has the disadvantage that it is necessary to qualify the new material, this process being particularly long.

[0006] The cost for a nuclear reactor operator would be particularly high.

[0007] In this context, the invention aims to propose a welding method which can be used to repair cracks in the safety injection system of pressurized water reactors, which does not have the above drawbacks.

[0008] To this end, the invention relates to a method of welding two conduits to each other, the conduits having respective conduit ends having respective free edges, the method comprising the following successive steps: - removing a layer from an inner surface of at least one of the conduit ends, thereby creating a recessed area in the inner surface extending to the free edge; - filling the hollow area by depositing stainless steel; - butt welding of the ends of the conduits to each other.

[0009] In general, the phenomenon of stress corrosion occurs when three main conditions are met:

[0010] - a material subject to the phenomenon of stress corrosion;

[0011] - a state of tensile stress;

[0012] - an aggressive environment.

[0013] For the circuits of the nuclear reactor safety injection system, the medium is the primary coolant of the nuclear reactor core, the composition of which cannot be changed without compromising the entire operation of the reactor.

[0014] The idea behind the invention is to ensure that the tensile stresses are concentrated only in the resurfacing zone, where the stainless steel has been deposited.

[0015] [Fig.2] is a numerical simulation of the axial stress level around the weld of [Fig.l].

[0016] Zones a, b and c are zones in which the material exhibits axial tensile stresses, these stresses decreasing from a to c. Zones d, e and f are zones in which the material exhibits axial compressive stresses, these stresses increasing from d to f.

[0017] Zones f to a correspond respectively to the following axial stress ranges: -150 to -100 MPa; -100 to -50 MPa; -50 to 0 MPa; 0 to 50 MPa; 50 to 100 MPa; 100 to 150 MPa.

[0018] The same notation is used in Figures 9, 10 and 13.

[0019] It is observed that the areas closest to the weld exhibit axial tensile stresses (stress levels a, b and c), while the areas further from the weld exhibit axial compressive stresses (stress levels d, e and f).

[0020] [Fig.3] is a numerical simulation showing the plastic deformation of the material around the weld of [Fig.l].

[0021] The deformation scale is graduated from a to f, the deformations decreasing from a to f. These deformations are expressed in % relative to the work hardening law of the material, here of the material of the conduit 1,3.

[0022] Zones f to a correspond respectively to the following plastic deformation ranges: 0 to 5%; 5 to 10%; 10 to 15%; 15 to 20%; 20 to 25%; 25 to 30%.

[0023] The same notation is used in Figures 11 and 12.

[0024] The greatest levels of plastic deformation are observed around the weld (levels a to d). The areas far from the weld show a very moderate level of plastic deformation (levels e and f).

[0025] According to the invention, a layer of stainless steel is deposited on either side of the weld, before welding the two pipe ends to each other. After welding the two pipe ends to each other, the areas of the internal surface having tensile stresses and / or significant levels of plastic deformation are localized in the deposited stainless steel. The areas of the surface internal joints further away from the weld, made of the original material of the pipe, only have compressive stresses.

[0026] In other words, in the invention, the areas of the inner surface exhibiting tensile stresses are made of the deposited stainless steel. This material has a structure such that it is not likely to develop stress corrosion.

[0027] The areas that are in the original material of the conduits have compressive stresses. Therefore, they are also not subject to the phenomenon of stress corrosion.

[0028] Similarly, the plastic deformations are concentrated in the stainless steel deposited in the hollow area. The areas of the internal surface made of the original material of the conduits undergo very moderate plastic deformations, and are therefore not likely to develop stress corrosion.

[0029] The method may further have one or more of the following characteristics, considered individually or in all technically possible combinations:

[0030] - stainless steel is an austenitic stainless steel or a nickel-based alloy;

[0031] - the filling step is carried out by depositing the stainless steel by a method welding with filler metal, for example a TIG welding method or a coated electrode welding method;

[0032] - the hollow area has a determined length chosen so that, after welding, only the deposited stainless steel exhibits tensile stresses, with an intermediate area of ​​the inner surface adjacent to the stainless steel exhibiting compressive stresses;

[0033] - the method comprises a step of determining the length determined by calculation ;

[0034] - the determination step comprises the following sub-steps:

[0035] * simulation of the stresses generated on the internal surface of the ends of conduits by butt welding said conduit ends to each other without the prior steps of removal and filling;

