A hydraulic bulging process for thick-walled cylinder instability control
By pre-treating the billet, conducting metal mechanical property tests and finite element simulation modeling, and combining real-time monitoring with visual displacement sensors, a two-stage loading hydraulic bulging process was adopted to solve the instability problem in the hydraulic bulging process of thick-walled cylinders. This achieved high-precision and high-efficiency bulging control, and is suitable for manufacturing forgings such as non-magnetic retaining rings for large generators.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- TAIYUAN UNIVERSITY OF SCIENCE AND TECHNOLOGY
- Filing Date
- 2026-06-23
- Publication Date
- 2026-07-21
AI Technical Summary
The existing hydraulic bulging process for thick-walled cylinders is prone to instability, resulting in low dimensional accuracy and uneven wall thickness of the products. Furthermore, it lacks effective real-time monitoring and active control methods, leading to insufficient production efficiency.
By employing a two-stage loading hydraulic bulging process—including billet pretreatment, metal mechanical property testing, finite element simulation modeling and parameter setting, and real-time monitoring using visual displacement sensors—the loading rate and mold structure can be adjusted to achieve real-time monitoring and active control of instability.
It effectively suppresses instability phenomena such as bulging, concave waist, and flared mouth during the forming process, improves product dimensional accuracy and pass rate, and enhances production efficiency. It is suitable for the localization of forgings such as non-magnetic retaining rings for large generators.
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Figure CN122425117A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of hydraulic bulging technology, specifically relating to a hydraulic bulging process for controlling instability in thick-walled cylinders. Background Technology
[0002] Thick-walled cylindrical forgings are core components in fields such as steam turbine generators, aerospace, and petrochemicals. Hydraulic bulging is a key forming process, which involves applying high pressure to a liquid inside the thick-walled cylinder to cause plastic deformation of the billet, thereby obtaining the target dimensions, shape, and properties. Especially for large generator non-magnetic retaining ring forgings, because the material is single-phase austenitic stainless steel, it can only be strengthened through cold deformation to meet technical requirements.
[0003] The hydraulic bulging method currently used for large retaining ring forgings is mostly the conical punch decompression method. Due to the thick wall of the thick-walled cylindrical blank, the short forming time, and the high deformation resistance, the stress distribution is prone to unevenness. During the forming process, instability phenomena such as bulging, concave waist, and flared mouth are very likely to occur, resulting in low dimensional accuracy and uneven wall thickness of the product. At the same time, the existing process lacks effective real-time monitoring methods for instability, and mostly detects instability after bulging. Once instability is severe, it cannot be repaired and may even be scrapped directly. In addition, unreasonable design of blank pretreatment and tooling selection can also exacerbate bulging instability from the source.
[0004] Therefore, to address the instability problem in conventional bulging processes, a hydraulic bulging process method based on instability control is needed to improve the dimensional accuracy and product qualification rate of the formed parts, while simultaneously achieving real-time monitoring and active control of instability. Summary of the Invention
[0005] The purpose of this invention is to overcome the shortcomings of existing thick-walled cylinder hydraulic bulging processes, such as easy instability, low dimensional accuracy, and insufficient production efficiency. This invention provides a hydraulic bulging process for controlling instability in thick-walled cylinders, achieving uniform deformation during the bulging process, effectively suppressing instability phenomena such as bulging, concave waist, and flared mouth, improving the dimensional accuracy and product qualification rate of the formed parts, and simultaneously realizing real-time monitoring and active control of instability to improve production efficiency.
[0006] This invention is achieved through the following technical solution: A hydraulic bulging process for instability control of thick-walled cylinders includes the following steps: S1. Pre-treatment of the billet includes the following steps: First, inspect the initial external dimensions, microscopic grain size, and grain size uniformity of the billet; second, remove oxide scale, burrs, and scratches from the surface of the billet to ensure a clean and smooth surface; third, perform ultrasonic testing on the billet to ensure that there are no obvious defects inside the billet and that the mechanical properties of the billet are uniform, thus avoiding instability caused by defect expansion during the bulging process; finally, machine C10 chamfers (10mm×45° chamfers) at the upper and lower inner circular ends of the billet to facilitate sealing with the punch of the special force-reducing hydraulic bulging die during the hydraulic bulging process. S2. Determine the criteria for judging the instability of the billet under hydraulic expansion, including the following steps: First, the mechanical properties of the metal are tested: standard tensile specimens are made from specimens of the same material and heat treatment state as the billet. Tensile tests are conducted using the standard tensile specimens to obtain the mechanical property parameters of the material and characterize the mechanical behavior of the material during cold deformation at room temperature. This provides accurate parameter support for the subsequent development of hydraulic bulging process and simulation modeling. Then, based on the material's mechanical property parameters, the actual stress-strain curve of the material is plotted, and a nonlinear power-law hardening model is used in