A mandrel for forming nuclear power plant bends and a forming method thereof

Abstract

The application discloses a core mold for forming a nuclear power elbow pipe and a forming method, which comprises n+1 bending core male molds, n bending core female molds and 2n supporting balls 12, wherein n is an odd number; each bending core male mold is composed of a bending core male mold inner half mold and a bending core male mold outer half mold, the bending core female mold is composed of a bending core female mold inner half mold and a bending core female mold outer half mold, and the supporting balls are installed in spherical holes formed by two large hemispherical hole platforms and two small hemispherical hole platforms in the bending core male mold and the bending core female mold after being connected in a plug-in mode. After the core mold is assembled, a straight pipe blank is loaded into the core mold before bending, and then the core mold and the straight pipe blank are fastened by a nut, the core mold is used for supporting the inner wall of the pipe during the bending process, and the core mold can ensure that the nuclear power elbow pipe is not wrinkled on the inner wall after being bent and formed, the inner hole at the bending position is not distorted, the inner wall of the elbow pipe is not dented due to improper design of the core mold, the bending and forming quality of the nuclear power elbow pipe is greatly improved, and the elbow pipe can be conveniently and quickly taken out after being bent.

Description

Technical Field

[0001] This invention belongs to the field of nuclear power plant tube bending technology, specifically relating to a core mold and a method for forming nuclear power plant tubes. Background Technology

[0002] Bends are large, thick-walled pressure-bearing pipes that connect the pressure vessel and steam generator of a nuclear power reactor. Bends are large in size and complex in structure. They require forging and machining before bending and forming, and there are strict requirements for technical parameters such as the amount of thinning and the ellipticity of the formed pipe. Therefore, the bending and forming process of bends and the design of supporting molds are key challenges in the field of nuclear power forging manufacturing.

[0003] Because the bending radius of pipe forgings is small and the relative wall thickness is relatively thick, the inner wall of the bend thickens during the bending process, making it prone to defects such as collapse or wrinkles. Simultaneously, the bending cross-section will also be distorted, becoming elliptical. If the mandrel design is unreasonable, wrinkles and distortion will still exist. Large wrinkles on the inner wall cannot be removed by machining; large distortion of the inner hole will also cause the pipe to be scrapped. Furthermore, an unreasonable mandrel design can cause the mandrel to get stuck in the inner wall of the pipe during bending, forming multiple pits and leading to pipe scrapping. Secondly, an unreasonable mandrel design can also make mandrel disassembly difficult, making it hard to remove from the bent pipe, resulting in the entire pipe being scrapped. Summary of the Invention

[0004] The purpose of this invention is to solve the above-mentioned technical problems and provide a core mold and forming method for forming nuclear power plant bent tubes that avoids wrinkles on the inner wall of the bent tube and distortion of the inner hole at the bend into an ellipse during bending.

[0005] To achieve the above objectives, the present invention provides a core mold for forming a nuclear power plant bend, comprising n+1 male core molds, n female core molds, and 2n support spheres 12, where n is an odd number; each male core mold is composed of an inner half-mold and an outer half-mold, the outer half-mold being a large semi-cylinder with a cuboid through hole recessed on its axial end face, the inner half-mold being a small semi-cylinder with a cuboid boss formed on its axial end face that fits into the cuboid through hole, and the outer diameter of the large semi-cylinder being the same as the outer diameter of the small semi-cylinder; both radial end faces of the outer half-mold have interconnected large hemispherical frustum holes, and both radial end faces of the inner half-mold have interconnected small hemispherical frustum holes, the diameter of the large hemispherical frustum holes being the same as the diameter of the small hemispherical frustum holes;

[0006] The bending core mold is composed of an inner half mold and an outer half mold. The outer half mold is a large semi-cylinder with a cuboid through hole recessed on its axial end face. The inner half mold is a small semi-cylinder with a cuboid boss formed on its axial end face that fits into the cuboid through hole. The outer diameter of the large semi-cylinder is the same as the outer diameter of the small semi-cylinder. Both radial end faces of the outer half mold have interconnected large hemispherical frustum holes, and both radial end faces of the inner half mold have interconnected small hemispherical frustum holes. The diameter of the large hemispherical frustum holes is the same as the diameter of the small hemispherical frustum holes.

[0007] The support ball is installed in the spherical hole formed by two large hemispherical holes and two small hemispherical holes after the male and female molds of the bending core are inserted and connected.

