Diesel engine high-pressure oil pipe sealing cone head cold heading forming die and method

Abstract

This invention discloses a cold forging die and method for sealing cone heads of high-pressure oil pipes for diesel engines, comprising an upper die, a lower die, and a mandrel that passes through the inner hole of the pipe along with the upper die. The mandrel has a precision support section, a gradual pressure relief section, and an introduction section sequentially arranged from its root to its free end. The outer diameter of the three sections decreases continuously, and the half-angle of the cone surface increases sequentially. A first single-sided gap is formed between the precision support section and the original inner wall of the pipe to limit inner wall buckling. The single-sided gaps between the gradual pressure relief section and the introduction section and the original inner wall of the pipe are both greater than the first single-sided gap, used to release the springback clamping force of the pipe during the return stroke of the upper die and to guide the mandrel coaxially. The effective length of the mandrel is determined by the ratio of the finished cone head volume to the original cross-sectional area of ​​the pipe, and the mandrel gap is determined by the inner diameter tolerance of the pipe, the die eccentricity, and the lubrication allowance. This invention, through a three-section mandrel that partitions the support, demolding, and introduction functions, is suitable for the cold forging of sealing cone heads for thick-walled high-pressure oil pipes.

Description

Technical Field

[0001] This invention belongs to the technical field of cold plastic forming of metal pipes and cold heading molds, specifically relating to a cold heading forming mold and method for a sealing cone of a high-pressure oil pipe for a diesel engine. Background Technology

[0002] The ends of high-pressure fuel lines in diesel engines typically require the formation of sealing cones to mate with injectors, high-pressure fuel pumps, or common rail interfaces. These sealing cones operate under the influence of high-pressure, pulsating fuel, and their cone shape, internal bore continuity, wall thickness distribution, and surface integrity directly affect sealing reliability and fatigue life. Cold forging can preserve continuous metal flow lines and reduce cutting volume. However, when axially forging thick-walled, small-bore tubing, the metal flow on the inner and outer surfaces is constrained differently, making the inner wall prone to inward buckling, folding, or localized shrinkage cavities.

[0003] When using existing molds to install a mandrel inside the tube, cylindrical mandrels or single small-tapered mandrels are typically used. Cylindrical mandrels can limit internal shrinkage, but the elastic rebound of the tube after molding generates a radial clamping force on the mandrel, leading to increased demolding force. While single large-tapered mandrels facilitate rapid release of contact, they rapidly increase the gap between the lower part of the mandrel and the inner wall of the tube, weakening support for the entire plastic deformation zone. Single small-tapered mandrels present another problem: when the half-angle of the taper is smaller than the friction angle of the contact pair, the taper may still be in a self-locking or near-self-locking state; therefore, the small taper alone cannot be used to determine if it necessarily facilitates demolding.

[0004] Furthermore, simply determining the mandrel diameter as a percentage of the tube wall thickness will result in excessive radial clearance in thick-walled tubes. For example, when the original tube has an outer diameter of 18mm and an inner diameter of 7mm, the wall thickness on one side is 5.5mm. If the clearance on one side is taken as 10% to 30% of the wall thickness, the clearance will reach 0.55 to 1.65mm, which cannot provide precise support for an inner hole with a diameter of 7mm. Therefore, the mandrel clearance should be based on the inner diameter tolerance of the tube, the coaxiality of the mold, the thickness of the lubricating film, and the allowable free deformation of the inner wall, rather than simply being determined by the wall thickness ratio.

[0005] Existing methods often directly use the axial length of the finished cone as the effective length of the mandrel, without considering the difference in axial length of the same pipe section before and after upsetting. During cold upsetting, the metal approximately satisfies the condition of constant volume. The length of the original pipe section involved in deformation should be calculated by inversely from the deformed volume of the finished product and the cross-sectional area of ​​the original pipe. Only when the length of the mandrel covers the original pipe section involved in deformation can the formation of an unsupported area at the end of the deformation zone be avoided. Summary of the Invention

[0006] The purpose of this invention is to provide a cold forging die for a high-pressure oil pipe sealing cone of a diesel engine. By dividing the mandrel into a precision support section, a gradual pressure relief section, and an introduction section, different sections respectively undertake the functions of inner hole support, clamping force release, and coaxial introduction. At the same time, the design relationship between the mandrel gap, cone angle, length, root fillet, and die holding force is established to solve the problem of mutual constraint between single-tapered mandrel support and demolding.