Origin software. s true = Yes true m Fit the actual stress-strain curve, where, K The hardening coefficient is... e true This refers to the hydraulic bulging deformation. m The enhancement coefficient; Finally, the criteria for determining the hydraulic bulging instability (tangential necking) of the material, based on the hydraulic bulging deformation amount ε (determined according to the requirements of bulging strengthening technology), are as follows: e true ≤2 m ;like e true ≤2 m If the billet is not prone to hydraulic bulging instability during subsequent hydraulic bulging, then it is considered that the billet meets the requirements for subsequent hydraulic bulging; if e true >2 m If the billet is not properly hydraulically bulged, it is prone to hydraulic bulging instability during the subsequent hydraulic bulging process, requiring an increase in the material's strengthening coefficient. m The value, until it is satisfied. e true ≤2 m ; S3. Finite Element Simulation Modeling and Parameter Setting: ABAQUS finite element simulation software was used to perform fluid-structure interaction (CEL) analysis (Coupled Eulerian-Lagrangian, a mainstream numerical method specifically designed to solve fluid-structure interaction problems involving large deformation, strong impact, and free liquid surfaces. Its core is to use Lagrangian meshes to calculate solids and Eulerian meshes to calculate fluids, achieving bidirectional interaction through interface coupling). A finite element model of hydraulic bulging of a thick-walled cylinder was established to simulate actual hydraulic bulging conditions, optimize and adjust process parameters, and determine the optimal loading rate. S4. Mold assembly and blank installation; S5. Hydraulic bulging: Based on the optimized process parameters from step S3, hydraulic bulging is performed using a two-stage loading method, including the following steps: S5-1, Initial Bulging Stage: Rapid loading, the pressure inside the hydraulic medium rises rapidly from 0 to 130~150MPa, and the loading rate of the upper punch is 10~20mm / s; the core of this stage is to allow the bulging device to quickly establish ultra-high pressure, so that the billet is subjected to force quickly and evenly and enters the plastic deformation state. Avoiding a slow pressure rise in the initial stage can lead to discontinuous local hydraulic medium spraying, causing hydraulic instability, uneven internal pressure on the billet, and thus inducing shape instability; S5-2, Stable Bulging Stage: The upper punch loading rate is maintained at 15~20mm / s, and the internal pressure continues to increase to 150~300MPa. This stage utilizes the work hardening characteristics of the material, controlling the loading speed to ensure uniform and stable internal pressure during thick-walled cylinder bulging, gradually increasing material strength, suppressing excessive local deformation, avoiding shape instability, and meeting dimensional accuracy requirements. The external dimensions of the blank during the stable bulging stage are monitored in real-time using a visual displacement sensor. If bulging, concave waisting, or trumpet-shaped instability is detected, the bulging process is immediately stopped, the blank is unloaded, and its external dimensions are accurately measured. Based on the blank's shape and dimensional measurement results, the upper punch loading rate is adjusted, and then the blank is reloaded onto the reducing hydraulic bulging die to continue bulging processing. The specific handling method after the aforementioned instability occurs is as follows: a) If the billet becomes unstable and forms a bulging shape after bulging: First, determine the deflection of the outer generatrix of the billet as δmm. If δmm < 7mm, proceed with the next steps. If δmm ≥ 7mm, terminate the bulging process. Then, reduce the loading rate of the upper punch by 1 to 2 times the deflection value δ. If the billet still forms a bulging shape after reducing the loading rate of the upper punch, repeat step a) to reduce the loading rate of the upper punch until the outer generatrix of the billet is straight. b) If the billet becomes unstable and forms a concave waist shape after bulging: First, determine the deflection of the outer generatrix of the billet as δmm. If δmm < 7mm, proceed with the next steps. If δmm ≥ 7mm, terminate the bulging process. Then, increase the loading rate of the upper punch by 1 to 2 times the deflection value δ. If the billet still forms a concave waist shape after increasing the loading rate of the upper punch, repeat step b) to increase the loading rate of the upper punch until the outer generatrix of the billet is straight. If the loading rate of the upper punch reaches the maximum limit or the billet still forms a concave waist shape after bulging after increasing the loading rate of the upper punch, add a 5 to 10mm chamfer to the chamfer of the upper and lower inner circle ends of the billet. c) If the billet becomes unstable and forms a flared shape after bulging: Measure the outer diameters of the large and small ends at the flared end of the billet, and denote the ratio of the outer diameter of the large end to that of the small end as c; if c ≥ 0.98, increase the chamfer angle of the inner circle port of the billet corresponding to the small end by 5°; if 0.96 ≤ c < 0.98, increase the chamfer angle of the inner circle port of the billet corresponding to the small end by 10°; if 0.94 ≤ c < 0.96, increase the chamfer angle of the inner circle port of the billet corresponding to the small end by 15°; if c < 0.94, increase the chamfer angle of the inner circle port of the billet corresponding to the small end by 20°. d) If the billet exhibits a mixed-type unstable shape after bulging: If the billet exhibits a flared shape with a bulging belly after bulging, adjust the loading rate of the upper punch and the chamfer angle of the inner circle port of the billet according to steps a) and c) until the outer generatrix of the billet is straight; If the billet exhibits a flared shape with a concave waist after bulging, adjust the loading rate of the upper punch and the chamfer angle of the inner circle port of the billet according to steps b) and c) until the outer generatrix of the billet is straight.
