[0001]The present invention relates to a method for producing a double-walled tube with an interface gap, and a double-walled tube produced with this method, wherein the double-walled tube is a type of heat-resistant and pressure-resistant tube which is used under high-temperature environments such as boilers for thermal power generation plants, chemical industries, nuclear power plants, and next-generation fast breeder reactors, and which requires heat exchange between inside and outside of the tube, and wherein the double-walled tube has an appropriate gap and yet a pressure bonding characteristic between the inner surface of outer wall tube and the outer surface of inner wall tube. [0002] Unless otherwise stated, the definitions of the terms in the present description are as follows. A "double-walled tube with an interface gap" is a double-walled tube in which metal surfaces of the inner surface of outer wall tube and the outer surface of inner wall tube are neither mechanically nor metallurgically bonded to each other (that is, there is no metallic bond between relevant metal surfaces, and there is a slight interface gap therebetween), and which fiirther has an appropriate pressure bonding characteristic. It is noted that the above described "pressure bonding characteristic" refers to the resistance to detachment and separation between the outer wall and inner wall tubes in a double-walled tube which is obtained by inserting the inner wall tube into the outer wall tube and applying pressure bonding to them by cold working. "Through-wall leakage" refers to a leakage of the fluid that passes inside the double-walled tube as a result of cracking which occurs either in the outer wall tube 2 i > or the inner wall tube and has developed and penetrated through the double-walls of the tube. "Percent %" represents the mass percentage (mass%) of each element contained in a product such as a double-walled tube. BACKGROUND ART [0003] Tube materials for steel tubes to be used under high-temperature environments such as boilers for thermal power generation plants, chemical industries, nuclear power plants, and next-generation fast breeder reactors are required to have excellent strength and corrosion resistance at high temperatures. Among them, a double-walled tube which is configured such that one of the walls is made of a material having an excellent corrosion resistance and the other is made of a material having excellent high-temperature strength is used as needed. [0004] For example. Patent Literature 1 describes a double-walled tube formed by hot rolling a multi-layer billet in which alloy powder containing Cr and Ni as principal elements is packed on the outer surface of a billet made of Fe-Ni-Cr austenitic heat-resistant steel with a Cr content of not more than 30%. Further, Patent Literature 2 describes a double-walled tube in which one of the walls is made up of an austenitic stainless steel tube containing not less than 30% of Cr, and the other is made up of an austenitic stainless steel tube containing not less than 25% of Cr. [0005] These double-walled tubes are of a type that is free of an interface gap, in which tubes of dissimilar metals having different properties, such as a corrosion resistance material and a high-strength material, are combined and the outer wall tube and the inner wall tube are physically or metallurgically bonded to each other (that is, the inner surface of outer tube and the outer surface of inner tube are brought into contact via metal surfaces, or in a metallic bond to each other). [0006] 3 However, since in a metallurgically bonded double-walled tube without the gap, the metal surfaces of the outer wall tube and the inner wall tube are physically or metallurgically bonded to each other, and it is not possible to prevent a throughwall leakage (a leakage which occurs as a result of cracking propagating through the double-walled tube) which can occur, even if in a worst case scenario, in a heat transfer tube (double-walled tube) in which liquid Na as coolant is circulated, for example, in a steam generator (SG) of a fast breeder reactor (hereafter referred to as "FBR"). [0007] If in any chance a through-wall leakage occurs in a double-walled tube to be used in a steam generator tube of a next-generation fast breeder reactor (FBR), reaction and explosion will occur as the result of contact between liqUid Na and water (vapor), which is very dangerous. Therefore, the prevention of through-wall leakage is the most important issue in a fast breeder reactor (FBR). [0008] In contrast, there has been developed a double-walled tube in which metal surfaces are not physically or metallurgically bonded to each