[0036] * determination on the internal surface of a limit between a traction zone of the an inner surface exhibiting tensile stresses in the simulation, and a compression zone of the inner surface exhibiting compressive stresses in the simulation, the tensile zone extending between a weld of the conduit ends to each other and said boundary, the compression zone extending beyond said boundary; * choice of length determined so that the hollow area covers the entire traction zone;

[0037] - the determination step comprises the following sub-steps:

[0038] * simulation of plastic deformations generated on the internal surface of the ends of conduits by butt welding said conduit ends to each other in the absence of the prior steps of removal and filling;

[0039] * determination on the internal surface of a boundary between a deformation zone of the internal surface having in the simulation a plastic deformation greater than a determined level, and a slightly deformed zone of the internal surface having in the simulation a plastic deformation less than said determined level, the deformation zone extending between a weld of the ends of the conduits to each other and said limit, the slightly deformed zone extending beyond said limit;

[0040] * choice of the length determined so that the hollow area covers the entire deformation zone;

[0041] - the welding step comprises the following sub-steps:

[0042] - machining of the free edges of the ends of conduits, the free edges defining between them a chamfer when the ends of the conduits are placed end to end; - filling the chamfer with said stainless steel;

[0043] - the ends of the conduits are welded to each other by an original weld, the method comprising, before the removal step, a step of separating the pipe ends and removing the original weld.

[0044] According to a second aspect, the invention relates to a pipe comprising two conduits welded to each other according to the method having the above characteristics.

[0045] Other characteristics and advantages of the invention will emerge from the detailed description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which:

[0046] - [Fig.l] [Fig.l] is a simplified schematic representation, in section, of a piping with cracks on its internal surface resulting from stress corrosion;

[0047] - [Fig.2] [Fig.2] is a numerical simulation of the piping of [Fig.l], showing axial stresses near the weld;

[0048] - [Fig.3] [Fig.3] is a numerical simulation of the piping of [Fig.l], showing the level of plastic deformation near the weld;

[0049] - [Fig.4], [Fig.5], [Fig.6], [Fig.7], [Fig.8] Figures 4 to 8 illustrate in a schematic of the successive stages of the method of the invention;

[0050] - [Fig.9] [Fig.9] is a numerical simulation illustrating the level of stress axial in the ends of the conduits at the stage of [Fig.6];

[0051] - [Fig. 10] [Fig. 10] is seen similar to that of [Fig.9], showing the level of axial stress at the end of the step in [Fig.8];

[0052] - [Fig. 11], [Fig. 12] Figures 11 and 12 are numerical simulations showing the level of plastic deformation at the step of [Fig.6] and at the step of [Fig.8] respec- tively;

[0053] - [Fig. 13] [Fig. 13] is a numerical simulation showing in the upper part the axial stress level in the conduit ends for the invention at the stage of [Fig.8], the lower part showing the axial stress level in the conduit ends for an alternative process in which the conduit ends are first butt welded and then the internal surface is hardfaced with stainless steel.

[0054] The welding process which will now be described is particularly suitable for repairing the piping illustrated in [Fig.l].

[0055] The main steps of this method are illustrated in Figures 4 to 7. In these figures, only the upper half of the conduit of [Fig.l] is shown, the lower half not being shown.

[0056] As described previously, this piping comprises two conduits 1, 3 having respective conduit ends 11, 13 joined to each other by the weld 5. It has a central axis C.

[0057] The piping is typically piping of a nuclear reactor. For example, it is piping of the safety injection system of the nuclear reactor.

[0058] The piping is typically intended to transport the primary heat transfer fluid of the nuclear reactor.

[0059] Alternatively, the piping is intended to transport a fluid other than the primary heat transfer fluid of the nuclear reactor.

[0060] The piping may also belong to an industrial installation other than a nuclear reactor.

[0061] The conduits 1 and 3 are typically made of stainless steel, for example 304L, 316L stainless steel or type 347 stainless steel.

[0062] The method comprises a step of separating the ends of conduits 11, 13 from each other, and of removing the original weld 5.

[0063] After the separation step, each end of conduit 11, 13 has a free edge 14.

[0064] The situation at the end of this stage is shown in [Fig.4].

[0065] The method then comprises a step of removing a layer from the surface internal 9 of each end of conduit 11, 13, thus creating a hollow zone 15 in the internal surface 9 extending to the free edge 14.