[0007] S5-3, Unloading stage: When the pressure equipment reaches the preset stroke, the billet expands to the specified size, the crossbeam of the pressure equipment is quickly lifted to release pressure quickly, avoiding instability caused by discontinuous hydraulic medium spray or uneven local leakage of liquid, thus completing the hydraulic expansion of the billet and producing a hydraulically expanded thick-walled cylinder.
[0008] Furthermore, in step S2, the mechanical property parameters include yield strength, tensile strength, uniform elongation, total elongation, elastic modulus, Poisson's ratio, hardening coefficient, and strengthening coefficient.
[0009] Further, step S3 includes the following steps: First, a geometric model of thick-walled cylinder hydraulic bulging was established and meshed in ABAQUS finite element simulation software. The geometric model includes the blank, hydraulic medium, and upper and lower punches and force-reducing columns of the force-reducing hydraulic bulging mold. When establishing the geometric model, it can be simplified into a two-dimensional axisymmetric model to reduce the computational scale, improve simulation efficiency, and ensure simulation accuracy. Secondly, the mechanical property parameters of the material obtained in step S2 are input into the simulation model as the core parameters of the material constitutive relationship to ensure that the model can truly reflect the mechanical response of the material. The hydraulic medium is set to water. Next, the actual stress-strain curve determined in step S2 is used to set the plastic flow criteria of the material, and the friction coefficient and loading boundary constraints between the mold and the blank are set to simulate the contact state and constraint conditions in the actual bulging process. Finally, by simulating the bulging process under different upper punch loading rates, the relationship between the loading rate of the pressure equipment, the volume of hydraulic medium spray, and the shape and size of the thick-walled cylinder was analyzed. The process parameters were optimized and adjusted to determine the optimal loading rate, ensuring that the billet does not experience instability phenomena such as bulging or trumpeting.
[0010] Furthermore, in step S5, if the hydraulic bulging deformation amount e true >0.2 or hydraulic press forming force F < P max π(r) 2 -r 减 2 ), P max The maximum internal pressure of the liquid is r, and the inner diameter of the billet is r. 减 To reduce the radius of the force-reducing column, increase its diameter and repeat steps S4 and S5 until the preset size is reached.
[0011] Furthermore, testing instruments are used to inspect the diameter, wall thickness, and roundness of the hydraulically bulged thick-walled cylinder. Defective parts are adjusted based on the instability phenomenon and then bulged again until the product passes inspection.
[0012] The beneficial effects of this invention are as follows: 1. This invention comprehensively characterizes the mechanical behavior of materials through preliminary metal mechanical property tests, and analyzes the plasticity and strengthening characteristics of materials by combining stress-strain curves, thus avoiding bulging necking instability caused by insufficient material properties in advance, and providing a foundation for the implementation of subsequent bulging processes; 2. This invention combines finite element simulation technology to accurately set material mechanical behavior parameters, simulate the shape change law and stress-strain distribution of thick-walled cylinder blanks during the bulging process, predict instability risk points, solve the problem of blind optimization of process formulation in the prior art, realize the accurate prediction and control of instability of thick-walled cylinder bulging, effectively suppress instability defects such as bulging belly, concave waist, and flared mouth, and improve the scientificity and rationality of the process from the source; 3. This invention employs a strategy of accurately controlling the loading speed of the pressure equipment and the size of the billet for measurement and control, achieving rapid pressurization and rapid unloading. This ensures a rapid and continuous uniform high-pressure response within the thick-walled cylinder bulging process, avoiding instability caused by uneven stress on the billet. Furthermore, this invention provides specific adjustment solutions for irregular shape instability issues such as bulging, concave waists, and flared openings in the thick-walled cylinder billet bulging process, improving product dimensional accuracy and forming quality, and increasing the product qualification rate to over 98%. 4. This invention is specifically designed for manganese-chromium austenitic stainless steel thick-walled cylindrical forgings such as non-magnetic retaining rings for 50MW~1000MW steam turbine generators. It has strong adaptability, solves the technical bottlenecks of difficult bulging and unstable control of such forgings, reduces production costs, promotes the localization of thick-walled cylindrical forgings, and has important engineering application value.