other and the interface gap is provided therebetween. A major feature of the gap of a double-walled tube is that even if cracking occurs either in the outer wall tube or the inner wall tube, the gap will deter the propagation of the cracking, thereby preventing an immediate occurrence of a through-wall leakage. Further, there has been developed a technique in which an inert gas such as helium is flowed by utilizing the interface gap of the double-walled tube, and even if, by any chance, either of the inner wall tube or the outer wall tube fails, the failure is promptly detected, thereby preventing a through-wall leakage accident. [0009] For example. Patent Literature 1 proposes a double-walled tube including a braided mesh wire, in which a braided mesh wire is inserted between the inner wall and outer wall tubes, the braided mesh wire being formed by bundling and weaving an element wire (100 [xm) made of the same material as that of the tube. By allowing inert gas such as helium to flow through the interface gap provided in the 4 i ( double-walled tube, it is possible to promptly detect the fracture of the inner wall tube or the outer wall tube thereby preventing a through-wall leakage. [0010] Fiirther, Non Patent Literature 2 describes a double-walled tube with a groove(s), in which a groove processing for detecting a leakage is performed on the inner svirface of the outer wall tube. [0011] However, the production of such a double-walled tube with the interface gap according to such prior art requires the repetition of very special, intricate and elaborate processing. Thus, in addition to considerable difficulty in production, the prior art suffers from complexities thereof so that a great deal of man-hours are required and a large number of defects may generate, and therefore it is not suitable for mass production and poor in economic efficiency. Further, since there is contact between metal surfaces, there still remains a risk that cracking may propagate and a leakage going through the double-walled tube may occur. Further, it is difficult in reality to produce a longer-length double-walled tube of 35 m or so in length which is envisioned in next-generation FBRs. [0012] As another prior art. Patent Literature 3 describes a cooling double-walled tube for a stave cooler which is installed in the inside of furnace wall of a blast fiimace, and the like. This double-walled tube is produced by placing a foreign layer of oxides etc. in the interface between the inner and outer wall tubes which are made of carbon steel, and subjecting them to a drawing process. However, such a double-walled tube cannot be used for heat exchanger tubes since the oxides etc. placed in the interface circumvent and shield the heat transfer between the outer and inner wall tubes. Besides, the tube cannot be used for the above, since the tube material is limited to carbon steel, the strength standard for a high-temperature and pressure-resistant member which is used under environments of not less than 450°C is not satisfied and corrosion resistance is also insufficient. CITATION LIST PATENT LITERATURE 5 L 4 • • [0013] Patent Literature 1: Japanese Patent Application Publication No. 64-17806 Patent Literature 2: Japanese Patent Application Publication No. 57-120002 Patent Literature 3: Japanese Patent Application Publication No. 2002-30315 NON PATENT LITERATURE [0014] Non Patent Literature 1: "TOSHIBA REVIEW", Vol. 47, No.l, (1992), PP. 75 to 78. Non Patent Literature 2: "SUMITOMO METAL", Vol. 40, No. 4, Oct. (1988), PP. 112 to 114. SUMMARY OF INVENTION TECHNICAL PROBLEM [0015] An object of the present invention, which has been made in view of the above described problems with a double-walled tube in which is provided with an interface gap for preventing a through-wall leakage in the contact/mating surfaces of the outer and inner wall tubes, is to provide a method for producing a double-walled tube with an interface gap, which satisfies the following conditions, and a double-walled tube with an interface gap produced with this method: (1) To have a high-temperature strength as a high-temperature heat-resistant and pressure-resistant member; (2) To be able to readily produce a double-walled tube having an interface gap between the inner surface of outer wall tube and the outer surface of inner wall tube without undergoing complex processes for preventing a through-wall leakage; (3) The thermal conductivity between outside and inside of the tube is not impaired, that is, the heat transfer between the outer and inner wall tubes of the double-walled tube is not isolated, or not notably hindered; (4) The outer and inner wall tubes of the double-walled tube have