A method for hot radial upsetting forming of a thick-walled variable-diameter pipe made of high-strength material

Abstract

The application discloses a kind of high-strength material thick-walled variable-diameter pipe hot radial upsetting forming method, thick-walled pipe blank is heated to hot forming temperature and then radial compression is upset, the deformation resistance of material is reduced by high temperature, the need of thick-walled pipe blank for support internal pressure required by large cross-section compression deformation is reduced, only auxiliary support internal pressure needs to be applied in the pipe blank inside to prevent instability wrinkling in forming process, and hot forming and auxiliary support internal pressure jointly act to make thick-walled material stable in large compression amount. Diameter reduction is realized to realize the high-precision forming of variable-diameter pipe fittings. The material deformation is large in the forming process, and the material strength of the component is improved. Compared with the traditional overall blank cutting method, the material utilization and production efficiency of the application are significantly improved, and are suitable for batch manufacturing of thick-walled variable-diameter pipes.

Description

Technical Field

[0001] This invention relates to a method for manufacturing a thick-walled reducing pipe, and more particularly to a method for hot radial upsetting forming of a high-strength material thick-walled reducing pipe. Background Technology

[0002] Thick-walled reducing pipes, compared to solid components, are hollow components with thick walls (wall thickness to inner diameter ratio ≥ 1:5), which can reduce weight. They also have advantages such as high specific strength and high specific stiffness. In actual use environments, they have the same excellent properties as solid components, such as fatigue resistance and impact resistance. In addition, they have high material utilization and low processing energy consumption, so their application scenarios are focused on high-end equipment fields, such as ball joints and support beams of heavy-duty vehicles, landing gear and wing main beams of aerospace, car body beams of rail transit, hydraulic columns of construction machinery, and crane telescopic booms.

[0003] However, during the forming process of high-strength, thick-walled variable-diameter tubes, the thick walls result in high resistance to plastic deformation, easily leading to poor metal flow and causing localized thick / thin walls, cavity collapse, or contour distortion. Common solutions include forging combined with turning, rotary forging, spinning, and powder metallurgy. Forging makes it difficult for the metal to uniformly fill the inner corners, easily resulting in defects such as missing material and folds. Turning also reduces material utilization. Rotary forging technology has a small transition cone angle (up to 20°). For parts with large cross-sectional differences, such as ball joints with transition angles close to 80°, forming is difficult. Spinning is generally only used for parts with large diameter-to-thickness ratios, i.e., thin-walled parts, and is difficult to form for thick-walled parts. Powder metallurgy methods do not require complex subsequent processing and are particularly suitable for hollow, thick-walled components that are difficult to form with traditional forging. However, powder metallurgy structural parts have a certain porosity after sintering, resulting in lower tensile strength, fatigue resistance, and impact toughness compared to forged parts, making it difficult to meet the requirements of heavy-load and impact-resistant scenarios such as aerospace and heavy equipment.

[0004] Therefore, the problems of forming methods, forming accuracy, and material strength of existing high-strength material thick-walled variable diameter tubes have been troubling those skilled in the art, and there is an urgent need to innovate a forming control method for high-strength material thick-walled variable diameter tubes. Summary of the Invention

[0005] To address the shortcomings of the aforementioned technologies, this invention provides a method for hot radial upsetting of a high-strength, thick-walled variable-diameter tube.

[0006] To solve the above technical problems, the technical solution adopted by the present invention is: a method for hot radial upsetting and shrinking of a high-strength material thick-walled variable diameter tube; Step 1: Heat the metal tube blank to a hot forming temperature range above the material's recrystallization temperature but below its melting point; Step 2: Place the metal tube blank heated to the target temperature range into the hot radial upsetting mold. The hot radial upsetting mold is initially closed until the mold contacts the metal tube blank, and both ends of the tube blank are sealed. Step 3: Inert gas is filled into the tube blank to bring the internal pressure to the target pressure value that is only used to provide auxiliary support. The mold closes to cause the thick-walled tube blank to be radially uptaken. When the diameter reduction reaches the target value, the mold stops moving, so that the thick-walled variable diameter tube blank reaches the target geometric profile.

[0007] Furthermore, the target pressure value ranges from ps ≤ p ≤ 10 ps; where ps is the yield pressure of the metal in the hot state.

[0008] Furthermore, the maximum compression value of the section with reduced diameter is set less than the material elongation of the metal tube blank in the hot forming temperature range.