[0013] 5. This invention clarifies the correlation logic between metal mechanical property testing, stress-strain curve analysis, and finite element simulation parameter settings, forming a complete technical system of "experimentation-simulation-process optimization," and providing a referable technical solution for the hydraulic bulging processing of similar thick-walled cylindrical forgings. Attached Figure Description
[0014] Figure 1 This is a flowchart of the method described in this invention. Detailed Implementation
[0015] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. Example 1
[0016] In this embodiment 1, a thick-walled cylinder of Mn18Cr18N high-nitrogen austenitic stainless steel is selected as the billet for the processing of a non-magnetic retaining ring for a 300MW steam turbine generator. The dimensions of the retaining ring after bulging are required to be an outer diameter of 1210 mm, an inner diameter of 990 mm, a wall thickness of 110 mm, and a height of 910 mm. The requirements of the initial billet meet the hot forging technology requirements for non-magnetic retaining rings of 50MW~1000MW steam turbine generators. like Figure 1 The hydraulic bulging process shown includes the following steps for controlling instability in thick-walled cylinders: S1. Pre-treatment of billet, including the following steps: First, the initial external dimensions, micrograin size, and grain size uniformity of the billet were tested; a Mn18Cr18N thick-walled cylindrical billet was selected with an outer diameter of 937 mm, an inner diameter of 672 mm, a wall thickness of 132.5 mm, and a height of 1034 mm. Secondly, remove the oxide scale, burrs, and scratches from the surface of the billet to ensure that the surface of the billet is clean and smooth; Next, ultrasonic testing is performed on the billet to ensure that there are no obvious defects inside the billet and that the mechanical properties of the billet are uniform. Finally, C10 chamfers are machined on the upper and lower inner circular ends of the blank.
[0017] S2. Determine the criteria for judging the instability of the billet under hydraulic expansion, including the following steps: First, the mechanical properties of the metal were tested: standard tensile specimens were prepared using samples of the same material and heat treatment state as the billet. Axial tension was applied using a WDW3200 micro-controlled electronic universal testing machine, and tensile tests were conducted using the standard tensile specimens (standard tensile specimens were prepared according to GB / T228-2002 "Metallic Materials - Tensile Testing at Room Temperature", axial tension was applied using an electronic universal testing machine, and load and elongation data were recorded simultaneously until the standard tensile specimens broke). The mechanical property parameters of the material were obtained as follows: yield strength was 640 MPa, tensile strength was 970 MPa, uniform elongation was 42%, total elongation was 58%, elastic modulus was 200 GPa, and Poisson's ratio was 0.3. Then, based on the material's mechanical property parameters, the actual stress-strain curve of the material is plotted, and a nonlinear power-law hardening model is used in Origin software. s true = Yes true m Fit the actual stress-strain curve to the hardening coefficient. K =1445, hydraulic bulging deformation amount e true =0.307, determine the strengthening coefficient of the material. m =0.216; Finally, e true =0.307 and m Substituting 0.216 into the criteria for judging material instability, we can see that... e true ≤2 m Therefore, it is believed that the billet provided in this embodiment 1 is not prone to hydraulic bulging instability during the subsequent hydraulic bulging process, and the billet has met the requirements of the subsequent hydraulic bulging, that is, the bulging does not cause tangential necking instability, the material has good plasticity, obvious work hardening characteristics, and is suitable for hydraulic bulging processing.
[0018] S3. Finite Element Simulation Modeling and Parameter Setting: Using ABAQUS finite element simulation software, a fluid-structure interaction (CEL) analysis was performed to establish a finite element model for the hydraulic bulging of the thick-walled cylinder. The actual hydraulic bulging conditions were simulated, process parameters were optimized and adjusted, and the optimal loading rate was determined. This includes the following steps: First, a geometric model of thick-walled cylinder hydraulic bulging was established and meshed in ABAQUS finite element simulation software. The geometric model includes the blank, hydraulic medium, and upper and lower punches and force-reducing columns of the force-reducing hydraulic bulging mold. Secondly, the mechanical property parameters of the material obtained in step S2 are input into the simulation model, and the hydraulic medium is set to water; Next, based on the actual stress-strain curve determined in step S2, the plastic flow criterion of the material is set. In this embodiment 1, the Mises yield criterion is selected to match the plastic deformation law of Mn18Cr18N steel under complex stress state, accurately predict the yield and plastic deformation process of the material; and the friction coefficient between the mold and the billet is set to 0.1, and the loading boundary constraint is set to simulate the contact state and constraint conditions in the actual bulging process. Finally, by simulating the bulging process under different upper punch loading rates, the relationship between the loading rate of the pressure equipment, the volume of hydraulic medium spray, and the shape and size of the thick-walled cylinder was analyzed, and the process parameters were optimized and adjusted to determine the optimal loading rate.
[0019] S4. Mold assembly and blank installation; First, based on the press tonnage and mold dimensions, the forming force is checked to ensure that the press tonnage provides sufficient forming force. An 8000-ton press is selected, and two sets of dedicated pressure-reducing hydraulic bulging molds are chosen for double-pass forming. The pressure-reducing column diameters are 650mm and 800mm respectively. The number of forming passes is calculated, and the corresponding number of dedicated pressure-reducing hydraulic bulging molds are prepared. The working dimensions of the mold are checked, assembled and measured, and the moving parts are cleaned to ensure flexible operation. Damaged parts are repaired to ensure the mold is in good working condition. Tooling and auxiliary equipment such as measuring tools, sealing rings, lifting tools, grease, cotton yarn, and protective covers are prepared. Next, clean the hydraulic press's work platform and place the lower die of the hydraulic bulging mold flat in the center of the platform. Apply grease evenly to the chamfered areas at the top and bottom of the blank, as well as the working cone surface and guide post surface of the mold. Use straps to lift the ring blank, center it, and place it on the guide post. Level it, then smoothly lift the upper punch and spherical pad together and place them on the guide post, gently pressing the ring blank. After alignment, lift it slightly and fill the gaps with tap water, ensuring no leaks, then lower it to tighten. Install a visual displacement sensor to detect the real-time dimensions of the blank during the hydraulic bulging process. Smoothly send the bulging mold and ring blank under the moving hydraulic press beam. Measure and record the initial flange distance between the upper and lower punches of the mold before bulging. After completing the above steps, prepare for pressure bulging.