appropriate pressxire bonding characteristic as a heat exchanger tube; and 6 (5) The double-walled tube with an interface gap can be stably produced ensuring a high quality (meaning that the gap has an appropriate spacing and least variation, that is, is uniform). SOLUTION TO PROBLEM [0016] The summaries of the present invention are as follow. [0017] (1) A method for producing a double-walled tube with an interface gap (this production method is also referred to as a "first invention"), including: preparing (a) an outer wall tube made of ferritic steel containing not less than 2% of Cr, in which an oxide scale layer containing Cr and having a thickness of 10 to 30 |im is formed on an inner surface thereof, and (b) an inner wall tube made of steel or alloy containing not less than 2% of Cr, in which the outer surface thereof is machined and/or ground by a thickness of not less than 0.1 mm including a scale layer; and, after the inner wall tube is inserted into the outer wall tube, performing cold working at an outside diameter reduction rate of 5 to 30%. [0018] (2) A method for producing a double-walled tube (this production method is referred to as a "second invention"), including: preparing (a) an inner wall tube made of ferritic steel containing not less than 2% of Cr, in which an oxide scale layer containing Cr and having a thickness of 10 to 30 fxm is formed on the outer surface thereof, and (b) an outer wall tube made of steel or alloy containing not less than 2% of Cr, in which an inner surface thereof is machined and/or ground by a thickness of not less than 0.1 mm including a scale layer; and, after the inner wall tube is inserted into the outer wall tube, performing cold working at an outside diameter reduction rateof5to30%. [0019] (3) A double-walled tube with an interface gap, wherein the gap thereof produced by the method according to the above described (1) or (2) is 10 to 100 \xm including the scale layer. [0020] 7 (4) The double-walled tube with an interface gap according to the above described (3), wherein an interfacial pressure between the inner and outer wall tubes of the double-walled tube with the gap is 5 to 70 MPa. [0021] (5) The double-walled tube with an interface gap according to the above described (3) or (4), wherein the double-walled tube with the gap is used for a steam generator tube of a fast breeder reactor. ADVANTAGEOUS EFFECTS OF INVENTION [0022] According to the double-walled tube with an interface gap of the present invention, it is possible to readily and stably produce a high-quality double-walled tube with an interface gap which has a high-temperature strength as a hightemperature heat-resistant and pressure-resistant member, and has an interface gap between the inner surface of outer wall tube and the outer surface of inner wall tube for preventing a through-wall leakage. The double-walled tube with the gap produced by the method of the present invention has the gap with an appropriate spacing between the inner surface of outer wall tube and the outer surface of inner wall tube so that the thermal conductivity between outside and inside of the double-walled tube is not impaired, and the outer wall tube and the inner wall tube have an appropriate pressure bonding characteristic. This double-walled tube with the gap can be used as a high-temperature heatresistant and pressure-resistant member for boilers of thermal power generation plants, chemical industries, and nuclear power, and particularly suitable for steam generator tubes of future fast breeder reactors. BRIEF DESCRIPTION OF DRAWINGS [0023] [FIG. 1] FIG. 1 is a diagram to illustrate the method for measuring an interfacial pressure (the Crampton method) of the interface between the inner and outer wall tubes of a double-walled tube with an interface gap. 8 « M DESCRIPTION OF EMBODIMENTS [0024] The present inventors have repeated studies on the method for efficiently and economically producing an interface gap with an appropriate spacing to allow the prevention of a through-wall leakage in a double-walled tube, which is a significant issue of prior art. As a result, the inventors have found that producing a scale layer in advance and allowing it to remain in a ferritic steel containing not less than 2% of Cr is very effective, and that heat trzuisfer (heat conduction of double-walled tube) between the outer and inner wall tubes is sufficiently conducted even if the scale layer remains in the contact surface between the outer and inner wall tubes. Further, it is found that machining and grinding of the outer surface or the inner surface of the relevant tube (that is, the counterpart tube) to mate with the surface where the scale layer