A variable diameter mandrel for a nuclear internal variable diameter guide tube and a processing method thereof

Abstract

The present invention discloses a variable diameter mandrel and processing method for a nuclear internal variable diameter guide tube. The variable diameter mandrel adopts a combined splicing structure, which is composed of a first mandrel and a second mandrel detachably connected, and threaded sections connected to the core rod are provided at both ends; the diameter of the first mandrel is the same as the inner diameter of the thin-walled section of the guide tube; the second mandrel is integrally formed by the first section, the second section and the intermediate transition section, the first section is connected to the first mandrel, and the diameter of the first section is the same as the inner diameter of the thin-walled section of the guide tube, and the diameter of the second section is the same as the inner diameter of the thick-walled section of the guide tube; its processing route is: material selection → sawing → rough turning → fine turning → salt bath heat treatment → grinding. The present invention adopts a method of breaking the whole into parts to solve the problems of low dimensional accuracy and great processing difficulty of the existing long variable diameter mandrel, which can not only prepare qualified internal variable diameter guide tubes, but also improve the qualified rate of mandrel products and reduce their manufacturing costs, and the tube is easy to demold during rolling, with good use effect, and is easy to promote and use.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation of special-shaped tubes for nuclear use, and in particular relates to a diameter-reducing mandrel of an inner diameter-reducing guide tube for nuclear use and a processing method thereof. Background Art

[0002] Zirconium and zirconium alloys have good corrosion resistance and mechanical properties, low thermal neutron absorption cross section, and are important structural materials for nuclear reactors. They are widely used in the core materials of water-cooled power reactors. Among them, zirconium alloy guide tubes are an important part of nuclear reactor fuel stack components. They not only have skeleton support functions, but also provide guidance, deceleration and buffering for control rods. If the shape design of the guide tubes is unreasonable or the dimensional accuracy is insufficient during processing, the lifting or falling action of the control rods will be blocked, which will affect the timely start and stop of the reactor. Therefore, the guide tubes play a vital role in the safety and reliability of the reactor.

[0003] At present, the common nuclear fuel assembly guide tubes in the industry are roughly divided into two categories in terms of structure: one is that the wall thickness remains unchanged, and the inner and outer diameters change in a step-like manner, such as the guide tube used in the AFA2G assembly; the other is that the outer diameter remains unchanged while the inner diameter changes, such as the Monobloc TM guide tube used in the AFA3G assembly. Figure 2 , 3 As shown, the internal variable diameter guide tube is adopted by more and more stack types due to its structural advantages. China has made a breakthrough in the three-roll cold rolling preparation technology for the internal variable diameter guide tube. Since the size of the rollers used in the multi-roll cold rolling process is consistent with the outer diameter of the finished pipe, the roller geometry is simple, and its design and processing are relatively easy. Therefore, the accuracy of the inner diameter size of the guide tube is completely determined by the variable diameter mandrel. However, the existing multi-roll cold rolling technology currently uses an integrated variable diameter mandrel. Because the variable diameter mandrel has high requirements for dimensional accuracy and is difficult to process, the integrated variable diameter mandrel will be limited by the length of the guide tube. If the length of the guide tube is long, it is difficult to process a variable diameter mandrel that meets the use requirements. Therefore, the design and processing of the variable diameter mandrel has become the core and key of the internal variable diameter guide tube preparation technology.

[0004] In view of this, the inventor proposes a variable diameter mandrel and a processing method for a nuclear inner variable diameter guide tube to overcome the defects of the prior art. Summary of the invention

[0005] The object of the present invention is to overcome the shortcomings of the above-mentioned prior art and to provide a reducing mandrel and a processing method for an internal reducing guide tube for nuclear use. The reducing mandrel of the present invention adopts a combined splicing design and segmented processing to solve the problems of low dimensional accuracy and great processing difficulty of the existing reducing mandrels. While being able to prepare qualified internal reducing guide tubes, the qualified rate of the reducing mandrel products is improved and the manufacturing cost of the reducing mandrel is reduced. The tube is easy to demold during rolling, the use effect is good, and it is easy to promote and use.