[0008] Furthermore, the outer half of the male bending mold has two axial through holes on its radial end face, and the inner half of the male bending mold has one axial through hole on its radial end face. The three axial through holes have the same diameter and are evenly distributed circumferentially. The outer half of the female bending mold has two axial through holes on its radial end face, and the outer half of the female bending mold has one axial through hole on its radial end face. The three axial through holes have the same diameter and are evenly distributed. The three axial through holes on the female bending mold correspond one-to-one with the three axial through holes on the male bending mold. Three double-ended stud assemblies are inserted into the three axial through holes of the male bending mold, the three axial through holes of the female bending mold, and the support ball, serving to connect and fix the male bending mold, the support ball, and the female bending mold.

[0009] Furthermore, it also includes two top shafts, each top shaft including a cylinder, a cylindrical protrusion located at the center of one end face of the cylinder, a ball-shaped protrusion located on the end face of the cylindrical protrusion, a rectangular blind hole located on one end face of the cylinder, and three axial mounting holes evenly distributed along the circumference on one end face of the cylinder. The three axial mounting holes have the same diameter as the three axial through holes on the bending core mold and are distributed in a one-to-one correspondence. The diameter of the ball-shaped protrusion is the same as the diameter of the large hemispherical hole. During installation, the ball-shaped protrusion is embedded in the ball-shaped hole formed by the mating of the large hemispherical hole and the small hemispherical hole of the bending core mold.

[0010] Furthermore, after the inner half of the bending core mold and the outer half of the bending core mold are assembled, there is a 4-8mm intermediate gap between the cuboid boss and the bottom surface of the cuboid through hole, and a 5-10mm side gap between the protrusions on both sides of the outer half of the bending core mold and the step surface of the inner half of the bending core mold.

[0011] Furthermore, after the inner half of the bending core mold and the outer half of the bending core mold are assembled, there is a 4-8mm intermediate gap between the cuboid boss and the bottom surface of the cuboid through hole, and a 5-10mm side gap between the protrusions on both sides of the outer half of the bending core mold and the stepped surface of the inner half of the bending core mold.

[0012] Furthermore, rectangular protrusions are formed on the two radial end faces of the outer half of the male mold and above the large hemispherical hole, and the cross-section of the rectangular protrusions is square; a rectangular through hole that matches the rectangular protrusions is opened on the radial end face of the outer half of the female mold and directly above the large hemispherical hole, the width of the rectangular through hole is the same as the width of the rectangular protrusion, and the height of the rectangular through hole is 10-15mm greater than the height of the rectangular protrusion.

[0013] Furthermore, the other end face of the cylinder is uniformly provided with multiple threaded holes along the circumference, and the diameter of the cylinder is 1-2 mm smaller than the inner diameter of the straight tube blank.

[0014] A method for forming a bent nuclear power tube is also provided. The forming method uses the aforementioned mandrel. The assembled mandrel is inserted into the straight tube blank at the bending position, with both ends of the mandrel extending beyond both ends of the straight tube blank. The center of the bending core female mandrel in the middle of the mandrel coincides with the bending center of the straight tube blank. The locking nuts of the double-ended stud assembly are adjusted so that the outer diameters of the bending core male mandrel and the bending core female mandrel are tightly fitted against the inner wall of the straight tube blank, and then the locking nuts are locked. The nuts are then installed and tightened onto the threads at both ends of the straight tube blank. After the nuts are tightened, the outer end face of the top shaft of the mandrel is tightly fitted against the inner plane of the nut. The locking nuts and washers of the three double-ended stud assemblies are removed from the inner hole of the nut, and then the double-ended studs are taken out. At this time, the mandrel and the straight tube blank are firmly secured together by the nuts.

[0015] As the press gradually applies pressure to the arc punch on the upper part of the straight tube blank, the male mold, support ball, and female mold of the core die will gradually bend in the bending direction. The support ball will rotate freely in the hollow spherical surface as it bends. The angle between the inner half of the male mold and the inner half of the female mold will gradually decrease as the bending angle increases until the bending ends. At the same time, the angle between the inner half of the male mold and the inner half of the female mold becomes 0°. Meanwhile, the gap between the outer half of the male mold and the outer half of the female mold becomes larger and larger.

[0016] After the straight tube blank is bent into a bent tube, the nuts at both ends of the bent tube are removed, and then the top shaft inside the bent tube is taken out. Subsequently, the male mold of the bending core, the support ball and the female mold of the bending core are taken out one by one.

