Manufacturing method of integral Christmas tree four-way joint with forged reinforcing ribs

Abstract

The invention belongs to the technical field of machine manufacturing, and particularly relates to a manufacturing method of an integral Christmas tree four-way joint with forged reinforcing ribs, which comprises the following steps: designing a near-net blank of a preset reinforcing rib prototype based on topological optimization; preparing an initial blank from high-purity alloy steel; pre-upsetting, radial shunting upsetting-extruding, axial reverse extruding forming and finish forging shaping are sequentially carried out through a multi-station die forging press, and a three-dimensional net-shaped variable cross-section reinforcing rib integrated with the body is generated in situ under the guidance of a convex mold core of the mold; and carrying out controlled cooling and heating treatment to obtain a complex phase structure mainly comprising tempered sorbite. According to the technical scheme, the overall structure without welding seams and casting defects is achieved, and the strength, rigidity and corrosion resistance of the intersection area of the four-way joint are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of mechanical manufacturing technology, specifically a method for manufacturing an integral oil well trunk with forged reinforcing ribs. Background Technology

[0002] In the field of oil and gas extraction, the wellhead cross-connector is a core pressure-bearing component of the wellhead control system, responsible for the convergence of multi-directional fluid channels. Its performance directly determines the safe operation and service life of oil and gas wells. With the advancement of deep wells, ultra-deep wells, and high-sulfur oil and gas fields, the risk of failure of traditional cross-connectors under extreme operating conditions has increased significantly, necessitating optimized design and manufacturing methods to improve mechanical performance and service reliability.

[0003] The current mainstream manufacturing path for the four-way joint of the oil wellhead is casting or separate forging followed by welding: The casting method has the advantages of one-time molding and adaptability to complex structures, but it is prone to metallurgical defects such as shrinkage cavities and porosity, and the material density is insufficient, making it difficult to meet the stringent requirements of API 6A and other standards for non-destructive testing and fatigue life of key pressure-bearing components; The separate forging and welding method improves the material properties by forging the main pipe section and the branch pipe section separately and then fully penetrating and welding them, but it inevitably introduces defects such as residual stress concentration and uneven microstructure at the weld joint, which are prone to stress corrosion cracking in hydrogen sulfide-containing corrosive environments, becoming a weak link in the structure.

[0004] The core contradiction of existing technologies lies in the inability to balance "overall structural continuity" and "local load-bearing reinforcement": casting, while providing overall integrity, sacrifices material uniformity; welding, while improving matrix performance, disrupts structural continuity. In high-pressure scenarios (e.g., 15,000 psi and above), conventional thickening or external reinforcing plates result in bulky structures, increased costs, and new stress concentrations. Furthermore, existing processes cannot simultaneously construct internal reinforcement structures during forming; subsequent machining adds costs and disrupts metal flow lines, weakening mechanical properties. Therefore, achieving directional reinforcement of key areas while ensuring a seamless, metallurgical-free overall structure has become a critical bottleneck restricting performance improvement, necessitating an integrated thermoforming method that can simultaneously form the external contour and internal reinforcement structure.

[0005] Therefore, the present invention provides a method for manufacturing an integral wellhead trunk with forged reinforcing ribs. Summary of the Invention

[0006] This invention provides a manufacturing method for an integral wellhead tee with forged reinforcing ribs, aiming to solve the technical problems of insufficient mechanical properties and reduced service reliability of existing cast or separately welded tee sections under extreme operating conditions due to structural discontinuities, metallurgical defects, or weak weld areas. To achieve the above-mentioned objective, this invention constructs a fully forged forming process system integrating preformed blank design, multi-directional composite upsetting, in-situ generation of internal reinforcing ribs, and thermo-mechanical coupling shape and property control. This system enables the tee body and internal reinforcing ribs to achieve complete integrated metallurgical bonding in a single hot forming process. This ensures that the entire structure is free of weld seams and casting defects, while directionally constructing a three-dimensional mesh reinforcing rib structure consistent with the principal stress path in the intersection area of ​​the branch and main pipes.