is formed, and giving appropriate working is important when inserting the inner wall tube into the outer wall tube and finishing them into a double-walled tube. [0025] The present invention has been made based on those findings. [0026] As described so far, in the method for producing a double-walled tube with an interface gap according to the present invention, a first invention is a method in which an oxide scale layer is formed in the inner surface of outer wall tube, the outer surface of the inner wall tube is machined and/or groxmd, and after the inner wall tube is inserted into the outer wall tube, cold working is performed at a predetermined reduction rate. On the other hand, a second invention is a method in which the processings applied to the outer wall tube and the inner wall tube are inversely arranged such that an oxide scale layer is formed on the outer surface of the irmer wall tube, while the inner surface of the outer wall tube is machined and/or ground, thereafter cold working is performed in a similar fashion. [0027] The purpose of using a ferritic steel containing not less than 2% of Cr as described above for the tube in which the oxide scale layer is formed is to build an interface gap with an appropriate spacing and least variation between mating surfaces; the inner surface of outer wall tube and the outer surface of irmer wall tube. 9 • • [0028] To build an interface gap with an appropriate spacing, a tight and stable scale layer is needed. Since the scale layer of carbon steel free of Cr has a large variation in thickness, is brittle and unstable, and easily exfoliates to come off during the working of the assembled tube, it is not possible to build the gap with an appropriate spacing when a double-walled tube is made. The scale layer of ferritic steel containing not less than 2% of Cr is stable and exhibits least variation, and therefore essentially needed for building the gap with an appropriate spacing. Although the upper limit of the amount of Cr is not defined, since the scale becomes less likely to beformed when the Cr content is not less than 15%, a ferritic steel having less than 13 % of Cr is preferable. [0029] On the other hand, since in an austenitic steel and Ni-based alloy, etc. other than ferritic steel, it is difficult to form a scale layer having a sufficient thickness, the tube in which a scale layer is formed is limited to be made of ferritic steel. [0030] The reason why the thickness of the scale layer to be formed on either the inner or outer surface of the relevant tube is made to be 10 to 30 |am as described above is that if the thickness of the scale layer is less than 10 |Lim, there is no effect of building an interface gap between mating surfaces: the inner surface of outer wall tube and the outer surface of inner wall tube; on the other hand, if the thickness is more than 30 f^m, the scale layer becomes more likely to exfoliate to come off during subsequent working, or the gap becomes too large so that the heat transfer capability between outside and inside of the product sharply deteriorates. [0031] The scale layer is formed such as by heating during the production of each of the outer and inner wall tubes before performing the drawing work and rolling work of the double-walled tube, and a heat treatment before assembling the outer and inner wall tubes that are processed as above for a double-walled tube. The thickness of the scale layer can be adjusted by investigating in advance the relationship among the temperature and time during heating, and the thickness of scale layer for each material. 10 [0032] The purpose of using a thickness of not less than 0.1 mm including the scale layer for the outer svirface or the inner surface of the counterpart tube which is to be mated with the surface where the scale layer is formed is machined and/or ground (hereafter, also referred to as "machining and grinding") is to build an interface gap with an appropriate spacing between mating surfaces: the inner svirface of outer wall tube and the outer surface of inner wall tube, when assembling the outer and inner wall tubes for making a double-walled tube,. [0033] When a scale layer is stuck on the surface of the covinterpart of the tube in which the scale layer is formed, or the surface of the relevant tube is not machined and/or ground, the gap to be built between mating surfaces: the irmer surface of outer wall tube and the outer surface of irmer wall tube will not have an appropriate spacing so that not only the uniformity (which means that the gap has an appropriate spacing and least variation) is impaired, but also a pressure bonding characteristic cannot be secured and the bonded interface