[0066] The hollow zone 15 extends over the entire periphery of the conduit end 11, 13. It has, along the central axis C of the conduit end, a determined length 1. It has a determined depth p. The length is taken from the free edge 14, along the central axis C, towards the inside of the conduit.

[0067] The method then comprises a step of filling the hollow zone 15, in depositing a 17 stainless steel. The 17 stainless steel is preferably an austenitic stainless steel, especially a low-carbon austenitic steel, or a nickel-based alloy.

[0068] For example, suitable stainless steel is type 316LSi, 316L, 308L or Inconel® 82 or Inconel® 52.

[0069] The stainless steel 17 preferably completely fills the recessed area 15.

[0070] The filling step is typically performed by depositing the stainless steel 17 in the recessed area 15 by a filler metal welding method.

[0071] This welding method is for example an orbital TIG welding method. Alternatively, the method is a coated electrode welding method.

[0072] The situation at the end of the filling step is illustrated in [Fig.5].

[0073] The method then preferably comprises a step of machining the deposited stainless steel layer 17. This step aims to obtain an equivalent internal diameter on the two ends of conduits 10, 11 to be assembled. The free surface of the stainless steel 17 is machined to a small depth.

[0074] A non-destructive test of the deposited material, in this case stainless steel 17, can be carried out.

[0075] The situation at the end of this step is shown in [Fig.6].

[0076] The method then comprises a step of butt welding the ends of conduits 11, 13 to each other.

[0077] This welding step includes the following sub-steps:

[0078] - machining of the free edges 14 of the conduit ends 11, 13, the free edges 14 of ending between them a chamfer 19 when the conduit ends 11,13 are placed end to end;

[0079] - filling the chamfer 19 with said stainless steel.

[0080] The situation at the end of the machining sub-step is shown in [Fig.7].

[0081] This figure shows that each free edge 14 is machined so as to have a truncated cone shape, coaxial with the central axis C. The diameter of this truncated cone surface decreases when following the central axis C towards the other end of the conduit.

[0082] The free edges 14 of the two ends of conduits 11, 13 therefore form frustoconical surfaces of opposite orientation, as visible in Figures 7 and 8.

[0083] The chamfer 19 has the shape of a groove with a closed contour, of substantially V-shaped section in a radial plane containing the central axis C. The width of the chamfer 19 decreases radially from the outside towards the inside of the conduit ends.

[0084] The chamfer filling sub-step is performed by depositing the stainless steel by a filler metal welding method, typically by an orbital TIG welding method.

[0085] As indicated above, the stainless steel 21 filling the chamfer 19 is the same as that deposited in the hollow area 15. In other words, it has the same composition.

[0086] Alternatively, the stainless steel 21 filling the chamfer 19 is of a different grade from the stainless steel 17 deposited in the recessed area 15.

[0087] The situation at the end of the sub-step of filling the chamfer 19 is shown in [Fig.8].

[0088] The two ends of conduits 11, 13 are welded to each other. The root 25 of the weld is flush between the layers of stainless steel 17 deposited in the hollow zones 15 of the two ends of conduits 11, 13.

[0089] These layers of stainless steel 17 extend over the internal surface 9, on either side of the root 25, substantially over an axial length 1. They extend over the entire periphery of the ends of conduits 11, 13.

[0090] The axial stresses in the two ends of conduits 11, 13 are shown in [Fig.9] in the situation of [Fig.6], i.e. after machining of the stainless steel layer 17 deposited in the hollow zone 15.

[0091] It appears in [Fig. 9] that the stainless steel 17 deposited in each hollow zone 15 has low compressive stresses towards the free end 14, the stainless steel 17 deposited opposite the free end 14 having moderate tensile stresses. The area of ​​the internal surface 9 adjoining the deposited stainless steel 17 has low tensile stresses, and beyond the hollow zone 15 has compressive stresses.

[0092] [Fig. 10] represents the axial stresses in the ends of conduits 11, 13 at the end of the welding step, i.e. in the situation of [Fig.8].

[0093] It can be seen that the tensile stresses are concentrated at the root 25 of the weld, and in the layer of stainless steel 17 deposited on either side of it. The internal surface areas 9 of the ends of conduits 11, 13 which are not constituted by the deposited stainless steel 17 only exhibit compressive stresses.