[0006] The purpose of the present invention is to be solved by the following technical solutions:

[0007] On the one hand, the present invention provides a variable diameter mandrel of a nuclear internal variable diameter guide tube, wherein the variable diameter mandrel adopts a combined splicing structure, which is composed of a first mandrel and a second mandrel that are detachably connected, and after the connection, both ends are respectively provided with threaded sections connected to the core rod;

[0008] Wherein, the diameter of the first core rod is the same as the inner diameter of the thin-walled section of the inner variable-diameter guide tube;

[0009] The second core rod is integrally formed by a first section, a second section and an intermediate transition section connecting the first section and the second section. The first section is detachably connected to the first core rod, and the diameter of the first section is the same as the inner diameter of the thin-walled section of the inner reducer guide tube, and the diameter of the second section is the same as the inner diameter of the thick-walled section of the inner reducer guide tube.

[0010] Furthermore, the first core rod is connected to the second core rod via threads.

[0011] Furthermore, the intermediate transition section is a truncated cone structure adapted to the transition zone of the inner variable diameter guide tube, and the length of the truncated cone is 3 mm to 30 mm, and the cone apex angle is 5° to 30°.

[0012] On the other hand, the present invention provides a method for processing the above-mentioned variable diameter mandrel, which specifically comprises the following steps:

[0013] Step 1: Material selection

[0014] Select a round steel billet that has been forged and annealed, and the nominal diameter of the round steel billet is ΦD 1 mm should be larger than the diameter of the largest cylindrical section in the reducing mandrel;

[0015] Step 2: Sawing

[0016] The round steel blank processed in step 1 is cut according to the length of the variable diameter mandrel to obtain the first mandrel and the second mandrel, and both ends of the mandrel are chamfered;

[0017] Step 3: Rough Turning

[0018] The bar material after the chamfering treatment in step 2 is rough turned. The diameter of the first core rod (1) after rough turning should meet The diameter of the first section of the second mandrel should meet Φ The diameter of the second section should meet The middle transition section is processed into a truncated cone with a cone vertex angle of 5° to 30°, and the length of the truncated cone is 3mm to 30mm;

[0019] Step 4: Finishing

[0020] The bar material after rough turning in step 3 is fine-turned on a CNC lathe. The diameter of the first core rod after fine turning should meet The diameter of the first section of the second mandrel should meet The diameter of the second section should meet and processing threads on both ends of the first mandrel and the second mandrel bar stock;

[0021] Step 5: Salt bath heat treatment

[0022] Perform quenching and tempering heat treatment on the bar material after finishing turning in step 4;

[0023] Step 6: Grinding

[0024] The first core rod and the second core rod after the salt bath heat treatment in step 5 are connected. The full length curvature of the connected rod should be less than 0.03mm. The connected rod is ground. The diameter of the first core rod after grinding should meet The diameter of the first section of the second mandrel should meet The diameter of the second section should meet

[0025] Among them, ΦD 01 is the inner diameter of the thin-walled section of the inner reducer guide tube, ΦD 02 It is the inner diameter of the thick-walled section of the internal reducer guide tube.

[0026] Furthermore, the material of the round steel blank in step 1 is GCr15 high carbon chromium bearing steel or H13 cold working die steel.

[0027] Furthermore, in the step 1, the nominal diameter ΦD1mm of the round steel billet is 2mm to 3mm larger than the diameter of the largest cylindrical section in the variable diameter mandrel.

[0028] Furthermore, in step 4, both ends of the first mandrel are processed with external threads; one end of the second mandrel is processed with external threads, and the other end is processed with internal threads, and the first mandrel is threadedly connected to the second mandrel;

[0029] When one end of the first mandrel external thread is threadedly connected to one end of the second mandrel internal thread, the external thread at the other end of the first mandrel is exactly the same as the external thread of the second mandrel, and is respectively threadedly connected to the internal thread of the core rod during operation.