[0007] The manufacturing method of this invention includes the following determination steps: First, based on the final geometric contour and load distribution characteristics of the target four-way connector, the spatial layout, cross-sectional shape, and fusion transition relationship between the internal reinforcing ribs and the walls of the main and branch pipes are determined using the finite element inverse topology optimization method, and a near-net-shape three-dimensional model of the blank with the pre-set reinforcing rib prototype is generated accordingly; Second, based on this model, a cylindrical initial blank made of high-purity alloy steel is prepared, the chemical composition of which meets API 6A. For PSL3 and higher grade materials used in pressure-bearing components, a dual refining process involving vacuum melting and electroslag remelting is employed to ensure an oxygen content below 20 parts per million and a sulfur content below 5 parts per million. Subsequently, the initial billet is heated to the austenitic single-phase temperature range and held at that temperature for homogenization to obtain a uniform and fine equiaxed grain structure. Next, the homogenized billet is placed in a multi-station die forging press and undergoes four consecutive hot forming processes: pre-upsetting, radial splitting upsetting, axial reverse extrusion forming, and final forging shaping. During this process, the die cavity is equipped with protrusions precisely matching the spatial trajectory of the reinforcing ribs. The core array applies local constraints and guidance to the billet during metal flow, forcing the metal to fill the reinforcing rib grooves along a preset flow direction, thereby generating a reinforcing rib structure that is completely integrated with the body on the inner wall of the four-way cavity. Finally, the formed forging is subjected to controlled cooling heat treatment, including first passing through the pearlite transformation zone at an air cooling rate, then passing through the bainite transformation nose temperature zone by forced air cooling, and finally completing isothermal residence above the martensite initiation transformation temperature to obtain a multiphase structure dominated by tempered sorbite with dispersed carbides, ensuring that the material has both high strength and good toughness.

[0008] Furthermore, the construction process of the near-net-shape blank three-dimensional model of the pre-formed reinforcing rib prototype specifically includes: based on the real load spectrum of the four-way connector under a working pressure of 15000 psi, a multi-physics field coupled simulation model containing the axial, radial, and circumferential stress components of the main pipe, the branch pipe, and the circumferential stress components is established; in this model, a topology optimization objective function driven by the von Mises equivalent stress gradient is defined, and volume constraints are applied so that the optimization results retain material only in areas where the stress concentration coefficient is higher than the preset safety threshold, while ensuring the structural stiffness; the optimized configuration generated thereby is the ideal spatial distribution form of the internal reinforcing rib, which is a three-dimensional mesh structure that extends along the principal tensile stress direction of the junction area of ​​the main pipe and the branch pipe, and forms an arc-shaped transition connection at the junction corner to avoid secondary stress concentration caused by sharp corners; after geometric smoothing, this configuration is embedded in the outer contour of the initial cylindrical blank to form a near-net-shape blank model with internal cavity pre-formation characteristics.

[0009] In a preferred embodiment of the present invention, the multi-station forging press is equipped with four independently servo-controlled punch systems, corresponding to the upper die main punch, the lower die anti-ejection punch, the left radial punch, and the right radial punch, respectively. In the pre-upsetting process, the upper die main punch and the lower die anti-ejection punch descend synchronously, compressing the initial billet to a height reduction of 30% to 40%, while simultaneously promoting preliminary grain refinement. In the radial splitting upsetting process, the left and right radial punches advance towards the center at the same rate, forcing the metal to split horizontally into the left and right branch tube pre-forming cavities. At the same time, the upper die main punch maintains a constant back pressure. To prevent excessive metal surge, in the axial reverse extrusion forming process, the lower die reverse punch pushes the bottom of the billet upward, causing the metal to flow in the opposite direction along the main pipe axis and fill the upper section cavity of the main pipe, while the left and right radial punches maintain their locked positions to ensure the stability of the branch pipe profile; in the final forging and shaping process, the four sets of punches work together to close the mold cavity in a micro-incremental manner, so that the metal completely fits all the detailed features, including the reinforcing rib groove. At the same time, the punch speed is adjusted in real time through the temperature sensor and pressure feedback unit built into the mold to ensure that the billet temperature is always maintained above the recrystallization temperature but below the grain coarsening critical temperature during the forming process.

