A method for constructing gradient structure on the surface layer of zirconium alloy thin-walled pipe

Abstract

This invention discloses a method for constructing a gradient microstructure on the surface of zirconium alloy thin-walled tubing. The method involves performing microparticle peening on the zirconium alloy thin-walled tubing in a cryogenic environment to form the gradient microstructure. This method, employing a cryogenic + microparticle peening process to construct a surface gradient microstructure on the zirconium alloy thin-walled tubing, can utilize low temperatures to suppress the activation of slip systems and introduce as many low-energy twin interfaces as possible during the microparticle peening process. This maintains good anisotropy in the zirconium alloy thin-walled tubing, constructs a gradient microstructure, and improves the mechanical, corrosion-resistant, and radiation-resistant properties of the zirconium alloy thin-walled tubing while maintaining low surface roughness and controlling surface quality. This method is applicable to fuel cladding tubes and key in-core structural materials in nuclear reactors.

Description

Technical Field

[0001] This invention belongs to the field of surface modification technology for thin-walled metal tubes, and in particular relates to a method for constructing a gradient structure on the surface of zirconium alloy thin-walled tubes. Background Technology

[0002] Zirconium alloys are widely used in key structural components such as nuclear reactor cladding tubes due to their low neutron absorption cross section, good corrosion resistance, and mechanical properties; these are their primary applications. However, driven by the development trends of high reactor burnup and extended lifespan, the mechanical properties, oxidation resistance, stress corrosion cracking resistance, and radiation resistance of zirconium alloy cladding, serving as the first line of defense against radioactive material leakage, are facing increasingly stringent challenges.

[0003] Extensive research on "gradient microstructure" in metallic materials has shown that it can not only improve the mechanical properties of alloys, such as strength-plasticity synergy, damage resistance, and fatigue resistance, but also demonstrates great potential in improving chemical properties such as corrosion resistance and oxidation resistance. Meanwhile, the introduction of "low-energy twin boundaries" has also been confirmed by researchers both domestically and internationally to improve the strength-plasticity, fatigue performance, work hardening, thermal stability, corrosion resistance, and irradiation resistance of metals and their alloys. The construction of gradient microstructure based on microstructure and grain boundary control strategies, along with the introduction of low-energy twin boundaries, not only provides reliable guarantees for the long service life and high safety of key metallic components under harsh operating conditions, but also provides a key solution for the further promotion of the application of metals and their alloys in extreme operating conditions such as nuclear energy and aerospace. Zirconium alloy cladding tubes have a thickness of approximately 0.57 mm, falling into the category of thin-walled tubes. Strict requirements for structural integrity and surface quality are fundamental factors for their application in the nuclear energy field. Therefore, traditional gradient microstructure construction processes such as surface mechanical grinding, ultrasonic surface rolling, laser shot peening, and ultrasonic shot peening, which have high plastic deformation or special requirements for the shape and size of the processed parts, are difficult to directly apply or learn from on thin-walled pipes, especially in order to simultaneously achieve gradient microstructure construction and pre-set low-energy twin boundaries in the micro-regions of the pipe surface.

[0004] Therefore, there is an urgent need to develop a novel surface modification process for zirconium alloy thin-walled tubes. This process should achieve a gradient microstructure on the surface of the thin-walled tube, with low roughness and no loss of structural integrity, while simultaneously introducing low-energy twin interfaces into the gradient microstructure that are beneficial to the mechanical, corrosion-resistant, and radiation-resistant properties of the metal and its alloys. The development of this process is not only a crucial entry point for the surface modification of zirconium alloys and even the development of high-performance zirconium alloys, but also a key link in achieving the ultimate goals of reactor life extension, high-reliability service, and small-scale modularization. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for constructing a gradient microstructure on the surface of zirconium alloy thin-walled tubes. This method treats the zirconium alloy thin-walled tubes using a cryogenic + microparticle peening process. The low temperature suppresses the opening of slip systems, and the microparticle peening process introduces as many low-energy twin interfaces as possible, maintaining good anisotropy in the tube and constructing a gradient microstructure. This gradient microstructure, with grain size increasing from the surface to the interior and twin density decreasing from high to low, improves the mechanical, corrosion-resistant, and radiation-resistant properties of the zirconium alloy thin-walled tubes while maintaining low surface roughness, thus solving the problem of surface roughness deterioration caused by traditional gradient microstructure preparation methods.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: a method for constructing a gradient structure on the surface of a zirconium alloy thin-walled tube, characterized in that the zirconium alloy thin-walled tube is shot-peened with microparticles in a cryogenic environment to form a gradient structure.