[0030] Furthermore, the salt bath heat treatment system in step five is: quenching temperature 800°C ~ 850°C, holding time 120min ~ 180min, oil cooling after furnace, protecting the threads at both ends of the bar from quenching during the quenching process; tempering is performed immediately after quenching to eliminate quenching stress, tempering is performed in an oil tank, the tempering temperature is 250°C ~ 300°C, the holding time is 120min ~ 180min, and air cooling is performed after furnace.

[0031] Furthermore, the cooling rate during the salt bath heat treatment in step 5 is ≤20°C / h, the hardness of the rod after heat treatment is between 60HRC and 62HRC, and the rod should be hung vertically during the salt bath heat treatment.

[0032] Furthermore, the surface roughness Ra of the reduced diameter mandrel after the grinding treatment in step six is ​​≤ 0.2 μm, and the reduced diameter mandrel should be suspended vertically after the grinding treatment.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. The variable diameter mandrel of the nuclear internal variable diameter guide tube provided by the present invention adopts a combined splicing structure, specifically consisting of a first mandrel and a second mandrel that are detachably connected. Compared with the existing one-piece variable diameter mandrel, the mandrels at both ends can be disassembled after rolling, so it is easy to separate from the mandrel when rolling the internal variable diameter guide tube, and the segmented mandrel is more convenient for transportation and storage; at the same time, the variable diameter mandrel can be disassembled and separated and can be used separately to produce internal variable diameter guide tubes of different shapes, which can achieve the purpose of one shape for multiple uses.

[0035] 2. The processing method of the variable diameter mandrel provided by the present invention has a process route of: material selection → sawing → rough turning → fine turning → salt bath heat treatment → grinding. Since the variable diameter mandrel is designed as a combined splicing structure, it can be processed in sections, thereby reducing the processing difficulty of the long mandrel. The long mandrel processed by this method has high dimensional accuracy and good straightness, and the pipe after rolling is easy to remove the mandrel. At the same time, the preparation process is easy to implement, simple to operate, and highly practical, and has a low degree of dependence on the skills and experience of personnel.

[0036] To sum up, the present invention has novel and reasonable design and strong practicality. It can meet the cold rolling preparation requirements of nuclear internal variable diameter guide tubes, while reducing the production cost of the long variable diameter mandrel manufacturing process and improving the processing qualification rate of the long variable diameter mandrel. It has good use effect and is easy to promote and use. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The accompanying drawings are incorporated in and constitute a part of this specification and, together with the description, serve to explain the principles of the present invention.

[0038] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0039] Figure 1 It is a schematic diagram of the structure of the variable diameter mandrel of the inner variable diameter guide tube for nuclear use of the present invention;

[0040] Figure 2 This is a schematic diagram of the structure of a single inner variable diameter guide tube of the present invention;

[0041] Figure 3 It is a schematic diagram of the structure of the double inner diameter reducing guide tube of the present invention.

[0042] Among them: 1 is the first core rod; 2 is the second core rod; 3 is the threaded section; 21 is the first section; 22 is the middle transition section; 23 is the second section; A is the thin-walled section of the inner reducer guide tube; B is the thick-walled section of the inner reducer guide tube; C is the transition zone of the inner reducer guide tube. DETAILED DESCRIPTION

[0043] Exemplary embodiments will be described in detail herein, examples of which are shown in the accompanying drawings. When the following description refers to the drawings, the same numbers in different drawings represent the same or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Instead, they are merely examples of devices consistent with some aspects of the present invention as detailed in the appended claims.