[0017] Furthermore, the nut has a lifting hole on its outer end face and a disassembly groove on its outer circle. The nut fixes the mandrel onto the straight tube blank, preventing the mandrel from moving in the length direction during the bending process.

[0018] Furthermore, the total number of the male and female bending core molds is determined by the inner arc length of the straight tube blank after bending. The thickness of the rectangular boss in the inner half of the male bending core mold is divided by the inner arc length, and the resulting number is rounded to obtain the total number of the male and female bending core molds.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: After the mandrel is assembled, it is inserted into the straight pipe blank before bending, and then the mandrel and the straight pipe blank are fastened with nuts. The mandrel is used to support the inner wall of the pipe during the bending process, which can ensure that the inner wall of the nuclear power plant bend is wrinkle-free and the inner hole at the bend will not be deformed after bending. At the same time, it ensures that the inner wall of the bend will not have pits due to improper mandrel design, which greatly improves the bending quality of the nuclear power plant bend and can be easily and quickly removed after bending. Attached Figure Description

[0020] Figure 1 This is a front view of the male mold for bending cores of the present invention;

[0021] Figure 2 for Figure 1 Side view;

[0022] Figure 3 This is a front view of the bending core mold of the present invention;

[0023] Figure 4 for Figure 3 Side view;

[0024] Figure 5 This is a schematic diagram of the assembly of the male mold, female mold, and support ball of the present invention.

[0025] Figure 6 This is the top-axis front view of the present invention;

[0026] Figure 7 for Figure 6 The diagram on the right;

[0027] Figure 8 for Figure 6 The diagram on the left;

[0028] Figure 9 This is a schematic diagram of the double-headed stud assembly structure of the present invention;

[0029] Figure 10 This is a schematic diagram of the core mold assembly of the present invention;

[0030] Figure 11 This is a schematic diagram of a straight tube blank;

[0031] Figure 12 This is a schematic diagram of the assembly of the straight tube blank and the core mold according to the present invention;

[0032] Figure 13 for Figure 12 Schematic diagram of the center nut;

[0033] Figure 14 This is a schematic diagram of the bending and forming process of the pipe according to the present invention. Detailed Implementation

[0034] The core mold for forming the nuclear power plant bend includes n+1 male core molds, n female core molds, 2n support balls 12, two top shafts and three double-ended stud assemblies, where n is an odd number.

[0035] Reference Figure 1 , 2 Each bending core die is composed of an inner half die 1 and an outer half die 2. The outer half die 2 is a large semi-cylinder with a cuboid through hole 2a recessed in its axial end face. The inner half die 1 is a small semi-cylinder with a cuboid boss 6 formed on its axial end face that fits into the cuboid through hole 2a. The outer diameter of the large semi-cylinder is the same as the outer diameter of the small semi-cylinder. During the bending process of the mandrel, the cuboid boss 6 and the cuboid through hole 2a act as guides, restricting the inner half of the mandrel 1 to rotate only in the bending direction; after the inner half of the mandrel 1 and the outer half of the mandrel 2 are aligned, there is a 4-8mm intermediate gap between the bottom surfaces of the cuboid boss 6 and the cuboid through hole 2a, and a 5-10mm side gap between the protrusions 7 on both sides of the outer half of the mandrel 2 and the step surface of the inner half of the mandrel 1. The larger gaps are beneficial for the mandrels to be removed from the straight tube blank one by one after the bending is completed.

[0036] Meanwhile, both radial end faces of the outer half of the bending core mold 2 have interconnected large hemispherical truncated holes 4, and both radial end faces of the inner half of the bending core mold 1 have interconnected small hemispherical truncated holes 5. The diameter of the large hemispherical truncated holes 4 is the same as the diameter of the small hemispherical truncated holes 5, and the large end faces of both the large and small hemispherical truncated holes 4 and 5 are arranged outwards. Rectangular protrusions 3 are formed on both radial end faces of the outer half of the bending core mold 2, above the large hemispherical truncated holes, and the cross-section of the rectangular protrusions 3 is square. Two axial through holes 8 are provided on the radial end faces of the outer half of the bending core mold 2, and one axial through hole 8 is provided on the radial end face of the inner half of the bending core mold 1. These three axial through holes 8 have the same diameter and are evenly distributed circumferentially.

