Molybdenum-rhenium alloy gradient-structured pipe prepared based on pilger rolling and method

Abstract

The application discloses a kind of molybdenum-rhenium alloy gradient organization pipe based on pilger warm rolling and method, belong to metal material processing technical field, solve molybdenum-rhenium alloy pipe full cross section uniform fine grain cannot give consideration to room temperature mechanical property and high temperature creep resistance, axial composition gradient exists interface failure risk, rolling process deformation resistance is big and process is long and pilger rolling technique still has no initiative construction radial organization gradient train of thought etc. Core problem, including the following steps: S1, preparation pipe blank;S2, pipe blank pretreatment;S3, pilger rolling mill warm rolling;S4, annealing;S5, finishing.The application makes the plastic deformation degree of inner wall lower than outer wall (outer wall violent plastic deformation) by one pass warm rolling, and the deformation energy storage difference is converted into grain size difference after annealing treatment, so that radial gradient organization from outer wall fine grain to inner wall coarse grain transition is realized on the single cross section of pipe, which fundamentally changes the regulation and control train of thought of "eliminating organization unevenness, pursuing full cross section uniformity" of prior art.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing technology, specifically relating to a method for preparing molybdenum-rhenium alloy gradient structure tubes based on Pilger warm rolling. Background Technology

[0002] Mo-Rhenium (Mo-Re) alloys are typical refractory metal alloys that combine high melting point, excellent high-temperature mechanical properties, good radiation resistance, and good compatibility with liquid metal coolants. They are the preferred structural material for core components such as cooling loop pipes, control rod sleeves, and thermion converter cladding in space nuclear reactors and fast neutron reactors.

[0003] Nuclear reactor operation places stringent dual performance requirements on molybdenum-rhenium alloy tubing: on the one hand, the tubing needs excellent room temperature strength, plasticity, and impact resistance during processing, installation, and reactor start-up and shutdown to prevent brittle cracking; on the other hand, the tubing needs excellent creep resistance during high-temperature (800~1200℃) and long-term steady-state operation to ensure long-term structural dimensional stability. However, there is a fundamental contradiction between the room temperature strength and plasticity of metallic materials and their high-temperature creep resistance requirements regarding grain size: fine-grained structures can significantly improve room temperature strength and plasticity, but grain boundary slip intensifies at high temperatures, leading to a significant decrease in creep resistance; coarse-grained structures can effectively suppress high-temperature grain boundary slip and improve creep life, but they are more brittle at room temperature, difficult to process and form, and have poor impact resistance.

[0004] Existing molybdenum-rhenium alloy tubing manufacturing technologies typically focus on achieving a uniform, fine-grained microstructure across the entire cross-section, emphasizing axial compositional gradients while neglecting radial microstructure gradient design. This approach fails to meet the service requirements of nuclear tubing. Furthermore, molybdenum-rhenium alloys are highly susceptible to oxidation at high temperatures, and current rolling processes lack specific atmosphere protection designs, easily introducing oxidation impurities and reducing the irradiation stability and service safety of the tubing.

[0005] Currently, the manufacturing process of molybdenum-rhenium alloy tubes mainly revolves around rolling, with the mainstream route being "powder metallurgy - sintering - forging - machining and piercing - multi-pass hot rolling or cold rolling - annealing". For example, the patent document "A rolling preparation method for molybdenum-rhenium alloy tubes" (application publication number: CN 111014654 A, application publication date: 2020.04.17) discloses a method for rolling molybdenum-rhenium alloy tubes. Specifically, it discloses the use of hot rolling at 1000~1350℃ combined with multi-pass annealing to obtain a uniform fine-grained structure across the entire cross-section, thereby improving the room temperature strength and plasticity of the tubes. Cold rolling, on the other hand, relies on multi-pass small deformation and intermediate annealing to avoid cracking due to the high room temperature brittleness of molybdenum-rhenium alloys. In addition, some studies have attempted to achieve gradient performance through composition design. For example, patent documents "A method for preparing molybdenum-rhenium alloy gradient materials" (application publication number: CN 111155018 A, application publication date: 2020.05.15) and "A molybdenum-rhenium alloy gradient material and its preparation method" (application publication number: CN111155017 A, application publication date: 2020.05.15) both disclose molybdenum-rhenium alloy gradient materials. They use powders with different rhenium contents (such as MoRe5, MoRe14, MoRe42) to be segmented, pressed, and sintered as a whole to prepare axially segmented composition gradient rods. However, the gradient direction is distributed along the length direction, but it cannot solve the problem of performance matching between the inner and outer walls on the same cross-section of the tube.