[0007] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: The pre-loading and overall structure of the mold for cold heading are as follows: Figure 1 As shown, a composite tapered mandrel is set at the center of the upper mold, and a positioning cavity and an upsetting tapered surface are set in the lower mold. From the root to the free end, the mandrel consists of a precision support section, a gradually decreasing pressure relief section, and an inlet section. The diameters of these three sections decrease continuously, while the half-angles of the tapered surfaces increase sequentially. The specific segmentation relationship is as follows: Figure 4 As shown.

[0008] The precision support section employs a small cone angle and maintains a small initial single-sided clearance with the original inner diameter of the tube. (Initial single-sided clearance) Determined according to the following formula:

[0009] at the same time

[0010] In the formula, Let be the range of variations in the inner diameter of the pipe, and e be the maximum eccentricity between the mandrel and the positioning cavity. This is the allowance required for the lubricating film and surface micro-undulations. Additional allowance for dimensional fluctuations during loading and heat treatment. The allowable free deformation gap will not cause significant buckling of the inner wall. By setting both the lower and upper limits, the mandrel will neither scratch the inner wall due to tolerance overlap nor lose its supporting function due to excessive gap.

[0011] The gradual pressure relief section uses a larger cone angle than the precision support section. Let N be the unit normal load on the mandrel surface, μ be the effective friction coefficient, and α be the cone half-angle. Then, the axial resistance component generated by contact in the return direction of the upper die can be approximately expressed as: when hour The component of the normal force on the conical surface in the return direction can offset frictional resistance, making it difficult for the conical surface to form a self-locking mechanism; however, when size limitations prevent it from fully satisfying this requirement... When the value is ≥μ, the total demolding force can be reduced by shortening the contact length, reducing the surface roughness, and using a lubricating film. Based on this mechanism, the present invention does not design the entire mandrel with a large cone angle, but only increases the cone angle in the gradual pressure relief section, thereby taking into account both deformation zone support and return pressure relief.

[0012] The inlet section features an increased cone angle and rounded ends, allowing the free end of the mandrel to enter the inner bore of the tube first. The inlet section is not a primary load-bearing support area; its dimensions are designed to avoid sharp corners damaging the inner wall, allow for a certain initial eccentricity, and enable the mandrel to automatically align itself.

[0013] Theoretical length of the original pipe segment involved in plastic deformation Determined based on the principle of constant volume:

[0014]

[0015] In the formula, and These are the original pipe's outer and inner diameters, respectively. This represents the original cross-sectional area of ​​the pipe. This refers to the solid volume formed by the tube segment within the finished cone head. The effective length of the mandrel entering the inner bore of the tube is also included. Should meet ≥ +ΔL, where ΔL is used to compensate for blanking tolerances, clamping position deviations, and allowance adjustments. This method of determination better reflects the volume transfer relationship during the upsetting process than directly multiplying the axial length of the finished cone by a fixed coefficient.

[0016] The strength of the mandrel root is simultaneously affected by axial load, eccentric bending moment, and abrupt changes in cross-section. A continuous transition fillet is used at the root, with its radius determined to be 0.15 to 0.30 times the maximum diameter of the precision support section. The ratio of the free length of the mandrel during insertion and forming, unsupported by the radial inner wall of the tube, to the corresponding minimum bearing diameter should not exceed 5, to reduce the risk of bending instability during the initial insertion and compression stages. The critical load of the mandrel should also be checked according to the Euler stability condition during design, and a safety factor of not less than 2.5 should be maintained relative to the calculated working load for the critical load.

[0017] In addition to radial positioning, the lower die's positioning cavity also features a straight-walled retaining section. During the upper die's return stroke, the lower die exerts an axial holding force on the formed part. It should be greater than the calculated demolding force of the mandrel. Preferred to satisfy This allows the workpiece to remain in the lower mold while the mandrel exits first, preventing the workpiece from moving upwards with the upper mold.

[0018] For cones with large deformation, a two-pass process using a pre-forming die and a final forming die can be employed. To ensure that the radial deformation in the two passes is similar, the maximum outer diameter of the pre-formed cone should be... Press Confirmed, among which The original outer diameter of the pipe. This is the final maximum outer diameter of the cone. This distribution method can avoid excessive concentration of deformation in the first or second pass.