[0010] Furthermore, the array of raised cores within the mold cavity is made of heat-resistant alloy, and its surface is treated with laser-coated tungsten carbide to improve wear resistance and anti-adhesion performance. The core array is embedded in the corresponding positions of the upper mold and side mold through a dovetail groove structure and is provided with cooling channels for forced heat dissipation by introducing compressed air during the forming interval, preventing the core from deforming due to thermal fatigue. The cross-sectional profile of each core is strictly consistent with the cross-section of the reinforcing rib it forms, and has a slight taper along the length direction to facilitate demolding. A gap of 0.5 mm to 1.5 mm is reserved between the end of the core and the inner wall of the main pipe or branch pipe. This gap is completely filled by metal in the final forging stage, thereby achieving seamless fusion between the reinforcing rib and the body wall.

[0011] As another key feature of the present invention, the reinforcing rib structure is not a simple protrusion, but has a variable cross-section characteristic: the cross-sectional area of ​​the reinforcing rib is the largest in the core area where the main pipe and the branch pipe meet, and gradually decreases in the direction away from the meeting area until it smoothly transitions with the inner wall of the main pipe or the branch pipe; this variable cross-section shape is achieved by the contour gradient design of the mold core, and strictly corresponds to the aforementioned topology optimization results; the arrangement density of the reinforcing ribs is the highest in the corner area of ​​the meeting, with one rib set every ten millimeters along the axial direction of the branch pipe, while it is sparse to one rib every thirty millimeters in the straight section area of ​​the main pipe, forming a gradient reinforcement distribution.

[0012] Furthermore, the isothermal dwell stage in the controlled cooling heat treatment process is carried out in a dedicated isothermal quenching furnace. The atmosphere inside the furnace is a nitrogen-hydrogen mixed protective gas, and the dew point is controlled below -40 degrees Celsius to prevent surface oxidation. The isothermal temperature is set in the middle range between the bainite transformation completion temperature and the martensite initiation temperature, and the dwell time is sufficient to allow the austenite to fully transform into a mixed structure of lower bainite and retained austenite. Subsequently, a high-temperature tempering treatment is performed, with the tempering temperature located within 50 degrees Celsius below the Ac1 critical point. The holding time is calculated based on the effective thickness of the forging at one hour per 25 mm to fully precipitate dispersed carbides and eliminate internal stress.

[0013] As another technical feature of the present invention, the formed forging must undergo dual non-destructive testing of ultrasonic flaw detection and magnetic particle testing before machining to ensure that there are no internal cracks, folds or incomplete filling defects; the machining process adopts a five-axis linkage CNC machine tool, and the tool path planning avoids the root area of ​​the reinforcing rib, and only the flange sealing surface, threaded hole and inner surface of the flow channel are precision machined to maximize the preservation of the forging flow line and the structural integrity of the reinforcing rib; the surface roughness of the inner surface of the flow channel is controlled below Ra three points two micrometers to reduce the risk of fluid erosion and corrosion.

[0014] Furthermore, the generation mechanism of the internal reinforcing ribs relies on the anisotropic flow behavior of metal in a high-temperature plastic state: during the radial splitting upsetting stage, the metal is squeezed by the lateral punch and flows upward along the side wall of the mold cavity. At this time, the protruding core applies local resistance to the flow front, forcing some of the metal to turn and fill the reinforcing rib groove below the core; during the axial back extrusion stage, the bottom metal pushes upward to further compact the root of the initially formed reinforcing rib and eliminate potential voids; the entire forming process ensures that the height tolerance of the reinforcing rib does not exceed ±0.3 mm through precise control of the mold closing height.