[0037] Reference Figure 3 , 4The bending core mold is composed of an inner half mold 9 and an outer half mold 10. The outer half mold 10 is a large semi-cylinder with a cuboid through hole recessed on its axial end face. The inner half mold 9 is a small semi-cylinder with a cuboid boss formed on its axial end face that fits into the cuboid through hole. The outer diameter of the large semi-cylinder is the same as that of the small semi-cylinder. Simultaneously, both radial end faces of the outer half mold 10 have interconnected large hemispherical frustum holes, and both radial end faces of the inner half mold 9 have interconnected small hemispherical frustum holes. The diameters of the large and small hemispherical frustum holes are the same, and the large end faces of both holes face outwards. Meanwhile, a rectangular through hole 11, which mates with the rectangular protrusion 3, is provided on the radial end face of the outer half of the bending core mold 10, directly above the hemispherical platform hole. The width of the rectangular through hole 11 is the same as the width of the rectangular protrusion 3, and the height of the rectangular through hole 11 is 10-15 mm greater than the height of the rectangular protrusion 3, ensuring that the rectangular protrusion 3 can rotate freely in the rectangular through hole 11 during bending without interference. At the same time, the rectangular protrusion 3 on the outer half of the bending core mold 2 is inserted into the rectangular through hole 11 on the outer half of the bending core mold 10, serving a connecting and guiding function. Two axial through holes and one axial through hole are provided on the radial end face of the outer half of the bending core mold 10. These three axial through holes have the same diameter and are evenly distributed. Similarly, after the inner half mold 9 and the outer half mold 10 of the bending core mold are assembled, there is a 4-8mm intermediate gap between the cuboid boss and the bottom surface of the cuboid through hole, and a 5-10mm side gap between the protrusions on both sides of the outer half mold 10 of the bending core mold and the step surface of the inner half mold 9 of the bending core mold. The larger gap is beneficial for the bending core male mold to be removed from the straight tube blank one by one after the bending is completed.

[0038] The outer diameter, the diameter of the large hemispherical hole, and the diameter of the axial through hole of the female bending core mold are all the same as the outer diameter, the diameter of the large hemispherical hole, and the diameter of the axial through hole of the male bending core mold A. The large and small hemispherical holes on the female bending core mold are arranged in a one-to-one correspondence with the large and small hemispherical holes on the male bending core mold. The three axial through holes on the female bending core mold are arranged in a one-to-one correspondence with the three axial through holes on the male bending core mold.

[0039] like Figure 5 As shown, each support ball 12 is a bearing steel ball, and the diameter of the support ball 12 is the same as the diameter of the large hemispherical platform hole. During assembly, the support ball 12 is installed in the spherical hole formed by the two large hemispherical platform holes and the two small hemispherical platform holes after the male mold and the female mold of the bending core are inserted and connected.

[0040] Reference Figure 6 , 78. Each top shaft includes a cylinder 21, a cylindrical protrusion 22 located at the center of one end face of the cylinder 21, a ball-shaped protrusion 13 located on the end face of the cylindrical protrusion 22, a rectangular blind hole 14 located on one end face of the cylinder 21, and three axial mounting holes 23 evenly distributed around the circumference of one end face of the cylinder 21. The three axial mounting holes 23 have the same diameter as the three axial through holes 8 on the bending core die and are distributed in a one-to-one correspondence. The diameter of the ball-shaped protrusion 13 is the same as the diameter of the large hemispherical hole 4. During installation, the ball-shaped protrusion 13 is embedded in the ball-shaped hole formed by the mating of the large hemispherical hole and the small hemispherical hole of the bending core die. In addition, multiple threaded holes 15 are evenly arranged around the other end face of the cylinder 21 in the circumferential direction, which serve to disassemble the top shaft. The diameter of the cylinder 21 is 1-2 mm smaller than the inner diameter of the straight tube blank 20.

[0041] Reference Figure 9 Each double-ended stud assembly includes a double-ended stud 24, with a washer 16 and a lock nut 17 fitted at both ends of the double-ended stud 24. The three double-ended stud assemblies are inserted into the three axial through holes of the male bending die, the three axial through holes of the female bending die, the support ball 12, and the three axial mounting holes of the top shaft, serving to connect and fix the male bending die, the support ball 12, the female bending die, and the top shaft.