[0006] In terms of Pilger rolling technology, although this technology has been applied to the processing of refractory metal tubes such as zirconium alloys and titanium alloys (e.g., patent document "A rolling method and tooling for improving the core quality of zirconium alloy bars" (application publication number: CN117583379 A, ​​application publication date: 2024.02.23)), its core objective is to eliminate the microstructure differences between the core and the edge (such as "white core" defects) by optimizing the die shape and deformation distribution, and to achieve uniform microstructure across the entire cross section, which is completely opposite to the idea of ​​actively constructing microstructure gradient.

[0007] In summary, the aforementioned technological status quo has resulted in the following core issues remaining unresolved for a long time: 1. The fundamental contradiction in performance matching: While existing fine-grained tubes with uniform cross-sections meet room temperature processing requirements, under the high-temperature, long-term service conditions of nuclear reactors, the fine-grained structure exhibits significant grain boundary slip, resulting in extremely poor creep resistance and making it difficult to meet long-life service requirements. Conversely, if only coarse-grained tubes are produced, they exhibit high room temperature brittleness, are difficult to process and form, and have low impact resistance. Existing technologies cannot simultaneously meet the requirements of high strength / fatigue resistance on the outer wall and high creep resistance / cracking resistance on the inner wall on the same cross-section of the same tube. 2. The inherent defects of existing gradient materials: Axial composition gradients can only provide performance differences along the length of the pipe and cannot be radially controlled according to the different service environments of the inner and outer walls of the pipe; and the interfaces of different compositions are prone to element diffusion and interface cracking under nuclear irradiation and high temperature cycling conditions, which pose serious service safety hazards. 3. Inherent defects of the rolling process: High temperature / warm rolling leads to severe oxidation of the alloy, introducing oxide inclusions and reducing the purity of the matrix and irradiation stability; cold rolling has high deformation resistance and is prone to cracking, so multi-pass rolling and intermediate annealing must be used, resulting in a long process flow, low yield and high cost; at the same time, multi-pass rolling often makes the deformation of the entire cross section of the tube tend to be uniform, making it difficult to retain the difference in radial deformation, thus making it impossible to achieve a gradient distribution of grain size; 4. Conflicting approaches to microstructure control: The current application of Pilger rolling technology in refractory alloy tubes focuses on eliminating microstructure inhomogeneity and achieving uniform microstructure across the entire cross section. There is no technical approach or process scheme that actively utilizes the radial attenuation characteristics of rolling deformation to construct a gradient microstructure.

[0008] Therefore, developing a method for preparing molybdenum-rhenium alloy tubes that can utilize the radial deformation difference of Pilger rolling and construct a stable radial grain gradient through controlled annealing, thereby simultaneously meeting the requirements of outer wall toughening and inner wall creep resistance, has urgent technical needs and significant engineering value. Summary of the Invention

[0009] The main objective of this invention is to overcome the shortcomings of the prior art and provide a molybdenum-rhenium alloy gradient microstructure tube and method based on Pilger warm rolling. This addresses the core problems in current molybdenum-rhenium alloy tube manufacturing technology, such as the inability to simultaneously achieve room temperature mechanical properties and high temperature creep resistance, the risk of interface failure due to axial compositional gradients, the large deformation resistance and long process of rolling, and the lack of an active approach to constructing radial microstructure gradients in Pilger rolling technology.