[0020] S5. Hydraulic bulging, including the following steps: S5-1, Initial bulging stage: Rapid loading, the pressure in the hydraulic medium rises rapidly from 0 to 150MPa, and the loading rate of the upper punch is 12mm / s; S5-2, Stable Expansion Stage: The upper punch loading rate is maintained at 15 mm / s, and the internal pressure continues to increase to 271 MPa. In this embodiment 1, when the press stroke reaches 223 mm, the billet size during the stable expansion stage is monitored in real time by a visual displacement sensor. An unstable phenomenon with a concave waist shape is detected in the billet. The pressure equipment beam is quickly raised to rapidly release pressure and stop expansion. The billet size is measured; if a slight concave waist is observed and the generatrix deflection δ is 3 mm, the upper punch loading rate is increased by 1 to 2 times the deflection value δ, i.e., the upper punch loading rate is increased to 18 mm / s, and expansion is repeated. Correspondingly, if the billet becomes unstable with an overall bulging shape after expansion, the upper punch loading rate is reduced by 1 to 2 times the deflection value δ. That is, assuming a slight bulge appears during the stable expansion stage and the generatrix deflection δ is 3 mm, the upper punch loading rate is reduced to 9 to 12 mm / s, and expansion is repeated. In this embodiment 1, when the press stroke reaches 348mm, the retaining ring expands to the calibrated size in the first pass, and the pressure equipment beam is quickly lifted to rapidly release the pressure. During the hydraulic expansion process, the billet size data is monitored in real time by a visual displacement sensor, and no shape instability phenomena such as bulging, concave waist, or flared shape occur. After unloading, the billet size is measured, and the billet size is 1115mm outer diameter, 882.4mm inner diameter, and 948mm height, with a straight generatrix. A second bulging pass was performed using a hydraulic bulging die with a damping column diameter of 800mm. The die installation was repeated using step S4 above. During hydraulic bulging, rapid loading was applied in the initial bulging stage, allowing the hydraulic medium pressure to rise quickly from 0 to 150MPa, with the upper punch loading rate controlled at 15mm / s. In the stable bulging stage, the upper punch loading rate was kept as stable as possible at 18mm / s, and the internal pressure gradually increased to 282MPa. When the press stroke reached 232mm, the retaining ring underwent its second bulging to the calibrated size, and the pressure equipment beam was quickly lifted to rapidly release the pressure. The billet size data was monitored in real time using a visual displacement sensor, and no instability occurred. S5-3, Unloading Stage: When the pressure equipment reaches the preset stroke, the billet expands to the calibrated size. The crossbeam of the pressure equipment is quickly lifted to rapidly release the pressure, completing the hydraulic expansion of the billet and producing a hydraulically expanded thick-walled cylinder. After unloading, the dimensions of the billet are measured. The billet generatrix is straight and reaches the preset dimensions, specifically an outer diameter of 1210mm, an inner diameter of 990mm, a wall thickness of 110mm, and a height of 910mm. The hydraulic expansion is then complete.
[0021] Based on this, testing instruments are used to test the diameter, wall thickness, and roundness of the hydraulically bulged thick-walled cylinder. Defective parts are adjusted according to the instability phenomenon and then bulged again until the product passes the inspection and enters the subsequent processing steps, finally obtaining thick-walled cylinder forgings such as non-magnetic retaining rings for 50MW~1000MW steam turbine generators that meet the requirements. Example 2
[0022] In this embodiment 2, a thick-walled cylinder of Mn18Cr18N high-nitrogen austenitic stainless steel is selected as the billet for the processing of a non-magnetic retaining ring for a 1000MW steam turbine generator. The dimensions of the retaining ring after bulging are required to be 2060 mm, with an inner diameter of 1830 mm, a wall thickness of 115 mm, and a height of 1016 mm. The requirements of the initial billet meet the hot forging technical requirements of the non-magnetic retaining ring for a 1000MW steam turbine generator. like Figure 1 The hydraulic bulging process shown includes the following steps for controlling instability in thick-walled cylinders: S1. Pre-treatment of billet, including the following steps: First, the initial external dimensions, micrograin size, and grain size uniformity of the billet were tested; a Mn18Cr18N thick-walled cylindrical billet was selected with an outer diameter of 1700mm, an inner diameter of 1450mm, a wall thickness of 125mm, and a height of 1120mm. Secondly, remove the oxide scale, burrs, and scratches from the surface of the billet to ensure that the surface of the billet is clean and smooth; Next, ultrasonic testing is performed on the billet to ensure that there are no obvious defects inside the billet and that the mechanical properties of the billet are uniform. Finally, C10 chamfers are machined on the upper and lower inner circular ends of the blank.