[0019] A method for cold forging a diesel engine high-pressure oil pipe sealing cone using the above-mentioned mold includes the following steps: Step 1: Loading and Positioning A fixed-length high-pressure oil pipe is inserted into the positioning cavity of the lower mold, so that the free pipe section to be formed extends to the preset position. The positioning cavity forms a radial positioning constraint on the outer wall of the pipe. Step 2: Inserting the mandrel The upper mold moves along the axis of the pipe towards the lower mold, and the free end of the mandrel fixed at the center of the upper mold passes through the inner hole of the pipe. During insertion, the guide section of the mandrel first enters the inner hole of the tube, and the half-angle of the conical surface of the guide section... Correct the eccentricity between the inner hole of the pipe and the mandrel; The upper mold continues to descend, the gradual pressure relief section then enters the inner hole of the pipe, and the precision support section finally enters the inner hole of the pipe; After insertion, a first unilateral gap is formed between the precision support section and the original inner wall of the pipe. The first single-sided gap satisfy:

[0020] and

[0021] in, Let e ​​be the range of variation in the inner diameter of the high-pressure oil pipe, and let e be the maximum eccentricity between the mandrel and the positioning cavity. This is the allowance required for the lubricating film and the micro-undulations of the inner wall surface of the pipe. Additional allowance for loading errors and dimensional fluctuations during tube heat treatment. The allowable free deformation clearance to prevent buckling of the inner wall of the pipe; Step 3: Cold heading The upper mold continues to descend to the forming endpoint, and the end of the tube undergoes plastic flow under the joint constraint of the upsetting cone surface of the lower mold and the precision support section of the mandrel, forming a sealing cone. During the forming process, the precision support section maintains the first single-sided gap. This restricts the free buckling of the inner wall of the pipe in the axial direction; The effective length of the mandrel entering the inner hole of the tube. The theoretical length of the pipe section covering plastic deformation The theoretical length of the pipe section Determined by the following formula:

[0022]

[0023] in, The solid volume formed by the deformed pipe section within the finished sealing cone. This represents the original cross-sectional area of ​​the high-pressure oil pipe material. The original outer diameter of the high-pressure oil pipe is given. The original inner diameter of the high-pressure oil pipe; Step 4: Depressurization and return demolding After forming is completed, the upper mold returns upward along the axis of the tube, causing the mandrel to exit from the inner hole of the tube; During the return journey, the cone half-angle of the gradual pressure relief section The effective coefficient of friction μ after lubrication satisfies:

[0024] The radial clamping force between the mandrel and the inner wall of the tube is gradually released along the gradual pressure relief section; Simultaneously, the positioning cavity of the lower mold applies an axial holding force to the formed tube. The axial demolding force on the mandrel is denoted as . And satisfy:

[0025] The formed tube remains in the lower mold until the mandrel is completely withdrawn; The precision support section, the gradual pressure relief section, and the guide section of the mandrel are sequentially withdrawn from the inner hole of the tube. After the mandrel is completely withdrawn, the formed part is taken out by the ejection mechanism.

[0026] The present invention has the following beneficial effects: Compared to cylindrical mandrels or single-tapered mandrels, this invention distributes support and demolding tasks through three different functional sections: a precision support section restricts the free deformation space of the inner wall, a gradual pressure relief section reduces the clamping resistance during the return stroke based on the friction angle relationship, and an introduction section improves the loading tolerance; therefore, the selection of the mandrel taper has a clear mechanical basis, rather than simply adopting an empirical small taper angle.

[0027] This invention determines the mandrel gap by using the inner diameter tolerance of the pipe and the eccentricity of the mold, avoiding excessive gap caused by taking the value according to the wall thickness ratio for thick-walled pipes; it uses volume conservation to back-calculate the length of the deformed pipe section, avoiding blind spots in support caused by the mandrel length only covering the axial length of the finished product; and it checks the strength of the mandrel by the root fillet, length-to-diameter ratio and stable load, so that a design system of mutual constraints is formed among the main dimensions. Attached Figure Description

[0028] Figure 1 This is a schematic cross-sectional view of the overall structure of the cold heading forming die of the present invention; Figure 2 This is a partially enlarged structural diagram of the core rod portion of the present invention; Figure 3 This is a cross-sectional schematic diagram of the cold heading working state of the present invention; Figure 4 This is a schematic diagram showing the relationship between the precision support section, the gradual pressure relief section, the inlet section, and the cone angle and length parameters of the composite tapered mandrel.