[0007] This invention combines cryogenic peening with microparticle shot peening. By performing microparticle shot peening in a cryogenic environment, it can not only offset the significant deterioration of surface roughness and the large and uneven distribution of surface residual stress caused by the temperature rise of the pipe surface during shot peening, but also suppress dislocation cross-slip using the low-temperature environment, strongly promoting the generation of deformation twins. The above process enables the acquisition of high-density low-energy twin interfaces even under milder shot peening parameters (reduced microparticle size and reduced shot peening pressure), thereby significantly reducing damage to the surface geometry and achieving high strength, high wear resistance, and low damage.

[0008] The above-mentioned method for constructing a gradient structure on the surface of a zirconium alloy thin-walled tube is characterized in that the method for forming the cryogenic environment is: using liquid nitrogen cold spraying before microparticle shot peening, or introducing vaporized low-temperature nitrogen gas during microparticle shot peening, wherein the temperature of the low-temperature nitrogen gas is -196℃ to -130℃.

[0009] This invention uses vaporized low-temperature nitrogen for cooling, which provides uniform cooling and reduces thermal shock. It not only counteracts the deterioration of surface quality caused by temperature rise during shot peening, but also effectively reduces residual stress on the surface during shot peening, thus effectively improving surface quality and the uniformity of gradient structure.

[0010] The above-mentioned method for constructing a gradient structure on the surface of a zirconium alloy thin-walled tube is characterized in that the particle size of the shot peening material is 20μm~100μm.

[0011] The reasons for controlling the particle size of the shot in this invention are as follows: (1) The impact energy of a single shot is small, resulting in low shot peening intensity and low damage to the substrate surface; (2) The local pits caused by the impact of small-diameter shot are small, resulting in low surface roughness and high surface quality; (3) Small-diameter shot impacts the substrate a large number of times per unit time, resulting in high coverage and shot peening efficiency; (4) Small-diameter shot is more suitable for shot peening of non-planar geometric materials.

[0012] The above-mentioned method for constructing a gradient structure on the surface of a zirconium alloy thin-walled tube is characterized in that the parameters of the microparticle shot peening include shot peening pressure and coverage, and the shot peening pressure is 2 bar to 4 bar. The shot peening pressure is matched with the coverage rate. When the shot peening pressure is 2 bar, the coverage rate is not less than 100%. When the shot peening pressure is 3 bar, the coverage rate is 100%~400%. When the shot peening pressure is 4 bar, the coverage rate is not higher than 200%.

[0013] As shot peening pressure or coverage increases, the surface quality of thin-walled pipes deteriorates more. Therefore, this invention matches shot peening pressure with coverage to achieve a surface roughness of less than 1.0 μm and to introduce a gradient structure rich in twin interfaces.

[0014] The above-mentioned method for constructing a gradient structure on the surface of a zirconium alloy thin-walled tube is characterized in that the specific process of the microparticle shot peening is as follows: shot peening is performed twice, using hard shot and soft shot respectively.

[0015] This invention utilizes hard shot peening in a cryogenic environment to rapidly pre-treat twins while preventing oxidation, forming nanoscale grains. This improves both fatigue strength and abrasion resistance, and the established high-density dislocation structure is in a more stable energy state, resisting recovery and coarsening under subsequent thermal / mechanical effects. Subsequently, soft shot peening with gentler materials does not damage the substrate and removes burrs from the pipe surface for reshaping. The cryogenic environment suppresses the thermal activation recovery process, and the secondary shot peening with soft shot continues to excite mechanical twins rather than disturb the existing dislocation structure. New twin boundaries are introduced into the refined nanocrystals, forming a composite reinforcement layer of nanocrystals + high-density twins + high dislocation density. This achieves simultaneous optimization of high strength and high smoothness without compromising the structural integrity of the thin-walled pipe.