of the resultant double-walled tube will be easily detached or separated. [0034] Further, since there are flaws, wrinkles, carburized and/ or nitrided layers, and the like on the surface of any tube at the time of production, and thus the surface is not uniform, they should be removed by performing machining and grinding to build the gap having an appropriate spacing and least variation. [0035] Performing machining and/or grinding to a thickness of not less than 0.1 mm allows the scale layer to be removed, thereby allows for removing flaws, wrinkles, carburized and/ or nitrided layers and the like on the surface of any tube at the time of production. Therefore, the thickness to be removed by machining and/or grinding is defined to be not less than 0.1 mm. The upper limit of the margin for machining and grinding is not particularly defined. This is because even if the margin for machining and grinding increases, there will be no problem in a technical sense fi-om the viewpoint of building an interface gap with an appropriate spacing, 11 and an excessive machining and grinding will be intrinsically restricted by the wall thickness etc. of the outer or inner wall tube. [0036] It is preferable that the surface roughness due to machining and/or grinding is arranged to be about 0.05 to 3 |am in arithmetic mean roughness (Ra). Since coarser surface roughness (Ra) will result in a non-uniform gap built between the inner surface of outer wall tube and the outer svirface of inner wall tube after they are processed into a double-walled tube, and therefore is not preferable. [0037] After machining and/or grinding is performed, compressive residual stress caused thereby is to be left therein to obtain a double-walled tube with an excellent pressure bonding characteristic, so heat treatment etc. will not be performed. [0038] Further, performing shotpeening processing in place of machining and grinding, or combining the both is also effective. On the other hand, performing a chemical treatment such as pickling in place of machining and grinding is not suitable since compressive residual stress on the surface is not generated, and therefore a double-walled tube with an excellent pressure bonding characteristic will not be obtained. [0039] The purpose of using steel or alloy containing not less than 2% of Cr as described above for the counterpart tube to mate with the surface where a scale layer is formed is to ensure a high-temperature strength specified by the Standard etc. as a high-temperature heat-resistant and pressure-resistant member. Although, as described above, the tube in which a scale layer is formed is limited to be made of ferritic steel fi"om the viewpoint of forming a tight and stable scale layer, the counterpart tube for the one having the scale layer will not be subject to such restriction, a wider variety of steels or alloys such as ferritic steels containing a higher amount of Cr, austenitic steels, Ni-based alloys may be used according to the condition of use as a high-temperature heat-resistant and pressure-resistant member. [0040] 12 The purpose of performing, in the first and second invention, cold working at an outside diameter reduction rate of 5 to 30% after the inner wall tube being inserted into the outer wall tube, the tubes being appropriately prepared as described above, is to easily and economically produce a double-walled tube which has an appropriate gap between the inner surface of outer tube and the outer surface of irmer tube for preventing a through-wall leakage, in which thermal conductivity between outside and inside of the double-walled tube is not impaired, and further the outer wall tube and the inner wall tube have an appropriate pressure bonding characteristic. [0041] Cold working is performed by a drawing process or a rolling process by a roll. The outside diameter reduction rate by cold working is 5 to 30%. When the outside diameter reduction rate is less than 5%, the outer wall tube and the irmer wall tube are not pressure bonded. On the other hand, by working at an outside diameter reduction rate of more than 30%, the gap will become small, thereby disabling sufficient prevention of through-wall leakage. Preferably, cold working is performed at an outside diameter reduction rate of 10 to 15%. More preferable is a drawing process at an outside diameter reduction rate of 10 to 15%, which uses only a die without using a plug. [0042] The treatment after cold working will not be particularly defined. The thusmade double-walled tube may be an as-worked product, or subjected to normalizing, tempering, annealing, solid solution heat treatment, stress-relief aimealing etc. to be an end product. When