[0009] Furthermore, the thermoforming temperature range is 600-1300℃.

[0010] Furthermore, during the process of applying radial compressive force to the metal tube blank during the hot radial upsetting die closing process, the distribution of the circumferential force of the metal tube blank along its arc segment to straight wall segment satisfies a decreasing or uniform distribution law determined by static equilibrium; the circumferential force of the arc segment of the metal tube blank... The distribution is shown in the following formula: ; in, The circumferential force at the top of the tube blank; For this infinitesimal element and Circular forces in force balance Let be the angle corresponding to the infinitesimal element at the arc. For internal support and internal pressure, The coefficient of friction, The outer radius of the arc segment of the tube blank is... This represents the thickness of the tube blank.

[0011] Furthermore, the circumferential force of the straight-walled section of the metal tube blank The distribution is shown in the following formula: ; in, This refers to the length of the straight wall section of the tube blank.

[0012] Furthermore, when radial upsetting is completed, the circumferential force of the arc segment of the tube blank tends to be consistent with the circumferential force of the straight wall segment, so that the tube blank as a whole is in a uniform stress state.

[0013] Furthermore, after the mold stops moving, the internal pressure of the metal tube blank is maintained for a predetermined time, and then the internal pressure of the metal tube blank is reduced to atmospheric pressure. The mold is then opened and the formed part is taken out. Cooling water is introduced into the mold to cool the surface of the formed part and form a martensitic structure. When the surface layer of the formed part cools and shrinks rapidly, it is pulled by the internal material to produce tensile plastic deformation. After cooling to room temperature, the surface volume increases relatively and is compressed by the internal material to form compressive stress. Cooling only stays on the surface of the formed part, so that the formed part forms a structure that is hard on the outside and tough on the inside.

[0014] Furthermore, the hot radial upsetting die includes an upper die and a lower die, which together constitute the final forming die cavity. The inner contour shape of the final forming die cavity matches the outer contour of the arc segment and the straight wall segment of the metal tube blank. The part of the final forming die cavity corresponding to the arc segment of the metal tube blank is provided with an arc-shaped surface that matches the outer radius of the arc segment of the metal tube blank, and the part of the final forming die cavity corresponding to the straight wall segment of the metal tube blank is provided with a flat surface that matches the length of the straight wall segment of the metal tube blank.

[0015] Furthermore, when the upper and lower molds are closed, the arc-shaped surface and the straight surface apply radial compressive force to the arc segment and straight wall segment of the metal tube blank, respectively, guiding the metal tube blank to compress the cross section to the set compression amount along the cavity of the final forming mold; under the guidance of the cavity of the final forming mold, the amount of compression deformation of each part of the metal tube blank tends to be consistent, realizing the uniformity of compressive stress.

[0016] This invention discloses a hot radial upsetting method for high-strength, thick-walled variable-diameter tubes. Addressing the technical challenges of low material utilization, poor forming accuracy, limited transition cone angle, and insufficient microstructure and properties in existing high-strength, thick-walled variable-diameter tube forming processes, this invention heats the thick-walled tube blank to the hot forming temperature and then performs radial compression upsetting. High temperature reduces the material's deformation resistance, thus reducing the required internal support pressure for large-section compression deformation of the thick-walled tube blank. During forming, only auxiliary internal support pressure is applied inside the tube blank to provide normal support to the straight wall of the tube blank to prevent instability and wrinkling. The combined effect of hot forming and auxiliary internal support pressure enables the thick-walled material to undergo stable diameter reduction under large compression, achieving high-precision forming of variable-diameter tubes. The compression forming method leaves residual compressive stress inside the component, which is beneficial for improving fatigue resistance and service performance. Attached Figure Description

[0017] Figure 1 This is an isometric schematic diagram showing the relative relationship between the part and the mold before upsetting.

[0018] Figure 2 This is an isometric schematic diagram showing the relative relationship between the part and the mold after upsetting.

[0019] Figure 3 This is an isometric half-section view showing the relative relationship between the part and the mold before upsetting.

[0020] Figure 4 This is an isometric half-section view showing the relative relationship between the part and the mold after upsetting.

[0021] Figure 5 This is a cross-sectional view showing the relative relationship between the parts and the mold before and after shrinkage.

[0022] Figure 6 This describes the circumferential stress distribution before upsetting.