[0015] As a system-level collaborative feature of the present invention, the multi-station die forging press communicates with the central control system in real time via industrial Ethernet. The central control system has an embedded forming process monitoring module based on digital twin technology. This module receives real-time data from thermocouples, strain gauges and displacement sensors built into the die, dynamically compares it with the preset process window, and immediately triggers the servo system to adjust the punch speed or pause forming once the billet temperature deviates from the allowable range or the punch load is abnormal, to prevent the generation of scrap. At the same time, this module records the complete process parameter chain of each product to form a traceable quality file.

[0016] Furthermore, the reinforcing rib structure performs multiple functions during service: firstly, as a local stiffness enhancer, it effectively suppresses the radial expansion deformation of the branch pipe under internal pressure; secondly, as a stress transfer bridge, it disperses the concentrated stress at the root of the branch pipe to a larger area of ​​the main pipe along the direction of the reinforcing rib; thirdly, as a support rib of the inner wall of the flow channel, it resists local buckling instability when encountering water hammer or transient pressure fluctuations; fourthly, the continuous metal flow lines formed on its surface are completely consistent with the body of the four-way connector, without any interface interruption, thereby avoiding galvanic corrosion or crevice corrosion that may be caused by traditional welded reinforcing plates.

[0017] As a feature of the material-structure-process integration of the present invention, the chemical composition of the high-purity alloy steel contains trace amounts of niobium, vanadium, and titanium composite additives, the total amount of which is controlled between 0.1% and 0.3%. These additives are used to precipitate fine carbonitride particles during hot forming, pinning grain boundaries and inhibiting austenite grain growth. At the same time, during the controlled cooling stage, these microalloying elements promote bainitic phase transformation, refine the final microstructure, and improve the fracture toughness and resistance to sulfide stress corrosion cracking of the material.

[0018] Furthermore, the mold cavity design takes into account the compensation mechanism for metal cooling and shrinkage: in the area where the main pipe and branch pipe intersect, the cavity size is enlarged by 0.8% to 1.2% in the opposite direction according to the coefficient of thermal expansion, while in the straight section area far from the intersection area, it is designed according to the conventional shrinkage rate, thereby ensuring that the reinforcing ribs still maintain the design height in the key area after forming, and avoiding the reduction of the strengthening effect due to uneven shrinkage.

[0019] As a closed-loop control feature of the manufacturing process of the present invention, the diameter-to-height ratio of the initial blank is strictly calculated so that it exactly fills the projected area of ​​the mold preforming cavity after pre-upsetting, which avoids metal overflow and flash formation and also prevents insufficient filling. This ratio is determined by the high-temperature flow stress curve of the material and the volume of the mold cavity, and is solidified into process parameters after trial molding verification.

[0020] The beneficial effects of this invention are as follows:

[0021] This invention discloses a method for manufacturing an integral well trunk with forged reinforcing ribs. Through a single hot forming process, a three-dimensional mesh-like reinforcing rib structure is generated in situ within the trunk cavity, completely integrated with the main body. This avoids residual stress and structural discontinuities introduced by welded joints. A multi-directional composite upsetting mechanism is utilized to ensure continuous distribution of metal streamlines along the reinforcing rib direction, significantly improving fatigue resistance in the intersection area. A gradient design of the reinforcing rib cross-section and arrangement density is achieved, highly matching the actual stress field, thereby enhancing local load-bearing capacity without increasing external dimensions. The method ensures material purity and structural uniformity throughout the entire process from billet preparation to final heat treatment, meeting corrosion resistance requirements in high-sulfur environments. Furthermore, closed-loop control of the forming process driven by digital twins guarantees structural consistency and process repeatability for each product. Attached Figure Description

[0022] The present invention will be further described below with reference to the accompanying drawings;

[0023] Figure 1 This is a flowchart of a method for manufacturing an integral oil well trunk with forged reinforcing ribs according to the present invention;

[0024] Figure 2 This is a schematic diagram of the process flow of the multi-station die forging forming process in the manufacturing method of the present invention. Detailed Implementation