[0023] S2. Determine the criteria for judging the instability of the billet under hydraulic expansion, including the following steps: First, the mechanical properties of the metal were tested: standard tensile specimens were prepared using samples of the same material and heat treatment state as the billet. Axial tension was applied using a WDW3200 micro-controlled electronic universal testing machine, and tensile tests were conducted using the standard tensile specimens (standard tensile specimens were prepared according to GB / T228-2002 "Metallic Materials - Tensile Testing at Room Temperature", axial tension was applied using an electronic universal testing machine, and load and elongation data were recorded simultaneously until the standard tensile specimens broke). The mechanical property parameters of the material were obtained as follows: yield strength 600 MPa, tensile strength 980 MPa, uniform elongation 38%, total elongation 51%, elastic modulus 200 GPa, and Poisson's ratio 0.3. Then, based on the material's mechanical property parameters, the actual stress-strain curve of the material is plotted, and a nonlinear power-law hardening model is used in Origin software. s true = Yes true m Fit the actual stress-strain curve to the hardening coefficient. K =1402, hydraulic bulging deformation amount e true =0.201, determine the strengthening coefficient of the material. m =0.202; Finally, e true =0.201 and m Substituting 0.202 into the criteria for judging material instability, we can see that... e true ≤2 m Therefore, it is believed that the billet provided in this embodiment 2 is not prone to hydraulic bulging instability during the subsequent hydraulic bulging process, and the billet has met the requirements of subsequent hydraulic bulging, that is, the bulging does not cause tangential necking instability, the material has good plasticity, obvious work hardening characteristics, and is suitable for hydraulic bulging processing.
[0024] S3. Finite Element Simulation Modeling and Parameter Setting: Using ABAQUS finite element simulation software, a fluid-structure interaction (CEL) analysis was performed to establish a finite element model for the hydraulic bulging of the thick-walled cylinder. The actual hydraulic bulging conditions were simulated, process parameters were optimized and adjusted, and the optimal loading rate was determined. This includes the following steps: First, a geometric model of thick-walled cylinder hydraulic bulging was established and meshed in ABAQUS finite element simulation software. The geometric model includes the blank, hydraulic medium, and upper and lower punches and force-reducing columns of the force-reducing hydraulic bulging mold. Secondly, the mechanical property parameters of the material obtained in step S2 are input into the simulation model, and the hydraulic medium is set to water; Next, based on the actual stress-strain curve determined in step S2, the plastic flow criterion of the material is set. In this embodiment 2, the Mises yield criterion is selected to match the plastic deformation law of Mn18Cr18N steel under complex stress state, accurately predict the yield and plastic deformation process of the material; and the friction coefficient between the mold and the billet is set to 0.1, and the loading boundary constraint is set to simulate the contact state and constraint conditions in the actual bulging process. Finally, the bulging process under different top punch loading rates was simulated to analyze the relationship between the loading rate of the pressure equipment, the volume of hydraulic medium spray, and the shape and size of the thick-walled cylinder. Process parameters were optimized and adjusted to determine the optimal loading rate. Specifically, the initial bulging speed was 14 mm / s, and the steady-state speed was 18 mm / s; the internal pressure was a maximum of 165 MPa for the first pass and 178 MPa for the second pass, ensuring that the thick-walled cylinder blank did not experience bulging, trumpeting, or other unstable phenomena.
[0025] S4. Mold assembly and blank installation; First, based on the press tonnage and mold dimensions, the forming force was checked to ensure that the press tonnage met sufficient forming force. An 18,500-ton press was selected, and two sets of dedicated pressure-reducing hydraulic bulging molds were chosen for double-pass forming. The pressure-reducing column diameters were 1130 mm and 1300 mm respectively. The number of forming passes was calculated, and the corresponding number of dedicated pressure-reducing hydraulic bulging molds were prepared. The working dimensions of the molds were checked, assembled and measured, and the moving parts were cleaned to ensure flexible operation. Damaged parts were repaired to ensure the molds were in good working condition. Measuring tools, sealing rings, lifting tools, grease, cotton yarn, protective covers, and other tooling and auxiliary equipment were prepared. Next, clean the hydraulic press's worktable and place the lower die of the hydraulic bulging mold flat in the center of the worktable. Apply grease evenly to the chamfered areas at the top and bottom of the blank, as well as the working cone surface and guide post surface of the mold. Use straps to lift the ring blank, center it, and place it on the guide post. Level it, then smoothly lift the upper punch and spherical pad together and place them on the guide post. Gently press the ring blank, align it, and lift it slightly. Fill the gaps with tap water, ensuring no leaks, then lower it to press firmly. Install a visual displacement sensor to detect the real-time dimensions of the blank during the hydraulic bulging process. Smoothly send the bulging mold and ring blank under the moving hydraulic press beam. Measure and record the initial flange distance between the upper and lower punches of the mold before bulging. After completing the above steps, prepare for pressure bulging.