[0023] Figure 7 This describes the circumferential stress distribution after upsetting.

[0024] Figure 8 This is a schematic diagram of the force distribution at any point in the circumferential section subjected to upsetting.

[0025] Figure 9 These are parts formed using this method.

[0026] In the diagram: 1. Upper mold; 2. Lower mold; 3. Metal tube blank; 4. Force balance section of the arc segment; 5. Force balance section of the straight wall segment. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0028] Example 1:

[0029] This invention uses the manufacturing of a DP600 duplex steel hollow ball joint as an example to specifically illustrate the hot radial upsetting forming method for thick-walled variable-diameter tubes made of high-strength materials. The tube blank parameters are: wall thickness t = 3 mm, outer radius of the arc segment r = 10 mm, and length of the straight wall segment l = 20 mm. The yield strength σ_s of DP600 steel at 900℃ is 300 MPa, and its initial yield pressure is calculated according to the formula for thin-walled cylinders. The coefficient of friction is μ=0.3.

[0030] Step 1: The tube blank is heated to 900℃ using electric heating. DP600 steel exhibits good plasticity at 900℃, significantly reducing the material's deformation resistance and facilitating subsequent forming. Simultaneously, the required internal pressure and equipment tonnage are reduced. The heating range of 600-1300℃ can be set depending on the metal tube material. Hot forming results in minimal springback, improving forming accuracy. It also provides the temperature conditions for dynamic recrystallization during subsequent upsetting, refining the grains under large deformation and improving the microstructure.

[0031] Note that the determination of the hot forming temperature range in the technical solution of this invention follows the following principles: the metal billet is heated to a temperature range above the critical phase transformation temperature in the solid state, but below the temperature of the solid-liquid mixture. For steel, the hot forming temperature needs to reach above the austenitizing temperature (727℃ for pure iron, but usually not lower than 800℃ in actual production due to heating efficiency and alloying elements); for titanium alloys, the hot forming temperature needs to reach above the β phase transformation temperature (882℃ for pure titanium, with the actual temperature adjusted according to the alloy composition); for materials with heat treatment strengthening effects such as aluminum alloys, magnesium alloys, and copper alloys, the temperature is heated to above the solution temperature. The upper limit of the temperature should be lower than the temperature of the solid-liquid mixture to ensure that the material has good plasticity in the solid state without melting or overheating. Experimental verification shows that for DP600 duplex steel and TC4 titanium alloy, hot radial upsetting can be achieved in the range of 600-1300℃, with the best results achieved in the range of 800-1000℃. Within this temperature range, the material has moderate deformation resistance, controllable oxide scale formation, and the highest forming accuracy.

[0032] Specifically, for different materials and different temperature conditions, the present invention provides the following typical embodiments:

[0033] Using DP600 duplex steel as the billet material, the billet is heated to 600℃ using electric heating. At this temperature, DP600 steel is in the initial stage of austenitization. The material's deformation resistance is significantly lower than at room temperature but higher than at 900℃. Therefore, the required internal support pressure is correspondingly increased, ps is approximately 25MPa, and the actual charging pressure p is taken as 40MPa (within the range of ps≤p≤10ps). Under these conditions, the billet can still achieve radial upsetting, but the required equipment tonnage is increased compared to 900℃, and the material flow uniformity is slightly inferior to that at high temperatures. This embodiment is suitable for scenarios where equipment tonnage is not sensitive but there are requirements for refined grain size after forming.

[0034] Using DP600 duplex steel as the billet material, the billet is heated to 900℃ using electric heating. At this temperature, the DP600 steel is fully austenitized, and the material's deformation resistance is significantly reduced, with ps approximately 18 MPa. The actual charging pressure p is taken as 30 MPa. Within this temperature range, the material exhibits good plasticity, moderate deformation resistance, controllable oxide scale formation rate, and significant grain refinement after forming, making it the preferred temperature range for this invention.

[0035] Using TC4 titanium alloy as the tube blank material, the tube blank is heated to 1200℃ using induction heating. At this temperature, TC4 titanium alloy is in the β phase region, exhibiting extremely low deformation resistance. The required internal support pressure is significantly reduced compared to 900℃, with ps approximately 12MPa, and the actual charging pressure p is taken as 20MPa. The material exhibits excellent fluidity at this temperature, allowing for radial upsetting with greater compression. However, careful control of the upper temperature limit is necessary to avoid excessive grain growth and oxide scale thickening. Consequently, heating and forming must be carried out in a protective atmosphere.