[0094] [Fig. 11] represents the plastic deformation in the ends of conduits 11, 13, after machining of the stainless steel layer 17 deposited in the hollow zone, i.e. in the situation of [Fig.6].

[0095] [Fig. 11] shows that the material constituting the conduit ends 11, 13 is only very little plastically deformed at this stage of the process.

[0096] [Fig.12] shows the plastic deformation in the ends of conduits 11,13 at the end of the welding step, i.e. in the situation of [Fig.8].

[0097] It appears that the areas of the internal surface 9 of the ends of the conduits exhibiting a plastic deformation greater than 10% relative to the work hardening law of the material of the conduit 1, 3 are all located in the stainless steel layer 17. deposited in the hollow zone 15. The zones of the internal surface 9 which do not belong to the stainless steel layer 17 have plastic deformations less than 10% compared to the work hardening law of the material of the conduit 1, 3. These plastic deformations are of level e or f.

[0098] The stainless steel 17 deposited in the hollow zone 15 is not susceptible to stress corrosion, due to its nature.

[0099] What distinguishes the base metal of the conduit 1, 3 and the deposited stainless steel 17 or the stainless steel 21 of the weld, from a stress corrosion point of view, is mainly:

[0100] - the microstructure of the materials considered, namely the grain size and the structure crystalline;

[0101] - the chemical nature of the materials considered;

[0102] - possibly, the presence of ferrite can be considered as favorable to the re resistance to stress corrosion cracking.

[0103] The conditions allowing resistance to stress corrosion are obtained by choosing appropriate welding parameters for the stainless steel deposition operation 17.

[0104] These parameters are determined experimentally. This determination is a routine operation for welders.

[0105] It should be noted that the austenitic steels used for stainless steel 17 are low carbon, generally less than 0.03%, to avoid intergranular corrosion by dechromization of the grain boundaries following the precipitation of chromium carbides.

[0106] There is therefore on the internal surface 9 of the ends of conduits 11, 13 an area presenting tensile stresses, entirely constituted by the layer of stainless steel 17 filling the hollow area 15. The areas of the internal surface 9 which are not constituted by the stainless steel 17 deposited in the filling step are only subjected to compressive stresses. As a result, they are also not likely to develop stress corrosion.

[0107] The plastic deformations of a level greater than 10% with respect to the work hardening law of the material of the conduit 1, 3 are concentrated in a zone of the internal surface 9 belonging to the stainless steel layer 17 filling the hollow zone 15. They cannot therefore contribute to the appearance of stress corrosion on the internal surface 9 of the conduits 1, 3.

[0108] [Fig. 13] compares the results obtained with the method of the invention and with an alternative method not in accordance with the invention. The upper part of [Fig. 13] is identical to [Fig. 10]. It shows the level of axial stresses in the pipe ends 11, 13 at the end of the welding step, i.e. in the situation of [Fig.8].

[0109] The lower part of [Fig. 13] represents the level of axial stresses in the conduit ends 11, 13, for the alternative welding method. In this, the removal, filling and welding steps are not carried out in the same order as in the method of the invention.

[0110] Butt welding of the conduit ends 11, 13 to each other is first performed. Then, the removal and filling steps are performed, in that order.

[0111] It appears in the lower part of [Fig. 13] that the deposited stainless steel layer 17 is subjected to a very high level of tensile stresses. The area of ​​the internal surface 9 located around the stainless steel layer 17 is also exposed to a very high level of tensile stresses. This latter area is therefore likely to develop stress corrosion, due to its level of tensile stresses.

[0112] According to an advantageous aspect of the invention, the determined length 1 of the hollow zone 15, as indicated above, is chosen so that, after welding, only the deposited stainless steel 17 presents tensile stresses.

[0113] An intermediate zone of the internal surface 9, adjoining the deposited stainless steel 17, only exhibits compressive stresses.

[0114] Advantageously, the method comprises a step of determining the determined length 1, by calculation.

[0115] This determination step comprises at least the following sub-steps:

[0116] - simulation of the stresses generated on the internal surface 9 of the conduit ends 11,13 by butt welding said conduit ends to each other, in the absence of the prior removal and filling steps;

[0117] - determination on the internal surface 9 of a limit L between a traction zone 27 of the inner surface 9 having tensile stresses in the simulation, and a compression zone 29 of the inner surface 9 having compressive stresses in the simulation;

[0118] - choice of the determined length 1 such that the hollow zone 15 covers the entire traction zone 27.