[0044] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0045] The present invention aims to prepare Figure 2 , 3 The nuclear inner reducer guide tube (total length greater than 1000mm) shown in the figure; wherein, Figure 2 It is a single inner reducer guide tube, including an inner reducer guide tube thin-walled section A at the upper end, an inner reducer guide tube thick-walled section B at the lower end, and an inner reducer guide tube transition zone C. The outer diameters of the entire inner reducer guide tube are the same, the inner reducer guide tube transition zone C is a cone vertex angle, and the inner diameter of the inner reducer guide tube thin-walled section A is greater than the inner diameter of the inner reducer guide tube thick-walled section B; Figure 3It is a double inner reducer guide tube, including an inner reducer guide tube thin-walled section A at the upper end, an inner reducer guide tube thin-walled section A at the lower end, and an inner reducer guide tube thick-walled section B between the upper and lower ends. The outer diameter of the whole inner reducer guide tube is the same, and the inner diameter of the inner reducer guide tube thin-walled section A at the upper and lower ends is the same. The inner reducer guide tube transition zone C is between the inner reducer guide tube thin-walled section A at the upper end and the inner reducer guide tube thick-walled section B, and between the inner reducer guide tube thick-walled section B and the inner reducer guide tube thin-walled section A at the lower end. The inner reducer guide tube transition zone C is a cone apex angle, and the inner diameter of the inner reducer guide tube thin-walled section A at the upper and lower ends is greater than the inner diameter of the inner reducer guide tube thick-walled section B. The present invention specially designs a reducer mandrel to assist in the production of the above-mentioned inner reducer guide tube, and its structure is as follows: Figure 1 As shown, the variable diameter mandrel adopts a combined splicing structure, which is composed of a first mandrel 1 and a second mandrel 2 that are detachably connected, and after the connection, the first and second ends are respectively provided with threaded sections 3 connected to the core rod; specifically, the diameter of the first mandrel 1 is the same as the inner diameter of the thin-walled section A of the inner variable diameter guide tube; the second mandrel 2 is integrally formed by a first section 21, a second section 23 and an intermediate transition section 22 that connects the first section 21 and the second section 23, the first section 21 is detachably connected to the first mandrel 1, and the diameter of the first section 21 is the same as the inner diameter of the thin-walled section A of the inner variable diameter guide tube, and the diameter of the second section 23 is the same as the inner diameter of the thick-walled section B of the inner variable diameter guide tube.

[0046] Among them, the intermediate transition section 22 of the present invention is a truncated cone structure adapted to the transition zone C of the inner variable diameter guide tube, and the length of the truncated cone is 3mm to 30mm, and the cone vertex angle is 5° to 30°. The length and the cone vertex angle can be adaptively adjusted according to the specifications of the actual inner variable diameter guide tube. The first mandrel 1 and the second mandrel 2 of the present invention are connected by threads, so that when the mandrel is removed after rolling the inner variable diameter guide tube, the first mandrel 1 and the second mandrel 2 can be disassembled from the threads to achieve the purpose of smoothly removing the mandrel from the rolled inner variable diameter guide tube.

[0047] On the other hand, the present invention also provides a method for processing the above-mentioned variable diameter mandrel, which specifically comprises the following steps:

[0048] Step 1: Material selection

[0049] Select round steel billets that have been forged and annealed. Generally, the material of the round steel billets is GCr15 high carbon chromium bearing steel or H13 cold working die steel, and the nominal diameter of the round steel billet is ΦD 1 mm should be 2mm to 3mm larger than the diameter of the largest cylindrical section in the reducing mandrel.

[0050] Step 2: Sawing

[0051] The round steel billet processed in step 1 is cut according to the length of the variable diameter mandrel to obtain the first mandrel 1 and the second mandrel 2, and both ends of the bar are chamfered; the lengths of the first mandrel 1 and the second mandrel 2 are L respectively. 1 mm and L 2 mm, chamfer is 1×45°.

[0052] Step 3: Rough Turning

[0053] The bar material after the chamfering treatment in step 2 is rough turned. The diameter size of the first core rod 1 after rough turning should meet The diameter of the first section 21 of the second mandrel 2 should satisfy The diameter of the second section 23 should meet The middle transition section 22 is processed into a truncated cone with a cone vertex angle of 5° to 30°, and the length of the truncated cone is 3 mm to 30 mm.

[0054] Step 4: Finishing

[0055] The bar material after rough turning in step 3 is fine-turned on a CNC lathe. The diameter size of the first core rod 1 after fine turning should meet The diameter of the first section 21 of the second mandrel 2 should satisfy The diameter of the second section 23 should meet Threads are processed at both ends of the first mandrel 1 and the second mandrel 2.