[0016] The above-mentioned method for constructing a gradient structure on the surface of a zirconium alloy thin-walled tube is characterized in that the hard shot has a hardness ≥80HRC and the soft shot has a hardness <60HRC; the shot peening angle when using the hard shot is 90° and the shot peening angle when using the soft shot is ≤60°.

[0017] This invention designs the shot peening angle of hard shot to 90°, resulting in a short shot rebound path perpendicular to the surface, making it less likely to embed into the surface and reducing excessive roughness. At the same time, all kinetic energy is used for plastic deformation, which can effectively form residual compressive stress and improve hardness. Combined with controlling the shot peening angle of soft shot to ≤60°, it can break up the directional processing texture formed by hard shot peening, induce mechanical twinning, and at the same time produce a scraping / micro-plowing effect, pushing the crest material to the trough, achieving precise shaping and further reducing surface roughness.

[0018] The above-mentioned method for constructing a gradient structure on the surface of a zirconium alloy thin-walled tube is characterized in that a vacuum heat treatment is performed before the microparticle shot peening, and the temperature of the vacuum heat treatment is 500℃~800℃.

[0019] This invention can significantly improve the plasticity of pipes by performing vacuum heat treatment, thus preventing the substrate from cracking under the strong impact during shot peening.

[0020] The above-mentioned method for constructing a gradient structure on the surface of a zirconium alloy thin-walled tube is characterized in that the zirconium alloy thin-walled tube rotates at a constant speed during the microparticle shot peening process.

[0021] The above-mentioned method for constructing a gradient structure on the surface of a zirconium alloy thin-walled tube is characterized in that the thickness of the gradient structure is 15μm~50μm.

[0022] The gradient structure thickness of the present invention is controlled based on the local plastic deformation of the microparticle shot peening process. If the thickness is too thin, it is difficult to achieve a good modification effect; if the layer thickness is too thick, the required shot peening pressure and coverage need to be increased, which not only significantly deteriorates the surface quality of the pipe, but also easily damages the structural integrity of the pipe itself.

[0023] The above-mentioned method for constructing a gradient structure on the surface of a zirconium alloy thin-walled tube is characterized in that, after the microparticle shot peening, the surface roughness Ra of the zirconium alloy thin-walled tube is not greater than 1.0 μm.

[0024] Compared with the prior art, the present invention has the following advantages: 1. This invention treats the surface of zirconium alloy thin-walled tubes using a cryogenic + microparticle shot peening process. This thermo-mechanical coupling utilizes low temperatures to suppress the opening of slip systems and introduces as many low-energy twin interfaces as possible during microparticle shot peening, maintaining good anisotropy of the tube and achieving gradient microstructure construction. This improves the mechanical, corrosion-resistant, and radiation-resistant properties of the zirconium alloy thin-walled tubes. Compared with traditional high-energy shot peening, microparticle shot peening, with its smaller particle size and lower energy, can significantly reduce surface roughness, reduce friction loss, improve precision, and lower the probability of deformation in zirconium alloy thin-walled tubes, thereby improving wear resistance and fatigue life.

[0025] 2. By adjusting parameters such as pressure and coverage during shot peening, this invention can achieve the construction of a gradient structure with a thickness of 15μm to 50μm on the surface of zirconium alloy thin-walled tubes. By constructing the gradient structure under low-temperature conditions, adverse phenomena such as increased roughness, shot embedding, and surface oxidation caused by surface temperature rise during microparticle shot peening can be avoided. Compared with traditional gradient structure preparation methods, this method has the advantages of less impact on the structural integrity of the material itself, faster processing speed, and wider applicability.

[0026] 3. Due to the extreme difficulty in controlling the surface uniformity during the shot peening process of pipes, factors such as pipe diameter, wall thickness, and radius of curvature will affect the energy distribution during the shot peening process. This invention controls the pipe to rotate at a uniform speed during the shot peening process, thereby avoiding poor surface uniformity and uneven energy distribution induced by uneven shot peening. At the same time, low temperature can effectively avoid poor energy distribution uniformity caused by high instantaneous energy and can offset the temperature rise during the shot peening process, which is beneficial to the control of the pipe surface uniformity.