[0042] Reference Figure 10 During assembly, the three double-ended studs first pass through the three axial mounting holes of a top shaft, then through the three axial through holes of the first bending core mold, so that the ball joint protrusion 13 of the top shaft is embedded in the ball joint hole of the first bending core mold where the large and small hemispherical holes on one side align. Simultaneously, the rectangular protrusion 3 of the first bending core mold is inserted into the rectangular blind hole 14 of the top shaft. Then, the first support ball 12 is embedded into the ball joint hole of the first bending core mold where the large and small hemispherical holes on the other side align. The three double-ended studs then pass through the three axial through holes of the first bending core mold, and the rectangular protrusion 3 of the first bending core mold is inserted into the rectangular through hole 11 of the first bending core mold, so that the other half of the first support ball 12 is embedded in the ball joint hole of the first bending core mold where the large and small hemispherical holes on the other side align. The first bending core mold has a ball joint hole where the large and small hemispherical holes align; then a second support ball 12 is inserted into the ball joint hole where the large and small hemispherical holes align on the other side. Three double-ended studs then pass through the second bending core mold. This process is repeated until the bending core mold, support ball 12, and bending core mold are all passed through, and until the three double-ended studs pass through the three axial mounting holes of another top shaft. After all components are passed through, the locking nuts 17 at both ends of the double-ended studs are tightened to form an integral core mold. After tightening, there is an included angle between the inner half mold 1 of each bending core mold and the inner half mold 9 of each bending core mold, and there is a gap between the outer half mold 2 of each bending core mold and the outer half mold 10 of each bending core mold, with a gap of 1-2 mm.

[0043] The forming method of the nuclear power plant bend includes a straight pipe blank 20 and the aforementioned mandrel, wherein threads are machined on the outer surfaces of both ends of the straight pipe blank to mate with nuts; refer to Figure 11 Considering that the inner arc surface 25 will thicken and the outer arc surface 26 will thin after bending, the inner arc surface 25 will be thinned and the outer arc surface 26 will be thickened at the corresponding bending point during the straight tube blank processing before bending. The thinning and thickening amounts are equal, both being 8-10% of the wall thickness. The outer diameter of the bending core die (i.e., the outer diameter of the semi-cylinder) is the same as the inner diameter of the straight tube blank.

[0044] Combination Figure 12 As shown, the forming process of the nuclear power plant bend is as follows: The assembled mandrel is inserted into the straight tube blank 20 at the bending position, with both ends of the mandrel extending beyond both ends of the straight tube blank 20. The center of the bending mandrel in the middle of the mandrel coincides with the bending center of the straight tube blank 20. The locking nuts of the double-ended stud assembly are adjusted so that the outer diameters of the bending mandrel male and female molds are tightly fitted against the inner wall of the straight tube blank 20, and then the locking nuts 17 are locked. Nuts 27 are then installed and tightened onto the threads at both ends of the straight tube blank 20. After the nuts 27 are tightened, the outer end face of the top shaft of the mandrel is tightly fitted against the inner plane of the nuts 27. The locking nuts 17 and washers 16 of the three double-ended stud assemblies are removed from the inner hole 28 of the nuts 27, and then the double-ended studs are taken out. At this time, the mandrel and the straight tube blank 20 are firmly secured together by the nuts 27.

[0045] As the press gradually applies pressure to the arc punch on the upper part of the straight tube blank 20, the male mold, support ball, and female mold of the core mold will gradually bend in the bending direction. The support ball 12 will rotate freely in the hollow spherical surface 4 as it bends. The included angle between the inner half mold 1 of the male mold and the inner half mold 9 of the female mold will gradually decrease as the bending angle increases until the bending ends. The included angle between the inner half mold 1 of the male mold and the inner half mold 9 of the female mold becomes 0°. At the same time, the gap between the outer half mold 2 of the male mold and the outer half mold 10 of the female mold becomes larger and larger.

[0046] After the straight tube blank 20 is bent into a bent tube, the nuts at both ends of the bent tube are removed, and then the top shaft inside the bent tube is taken out. Subsequently, the male mold of the bending core, the support ball and the female mold of the bending core are taken out one by one.

[0047] Reference Figure 13 The outer end face of nut 27 has a lifting hole 18, and the outer circle of nut has a disassembly groove 19. The nut fixes the mandrel on the straight tube blank, so that the mandrel does not move in the length direction during the bending process.