[0029] Explanation of reference numerals in the attached figures: 1 - Upper mold; 2 - Lower mold; 3 - Mandrel; 31 - Precision support section; 32 - Gradual pressure relief section; 33 - Inlet section; 4 - High-pressure oil pipe; 5 - Transition fillet; 6 - Positioning cavity; 7 - Upsetting conical surface. Detailed Implementation

[0030] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0031] Example 1: Composite Tapered Mandrel Size Design This embodiment uses 42CrMo cold-drawn seamless steel pipe, with an original outer diameter of... It is 18mm, the original inner diameter. The thickness is 7mm, and the original single-sided wall thickness is 5.5mm. The pipe undergoes softening treatment before cold heading to give it the plasticity required for cold plastic deformation, and end burrs are removed. The maximum outer diameter of the finished 3D model is 29mm, the half-angle of the generatrix of the conical surface relative to the axis is 26°, the total included angle corresponding to the sealing conical surface is 52°, the axial reference dimension of the conical head is 22.5mm, and the calculated volume of the model is... It is 8545.746mm 3 .

[0032] The original pipe cross-sectional area is:

[0033] According to the law of conservation of volume, the theoretical length of the original pipe segment that participated in the formation of the cone is:

[0034] Considering end face finishing, blanking tolerances, and clamping position deviations, the initial length of the free tube segment involved in forming is selected to be 40mm to 42mm; the effective length of the mandrel entering the inner hole of the tube is initially selected to be 41mm to 43mm, and the precision support section is ensured to cover the main plastic flow area based on the actual clamping structure. This length is significantly larger than the axial dimension of the finished cone head of 22.5mm, because the tube segment undergoes radial expansion and axial shortening after cold heading.

[0035] This embodiment takes into account the range of pipe inner diameter variation, coaxiality error, and lubrication allowance, and considers the first single-sided gap. Set to 0.10mm, the diameter of the precision support section near the root. for:

[0036] When the actual pipe inner diameter tolerance has not yet been finalized, the diameter of the precision support section can be selected within the range of 6.70mm to 6.90mm, but the insertion conditions must be checked according to the minimum measured inner diameter. The coaxiality error between the mandrel and the lower mold positioning cavity is controlled within 0.03mm and does not exceed one-third of the first single-sided gap.

[0037] Precision support section length Take 24mm, half angle of the cone. A cone angle of 0.4° is used only to compensate for axial tolerances and to gradually release contact after the return stroke; it is not a primary anti-locking measure. (Length of the gradual pressure relief section) Take 10mm, half angle of the cone surface Take 5°; Length of the introductory segment Take 4mm, half angle of the cone surface Take a 15° angle, and use a rounded transition of R0.5mm to R1.0mm at the free end. Use tangent arcs at the joints of each section to avoid leaving step marks on the inner wall of the pipe due to abrupt changes in diameter.

[0038] The working surface of the mandrel is polished to a thickness Ra not exceeding 0.4 μm, and a uniform cold forging lubricating film is formed on the inner wall of the tube or the surface of the mandrel. When the effective coefficient of friction μ after lubrication is not greater than 0.08, the friction angle... The angle is approximately 4.57°. In this embodiment, the half-angle of the gradual pressure relief section is 5°, which is greater than this friction angle, thus reducing the possibility of conical self-locking in this section. If actual lubrication conditions increase μ, it should be increased accordingly. Alternatively, the contact length of the gradual pressure relief section can be shortened and a mechanical material unloading structure can be added; the fixed small cone angle should not be continued.

[0039] The radius of the transition fillet at the root of the mandrel is 1.2mm to 1.8mm, preferably 1.5mm. The ratio of the effective length to the diameter of the mandrel is checked against the minimum load-bearing diameter, and a combined check is performed for axial pressure and bending moment caused by assembly eccentricity. The mandrel can be manufactured using DC53 or Cr12MoV, and its hardness is controlled to HRC58 to HRC62 after heat treatment. To reduce the risk of brittle fracture under high hardness conditions, grinding marks or sharp corners must not be retained in the root transition zone.