[0036] Using DP600 duplex steel as the tube blank material, the tube blank is heated to 1300℃ using electric heating. This temperature is close to the lower limit of the material's solid-liquid mixed state temperature, resulting in extremely low resistance to deformation. However, the heating time must be strictly controlled to avoid overheating. Forming at this temperature can achieve extremely low equipment tonnage requirements, but it places high demands on the heat resistance and oxidation protection of the mold, making it suitable for special production scenarios with limited equipment capacity.

[0037] The invention can achieve hot radial upsetting of high-strength material thick-walled variable diameter tubes within a wide temperature range of 600-1300℃, with the best overall effect at 800-1000℃. However, the specific temperature selection can be flexibly adjusted according to the type of material, equipment conditions, and product quality requirements.

[0038] Step 2: Place the heated metal tube blank into the hot radial upsetting die. (Example:...) Figures 1-4 The diagram shows the isometric view of the relative relationship between the parts and the mold before and after upsetting, and the isometric half-section view of the relative relationship between the parts and the mold before and after upsetting. The mold consists of an upper mold 1 and a lower mold 2. The two ends of the metal tube blank 3 are sealed by sealing punches. The sealing punches have gas channels for filling the metal tube blank 3 with inert gas. Argon is used as the inert gas to avoid reaction with DP600 steel at room temperature and high temperature. The mold is initially closed until it contacts the metal tube blank 3. The cross-section of the metal tube blank 3 includes a circular arc section and a straight wall section. The upper mold 1 is located above the metal tube blank 3, and the lower mold is located below the metal tube blank 3. The mold closing direction is up and down.

[0039] During the forming process, the temperature of the metal tube blank 3 material should be kept within the target temperature range (900℃ in this embodiment). The mold should be cooled in time to ensure strength. For example, water cooling or gas blowing can be used to cool the mold. At the same time, the fallen oxide scale should be cleaned up in time.

[0040] Step 3: Argon gas is injected into the metal tube blank 3 through the gas channel of the sealed punch via the pressurization system, so that the internal pressure reaches the target pressure value p. In this embodiment, p is 30 MPa; this pressure value is within the range of p_s ≤ p ≤ 10p_s (p_s = 18 MPa), and is only used to provide auxiliary support, that is, to provide normal support for the straight wall section of the metal tube blank 3 during the subsequent radial compression process to prevent instability and wrinkling, rather than as the main driving force for forming. The target pressure value is applied by gas, which should avoid reacting with the high-strength material tube blank at room temperature and high temperature.

[0041] The upper die 1 is then driven to move to the lower die, applying radial compressive force to the metal billet 3. The upper die 1 continues to feed, causing compressive plastic deformation of the cross-section of the metal billet 3. The stroke of the upper die 1 is controlled by a displacement sensor; the maximum compression value of the cross-section should be less than the elongation of DP600 steel at 900℃. When the cross-sectional compression reaches the target value (the straight segment is compressed from its initial size to l=20mm), the upper die 1 stops moving. Figure 2 and Figure 4 As shown, at this point, the upper mold 1 and the lower mold are completely closed, and the final forming mold cavity formed by the two together perfectly matches the outer contour of the target ball joint component. Comparing the isometric schematic diagram of the relative relationship between the part and the mold after upsetting and the isometric half-section view after upsetting, it can be seen that the cross-section of the metal tube blank 3 is compressed from the initial racetrack shape to a near-annular shape, and the wall thickness is increased, thus achieving radial upsetting forming.

[0042] at the same time, Figure 5 A cross-sectional view showing the relative relationship between the part and the die before and after upsetting is provided. Figure 5 As can be clearly seen, this method ultimately leads to compression deformation of the metal tube blank 3 in both the circumferential and radial directions, manifested as a reduction in the circumferential circumference and an increase in the radial wall thickness.