[0048] Reference Figure 14 The total number of male and female bending core molds is determined by the inner arc length of the straight tube blank after bending. The thickness of the rectangular boss in the inner half of the male bending core mold is divided by the inner arc length, and the resulting number is rounded to get the total number of male and female bending core molds.

[0049] This invention can bend and form nuclear power plant bent tubes of different specifications with an inner diameter of up to 1 meter. The inner arc of the straight tube blank at the bend is reduced, while the outer arc is increased; only the mandrel and nut of different sizes need to be replaced. Using this invention ensures that the inner wall of the bent nuclear power plant tube is wrinkle-free and the inner hole at the bend is not distorted after bending. At the same time, it ensures that the inner wall of the bent tube will not have pits due to improper mandrel design, greatly improving the bending quality of the nuclear power plant bent tube. This invention, for nuclear power plant bent tubes of different specifications, whether cold or hot bent, obtains dimensionally accurate nuclear power plant bent tubes by eccentric processing at the bend, installing and securing the mandrel, inserting the mandrel into the straight tube blank, and bending it.

Claims

1. A core for forming a nuclear elbow, characterized by: The application relates to a core-bending mold, which comprises n+1 core-bending male molds, n core-bending female molds, 2n supporting balls (12), and n is an odd number; each core-bending male mold is composed of a core-bending male mold inner half mold (1) and a core-bending male mold outer half mold (2) which are matched together, the core-bending male mold outer half mold (2) is a large semi-cylinder with a rectangular through hole (2a) concaved on an axial end surface, the core-bending male mold inner half mold (1) is a small semi-cylinder with a rectangular boss (6) formed on an axial end surface and matched with the rectangular through hole (2a), and the outer diameter of the large semi-cylinder is the same as that of the small semi-cylinder; two radial end surfaces of the core-bending male mold outer half mold (2) are both inwardly provided with large semi-spherical platform holes (4) which are communicated with each other, two radial end surfaces of the core-bending male mold inner half mold (1) are both inwardly provided with small semi-spherical platform holes (5) which are communicated with each other, and the diameter of the large semi-spherical platform hole (4) is the same as that of the small semi-spherical platform hole (5); the core-bending female mold is composed of a core-bending female mold inner half mold (9) and a core-bending female mold outer half mold (10) which are matched together, the core-bending female mold outer half mold (10) is a large semi-cylinder with a rectangular through hole concaved on an axial end surface, the core-bending female mold inner half mold (9) is a small semi-cylinder with a rectangular boss formed on an axial end surface and matched with the rectangular through hole, and the outer diameter of the large semi-cylinder is the same as that of the small semi-cylinder; two radial end surfaces of the core-bending female mold outer half mold (10) are both inwardly provided with large semi-spherical platform holes which are communicated with each other, two radial end surfaces of the core-bending female mold inner half mold (9) are both inwardly provided with small semi-spherical platform holes which are communicated with each other, and the diameter of the large semi-spherical platform hole is the same as that of the small semi-spherical platform hole; the supporting ball (12) is installed in a spherical hole formed by two large semi-spherical platform holes and two small semi-spherical platform holes after the core-bending male mold and the core-bending female mold are inserted and connected; after the core-bending male mold inner half mold (1) and the core-bending male mold outer half mold (2) are matched together, a middle gap of 4-8mm is left between the rectangular boss (6) and the bottom surface of the rectangular through hole (2a), and a side gap of 5-10mm is left between the convex (7) on the two sides of the core-bending male mold outer half mold (2) and the stepped surface of the core-bending male mold inner half mold (1); after the core-bending female mold inner half mold (9) and the core-bending female mold outer half mold (10) are matched together, a middle gap of 4-8mm is left between the rectangular boss and the bottom surface of the rectangular through hole, and a side gap of 5-10mm is left between the convex on the two sides of the core-bending female mold outer half mold (10) and the stepped surface of the core-bending female mold inner half mold (9).

2. The core for forming a nuclear elbow according to claim 1, wherein: two axial through holes (8) are arranged on the radial end surface of the core-bending male mold outer half mold (2), one axial through hole (8) is arranged on the radial end surface of the core-bending male mold inner half mold (1), the three axial through holes (8) have the same diameter and are uniformly distributed in a circle; two axial through holes are arranged on the radial end surface of the core-bending female mold outer half mold (10), one axial through hole is arranged on the radial end surface of the core-bending female mold inner half mold (9), the three axial through holes have the same diameter and are uniformly distributed; the three axial through holes on the core-bending female mold are arranged in one-to-one correspondence with the three axial through holes on the core-bending male mold, three double-headed stud assemblies are inserted into the three axial through holes of the core-bending male mold and the three axial through holes of the core-bending female mold, and the three double-headed stud assemblies play the roles of series connection and fixation of the core-bending male mold, the supporting ball (12) and the core-bending female mold.