[0119] The result of the simulation sub-step is shown in [Fig.2]. The traction zone 27 extends between the weld 30 connecting the conduit ends 11, 13 to each other and said boundary L.

[0120] The compression zone 29 extends beyond said limit.

[0121] The traction zone 27 only comprises portions of the internal surface 9 undergoing tensile stresses, or on the contrary may comprise both portions of the internal surface 9 undergoing tensile stresses and others undergoing compressive stresses.

[0122] On the other hand, the compression zone 29 only comprises portions of the internal surface 9 exhibiting compressive stresses.

[0123] Preferably, the limit L is placed as close as possible to the weld 30.

[0124] Advantageously, the determination step comprises, in addition to or instead of the sub- steps defined above, the following other sub-steps:

[0125] - simulation of the plastic deformations generated on the internal surface 9 of the ex conduit ends 11,13 by butt welding said conduit ends 11,13 to each other in the absence of the prior steps of removal and filling;

[0126] - determination on the internal surface 9 of a limit L' between a deformation zone 31 of the internal surface 9 presenting in the simulation a plastic deformation greater than a determined level, and a slightly deformed zone 33 of the internal surface 9 presenting in the simulation a plastic deformation lower than said determined level;

[0127] - choice of the determined length 1 such that the hollow zone 15 covers the entire deformation zone 31.

[0128] The determined level is typically 10% relative to the work hardening law of the conduit material 1,3.

[0129] The result of the simulation step is shown in [Fig.3]. The deformation zone 31 extends between the weld 30 and the limit L'. The slightly deformed zone 33 extends beyond the limit L'.

[0130] The deformation zone 31 only comprises portions of the internal surface 9 in which the level of plastic deformation is greater than the determined level, or on the contrary comprises both portions in which the plastic deformation is greater than the determined level and other portions for which the plastic deformation is less than the determined level. On the other hand, the slightly deformed zone 33 only comprises portions of the internal surface 9 in which the plastic deformation is less than the determined level.

[0131] This method is particularly suitable, because the applicant has found that the level of axial stresses in the conduit ends is little impacted by the presence of the stainless steel layer deposited in the hollow zone. The tensile stresses are essentially generated by the butt welding of the conduit ends 11, 13 to each other.

[0132] This is also true for plastic deformations.

[0133] Alternatively, the determined length 1 of the hollow zone 15 is fixed arbitrarily, that is to say on the basis of the operator's experience, without prior simulation.

[0134] The depth p of the hollow zone 15 is chosen so as to guarantee, at the end of the different stages of the process, the metallurgical and chemical resistance properties to expected stress corrosion cracking. This depth must also allow for the plastic deformations that will be generated during the butt welding stage to be absorbed.

[0135] Typically, the determined length 1 is between 20 mm and 40 mm. The external diameters of the pipes are generally between 200 mm and 350 mm and have a thickness between 20 mm and 37 mm.

[0136] The depth p in this case is typically between 2 mm and 6 mm.

[0137] The welding process can have multiple variants.

[0138] In the preceding description, it was specified that a hollow zone was provided in the internal surface of the two ends of the conduit, and that these two hollow zones were filled by depositing a stainless steel.

[0139] Alternatively, the hollow area is created only on the internal surface of one of the two ends of the conduits, then filled with stainless steel. The other end of the conduit does not undergo the stainless steel resurfacing operation.

[0140] In the foregoing description, the chamfer has been described as having a general groove shape, of V-shaped section. Alternatively, the chamfer may have any other suitable shape. The section may be U-shaped, or rectangular, etc.

[0141] As described above, the method is typically applied to repair a pipe consisting of two conduits whose ends are welded to each other.

[0142] It also applies to the production of new piping, for welding two pipes of this piping together. The process in this case does not include the step of separating the ends of the pipes and removing the original weld.

[0143] The method described above has multiple advantages.

[0144] Using austenitic stainless steel or a nickel-based alloy to fill the hollow area ensures excellent resistance to stress corrosion on the internal surface of the conduits.

[0145] Carrying out the filling step by depositing the molten stainless steel by a welding method with filler metal is particularly convenient, this method being well mastered. The molten stainless steel, after solidification, has a high resistance to stress corrosion.