[0056] Specifically, both ends of the first mandrel 1 are processed with external threads; one end of the second mandrel 2 is processed with external threads, and the other end is processed with internal threads, and the first mandrel 1 is threadedly connected to the second mandrel 2; wherein, when one end of the external thread of the first mandrel 1 is threadedly connected to one end of the internal thread of the second mandrel 2, the external thread at the other end of the first mandrel 1 is exactly the same as the external thread of the second mandrel 2 (equivalent to thread segment 3), and are respectively threadedly connected to the internal thread of the core rod during operation.

[0057] Or one end of the first mandrel 1 is processed with an external thread, and the other end is processed with an internal thread; one end of the second mandrel 2 is processed with an external thread, and the other end is processed with an internal thread, and the first mandrel 1 is threadedly connected to the second mandrel 2; wherein, when one end of the external thread of the first mandrel 1 is threadedly connected to one end of the internal thread of the second mandrel 2, one end of the internal thread of the first mandrel 1 is connected to a thread segment 3, and the specifications of one end of the external thread of the second mandrel 2 are exactly the same as those of the thread segment 3; when one end of the internal thread of the first mandrel 1 is threadedly connected to one end of the external thread of the second mandrel 2, one end of the internal thread of the second mandrel 2 is connected to a thread segment 3, and the specifications of one end of the external thread of the first mandrel 1 are exactly the same as those of the thread segment 3.

[0058] Step 5: Salt bath heat treatment

[0059] The bar material after the fine turning in step 4 is subjected to quenching and tempering heat treatment (quenching + tempering, quenching and tempering heat treatment is performed on quenching and tempering equipment), and the heat treatment system is: quenching temperature 800℃~850℃, holding time 120min~180min, oil cooling after leaving the furnace, and protecting the threads at both ends of the bar material from quenching during the quenching process; tempering is performed immediately after quenching to eliminate quenching stress, and tempering is performed in an oil tank, the tempering temperature is 250℃~300℃, the holding time is 120min~180min, and air cooling is performed after leaving the furnace. Furthermore, the cooling rate during the salt bath heat treatment process is ≤20℃ / h, the hardness of the bar material after heat treatment is between 60HRC~62HRC, and the bar material should be suspended vertically during the salt bath heat treatment process to ensure the straightness of the entire variable diameter mandrel.

[0060] Step 6: Grinding

[0061] The first core rod 1 and the second core rod 2 after the salt bath heat treatment in step 5 are connected. The total length curvature of the connected rods should be less than 0.03 mm. The connected rods are ground. The diameter of the first core rod 1 after the grinding treatment should meet The diameter of the first section 21 of the second mandrel 2 should satisfy The diameter of the second section 23 should meet The surface roughness of the reducing mandrel after grinding is Ra≤0.2μm, and the reducing mandrel after grinding should be suspended vertically.

[0062] Among them, ΦD in the above steps 01 ΦD is the inner diameter of the thin-walled section A of the inner reducer guide tube, 02 It is the inner diameter of the thick-walled section B of the inner reducer guide tube.

[0063] In order to verify the efficacy of the present invention, the present invention specifically carried out the following examples:

[0064] Example

[0065] The outer diameter of the Zr-4 inner reducer guide tube rolled in this embodiment is 19 mm, the length of the thin-walled section from the tube end is 1200 mm, the length of the thick-walled section from the tube end is 200 mm, the inner diameter of the thin-walled section of the inner reducer tube is 17 ± 0.04 mm, the inner diameter of the thick-walled section of the tube is 15 ± 0.04 mm, the cone vertex angle of the inner reducer transition section is 5°, the length of the cone transition section is 23 mm, and the tube structure is as follows: Figure 2 , 3 shown.

[0066] The method for processing the core inner reducing guide tube reducing mandrel of this embodiment comprises the following steps:

[0067] 1) Material selection: In this embodiment, the cold-rolled variable diameter mandrel of the variable diameter guide tube is a forged + annealed bar of GCr15 high carbon chromium bearing steel with a nominal diameter of 19 mm.

[0068] 2) Sawing: The length L of the first core rod 1 after sawing 1 The length L of the second mandrel 2 after sawing is 1215 mm. 2 The bar is 405mm, and after cutting, both ends of the bar are chamfered according to 1×45°.