[0025] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0026] like Figure 1 - Figure 2As shown in the embodiment of the present invention, a method for manufacturing an integral wellhead trunk with forged reinforcing ribs includes establishing a multiphysics coupled finite element simulation model containing three-dimensional stress components (axial, radial, and circumferential) of the main pipe, branch pipe, and branch pipe, based on the final geometric profile of the target trunk product and its actual load spectrum under a working pressure of 15,000 psi (approximately 103.4 MPa). The model uses typical wellhead equipment operating conditions specified in the API 6A standard as boundary conditions, applying internal pressure, temperature gradient, and external constraint loads. Based on this, a topology optimization objective function driven by the von Mises equivalent stress gradient is defined, and a volume fraction upper limit constraint (usually set to 65% to 75% of the original solid volume) is introduced, ensuring that the optimization algorithm retains material only in regions where the stress concentration factor is higher than a preset safety threshold (e.g., 2.8). After iterative calculation, the generated optimized configuration presents a three-dimensional mesh structure extending along the principal tensile stress direction of the junction area between the main pipe and branch pipe, forming a smooth arc transition connection at the junction corner to avoid secondary stress concentration caused by geometric abrupt changes. After NURBS surface fitting and geometric smoothing, this configuration is embedded into the outer contour of the initial cylindrical blank, forming a near-net-shape three-dimensional model of the blank with pre-formed internal cavities. This model accurately reflects the spatial trajectory, cross-sectional shape, and fusion transition relationship between the reinforcing ribs and the walls of the main and branch pipes.

[0027] Based on the aforementioned near-net-shape blank model, a cylindrical initial blank made of high-purity alloy steel was prepared. The chemical composition of this alloy steel meets the requirements of API 6A PSL3 and above for pressure-bearing components, specifically: carbon (C) 0.28%–0.35%, manganese (Mn) 0.80%–1.10%, silicon (Si) 0.15%–0.35%, chromium (Cr) 0.80%–1.20%, molybdenum (Mo) 0.15%–0.25%, nickel (Ni) 0.40%–0.70%, sulfur (S) ≤0.005%, phosphorus (P) ≤0.010%, oxygen (O) ≤20 ppm, and hydrogen (H) ≤2 ppm. In addition, trace amounts of niobium (Nb), vanadium (V), and titanium (Ti) composite microalloying elements are added, with the total amount controlled between 0.10% and 0.30%, to precipitate fine carbonitride particles during hot forming, pinning austenite grain boundaries and inhibiting grain growth. The initial billet is prepared by a dual refining process of vacuum induction melting (VIM) and electroslag remelting (ESR) to ensure extremely low and uniform inclusion content. The billet diameter-to-height ratio is rigorously calculated to ensure that it precisely fills the projected area of ​​the mold preforming cavity after the subsequent pre-upsetting process. This ratio is determined by the high-temperature flow stress curve of the material at 1150°C (the measured peak flow stress is approximately 85 MPa) and the mold cavity volume, and is solidified into process parameters after trial molding verification. In a specific embodiment, the target four-way nominal diameter is DN100 (4 inches), the initial billet diameter is Φ220 mm, the height is 380 mm, and the volume is 14.42 × 10⁻⁶ mm. 6 mm³.

[0028] Subsequently, the initial billet was placed in a walking beam furnace and heated to the austenitic single-phase temperature range, specifically 1150℃±10℃, and held at that temperature for 2.5 hours to achieve a uniform and fine equiaxed austenitic grain structure, with an average grain size reaching ASTM grade 7 or finer. During the heating process, the furnace atmosphere was a nitrogen-based protective gas, and the dew point was controlled below -40℃ to prevent surface decarburization and oxidation.

[0029] After homogenization, the billet is rapidly transferred to a multi-station die forging press equipped with four independent servo-controlled punch systems. This press has a nominal pressure of 25 MN and real-time closed-loop control capability. The forming process involves four consecutive hot forming steps: pre-upsetting, radial splitting upsetting, axial reverse extrusion forming, and final forging shaping.