there is required a double-walled tube in which interfacial pressure between the inner and outer wall tubes as an end product is high, working conditions suitable for that may be appropriately selected. Further, when placing importance on the high-temperature creep characteristic, heat treatment specified by Standards is performed on either or both of the outer and inner wall tubes. [0043] According to the method for producing a double-walled tube with an interface gap of the present invention described above, it is possible to readily and stably produce a double-walled tube having a uniform interface gap between the inner surface of outer wall tube and the outer surface of irmer wall tube without 13 undergoing complex processes. It is also possible to produce a longer-length product of about 35 m long. [0044] The double-walled tube with an interface gap of the present invention is a double-walled tube with the gap wherein the gap between the inner surface of outer wall tube and the outer surface of inner wall tube is 10 to 100 |am including the scale layer. [0045] When the gap is less than 10 j^m, it is not possible to sufficiently arrest the propagation of cracking that occurs in the outer wall tube or the inner wall tube, and thus the cracking will develop and the double-walled tube will become liable to through-wall fi-acture. On the other hand, when the gap is more than 100 |am, sufficient heat transfer between the outer and inner wall tubes cannot be ensured and the outer wall tube and the inner wall tube becomes less likely to be pressure bonded. The spacing of the gap is preferably 10 to 50 |j,m. More preferably, the spacing is 10to35|im. [0046] In the double-walled tube with an interface gap of the present invention, the interfacial pressure in the interface between the inner and outer tubes is preferably 5 to 70 MPa. [0047] The interfacial pressvire in the interface between the inner and outer tubes of the double-walled tube is needed to ensvire a pressure bonding characteristic so that the inner and outer wall tubes will not be easily detached or separated from each other. When the interfacial pressure in the interface of the double-walled tube is less than 5 MPa, an excellent pressure bonding characteristic cannot be ensured, and when more than 70 MPa, there is a risk that the defective such as curved tube occur in the double-walled tube. [0048] The double-walled tube with the gap of the present invention has a versatility as a heat-resistant and pressure-resistant member, and can be widely used under high-temperature environments such as boilers for thermal power generation plants, 14 chemical industries, and nuclear power plants. In particular, it is suitable as a steam generator tube of next-generation fast breeder reactors in which the prevention of through-wall leakage is a crucial issue, and can be provided as a longer-length product of about 35 m long. EXAMPLES [0049] As outer and inner wall tubes for producing a double-walled tube, seamless tubes of JIS standard STBA24 (2.25Cr - IMo) which is a conventional heat-resistant steel containing not less than 2% of Cr, ASME standard T91 (9Cr -1 Mo - V - Nb), and JIS standard SUS321HTB (18Cr - 1 ONi - Ti) were prepared. The dimensions of respective tubes were as follows. Outer wall tube: Outside diameter 20.4 mm, wall thickness 1.9 mm, and length 15m Inner wall tube: Outside diameter 15.7 mm, wall thickness 1.6 mm, and length 15m [0050] Using these seamless steel tubes, tubes in which an oxide scale is formed in advance in the inner surface or the outer surface thereof, and tubes in which the surface (inner or outer surface) thereof to be the counterpart of the aforementioned tube was machined and ground were prepared in combination of the outer and inner wall tubes as shown in Table 1. [0051] [Table 1] 15 I 1 \ i \ 1 1 1 i 1 1 1 S^Q) ta-'=<; i « ^ ca^ ea •"= <; K H I S<« E.Sizi S-oi £"5 E.Sw3 S 2 | "co -a "ta c c c c c 3 c c c3 g ' 5 > u i i iJ ii Ji M « ii ii iJ "M H 'ft n', -S "u "u 'u "o "u '3 "u 13 "u '3 £ 5 3 X X X X x g x x X c o-S ^ ^ W5 60-3 y k- «> y t- u | o | S § § g J ^ 1 § ^ ^ > . 1 1 ^ ^ £ l .2 -S-S-8 1 I I i i la -gl gtl |l i^^ S S p " w w w w a . g « l g | l 2 l «§l-2£-Sl % 2 a o S < N O V O ( N • * O o i r > -2 2 S3 . S ^-^ I I I I I ' . ^ O 1> 'T T5 & ! - - - - --°° ^°o C3 « .2S _ _ a \ ( ^ l „ t ^ _ , < N » T3 "^ ^ 2 O c t * - ,, 3 c^?Pa - i i ui 5 S - S - S ^ a c g CC e g | c fiC oo e g C X I g g CC -M c .s-gs z--^ z - z^ ^ z ^z z z ;|^ =.2-ot^^„ =«:> § .2 •g-c^'^o ^5 '^d d-^ d'^ * » •^d ' ^ « > . S d d •^g ° I _^ ^IB J > U . S ' g ' j i U s; i i j ; i i a j l — ' ^ ' ^ u ;;4> ® ° i;i> "5 Knx>"- CO So So -o -O 5 3 cO =0 =3 £0 =3 s o s SO ^ ^g fN