[0043] During the aforementioned hot radial upsetting process, the distribution of the circumferential force of the metal tube blank 3 along its arc segment to straight wall segment satisfies the distribution law determined by static equilibrium. Figure 6 and Figure 7 This illustrates the circumferential stress distribution before and after upsetting. At the very beginning of upsetting, as... Figure 6 As shown, the stress reaches its maximum value in the middle of the straight wall section. However, due to the homogenization of compressive stress, the actual stress difference mainly comes from friction, and the stress values ​​are not significantly different. After upsetting is completed, as... Figure 7 As shown, at this point, the cross-section of the metal tube blank is nearly annular, and the mechanical conditions are the same everywhere. The influence of friction disappears, the internal stress is redistributed, and the overall stress is further homogenized, resulting in a more uniform thickness. At the same time, under the combined influence of the homogenization of compressive stress and hot forming, springback is further reduced, and forming accuracy is high.

[0044] From a mechanical perspective, during the radial compression process, the wall thickness of the component should tend to become uniform throughout the cross-section. For the same material, if wall thickness differences occur, under the same circumferential force, the thickness increase of the thinner-walled portion will be greater than that of the thicker-walled portion until the wall thicknesses are equal. The following theoretical analysis demonstrates the influence of the radial compression circumferential force distribution on wall thickness uniformity, using a racetrack-shaped initial tube blank as an example.

[0045] like Figure 8 As shown, this is a schematic diagram of the mechanical equilibrium of any infinitesimal element at the arc segment and the straight-wall segment. The straight-wall segments on both sides are fixed, and the diameter is reduced by using two crescent-shaped molds at the top and bottom. Figure 8 The diagram shows the mechanical equilibrium of any micro-element at arc segment 4 and straight wall segment 5. The straight wall segments on both sides are fixed, and the tube is reduced in diameter using two crescent-shaped molds. The diagram illustrates the frictional force on the tube blank, the supporting force provided by the molds, the circumferential force inside the tube blank, and the supporting internal pressure on the tube blank. The circumferential force at the top of the tube blank. For this infinitesimal element and The circumferential forces in force equilibrium, by analogy, lead to the circumferential force in the middle of the straight wall section. , Let the angle be the infinitesimal element at the arc. For internal support and internal pressure, For mold support force, The coefficient of friction, The outer radius of the arc segment of the tube blank is... For the thickness of the tube blank, This refers to the length of the straight wall section of the tube blank.

[0046] This leads to the following static equilibrium formula: Circular force and The difference is due to friction, so the following equation can be established: Substituting formula (1) into (2) and simplifying, we get: Then, substitute it into formula (1), where It can be used ( ): By analogy, we can conclude that: right Taking the limit, we can obtain the distribution of circumferential force at the arc: The distribution of circumferential forces on each cross-section of the straight wall can be obtained using the following formula: The distribution of circumferential force on any cross section can be obtained using the formula above.

[0047] Taking DP600 duplex steel as an example, at 900℃, assuming the yield strength is... The tube blank parameters are , , Support internal pressure coefficient of friction For a unit length of tube, assuming the top of the tube blank just enters the yielding state, at this point... Then the position of the arc

[0048] At this time, the middle part of the straight wall section of the tube blank At this point, both are under compressive stress and are relatively uniform, achieving stress homogenization compression. When compression is nearing completion, according to the principle of constant volume, the wall thickness increases by [value missing]. At this point, the overall stress is

[0049] After upsetting, the overall stress state is closer to uniform. Figure 9 The exhibit showcases parts formed by hot radial upsetting. The billet material is DP600 steel, and the radius is compressed from 31mm to 16.5mm, achieving a compression rate of approximately 47%. This method has already enabled large-scale mass production. Inspection using a fixture yields an accuracy of 0.05mm.

[0050] Example 2:

[0051] Based on Example 1, after the mold stops moving, the internal pressure of the tube blank is maintained for a predetermined time. Then, the internal pressure of the tube blank is reduced to atmospheric pressure, and the mold is opened to remove the formed part. Cooling water is circulated inside the mold, causing the surface of the formed part to cool and form a martensitic structure. During the rapid cooling and shrinkage of the surface layer of the formed part, it undergoes tensile plastic deformation due to the pull of the internal material. After cooling to room temperature, the surface volume relatively increases, and compressive stress is formed due to the compression of the internal material. Because the cooling only occurs on the surface of the formed part, a structure that is hard on the outside and tough on the inside is ultimately formed, achieving an excellent balance between strength and toughness.

[0052] Figure 9 The demonstration showcased a part (DP600 steel ball joint) manufactured using the aforementioned hot radial upsetting method, where the tube blank radius was compressed from 31 mm to 16.5 mm, a compression rate of approximately 47%. Measurements showed that the ball joint component formed using this method achieves an accuracy of 0.05 mm, and mass production has already been achieved.