[0040] The lower mold positioning cavity is matched with the original 18mm outer diameter tube and a straight wall retaining section of sufficient length is provided. The half angle of the final formed upset cone surface is 26°, which allows for fine adjustment within the range of 25.5° to 26.5° according to the elastic rebound direction after trial production.

[0041] Example 2: Two-pass forming mold configuration For a pipe with an initial outer diameter of 18mm and a final maximum outer diameter of 29mm at the cone tip, the ratio of the final outer diameter to the initial outer diameter is approximately 1.61. To reduce localized deformation concentration in a single pass, a pre-forming die and a final-forming die can be configured. Based on the principle of equal logarithmic diameter deformation, the maximum outer diameter of the pre-formed pipe is:

[0042] Therefore, the maximum cavity diameter of the preforming mold is initially selected to be 22.5mm to 23.5mm, and the final forming mold then forms a sealing conical surface with a maximum outer diameter of 29mm and a total included angle of 52°. Both passes use composite tapered mandrels. The preforming mandrel focuses on limiting the initial buckling of the inner hole, while the final forming mandrel focuses on controlling the final inner hole continuity and demolding. Between passes, the ends are checked for cracks, folds, and significant eccentricity. If defects are found, priority should be given to adjusting the deformation distribution, lubrication, and pipe softening state of each pass, rather than simply increasing the tonnage of the equipment.

[0043] Work process During work, according to Figure 1 The state shown is as follows: A fixed-length tube is inserted into the lower mold positioning cavity, allowing the free tube section to be formed to extend to the calculated position. The upper mold moves downwards; the mandrel guide section first enters the inner hole of the tube and corrects slight eccentricity, followed by the gradual pressure relief section and the precision support section entering their respective areas. The local support state after the mandrel insertion is as shown... Figure 2 As shown, as the upper die continues to descend, the end of the tube undergoes plastic flow under the combined constraint of the upsetting cone surface and the mandrel, forming... Figure 3 The sealing cone shape shown is such that the precision support section restricts the inner wall from freely buckling in the axial direction.

[0044] After forming is complete, the upper die returns. As the outer diameter of the mandrel continuously decreases from the root to the free end, as the mandrel moves upward, the original contact position transitions sequentially from the precision support section to the smaller diameter gradual pressure relief section and the guide section, gradually releasing the radial clamping force. Simultaneously, the straight wall retaining section of the lower die provides axial holding force to the formed part, keeping it in the lower die. After the mandrel is completely withdrawn, the formed part is removed by the equipment's ejection mechanism or manually.

[0045] This invention is not limited to single-pass or two-pass manufacturing, nor is it limited to the mandrel and upper die being manufactured as a single unit. The mandrel can be detachably connected to the upper die via end face positioning and threaded locking, but the coaxiality requirement must be met after assembly. For other pipe diameter specifications, the actual outer diameter, inner diameter, finished product volume, inner diameter tolerance, and friction coefficient should be recalculated, and the absolute dimensions of this embodiment should not be directly copied.

[0046] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A cold forging die for a sealing cone of a high-pressure oil pipe for a diesel engine, comprising an upper die (1) and a lower die (2) that cooperate with each other, wherein the lower die (2) is provided with a positioning cavity (6) for positioning a high-pressure oil pipe (4) and an upsetting cone surface (7) for forming the outer contour of the sealing cone, and a mandrel (3) is provided at the center of the upper die (1), wherein the mandrel (3) moves along the axial direction of the high-pressure oil pipe (4) with the upper die (1) and penetrates the inner hole of the pipe, characterized in that: The mandrel (3) includes, in sequence from the root to the free end, a precision support section (31), a gradual pressure relief section (32), and an inlet section (33). The outer diameters of the precision support section (31), the gradual pressure relief section (32), and the inlet section (33) decrease continuously; The cone half-angle of the precision support section (31) relative to the mandrel axis is: The half-angle of the tapered surface of the gradually reducing pressure relief section (32) relative to the mandrel axis is . The half-angle of the conical surface of the inlet segment (33) relative to the mandrel axis is . And satisfy ; The precision support section (31) forms a first single-sided gap with the original inner wall of the pipe, the single-sided gap between the gradual pressure relief section (32) and the original inner wall of the pipe is greater than the first single-sided gap, and the single-sided gap between the inlet section (33) and the original inner wall of the pipe is greater than the first single-sided gap. The precision support section (31) is used to limit the inward buckling of the inner wall of the cold heading deformation section. The gradual pressure relief section (32) is used to gradually release the radial clamping force formed by the springback of the tube when the upper die (1) returns. The guide section (33) is used for the coaxial introduction of the mandrel (3) and the inner hole of the tube.