[0010] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for preparing molybdenum-rhenium alloy gradient microstructure tubing based on Pilger warm rolling includes the following steps: S1. Preparation of tube blank: Hot-extruded molybdenum-rhenium alloy bars are selected as raw materials. The extruded bars are machined (first the outer circle is turned to remove the surface oxide scale and macro defects, and then a deep hole drilling machine is used to make concentric holes) to prepare tube blanks for Pilger warm rolling. S2. Tube blank pretreatment: Set the vacuum degree to no more than 5×10 -3Pa, annealing temperature is 1000~1200℃, annealing time is 30~120min, after holding is cooled to room temperature in furnace to eliminate residual stress from machining and homogenize the initial structure, then the tube blank is ultrasonically cleaned with anhydrous ethanol for 10~20min, and then dried with hot air for later use. S3. Pilger Mill Warm Rolling: The pretreated billet from step S2 is inserted into a molybdenum alloy mandrel and then fed into the Pilger mill for rolling. The entire rolling process utilizes a hydrogen flame reducing atmosphere with dynamic protection. The hydrogen flame rapidly heats the deformation zone of the billet to the target warm rolling temperature. Simultaneously, the unburned hydrogen and the reducing atmosphere created by combustion in the flame locally form dynamic reducing protection in the deformation zone, inhibiting the oxidation of the molybdenum-rhenium alloy at high temperatures. The hydrogen purity is ≥99.999%, and the flame temperature can be precisely controlled by adjusting the hydrogen flow rate (5~15 L / min) and the air ratio. The preheating temperature and warm rolling temperature are both set to 700~900℃ (target temperature ±20℃), the number of mill stand strokes is set to 60~120 times / min, the feed rate is 0.5~4.0mm / pass, the area reduction rate per pass is 50%~75%, and the ratio of wall reduction rate to diameter reduction rate is 0.8~2.0. After warm rolling, the tube blank is naturally cooled to room temperature to obtain a warm rolled tube blank. The cumulative deformation of the outer wall of the warm rolled tube blank is not less than 80%, and the cumulative deformation of the inner wall is not greater than 60%. A deformation energy storage gradient is established along the wall thickness direction of the warm rolled tube blank. S4. Annealing: After warm rolling at the Pilger mill, the warm-rolled tube blank undergoes annealing to stabilize the gradient microstructure. First, the annealing furnace is evacuated to a vacuum level ≤ 5 × 10⁻⁶. -3 Pa; then, high-purity hydrogen is introduced into the annealing furnace cavity to form a hydrogen protective atmosphere; finally, the warm-rolled tube blank obtained in step S3 is sent into the annealing furnace, the annealing temperature is 1000~1400℃, the annealing holding time is 30~90min, and after the annealing holding is completed, it is cooled to room temperature with the furnace to obtain the intermediate finished tube blank; during the annealing process, the high deformation energy storage zone of the outer wall of the warm-rolled tube blank has an extremely high density of recrystallization nucleation sites, and complete static recrystallization occurs during annealing, the grains compete with each other and growth is restricted, and finally evolves into a fine equiaxed grain layer; the low deformation energy storage zone of the inner wall mainly undergoes recovery and strain-induced grain boundary movement because the strain is lower than the critical recrystallization deformation, and the original grains undergo controlled coarsening, and finally obtains a radial gradient structure of "fine grains on the outer wall and coarse grains on the inner wall"; S5. Finishing: First, a two-roll straightener is used to straighten the intermediate finished tube blank obtained in step S4, controlling the straightness of the intermediate finished tube blank to ≤0.5mm / m; second, it is successively washed with NaOH+KNO3 mixed molten salt and then with HF+HNO3 mixed acid, with a total wall thickness removal of 0.03~0.08mm, removing oxide scale and processing defects from the surface of the intermediate finished tube blank; third, the inner hole of the intermediate finished tube blank is honed to make the inner wall roughness Ra≤0.8μm; finally, an ultrasonic flaw detector is used to perform full-length non-destructive testing on the intermediate finished tube blank to meet the requirements of nuclear tube materials, thus producing molybdenum-rhenium alloy gradient structure tubes.