[0053] Therefore, the tube blank material is a cold-formable metal material such as steel or titanium alloy, which possesses good high-temperature plasticity. This invention achieves high-precision and stable forming of large-section differential diameter tubes by heating the thick-walled tube blank to its hot-forming temperature, filling it with auxiliary support air pressure, and using a mold for radial compression upsetting. This method significantly reduces the material's deformation resistance at high temperatures, greatly reducing the need for internal support pressure. Simultaneously, the internal air pressure acts as a flexible force transmission medium, providing uniform normal support and effectively preventing instability and wrinkling of the straight-walled section of the tube blank during radial compression, allowing the thick-walled material to achieve uniform diameter reduction under large compression. High-temperature, large deformation promotes dynamic recrystallization of the material, resulting in a fine-grained, uniformly distributed microstructure, effectively improving component strength. Furthermore, the compression forming method leaves residual compressive stress inside the component, which is beneficial for improving fatigue resistance and service performance. This invention overcomes the technical difficulties of traditional forging, rotary forging, spinning, and powder metallurgy methods in forming high-strength thick-walled variable diameter tubes, such as low material utilization, poor forming accuracy, limited transition cone angle, and insufficient microstructure and properties. It has outstanding advantages such as wide material applicability, high forming accuracy, large compression ratio, small equipment tonnage required, low internal pressure requirement, and simple and reliable process.

[0054] This invention lies in the coordination between hot forming temperature, auxiliary support internal pressure, and radial compression deformation, rather than a simple superposition of single process parameters. It is manifested in heating the thick-walled tube blank to the hot forming temperature, thereby significantly reducing the material's deformation resistance and drastically reducing the internal support pressure required for large-section compression deformation of the thick-walled tube blank.

[0055] In other words, it is precisely because of the effective reduction of the material's yield strength by high temperature that only the application of auxiliary support internal pressure is needed to prevent the straight wall section of the tube blank from becoming unstable and wrinkling during radial compression. In other words, high pressure is not necessary, and low pressure is sufficient to support it.

[0056] The combined effect of hot forming and auxiliary support internal pressure enables thick-walled materials to undergo stable diameter reduction under large compression, achieving high-precision forming of variable-diameter pipe fittings. During radial upsetting, the thick-walled tube blank undergoes severe plastic deformation under large compression. This deformation causes the metal tube blank to increase wall thickness and shrink cross-section under radial compressive force, achieving the target geometric profile. On the other hand, the large deformation induces dynamic recrystallization of the tube blank material within the hot forming temperature range, causing the internal grains of the metal to break down and recrystallize, forming a fine and uniformly distributed equiaxed grain structure, effectively improving the strength of the component. Deformation and recrystallization occur simultaneously and are mutually conditional. Large deformation provides the driving force for recrystallization, and the hot forming temperature provides the activation energy for recrystallization. The two work together to achieve the simultaneous completion of forming and strengthening.

[0057] From a mechanical perspective, during the hot radial upsetting process, the circumferential force of the tube blank is distributed in a specific pattern along the arc segment to the straight wall segment, eventually becoming uniform, so that the tube blank is in a uniform compressive stress state. This compressive stress homogenization effect is superimposed with the inherent advantages of low springback and high precision in hot forming, which further reduces springback and improves forming accuracy.

[0058] Meanwhile, the compression forming method leaves residual compressive stress inside the component. This compressive stress, combined with the fine-grained structure obtained from hot forming, helps improve the component's fatigue resistance and service performance. Ultimately, this method solves the technical challenges of high-strength, thick-walled, variable-diameter tubes during cold forming, which involve large deformation resistance, poor plasticity, and difficulty in achieving large compression reduction. It also overcomes the shortcomings of traditional forging, rotary forging, spinning, and powder metallurgy methods in terms of material utilization, forming accuracy, transition cone angle, and microstructure properties.

[0059] The above embodiments are not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the technical solution of the present invention are also within the protection scope of the present invention.