2. The cold forging die for the sealing cone of a diesel engine high-pressure oil pipe according to claim 1, characterized in that: The cone half angle α1 of the precision support section (31) is 0.2° to 0.8°, the cone half angle α2 of the gradual pressure relief section (32) is 3° to 8°, and the cone half angle α3 of the inlet section (33) is 10° to 20°. The transition between adjacent sections is either a circular arc or a tangent curve.

3. The cold forging die for the sealing cone of a diesel engine high-pressure oil pipe according to claim 1, characterized in that: The first single-sided gap is denoted as The first single-sided gap satisfy: ； and ; in, Let e ​​be the range of inner diameter variation of the high-pressure oil pipe (4), and let e be the maximum eccentricity between the mandrel (3) and the positioning cavity (6). This is the allowance required for the lubricating film and the micro-undulations of the inner wall surface of the pipe. Additional allowance for loading errors and dimensional fluctuations during tube heat treatment. The allowable free deformation clearance to prevent buckling of the inner wall of the pipe; First single-sided gap The thickness ranges from 0.05 mm to 0.15 mm. The coaxiality error between the axis of the mandrel (3) and the axis of the positioning cavity (6) is no greater than 0.03 mm, and the coaxiality error is no greater than the first single-sided gap. one-third; When the original inner diameter of the high-pressure oil pipe (4) is 7 mm, the diameter of the precision support section (31) near the root is 6.70 mm to 6.90 mm.

4. The cold forging die for the sealing cone of a diesel engine high-pressure oil pipe according to claim 1, characterized in that: The theoretical length of the pipe segment involved in plastic deformation is denoted as The Determined by the following formula: ； in, The solid volume formed by the deformed pipe section within the finished sealing cone. The original cross-sectional area of ​​the high-pressure oil pipe (4) is given, and the original cross-sectional area is given. Determined by the following formula: ； in, The original outer diameter of the high-pressure oil pipe (4) is given. The original inner diameter of the high-pressure oil pipe (4); With the theoretical length Based on this, the length of the precision support section (31) It is 0.45 Up to 0.70 The length of the gradual pressure relief section (32) It is 0.15 Up to 0.30 The length of the import segment (33) It is 0.05 Up to 0.15 ; The effective length of the mandrel (3) entering the inner hole of the tube. satisfy ≥ +ΔL, where ΔL is the axial safety margin.

5. The cold forging die for the sealing cone of a diesel engine high-pressure oil pipe according to claim 1, characterized in that: A lubricating film is provided between the mandrel (3) and the high-pressure oil pipe (4), and the effective friction coefficient after lubrication is denoted as μ. The effective friction coefficient μ is related to the half angle of the cone surface of the gradual pressure relief section (32). satisfy: ； The axial holding force of the lower die (2) on the formed high-pressure oil pipe is denoted as: The axial demolding force of the mandrel (3) is denoted as The axial holding force With the axial demolding force satisfy: 。 6. The cold forging die for the sealing cone of a diesel engine high-pressure oil pipe according to claim 1, characterized in that: The root of the mandrel (3) connected to the upper mold (1) is provided with a transition fillet (5), the radius of which is 0.15 to 0.30 times the maximum diameter of the precision support section (31); The free length of the mandrel (3) during insertion and forming, which is not radially supported by the inner wall of the tube, is denoted as . The mandrel (3) is in relation to the free length The minimum bearing diameter at the corresponding location is denoted as And satisfy: 。 7. The cold forging die for the sealing cone of a diesel engine high-pressure oil pipe according to claim 1, characterized in that: The material of the mandrel (3) is Cr12MoV, DC53 or high-speed steel, and the hardness after heat treatment is HRC58 to HRC62. The surface roughness Ra of the precision support section (31) is no greater than 0.4 μm, and the surface roughness Ra of the gradual pressure relief section (32) is no greater than 0.4 μm; The surface of the mandrel (3) is provided with a cold forging lubricating film, which is used to reduce the effective friction coefficient between the mandrel (3) and the inner wall of the tube.