[0011] Conventional Pilger rolling is mostly used in cold forming processes with the design goal of "uniform deformation penetrating the wall thickness". It usually adopts multiple passes (3 to 6 passes), large feed rate (3 to 6 mm / pass), and high stroke rate (100 to 150 times / min). By optimizing the pass profile curve, the cumulative deformation of the inner and outer walls is made similar (the difference is usually less than 10%). After annealing, a uniform fine-grained structure is obtained throughout the cross section.

[0012] In contrast, this invention employs a single-pass rolling process, using a hydrogen flame to preheat the billet and dynamically heat the deformation zone during rolling. The hydrogen flame creates a reducing atmosphere on the tube surface, effectively isolating it from air and preventing high-temperature oxidation. Furthermore, it keeps the deformation zone in a warm rolling environment (700-900°C) below the alloy recrystallization temperature. Under these conditions, by controlling parameters such as the feed rate (1-2 mm / pass), this invention concentrates the rolling deformation energy primarily on the outer wall of the tube, while the cumulative deformation on the inner wall does not exceed 60%. This actively preserves and amplifies the energy difference in deformation storage between the inner and outer walls, which is then converted into a grain size gradient through conventional uniform annealing.

[0013] The core of this invention lies in utilizing the periodic radial forced compression of the outer wall of the tube blank by the die during Pilger rolling, and controlling the temperature of the tube deformation zone at 700~900℃ (below the alloy recrystallization temperature) under hydrogen flame follow-up heating. This places the outer metal in a triaxial stress state dominated by compressive stress, inducing rapid dislocation proliferation and severe grain fragmentation on the outer wall. Simultaneously, this warm rolling condition effectively avoids dynamic recrystallization, allowing for sufficient accumulation of deformation energy. As the rolling force propagates inward along the wall thickness, it undergoes radial nonlinear decay due to the resistance to metal flow. This controlled non-uniform deformation establishes a stable radial deformation energy storage gradient field on the tube cross-section: the outer wall acquires a high-density deformed fragmented structure, while the inner wall retains the original crystalline state with lower strain, thus laying the physical foundation for the subsequent formation of the gradient structure.

[0014] Furthermore, in step S1, the mass percentage of rhenium in the molybdenum-rhenium alloy rod is 3% to 14%, with the remainder being molybdenum and unavoidable impurities.

[0015] Further, in step S1, the outer diameter of the tube blank is Φ20~Φ30mm, the wall thickness is 3~5mm, and the length is 500~1500mm; the surface roughness of the inner and outer surfaces of the tube blank is Ra≤1.6μm, the concentricity of the inner and outer circles is ≤0.5mm, and the perpendicularity of the end face is ≤0.5mm.

[0016] Furthermore, in step S4, fine equiaxed crystals with a particle size of 1~30μm are formed on the outer wall of the intermediate finished tube blank, and coarse equiaxed crystals with a particle size of 50~100μm are formed on the inner wall.

[0017] Molybdenum-rhenium alloy gradient structure tubing suitable for extreme service environments such as nuclear reactors, prepared using the method described above.