[0027] 4. This invention utilizes a secondary shot peening process involving both hard and soft shot materials in a cryogenic environment to form a composite reinforcement layer of nanocrystals, high-density twins, and high dislocation density on the surface of zirconium alloy thin-walled tubes, achieving simultaneous optimization of high strength and high surface finish.

[0028] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0029] Figure 1 This is a microstructure diagram of a zirconium alloy thin-walled tube after microparticle shot peening in Example 1 of the present invention.

[0030] Figure 2 This is a microstructure diagram of a zirconium alloy thin-walled tube after microparticle shot peening in Example 2 of the present invention.

[0031] Figure 3 This is a microstructure diagram of a zirconium alloy thin-walled tube after microparticle shot peening in Example 3 of the present invention.

[0032] Figure 4 This is a microstructure diagram of a zirconium alloy thin-walled tube after microparticle shot peening in Example 4 of the present invention.

[0033] Figure 5 The figures show the flattening mechanical properties of the zirconium alloy thin-walled tubes after microparticle shot peening in Examples 1 and 6 of the present invention, and the zirconium alloy thin-walled tubes after vacuum heat treatment in Comparative Example 1. Detailed Implementation

[0034] Example 1 The method in this embodiment is as follows: a zirconium alloy thin-walled tube with an outer diameter of 9.5 mm, a wall thickness of 0.57 mm, and a length of 30 mm is subjected to vacuum heat treatment at 800℃ for 24 h, followed by cold blasting of the tube with liquid nitrogen for 60 s, and then shot peening of the tube with microparticles at a rotation speed of 15 r / min to form a gradient structure; the shot material for microparticle peening is tungsten steel with a particle size of 30 μm to 50 μm, and the parameters for microparticle peening are: shot peening pressure 2 bar, coverage 200%, flow rate 3 kg / min, shot peening angle 90°, and distance between the spray gun and the target material 100 mm.

[0035] Microstructure analysis was performed on the zirconium alloy thin-walled tube after microparticle shot peening in this embodiment, such as... Figure 1 As shown, the zirconium alloy thin-walled tube exhibits a gradient microstructure on its surface, with a layer thickness of 17.5 μm; the measured surface roughness of the zirconium alloy thin-walled tube is 0.67 μm. Figure 5 As shown, the strength when flattened is 0.74 kN.

[0036] Example 2 The difference between this embodiment and Embodiment 1 is that the shot peening pressure of the microparticle shot peening is 3 bar and the coverage is 100%.

[0037] Microstructure analysis was performed on the zirconium alloy thin-walled tube after microparticle shot peening in this embodiment, such as... Figure 2 As shown, the zirconium alloy thin-walled tube has a gradient structure on its surface with a layer thickness of 21.3 μm; the surface roughness of the zirconium alloy thin-walled tube is measured to be 0.65 μm, and the flattening strength is 0.73 kN.

[0038] Example 3 The difference between this embodiment and Embodiment 2 is that the shot peening pressure of the microparticle shot peening is 3 bar and the coverage is 400%.

[0039] Microstructure analysis was performed on the zirconium alloy thin-walled tube after microparticle shot peening in this embodiment, such as... Figure 3 As shown, the zirconium alloy thin-walled tube has a gradient structure on its surface with a layer thickness of 36.9 μm; the surface roughness of the zirconium alloy thin-walled tube is measured to be 0.83 μm, and the flattening strength is 0.78 kN.

[0040] Example 4 The difference between this embodiment and Embodiment 1 is that the shot peening pressure of the microparticle shot peening is 4 bar and the coverage is 200%.

[0041] Microstructure analysis was performed on the zirconium alloy thin-walled tube after microparticle shot peening in this embodiment, such as... Figure 4As shown, the zirconium alloy thin-walled tube has a gradient structure on its surface with a layer thickness of 48.5 μm; the surface roughness of the zirconium alloy thin-walled tube is measured to be 0.92 μm, and the flattening strength is 0.79 kN.