[0069] 3) Rough turning: The first mandrel 1 and the second mandrel 2 are rough turned after the chamfering treatment in step 2) on a conventional lathe or a CNC lathe. The diameter of the first mandrel 1 after rough turning should meet The diameter of the first section 21 of the second mandrel 2 should satisfy The diameter of the second section 23 should meet The middle transition section 22 is processed into a truncated cone with a cone apex angle of 5°, and the length of the truncated cone is 23 mm.

[0070] 4) Finish turning: The rough-turned bar obtained in step 3) is finished on a CNC lathe, and the threads at both ends of the variable diameter mandrel are processed according to the mandrel drawing. The diameter size of the first mandrel 1 should meet The diameter of the first section 21 of the second mandrel 2 should satisfy The diameter of the second section 23 should meet The middle transition section 22 is processed into a conical transition section with a cone vertex angle of 5°, and the length of the conical transition section is 23 mm.

[0071] 5) Salt bath heat treatment: the first core rod 1 and the second core rod 2 in step 4) are subjected to quenching and tempering heat treatment (quenching + tempering) on ​​a quenching and tempering device, and thread sleeves are installed on the threaded segments 3 at both ends to protect the threads before the salt bath heat treatment;

[0072] The specific heat treatment system is: quenching temperature 820℃~840℃, holding time 120min~180min, the reducing mandrel is vertically hung in the furnace, and oil-cooled after being taken out of the furnace; low-temperature tempering is carried out immediately after quenching to eliminate quenching stress. The tempering is carried out in an oil tank, the tempering temperature is 260℃~270℃, the holding time is 120min~180min, and air-cooling is carried out after being taken out of the furnace.

[0073] 6) Grinding: The first core rod 1 and the second core rod 2 after the salt bath heat treatment in step 5) are threadedly connected. The curvature of the entire length of the connected rod should be less than 0.03 mm. The connected rod is ground on a centerless grinder. The diameter of the first core rod 1 after the grinding treatment should meet The diameter of the first section 21 of the second mandrel 2 should satisfy The diameter of the second section 23 should meet The middle transition section 22 is processed into a truncated cone with a cone apex angle of 5°, and the length of the truncated cone is 23 mm.

[0074] The above description is only a specific embodiment of the present invention, so that those skilled in the art can understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention.

[0075] It should be understood that the present invention is not limited to what has been described above and that various modifications and changes may be made without departing from its scope. The scope of the present invention is limited only by the appended claims.

Claims

1. A variable diameter mandrel of a nuclear internal variable diameter guide tube, characterized in that: The variable diameter mandrel adopts a combined splicing structure, which is composed of a first mandrel (1) and a second mandrel (2) which are detachably connected, and after the connection, both ends are respectively provided with threaded sections (3) connected to the core rod; Wherein, the diameter of the first core rod (1) is the same as the inner diameter of the thin-walled section (A) of the inner variable-diameter guide tube; The second mandrel (2) is integrally formed by a first section (21), a second section (23) and an intermediate transition section (22) connecting and transitioning the first section (21) and the second section (23); the first section (21) is detachably connected to the first mandrel (1); the diameter of the first section (21) is the same as the inner diameter of the thin-wall section (A) of the inner reducer guide tube; and the diameter of the second section (23) is the same as the inner diameter of the thick-wall section (B) of the inner reducer guide tube; The end of the first mandrel (1) away from the self-threaded section (3) is provided with an external thread, and the end of the second mandrel (2) away from the self-threaded section (3) is provided with an internal thread. The first mandrel (1) and the second mandrel (2) are connected by the external thread and the internal thread, and the connection is located in the non-rolling deformation area. The intermediate transition section (22) is a truncated cone structure adapted to the transition zone (C) of the inner variable diameter guide tube, and the length of the truncated cone is 3 mm to 30 mm, and the cone vertex angle is 5° to 30°; The first core rod (1) and the second core rod (2) are used to realize the rolling processing of an inner diameter reducing guide tube for a core having a single-end or two-end transition zone in a truncated cone structure.