[0018] Compared with the prior art, the present invention has the following technical effects: This invention is based on the physical characteristic that the rolling force attenuates along the wall thickness direction during Pilger rolling. It induces severe plastic deformation of the outer wall through a single warm rolling pass, while the inner wall undergoes less plastic deformation. Annealing then transforms the difference in deformation energy storage into a difference in grain size, thereby achieving a radial gradient microstructure from fine grains on the outer wall to coarse grains on the inner wall across a single cross-section of the tube. This fundamentally changes the existing technology's approach of "eliminating microstructure inhomogeneity and pursuing uniformity across the entire cross-section." Specifically, this invention controls the reduction rate, wall thickness reduction rate, rolling temperature, and feed rate during a single rolling pass. The cumulative deformation of the outer wall of the warm-rolled tube is not less than 80%, and the cumulative deformation of the inner wall is not greater than 60%. Annealing at 1100~1400℃ causes complete recrystallization in the high-energy-storage zone of the outer wall, forming fine equiaxed grains (1~30μm), while the low-energy-storage zone of the inner wall primarily undergoes grain coarsening, forming coarse equiaxed grains (50~100μm). The grain size transitions continuously in the intermediate region. Meanwhile, the present invention adopts hydrogen flame reducing atmosphere follow-up temperature rolling (700~900℃), which effectively reduces deformation resistance, reduces cracking tendency, avoids the introduction of high temperature oxidation impurities, simplifies the process flow, and improves the yield. It provides a process controllable, repeatable, and industrially mass-producible molybdenum rhenium alloy gradient structure tube preparation scheme for extreme service environments such as nuclear reactors. Attached Figure Description

[0019] Figure 1 The figures show the metallographic structure of the cross section of the hot-extruded molybdenum-rhenium alloy bar. Figure (a) shows the metallographic structure at the core of the cross section of the hot-extruded molybdenum-rhenium alloy bar, and Figure (b) shows the metallographic structure at the edge of the cross section of the hot-extruded molybdenum-rhenium alloy bar. Figure 2 This is a microscopic morphology image of the cross-section of the pretreated tube blank; Figure 3 A schematic diagram of the Pilger warm rolling mill and the tube blank deformation zone; Figure 4 The metallographic diagrams are of the warm-rolled tube blanks prepared in Example 1; in the figures, Figure (a) is the metallographic diagram of the cross section of the warm-rolled tube blank, and Figure (b) is the metallographic diagram of the longitudinal section of the warm-rolled tube blank. Figure 5 The stress-strain curve of the molybdenum-rhenium alloy gradient structure tube prepared in Example 1 is shown. Figure 6 Photographs of the hardness measurement points of the molybdenum-rhenium alloy gradient structure tube prepared in Example 1; Figure 7 The bar chart shows the hardness test results of the molybdenum-rhenium alloy gradient structure tube prepared in Example 1. Detailed Implementation

[0020] It should be noted that, unless otherwise specified, all components and materials used in this invention are those known in the art.

[0021] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments. All equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention. Example 1