[0026] S5. Hydraulic bulging, including the following steps: S5-1, Initial bulging stage: Rapid loading, the pressure in the hydraulic medium rises rapidly from 0 to 150MPa, and the loading rate of the upper punch is 14mm / s; S5-2, Stable bulging stage: The loading rate of the upper punch is maintained at 18mm / s, and the internal pressure is continued to be increased to 165MPa; In this embodiment 2, when the press stroke reaches 704mm, the retaining ring expands to the calibrated size in the first pass, and the pressure equipment beam is quickly lifted to rapidly release pressure, avoiding instability caused by discontinuous hydraulic medium spraying or uneven local liquid leakage; the billet size data during the stable expansion stage is monitored in real time by a visual displacement sensor, and no shape instability phenomena such as bulging, concave waist, or horn occur; after unloading, the billet size is measured, and the generatrix is straight; After the first pass of bulging is completed, a hydraulic bulging die with a 1300mm diameter pressure-reducing column is used for the second pass of bulging. The die installation is repeated using step S4 above. During hydraulic bulging, rapid loading is applied in the initial bulging stage, allowing the pressure in the hydraulic medium to rise quickly from 0 to 150 MPa, with the upper punch loading rate controlled at 14 mm / s. In the stable bulging stage, the upper punch loading rate is kept as stable as possible at 18 mm / s, and the internal pressure gradually increases to 178 MPa. When the press stroke reaches 500 mm, the retaining ring is bulged to the calibrated size in the second pass, and the pressure equipment beam is quickly lifted to rapidly release the pressure. The billet size data is monitored in real time using a visual displacement sensor, and no instability occurs. S5-3, Unloading Stage: When the pressure equipment reaches the preset stroke, the billet expands to the calibrated size. The crossbeam of the pressure equipment is quickly lifted to rapidly release the pressure, completing the hydraulic expansion of the billet and producing a hydraulically expanded thick-walled cylinder. After unloading, the dimensions of the billet are measured. The billet generatrix is straight and reaches the preset dimensions, specifically an outer diameter of 2060 mm, an inner diameter of 1830 mm, a wall thickness of 115 mm, and a height of 1016 mm. The hydraulic expansion is then complete.
[0027] Based on this, testing instruments are used to test the diameter, wall thickness, and roundness of the hydraulically bulged thick-walled cylinder. Defective parts are adjusted according to the instability phenomenon and then bulged again until the product passes the inspection and enters the subsequent processing steps, finally obtaining the 1000MW steam turbine generator non-magnetic retaining ring and other thick-walled cylinder forgings that meet the requirements.
[0028] The above description is merely 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 technical scope 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 hydraulic bulging process for controlling instability in thick-walled cylinders, characterized in that, Includes the following steps: S1. Pre-treatment of billet, including the following steps: First, inspect the initial external dimensions, micro-grain size, and grain size uniformity of the billet; second, remove oxide scale, burrs, and scratches from the surface of the billet to ensure that the surface of the billet is clean and smooth; third, perform ultrasonic testing on the billet to ensure that there are no obvious defects inside the billet and that the mechanical properties of the billet are uniform; finally, machine C10 chamfers on the upper and lower inner circular ends of the billet. S2. Determine the criteria for judging the instability of the billet under hydraulic expansion, including the following steps: First, metal mechanical property test: Select a sample with the same material and heat treatment state as the billet to make a standard tensile test sample, and use the standard tensile test sample to conduct a tensile test to obtain the mechanical property parameters of the material. Then, based on the material's mechanical property parameters, the actual stress-strain curve of the material is plotted, and a nonlinear power-law hardening model is used in Origin software. σ true = Kε true m Fit the actual stress-strain curve, where, K The hardening coefficient is... ε true This refers to the hydraulic bulging deformation. m The enhancement coefficient; Finally, based on the hydraulic bulging deformation amount ε true The criteria for determining the hydraulic bulging instability of materials are as follows: ε true ≤2 m ;like ε true ≤2 m If the billet is not prone to hydraulic bulging instability during subsequent hydraulic bulging, then it is considered that the billet meets the requirements for subsequent hydraulic bulging; if ε true >2 m If the billet is not properly hydraulically bulged, it is prone to hydraulic bulging instability during the subsequent hydraulic bulging process, requiring an increase in the material's strengthening coefficient. m The value, until it is satisfied. ε true ≤2 m ; S3. Finite element simulation modeling and parameter setting: The fluid-structure interaction CEL analysis was performed using ABAQUS finite element simulation software to establish a finite element model of hydraulic bulging of thick-walled cylinder, simulate the actual hydraulic bulging working condition, optimize and adjust the process parameters, and determine the optimal loading rate. S4. Mold assembly and blank installation; S5. Hydraulic bulging, including the following steps: S5-1, Initial bulging stage: The pressure in the hydraulic medium rises rapidly from 0 to 130~150MPa, and the loading rate of the upper punch is 10~20mm / s; S5-2, Stable Bulging Stage: The loading rate of the upper punch is maintained at 15~20mm / s, and the internal pressure continues to increase to 150~300MPa; the external dimensions of the blank during the stable