[0042] Comparative Example 1 This comparative example only involves vacuum heat treatment at 800℃ for 24 hours on thin-walled zirconium alloy tubing.

[0043] Testing revealed that the average grain size of the zirconium alloy thin-walled tube was approximately 28.9 μm. Figure 5 As shown, the strength when flattened is 0.50 kN, which is significantly lower than that in Example 1.

[0044] Comparative Example 2 The difference between this comparative example and Example 1 is that liquid nitrogen cold spraying is not performed before microparticle shot peening.

[0045] Testing revealed that the zirconium alloy thin-walled tube had a surface roughness of 1.83 μm, a gradient structure layer thickness of 7.6 μm, and a flattening strength of 0.68 kN. Compared to Example 1, the surface roughness of the tube was significantly worse, the gradient structure layer thickness was greatly reduced, and the flattening strength was significantly lower. This indicates that liquid nitrogen cold blasting before microparticle shot peening can significantly increase the gradient structure layer thickness, improve the smoothness of the tube surface, increase the low-energy twin interface, and improve mechanical properties.

[0046] Example 5 The method of this embodiment is as follows: a zirconium alloy thin-walled tube with an outer diameter of 9.5 mm, a wall thickness of 0.57 mm, and a length of 30 mm is subjected to vacuum heat treatment at 800℃ for 24 h. The tube is continuously cooled by vaporized low-temperature nitrogen gas at a temperature of -196℃ to -130℃ during the shot peening process. The tube surface is shot peened with microparticles at a rotation speed of 15 r / min using a shot peening machine to form a gradient structure. The shot material for the microparticle peening is tungsten steel with a particle size of 30 μm to 50 μm. The parameters for the microparticle peening are: shot peening pressure 2 bar, coverage 200%, flow rate 3 kg / min, shot peening angle 90°, and distance between the spray gun and the target material 100 mm.

[0047] Upon inspection, the surface roughness of the zirconium alloy thin-walled tube after microparticle shot peening in this embodiment was 0.54 μm, which is an improvement compared to the roughness in Example 1. The flattening strength was measured to be 0.73 kN, and the gradient structure layer thickness was comparable to that in Example 1, but its dimensional uniformity in the thickness direction was better. This indicates that using a low-temperature environment, compared to deep cooling before shot peening, can optimize surface roughness, improve the uniformity of shot peening, and thus optimize the dimensional uniformity in the thickness direction.

[0048] Example 6 The method in this embodiment is as follows: a zirconium alloy thin-walled tube with an outer diameter of 9.5 mm, a wall thickness of 0.57 mm, and a length of 30 mm is subjected to vacuum heat treatment at 800℃ for 24 h. The tube is continuously cooled during the shot peening process using vaporized low-temperature nitrogen gas at a temperature of -196℃ to -130℃. The surface of the tube is shot peened with microparticles at a rotation speed of 15 r / min using a shot peening machine. Then, ceramic shot (hardness ≥ 80 HRC) with a particle size of 20 μm to 100 μm and glass shot (hardness < 60 HRC) are used for shot peening twice, respectively, to form a gradient structure. The shot peening pressure for both shots is 2 bar, the coverage is 200%, the flow rate is 3 kg / min, and the distance between the spray gun and the target is 100 mm. The shot peening angle for the ceramic shot is 90°, and the shot peening angle for the glass shot is 60°.

[0049] Upon inspection, the surface roughness of the zirconium alloy thin-walled tube in this embodiment is approximately 0.49 μm. Figure 5 As shown, the strength when flattened is 0.81 kN.

[0050] Example 7 The method of this embodiment is as follows: a zirconium alloy thin-walled tube with an outer diameter of 9.5 mm, a wall thickness of 0.57 mm, and a length of 30 mm is subjected to vacuum heat treatment at 800℃ for 24 h. The tube is continuously cooled during the shot peening process using vaporized low-temperature nitrogen gas at a temperature of -196℃ to -130℃. The surface of the tube is shot peened with microparticles at a rotation speed of 15 r / min using a shot peening machine. Then, tungsten carbide shot (hardness ≥ 80 HRC) with a particle size of 20 μm to 100 μm and glass shot (hardness < 60 HRC) are used for shot peening twice to form a gradient structure. The shot peening pressure for both shots is 2 bar, the coverage is 200%, the flow rate is 3 kg / min, and the distance between the spray gun and the target is 100 mm. The shot peening angle for the tungsten carbide shot is 90°, and the shot peening angle for the glass shot is 60°.