[0022] A method for preparing molybdenum-rhenium alloy gradient structure tubing based on Pilger warm rolling includes the following steps; S1. Preparation of tube blank: Hot-extruded molybdenum-rhenium alloy rods are selected (the metallographic structure of different regions of the cross-section is as follows...) Figure 1 The raw material (shown) is a bar with an outer diameter of Φ28.5mm and a length of 1000mm. The rhenium content in the molybdenum-rhenium alloy bar is 3% by mass, with the balance being molybdenum and unavoidable impurities. The extruded bar is machined to prepare a tube blank for Pilger warm rolling. The tube blank has the following dimensions: outer diameter of Φ25.4mm, wall thickness of 3mm, length of 500mm, surface roughness Ra of the inner and outer surfaces of the tube blank of 1.2μm, concentricity of the inner and outer circles of 0.3mm, and perpendicularity of the end face of 0.5mm. S2. Tube blank pretreatment: Set the vacuum degree to 3×10 -3 The tube blank was annealed at 1100℃ for 60 min, and then cooled to room temperature in the furnace after holding. It was then ultrasonically cleaned with anhydrous ethanol for 15 min, and dried with hot air for later use. The initial microstructure of the tube blank was equiaxed coarse grains (average grain size 70 μm), and its metallographic structure was as follows: Figure 2 As shown; S3. Pilger Warm Rolling: The pretreated billet from step S2 is inserted into a molybdenum alloy mandrel and then fed into an LG15 Pilger mill for rolling. A schematic diagram of the Pilger warm rolling equipment and the billet deformation zone is shown below. Figure 3As shown in the figure (1 is the tube blank, 2 is the roll, 3 is the roll shaft, 4 is the mandrel, and 5 is the finished tube), the entire rolling process adopts a hydrogen flame reducing atmosphere with follow-up protection, and the flow rate of high-purity hydrogen is 10L / min; the preheating temperature and warm rolling temperature are both set to 850℃, the number of mill stand strokes is set to 60 times / min, the feed rate is 4.0mm / pass, the pass area reduction rate is 61%, and the ratio of wall reduction rate to diameter reduction rate is 0.8; after one pass of warm rolling, the tube blank is naturally cooled to room temperature, and after head and tail cutting and sampling, a warm rolled tube blank is obtained. The dimensions of the warm rolled tube blank are: outer diameter Φ15mm, wall thickness 2mm, and length 1100mm; S4. Annealing: First, the annealing furnace cavity is evacuated to a vacuum level ≤3×10⁻⁶. -3 Pa; then, high-purity hydrogen is introduced into the annealing furnace cavity to form a hydrogen protective atmosphere; finally, the warm-rolled tube blank obtained in step S3 is sent into the annealing furnace, the annealing temperature is 1200℃, the annealing holding time is 60min, and after the annealing holding time is completed, it is cooled to room temperature with the furnace to obtain the intermediate finished tube blank; the outer wall of the intermediate finished tube blank forms fine equiaxed crystals with an average grain size of 10μm, and the inner wall forms coarse equiaxed crystals with a grain size of about 50μm, thereby forming a stable radial gradient structure; S5. Finishing: First, a two-roll straightener is used to straighten the intermediate finished tube blank obtained in step S4, controlling the straightness of the intermediate finished tube blank to 0.3 mm / m; second, it is successively washed with NaOH+KNO3 mixed molten salt and then with HF+HNO3 mixed acid, removing a total wall thickness of 0.05 mm to remove oxide scale and processing defects from the surface of the intermediate finished tube blank; third, the inner hole of the intermediate finished tube blank is honed to achieve an inner wall roughness Ra of 0.6 μm; finally, an ultrasonic flaw detector is used to perform full-length non-destructive testing on the intermediate finished tube blank to meet the requirements of nuclear pipe standards.

[0023] like Figure 5 As shown in Table 1, standard tensile specimens were prepared from samples taken from the molybdenum-rhenium alloy gradient structure tube prepared in Example 1. The results of the room temperature tensile tests are shown in Table 1. The repeatability of the three parallel tests from specimen #1 to specimen #3 is good. The specimens exhibit continuous yield characteristics, and the yield strength (σ) is... 0.2 The strength is approximately 480-500 MPa, the tensile strength is approximately 550-580 MPa, and the elongation at break reaches 50%-60%, demonstrating excellent strength-plasticity matching. Therefore, the molybdenum-rhenium alloy gradient structure tube prepared in Example 1 exhibits stable overall forming quality and good plasticity, making it suitable for subsequent secondary forming processes.

[0024] .