bulging stage are monitored in real time using a visual displacement sensor. If the blank is found to bulge, concave, or exhibit a trumpet-shaped instability, the bulging process is immediately stopped, the blank is unloaded, and its external dimensions are accurately measured. Based on the blank's shape and dimensional measurement results, the loading rate of the upper punch is adjusted, and then the blank is reloaded onto the reducing hydraulic bulging die to continue the bulging process. The specific handling method after the aforementioned instability occurs is as follows: a) If the billet becomes unstable and forms a bulging shape after bulging: First, determine the deflection of the outer generatrix of the billet as δmm. If δmm < 7mm, proceed with the next steps. If δmm ≥ 7mm, terminate the bulging process. Then, reduce the loading rate of the upper punch by 1 to 2 times the deflection value δ. If the billet still forms a bulging shape after reducing the loading rate of the upper punch, repeat step a) to reduce the loading rate of the upper punch until the outer generatrix of the billet is straight. b) If the billet becomes unstable and forms a concave waist shape after bulging: First, determine the deflection of the outer generatrix of the billet as δmm. If δmm < 7mm, proceed with the next steps. If δmm ≥ 7mm, terminate the bulging process. Then, increase the loading rate of the upper punch by 1 to 2 times the deflection value δ. If the billet still forms a concave waist shape after increasing the loading rate of the upper punch, repeat step b) to increase the loading rate of the upper punch until the outer generatrix of the billet is straight. If the loading rate of the upper punch reaches the maximum limit or the billet still forms a concave waist shape after bulging after increasing the loading rate of the upper punch, add a 5 to 10mm chamfer to the chamfer of the upper and lower inner circle ends of the billet. c) If the billet becomes unstable and forms a flared shape after bulging: Measure the outer diameters of the large and small ends at the flared end of the billet, and denote the ratio of the outer diameter of the large end to that of the small end as c; if c ≥ 0.98, increase the chamfer angle of the inner circle port of the billet corresponding to the small end by 5°; if 0.96 ≤ c < 0.98, increase the chamfer angle of the inner circle port of the billet corresponding to the small end by 10°; if 0.94 ≤ c < 0.96, increase the chamfer angle of the inner circle port of the billet corresponding to the small end by 15°; if c < 0.94, increase the chamfer angle of the inner circle port of the billet corresponding to the small end by 20°. d) If the billet exhibits a mixed-shape instability after bulging: If the billet exhibits a flared shape with bulging instability, adjust the loading rate of the upper punch and the chamfer angle of the inner circle port of the billet according to steps a) and c) until the outer generatrix of the billet is straight; If the billet exhibits a flared shape with concave waist instability after bulging, adjust the loading rate of the upper punch and the chamfer angle of the inner circle port of the billet according to steps b) and c) until the outer generatrix of the billet is straight. S5-3, Unloading stage: When the pressure equipment reaches the preset stroke, the billet expands to the specified size, the crossbeam of the pressure equipment is quickly lifted, the pressure is quickly released, the hydraulic expansion of the billet is completed, and a hydraulically expanded thick-walled cylinder is obtained.
2. The hydraulic bulging process for controlling instability of thick-walled cylinders according to claim 1, characterized in that, In step S2, the mechanical property parameters include yield strength, tensile strength, uniform elongation, total elongation, elastic modulus, Poisson's ratio, hardening coefficient, and strengthening coefficient.
3. The hydraulic bulging process for controlling instability of thick-walled cylinders according to claim 1, characterized in that, Step S3 includes the following steps: First, a geometric model of thick-walled cylinder hydraulic bulging was established and meshed in ABAQUS finite element simulation software. The geometric model includes the blank, hydraulic medium, and upper and lower punches and force-reducing columns of the force-reducing hydraulic bulging mold. Secondly, the mechanical property parameters of the material obtained in step S2 are input into the simulation model, and the hydraulic medium is set to water; Next, the actual stress-strain curve determined in step S2 is used to set the plastic flow criteria of the material, and the friction coefficient and loading boundary constraints between the mold and the blank are set to simulate the contact state and constraint conditions in the actual bulging process. Finally, by simulating the bulging process under different upper punch loading rates, the relationship between the loading rate of the pressure equipment, the volume of hydraulic medium spray, and the shape and size of the thick-walled cylinder was analyzed, and the process parameters were optimized and adjusted to determine the optimal loading rate.
4. The hydraulic bulging process for controlling instability of thick-walled cylinders according to claim 1, characterized in that, In step S5, if the hydraulic bulging deformation amount ε true >0.2 or hydraulic press forming force F < P max π(r) 2 -r 减 2 ), P max The maximum internal pressure of the liquid is r, and the inner diameter of the billet is r. 减 To reduce the radius of the force-reducing column, increase its diameter and repeat steps S4 and S5 until the preset size is reached.
5. The hydraulic bulging process for controlling instability in thick-walled cylinders according to claim 1, characterized in that, The diameter, wall thickness, and roundness of the hydraulically bulged thick-walled cylinder are tested using testing instruments. Defective parts are adjusted based on the instability phenomenon and then bulged again until the product passes the inspection.