[0146] Choosing the determined length of the hollow zone such that, after welding, only the deposited stainless steel has tensile stresses, with the intermediate zone of the internal surface adjoining the hollow zone having compressive stresses, ensures high resistance to stress corrosion at the internal surface of the conduits.

[0147] Providing in the process a step of determining the length determined by calculation makes it possible to guarantee that the tensile stresses are entirely localized. in the deposited stainless steel.

[0148] Determining this length by simulating the stresses generated on the internal surface of the conduit ends by butt welding is particularly convenient.

[0149] Determining this length by simulating plastic deformations is also particularly convenient.

[0150] When the determined length is chosen taking into consideration both stresses and plastic deformations, the choice of the determined length is particularly robust.

[0151] The fact that the welding step comprises both the machining of the free edges of the pipe ends and the filling of the chamfer thus formed with said stainless steel makes it possible to obtain a weld of the pipe ends of particularly good quality. In particular, the bond between the weld and the layer of stainless steel deposited in the hollow zone is particularly good. The machining of the free edges makes it possible to adapt the chamfer to the welding process by promoting the bond between the ends of the pipes. The filling area of ​​the chamfer carried out by depositing the molten stainless steel by the welding method with filler metal can be easily controlled by ultrasound in order to ensure the good quality of the weld.

Claims

Claims

1. A method of welding two conduits (1, 3) to each other, the conduits (1, 3) having respective conduit ends (11, 13) having respective free edges (14), the method comprising the following successive steps: - removing a layer from an inner surface (9) of at least one of the conduit ends (11, 13), thereby creating a recessed area (15) in the inner surface (9) extending to the free edge (14); - filling the recessed area (15) by depositing a stainless steel (17); - butt welding of the ends of conduits (11, 13) to each other, in which the hollow zone (15) has a determined length (1) chosen so that, after welding, only the stainless steel (17) deposited has tensile stresses, an intermediate zone of the internal surface (9) adjoining the stainless steel (17) having compressive stresses.

2. The method of claim 1, wherein the stainless steel (17) is an austenitic stainless steel or a nickel-based alloy.

3. A method according to claim 1 or 2, wherein the filling step is carried out by depositing the stainless steel (17) by a filler metal welding method, for example a TIG welding method or a coated electrode welding method.

4. A method according to any preceding claim, wherein the method comprises a step of determining the determined length (1) by calculation.

5. A method according to claim 4, wherein the determining step comprises the following sub-steps: - simulating the stresses generated at the inner surface (9) of the conduit ends (11, 13) by butt welding said conduit ends (11, 13) to each other in the absence of the prior removal and filling steps; - determining on the inner surface (9) a boundary (L) between a tensile zone (27) of the inner surface (9) exhibiting tensile stresses in the simulation, and a compression zone (29) of the inner surface (9) exhibiting compressive stresses in the simulation, the tensile zone (27) extending between a weld (30) of the ends of conduits (11, 13) to each other and said limit (L), the compression zone (29) extending beyond said limit (L); - choice of the determined length (1) such that the hollow zone (15) covers the entire traction zone (27).

6. A method according to claim 4 or 5, wherein the determining step comprises the following sub-steps: - simulating the plastic deformations generated at the inner surface (9) of the conduit ends (11, 13) by butt welding said conduit ends (11, 13) to each other in the absence of the prior removal and filling steps; - determination on the internal surface (9) of a limit (L') between a deformation zone (31) of the internal surface (9) having in the simulation a plastic deformation greater than a determined level, and a slightly deformed zone (33) of the internal surface (9) having in the simulation a plastic deformation less than said determined level, the deformation zone (31) extending between a weld (30) of the ends of conduits (11, 13) to each other and said limit (L'), the slightly deformed zone (33) extending beyond said limit (L');- choice of the determined length (1) such that the hollow zone (15) covers the entire deformation zone (31).;

7. A method according to any preceding claim, wherein the welding step comprises the following sub-steps: - machining the free edges (14) of the conduit ends (11, 13), the free edges (14) defining between them a chamfer (19) when the conduit ends (11, 13) are placed end-to-end; - filling the chamfer (19) with said stainless steel.

8. A method according to any preceding claim, wherein the conduit ends (11, 13) are welded to each other by an original weld (5), the method comprising, before the removing step, a step of separating the conduit ends (11, 13) and removing the original weld (5).

9. Piping comprising two conduits (1, 3) welded to each other according to the method of any one of claims 1 to 8.

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