Claims

1. A method for hot radial upsetting and shrinking of a high-strength material thick-walled variable-diameter tube, characterized in that: Step 1: Heat the metal tube blank to a hot forming temperature range above the material's recrystallization temperature but below its melting point; Step 2: Place the metal tube blank heated to the target temperature range into the hot radial upsetting mold. The hot radial upsetting mold is initially closed until the mold contacts the metal tube blank, and both ends of the tube blank are sealed. Step 3: Inert gas is filled into the tube blank to bring the internal pressure to the target pressure value that is only used to provide auxiliary support. The mold closes to cause the thick-walled tube blank to be radially uptaken. When the diameter reduction reaches the target value, the mold stops moving, so that the thick-walled variable diameter tube blank reaches the target geometric profile.

2. The method for hot radial upsetting and shrinking of a high-strength material thick-walled variable-diameter tube according to claim 1, characterized in that: The target pressure value is in the range of ps ≤ p ≤ 10 ps; where ps is the yield pressure of the metal in the hot state.

3. The method for hot radial upsetting and shrinking of a high-strength material thick-walled variable-diameter tube according to claim 1, characterized in that: The maximum compression value of the section with the reduced diameter is set to be less than the material elongation of the metal tube blank in the hot forming temperature range.

4. The method for hot radial upsetting and shrinking of a high-strength material thick-walled variable-diameter tube according to claim 1, characterized in that: The thermoforming temperature range is 600-1300℃.

5. The method for hot radial upsetting and shrinking of a high-strength material thick-walled variable-diameter tube according to claim 1, characterized in that: During the process of applying radial compressive force to the metal tube blank by the hot radial upsetting die closing, the distribution of the circumferential force of the metal tube blank along its arc segment to straight wall segment satisfies a decreasing or uniform distribution law determined by static equilibrium; the circumferential force of the arc segment of the metal tube blank The distribution is shown in the following formula: ； in, The circumferential force at the top of the tube blank; For this infinitesimal element and Circular forces in force balance Let the angle be the infinitesimal element at the arc. For internal support and internal pressure, The coefficient of friction, The outer radius of the arc segment of the tube blank is... This represents the thickness of the tube blank.

6. The method for hot radial upsetting of a high-strength material thick-walled variable-diameter tube according to claim 5, characterized in that: The circumferential force of the straight-walled section of the metal tube blank The distribution is shown in the following formula: ； in, This refers to the length of the straight wall section of the tube blank.

7. The method for hot radial upsetting of a high-strength material thick-walled variable-diameter tube according to claim 6, characterized in that: When radial upsetting is completed, the circumferential force of the arc segment of the tube blank and the circumferential force of the straight wall segment tend to be consistent, so that the tube blank as a whole is in a uniform stress state.

8. The method for hot radial upsetting of a high-strength material thick-walled variable-diameter tube according to any one of claims 1-7, characterized in that: After the mold stops moving, the internal pressure of the metal tube blank is maintained for a predetermined time. Then, the internal pressure of the metal tube blank is reduced to atmospheric pressure, the mold is opened, and the formed part is taken out. Cooling water is introduced into the mold to cool the surface of the formed part and form a martensitic structure. When the surface of the formed part cools and shrinks rapidly, it is pulled by the internal material and undergoes tensile plastic deformation. After cooling to room temperature, the surface volume increases relatively and is compressed by the internal material to form compressive stress. The cooling only stays on the surface of the formed part, so that the formed part forms a structure that is hard on the outside and tough on the inside.

9. The method for hot radial upsetting and shrinking of a high-strength material thick-walled variable-diameter tube according to claim 8, characterized in that: The hot radial upsetting die includes an upper die and a lower die, which together form a final forming die cavity. The inner contour shape of the final forming die cavity matches the outer contour of the arc segment and the straight wall segment of the metal tube blank. The final forming die cavity is provided with an arc-shaped surface that matches the outer radius of the arc segment of the metal tube blank at the corresponding part of the arc segment, and a flat surface that matches the length of the straight wall segment of the metal tube blank at the corresponding part of the straight wall segment.

10. The method for hot radial upsetting and shrinking of a high-strength material thick-walled variable-diameter tube according to claim 9, characterized in that: When the upper and lower molds are closed, the arc-shaped surface and the straight surface apply radial compressive force to the arc segment and straight wall segment of the metal tube blank, respectively, guiding the metal tube blank to compress the cross section along the cavity of the final forming mold to the set compression amount. Guided by the final forming mold cavity, the amount of compressive deformation of each part of the metal tube blank tends to be consistent, thus achieving uniform compressive stress.

Images