3. The core for forming a nuclear elbow according to claim 2, wherein: The rectangular protrusion (3) is formed on the two radial end faces of the male outer half die (2) above the large hemisphere platform hole, and the cross section of the rectangular protrusion (3) is a square; a rectangular through hole (11) is formed on the radial end face of the female outer half die (10) directly above the large hemisphere platform hole, and the rectangular through hole (11) is matched with the rectangular protrusion (3); the width of the rectangular through hole (11) is the same as the width of the rectangular protrusion (3), and the height of the rectangular through hole (11) is 10-15 mm larger than the height of the rectangular protrusion (3); The two top shafts each include a cylindrical body (21), a cylindrical protrusion (22) arranged at the center of one end face of the cylindrical body (21), a spherical platform protrusion (13) arranged at the end face of the cylindrical protrusion (22), a rectangular blind hole (14) arranged on one end face of the cylindrical body (21) for inserting the rectangular protrusion (3), and three axial mounting holes (23) arranged uniformly along the circumference of one end face of the cylindrical body (21), the three axial mounting holes (23) have the same diameter as the three axial through holes (8) of the male die and are distributed in one-to-one correspondence, and the diameter of the spherical platform protrusion (13) is the same as the diameter of the large hemisphere platform hole (4); during installation, the spherical platform protrusion (13) is embedded in the spherical platform hole formed by the large hemisphere platform hole and the small hemisphere platform hole of the male die.

4. The core for forming a nuclear elbow according to claim 3, wherein: A plurality of threaded holes (15) are arranged uniformly along the circumferential direction of the other end face of the cylindrical body (21), and the diameter of the cylindrical body (21) is 1-2 mm smaller than the inner diameter of the straight pipe blank (20).

5. A method of forming a formed nuclear elbow, the method comprising: The core die used in the forming method is the core die of claim 3; each double-headed stud assembly includes a double-headed stud (24), and a gasket (16) and a locking nut (17) are sleeved on both ends of the double-headed stud (24); the assembled core die is installed in the position to be bent of the straight pipe blank (20), and both ends of the core die protrude out of both ends of the straight pipe blank (20), the center of the female die in the middle of the core die coincides with the bending center of the straight pipe blank (20); the locking nuts of the double-headed stud assemblies are adjusted to tightly fit the outer diameters of the male die and the female die to the inner wall of the straight pipe blank (20), and then the locking nuts (17) are locked; then nut one (27) is installed and fastened on the threads at both ends of the straight pipe blank (20), after the nut one (27) is fastened, the outer end face of the top shaft of the core die tightly fits the inner flat surface of the nut one (27); the locking nuts (17) and the gaskets (16) of the three double-headed stud assemblies are removed from the inner hole (28) of the nut one (27), and then the double-headed studs are taken out, at this time, the core die and the straight pipe blank (20) are tightly and stably fixed together by the nut one (27); With the press gradually pressurizing the arc punch on the upper part of the straight pipe blank (20), the bending core male die, the supporting ball and the bending core female die will gradually bend in the bending direction, the supporting ball (12) will rotate freely in the hollow spherical surface along with the bending, the included angle between the inner half die (1) of the bending core male die and the inner half die (9) of the bending core female die will gradually decrease along with the increase of the bending angle until the bending is finished, the included angle between the inner half die (1) of the bending core male die and the inner half die (9) of the bending core female die becomes 0°, at the same time, the gap between the outer half die (2) of the bending core male die and the outer half die (10) of the bending core female die becomes larger and larger; After the straight pipe blank (20) is bent into a bent pipe, the nuts at the two ends of the bent pipe are disassembled, the top shaft inside the bent pipe is taken out, then, the bending core male die, the supporting ball and the bending core female die are taken out one by one.

6. The method of claim 5, wherein the forming of the nuclear elbow is performed by: The outer end surface of the nut one (27) has a lifting hole (18), the outer circle of the nut one has a disassembly groove (19), the nut one fixes the core die on the straight pipe blank, so that the core die does not move in the length direction during the bending.

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