[0051] Upon inspection, the surface roughness of the zirconium alloy thin-walled tube in this embodiment is approximately 0.49 μm, and its strength when flattened is 0.77 kN.

[0052] Example 8 The difference between this embodiment and Embodiment 7 is that the particle size of the tungsten carbide shot is 30μm~50μm.

[0053] Upon inspection, the surface roughness of the zirconium alloy thin-walled tube in this embodiment is approximately 0.46 μm, and its strength when flattened is 0.75 kN.

[0054] Example 9 The difference between this embodiment and Embodiment 1 is that the vacuum heat treatment temperature is 500℃.

[0055] Upon inspection, the surface roughness of the zirconium alloy thin-walled tube in this embodiment is approximately 0.68 μm, and its strength when flattened is 0.72 kN.

[0056] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention. Any simple modifications, alterations, or equivalent structural transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for constructing a gradient microstructure on the surface of a zirconium alloy thin-walled tube, characterized in that, Zirconium alloy thin-walled tubes are shot-peened with microparticles in a cryogenic environment to form a gradient structure.

2. The method for constructing a gradient microstructure on the surface of a zirconium alloy thin-walled tube according to claim 1, characterized in that, The method for forming the cryogenic environment is as follows: liquid nitrogen is used for cold spraying before microparticle shot peening, or vaporized low-temperature nitrogen gas is introduced during microparticle shot peening, wherein the temperature of the low-temperature nitrogen gas is -196℃ to -130℃.

3. The method for constructing a gradient microstructure on the surface of a zirconium alloy thin-walled tube according to claim 1, characterized in that, The particle size of the shot peening material is 20μm~100μm.

4. The method for constructing a gradient microstructure on the surface of a zirconium alloy thin-walled tube according to claim 1, characterized in that, The parameters of the microparticle shot peening include shot peening pressure and coverage, wherein the shot peening pressure is 2 bar to 4 bar; The shot peening pressure is matched with the coverage rate. When the shot peening pressure is 2 bar, the coverage rate is not less than 100%. When the shot peening pressure is 3 bar, the coverage rate is 100%~400%. When the shot peening pressure is 4 bar, the coverage rate is not higher than 200%.

5. The method for constructing a gradient microstructure on the surface of a zirconium alloy thin-walled tube according to claim 1, characterized in that, The specific process of the microparticle shot peening is as follows: shot peening is performed twice, using hard shot and soft shot respectively.

6. The method for constructing a gradient microstructure on the surface of a zirconium alloy thin-walled tube according to claim 5, characterized in that, The hard shot has a hardness ≥ 80 HRC, and the soft shot has a hardness < 60 HRC; the shot peening angle when using the hard shot is 90°, and the shot peening angle when using the soft shot is ≤ 60°.

7. The method for constructing a gradient microstructure on the surface of a zirconium alloy thin-walled tube according to claim 1, characterized in that, The microparticles undergo vacuum heat treatment before shot peening, and the temperature of the vacuum heat treatment is 500℃~800℃.

8. The method for constructing a gradient microstructure on the surface of a zirconium alloy thin-walled tube according to claim 1, characterized in that, The zirconium alloy thin-walled tube rotates at a constant speed during the microparticle shot peening process.

9. The method for constructing a gradient microstructure on the surface of a zirconium alloy thin-walled tube according to claim 1, characterized in that, The thickness of the gradient structure is 15 μm to 50 μm.

10. The method for constructing a gradient microstructure on the surface of a zirconium alloy thin-walled tube according to claim 1, characterized in that, After the microparticle shot peening, the surface roughness Ra of the zirconium alloy thin-walled tube is no greater than 1.0 μm.

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