8. The cold forging die for the sealing cone of a diesel engine high-pressure oil pipe according to claim 1, characterized in that: The material of the mandrel (3) is Cr12MoV, DC53 or high-speed steel, and the hardness after heat treatment is HRC58 to HRC62. The surface roughness Ra of the precision support section (31) is no greater than 0.4 μm, and the surface roughness Ra of the gradual pressure relief section (32) is no greater than 0.4 μm; The surface of the mandrel (3) is provided with a cold heading lubricating film, which is used to reduce the effective coefficient of friction between the mandrel (3) and the inner wall of the tube.

9. The cold forging die for the sealing cone of a diesel engine high-pressure oil pipe according to claim 1, characterized in that: The upsetting cone surface (7) has a cone half angle of 25.5° to 26.5°, and the upsetting cone surface (7) makes the total included angle of the sealing cone head 51° to 53°; The positioning cavity (6) includes a straight wall retaining section, which is used to restrict the high-pressure oil pipe material after forming from moving upward with the mandrel (3).

10. A method for cold forging a diesel engine high-pressure oil pipe sealing cone using the mold described in any one of claims 1 to 9, characterized in that, Includes the following steps: Step 1: Loading and positioning; The fixed-length high-pressure oil pipe (4) is inserted into the positioning cavity (6) of the lower mold (2), so that the free pipe section to be formed extends to the preset position. The positioning cavity (6) forms a radial positioning constraint on the outer wall of the pipe. Step 2: Insert the mandrel; The upper mold (1) moves along the pipe axis towards the lower mold (2), and the free end of the mandrel (3) fixed at the center of the upper mold (1) is inserted into the inner hole of the pipe. During insertion, the guide section (33) of the mandrel (3) first enters the inner hole of the tube, and the cone half-angle of the guide section (33) Correct the eccentricity between the inner hole of the pipe and the mandrel (3); The upper mold (1) continues to descend, the gradual pressure relief section (32) then enters the inner hole of the pipe, and the precision support section (31) finally enters the inner hole of the pipe; After insertion, a first unilateral gap is formed between the precision support section (31) and the original inner wall of the pipe. The first single-sided gap satisfy: ； and ; in, Let e ​​be the range of inner diameter variation of the high-pressure oil pipe (4), and let e be the maximum eccentricity between the mandrel (3) and the positioning cavity (6). This is the allowance required for the lubricating film and the micro-undulations of the inner wall surface of the pipe. Additional allowance for loading errors and dimensional fluctuations during tube heat treatment. The allowable free deformation clearance to prevent buckling of the inner wall of the pipe; Step 3: Cold heading; The upper mold (1) continues to descend to the forming end point, and the end of the tube undergoes plastic flow under the joint constraint of the upsetting cone surface (7) of the lower mold (2) and the precision support section (31) of the mandrel (3) to form a sealed cone head; During the forming process, the precision support section (31) maintains the first single-sided gap. This restricts the free buckling of the inner wall of the pipe in the axial direction; The effective length of the mandrel (3) entering the inner hole of the tube. The theoretical length of the pipe section covering plastic deformation The theoretical length of the pipe section Determined by the following formula: ； ； in, The solid volume formed by the deformed pipe section within the finished sealing cone. The original cross-sectional area of ​​the high-pressure oil pipe (4) is given. The original outer diameter of the high-pressure oil pipe (4) is given. The original inner diameter of the high-pressure oil pipe (4); Step 4: Depressurization and return demolding; After the forming is completed, the upper mold (1) returns upward along the axis of the tube, driving the mandrel (3) to exit from the inner hole of the tube; During the return journey, the cone half-angle of the gradually reducing pressure relief section (32) The effective friction coefficient μ after lubrication satisfies: ； The radial clamping force between the mandrel (3) and the inner wall of the tube is gradually released along the gradual pressure relief section (32); At the same time, the positioning cavity (6) of the lower mold (2) applies an axial holding force to the formed tube. The axial demolding force on the mandrel (3) is denoted as And satisfy: ； The formed tube is kept in the lower mold (2) until the mandrel (3) is completely withdrawn; The precision support section (31), the gradual pressure relief section (32) and the inlet section (33) of the mandrel (3) are sequentially withdrawn from the inner hole of the tube. After the mandrel (3) is completely withdrawn, the forming part is taken out by the ejection mechanism.

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