[0025] like Figure 6 and Figure 7As shown, Vickers hardness tests were performed radially (from the inner wall to the outer wall, with a measuring point spacing of 300 μm) on the molybdenum-rhenium alloy gradient structure tube prepared in Example 1. The test results showed that the three parallel tests exhibited good repeatability. The overall hardness distribution was between 190 and 225 HV, with the fifth measuring point near the outer wall showing higher hardness, indicating slight work hardening characteristics of the surface layer. The moderate hardness level matches the medium-to-high strength and high plasticity characteristics shown in the tensile test, indicating that the tube has good potential for subsequent plastic forming. Example 2

[0026] A method for preparing molybdenum-rhenium alloy gradient structure tubing based on Pilger warm rolling includes the following steps; S1. Preparation of tube blanks: Hot-extruded Mo-14Re alloy bars are selected as raw materials. The bar dimensions are: outer diameter Φ28mm, length 1000mm. The mass percentage of rhenium in the molybdenum-rhenium alloy bar is 14%, with the balance being molybdenum and unavoidable impurities. The extruded bars are machined to prepare tube blanks for Pilger warm rolling. The tube blank dimensions are: outer diameter Φ22mm, wall thickness 3mm, length 500mm, inner and outer surface roughness Ra of 1.2μm, inner and outer circle concentricity of 0.3mm, and end face perpendicularity of 0.5mm. S2. Tube blank pretreatment: Set the vacuum degree to 3×10 -3 Pa, annealing temperature is 1150℃, annealing time is 60min, after holding at the temperature is cooled to room temperature in the furnace, then the tube blank is ultrasonically cleaned with anhydrous ethanol for 15min, and then dried with hot air for use; the initial microstructure of the tube blank is equiaxed coarse grain. S3. Pilger Warm Rolling: The pretreated billet from step S2 is inserted into a molybdenum alloy mandrel and then fed into an LG15 Pilger mill for rolling. A schematic diagram of the Pilger warm rolling equipment and the billet deformation zone is shown below. Figure 3 As shown, the entire rolling process employs a hydrogen flame reducing atmosphere with dynamic protection, and the flow rate of high-purity hydrogen is 12 L / min. The preheating temperature and warm rolling temperature are both set at 850℃, the mill stand stroke is set at 100 strokes / min, the feed rate is 1.2 mm / stroke, the pass area reduction rate is 70%, and the ratio of wall reduction rate to diameter reduction rate is 1.8. After one pass of warm rolling, the tube blank is naturally cooled to room temperature, and after head and tail trimming and sampling, a warm-rolled tube blank is obtained. The dimensions of the warm-rolled tube blank are: outer diameter Φ15 mm, wall thickness 1.2 mm, and length 1100 mm. A certain radial gradient structure of grain size is formed on the inner and outer walls of the warm-rolled tube blank, which accumulates deformation energy storage gradient for the next process. S4. Annealing: First, the annealing furnace cavity is evacuated to a vacuum level ≤3×10⁻⁶. -3Pa; then, high-purity hydrogen is introduced into the annealing furnace cavity to form a hydrogen protective atmosphere in the annealing furnace cavity; finally, the warm rolled tube blank obtained in step S3 is sent into the annealing furnace, the annealing temperature is 1100℃, the annealing holding time is 60min, and after the annealing holding time is completed, it is cooled to room temperature with the furnace to obtain the intermediate finished tube blank. The outer wall of the intermediate finished tube blank forms fine equiaxed crystals with a grain size of 20μm, and the inner wall forms coarse equiaxed crystals with a grain size of 80μm, thereby forming a stable radial gradient structure; S5. Finishing: First, a two-roll straightener is used to straighten the intermediate finished tube blank obtained in step S4, controlling the straightness of the intermediate finished tube blank to 0.3 mm / m; second, it is successively washed with NaOH+KNO3 mixed molten salt and then with HF+HNO3 mixed acid, removing a total wall thickness of 0.05 mm to remove oxide scale and processing defects from the surface of the intermediate finished tube blank; third, the inner hole of the intermediate finished tube blank is honed to achieve an inner wall roughness Ra of 0.6 μm; finally, an ultrasonic flaw detector is used to perform full-length non-destructive testing on the intermediate finished tube blank, producing Mo-14Re molybdenum-rhenium alloy gradient structure tube that meets the requirements for nuclear reactor service.