2. A method for processing a variable diameter mandrel, characterized in that: The processing method is used to process the variable diameter mandrel of the nuclear internal variable diameter guide tube according to claim 1, and the processing method specifically comprises the following steps: Step 1: Material selection Select a round steel billet that has been forged and annealed, and the nominal diameter ΦD1mm of the round steel billet should be larger than the diameter of the largest cylindrical section in the variable diameter mandrel; Step 2: Sawing The round steel blank processed in step 1 is cut according to the length of the variable diameter mandrel to obtain the first mandrel (1) and the second mandrel (2) rods, and both ends of the rods are chamfered; Step 3: Rough Turning The bar material after the chamfering treatment in step 2 is rough turned. The diameter of the first core rod (1) after rough turning should meet The diameter of the first section (21) of the second mandrel (2) should satisfy The diameter of the second section (23) should satisfy The middle transition section (22) is processed into a truncated cone with a cone vertex angle of 5° to 30°, and the length of the truncated cone is 3mm to 30mm; Step 4: Finishing The bar material after rough turning in step 3 is fine-turned on a CNC lathe. The diameter of the first core rod (1) after fine turning should meet The diameter of the first section (21) of the second mandrel (2) should satisfy The diameter of the second section (23) should satisfy and processing threads on both ends of the first mandrel (1) and the second mandrel (2); Step 5: Salt bath heat treatment Perform quenching and tempering heat treatment on the bar material after finishing turning in step 4; Step 6: Grinding The first core rod (1) and the second core rod (2) after the salt bath heat treatment in step 5 are connected. The full-length curvature of the connected rods should be less than 0.03 mm. The connected rods are ground. The diameter of the first core rod (1) after the grinding treatment should meet The diameter of the first section (21) of the second mandrel (2) should satisfy The diameter of the second section (23) should satisfy Among them, ΦD 01 is the inner diameter of the thin-walled section of the inner reducer guide tube, ΦD 02 It is the inner diameter of the thick-walled section of the internal reducer guide tube.

3. The method for processing a variable diameter mandrel according to claim 2, characterized in that: The material of the round steel blank in step 1 is GCr15 high carbon chromium bearing steel or H13 cold working die steel.

4. The method for processing a variable diameter mandrel according to claim 2, characterized in that: In the step 1, the nominal diameter ΦD1mm of the round steel billet is 2mm to 3mm larger than the diameter of the largest cylindrical section in the variable diameter mandrel.

5. The method for processing a variable diameter mandrel according to claim 2, characterized in that: In the step 4, both ends of the first mandrel (1) are processed with external threads; one end of the second mandrel (2) is processed with external threads, and the other end is processed with internal threads, and the first mandrel (1) and the second mandrel (2) are threadedly connected; When one end of the external thread of the first mandrel (1) is threadedly connected with one end of the internal thread of the second mandrel (2), the external thread of the other end of the first mandrel (1) has exactly the same specifications as the external thread of the second mandrel (2), and they are respectively threadedly connected with the internal thread of the core rod during operation.

6. The method for processing a variable diameter mandrel according to claim 2, characterized in that: The salt bath heat treatment system in step 5 is as follows: quenching temperature 800°C to 850°C, holding time 120min to 180min, oil cooling after furnace discharge, and protecting the threads at both ends of the bar from quenching during quenching; tempering is performed immediately after quenching to eliminate quenching stress, and tempering is performed in an oil tank, the tempering temperature is 250°C to 300°C, the holding time is 120min to 180min, and air cooling is performed after furnace discharge.

7. The method for processing a variable diameter mandrel according to claim 2, characterized in that: The cooling rate during the salt bath heat treatment in step 5 is ≤20°C / h, the hardness of the bar after heat treatment is between 60HRC and 62HRC, and the bar should be hung vertically during the salt bath heat treatment.

8. The method for processing a variable diameter mandrel according to claim 2, characterized in that: The surface roughness Ra of the reducing mandrel after the grinding treatment in step 6 is ≤ 0.2 μm, and the reducing mandrel should be suspended vertically after the grinding treatment.

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