[0027] This is a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing molybdenum-rhenium alloy gradient structure tubing based on Pilger warm rolling, characterized in that, Includes the following steps: S1. Preparation of tube blank: Hot-extruded molybdenum-rhenium alloy rods are selected as raw materials. The mass percentage of rhenium in the molybdenum-rhenium alloy rods is 3%~14%, and the balance is molybdenum and unavoidable impurities. The extruded rods are machined to prepare tube blanks for Pilger warm rolling. S2, pipe blank pretreatment: set the vacuum degree is not more than 5x10 -3 Pa, annealing temperature is 1000~1200℃, annealing time is 30~120min, after the heat preservation ends, the furnace is cooled to room temperature, then the pipe blank is ultrasonic cleaned for 10~20min with anhydrous ethanol, and the pipe blank is dried with hot air and is ready for use. S3. Pilger Mill Warm Rolling: The pretreated tube blank from step S2 is inserted into a molybdenum alloy mandrel and then fed into the Pilger mill for rolling. The entire rolling process is protected by a hydrogen flame reducing atmosphere. The preheating temperature and warm rolling temperature are set to 700~900℃, the mill stand stroke rate is set to 60~120 times / min, the feed rate is 0.5~4.0mm / pass, the area reduction rate per pass is 50%~75%, and the ratio of wall reduction rate to diameter reduction rate is 0.8~2.

0. After warm rolling, the tube blank is naturally cooled to room temperature to obtain a warm-rolled tube blank. The cumulative deformation of the outer wall of the warm-rolled tube blank is not less than 80%, and the cumulative deformation of the inner wall is not greater than 60%. A deformation energy storage gradient is established along the wall thickness direction of the warm-rolled tube blank. S4. Annealing: First, the annealing furnace cavity is evacuated to a vacuum level ≤5×10⁻⁶. -3 Pa; then, high-purity hydrogen is introduced into the annealing furnace chamber to form a hydrogen protective atmosphere in the annealing furnace chamber; finally, the warm rolled tube blank obtained in step S3 is sent into the annealing furnace, the annealing temperature is 1000~1400℃, the annealing holding time is 30~90min, and after the annealing holding is completed, it is cooled to room temperature with the furnace to obtain the intermediate finished tube blank. S5. Finishing: First, a two-roll straightener is used to straighten the intermediate finished tube blank obtained in step S4, controlling the straightness of the intermediate finished tube blank to ≤0.5mm / m; second, it is successively washed with NaOH+KNO3 mixed molten salt and then with HF+HNO3 mixed acid, with a total wall thickness removal of 0.03~0.08mm, removing oxide scale and processing defects from the surface of the intermediate finished tube blank; third, the inner hole of the intermediate finished tube blank is honed to make the inner wall roughness Ra≤0.8μm; finally, an ultrasonic flaw detector is used to perform full-length non-destructive testing on the intermediate finished tube blank to meet the requirements of nuclear tube materials, thus producing molybdenum-rhenium alloy gradient structure tubes.

2. The method for preparing molybdenum-rhenium alloy gradient structure tubing based on Pilger warm rolling according to claim 1, characterized in that, In step S1, the outer diameter of the tube blank is Φ20~Φ30mm, the wall thickness is 3~5mm, and the length is 500~1500mm; the surface roughness of the inner and outer surfaces of the tube blank is Ra≤1.6μm, the concentricity of the inner and outer circles is ≤0.5mm, and the perpendicularity of the end face is ≤0.5mm.

3. The method for preparing molybdenum-rhenium alloy gradient structure tubing based on Pilger warm rolling according to claim 1, characterized in that, In step S4, fine equiaxed crystals with a particle size of 1~30μm are formed on the outer wall of the intermediate finished tube blank, and coarse equiaxed crystals with a particle size of 50~100μm are formed on the inner wall.

4. A molybdenum-rhenium alloy gradient structure tube prepared by the method described in any one of claims 1 to 3.

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