[0030] In the pre-upsetting process, the upper die main punch and the lower die counter-upsetting punch move downwards synchronously, compressing the initial billet to reduce its height by 35%, from 380 mm to 247 mm, while simultaneously promoting the initial crushing and refinement of the grains. During this stage, the billet temperature is maintained above 1120℃ to ensure that dynamic recrystallization occurs fully.

[0031] In the radial splitting upsetting process, the left and right radial punches advance towards the center at the same rate (set to 80 mm / s), forcing the metal to split horizontally into the preforming cavities of the left and right branch tubes. Simultaneously, the upper die main punch applies a constant back pressure (approximately 30% of the main punch's maximum load, i.e., 7.5 MN) to prevent excessive upward flow of metal, which could lead to insufficient filling at the top of the main tube. At this stage, a raised core array within the die cavity begins to exert localized constraint on the flowing metal. This core array is made of Inconel 718 heat-resistant alloy, with a 0.3 mm thick tungsten carbide (WC-Co) coating laser-clad on its surface, achieving a microhardness of 1600 HV. The cores are embedded in the corresponding positions of the upper and side dies via dovetail groove structures and incorporate a Φ6 mm diameter cooling channel. Compressed air at 0.6 MPa is introduced during the forming interval for forced heat dissipation, preventing thermal fatigue deformation. The cross-sectional profile of each core is strictly consistent with the cross-section of the reinforcing rib it forms, and it has a 0.5° micro-taper along its length to facilitate demolding. A 1.0 mm gap is reserved between the end of the core and the inner wall of the main or branch pipe. This gap is completely filled with metal during the final forging stage to achieve seamless fusion between the reinforcing rib and the body wall.

[0032] In the axial reverse extrusion forming process, the lower die's reverse ejector punch pushes the bottom of the billet upwards at a speed of 60 mm / s, causing the metal to flow in the reverse direction along the main pipe axis and fill the upper section cavity of the main pipe. At this time, the left and right radial punches maintain their locked positions to ensure that the already formed branch pipe profile remains stable. During this stage, the bottom metal pushes upwards to further compact the root area of ​​the preliminarily formed reinforcing ribs, eliminating potential voids or incomplete filling defects. Throughout the forming process, the billet temperature is monitored in real time using an infrared thermometer to ensure that it is not lower than 1050℃ (i.e., above the recrystallization temperature but below the grain coarsening critical temperature of 1180℃).

[0033] In the final forging and shaping process, four sets of punches work in concert, gradually closing the die cavity in incremental increments (0.5 mm per step), ensuring the metal perfectly conforms to all details, including the reinforcing rib grooves. The die's built-in temperature sensor and pressure feedback unit transmit real-time data to the central control system, which dynamically adjusts the punch speed based on a digital twin model. For example, if an abnormal increase in metal flow resistance is detected in a certain area, the system automatically reduces the punch speed in that direction by 5%–10% to prevent localized overload cracking. After final forging, the height tolerance of the reinforcing ribs is controlled within ±0.3 mm.

[0034] The forgings, once formed, immediately enter the controlled cooling heat treatment process. First, they are cooled naturally at a rate of approximately 5°C / s through the pearlite transformation zone (approximately 700°C–550°C) to prevent proeutectoid ferrite precipitation along grain boundaries. Then, they are rapidly cooled by forced air (wind speed 15 m / s, cooling rate approximately 20°C / s) through the bainite transformation nose temperature zone (approximately 550°C–400°C) to suppress pearlite formation. Finally, they undergo isothermal treatment in an isothermal quenching furnace at 380°C. The furnace atmosphere is a 95% N₂ + 5% H₂ mixed protective gas with a dew point ≤ -40°C. The isothermal treatment time is 90 minutes, sufficient to allow the retained austenite to fully transform into a mixed microstructure of lower bainite and a small amount of stable retained austenite. The workpiece was then subjected to high-temperature tempering at a temperature of 620℃ (107℃ below the Ac1 critical point of 727℃) for 5.5 hours (calculated based on an effective forging thickness of 138 mm, with 1 hour of holding time per 25 mm of thickness). The resulting microstructure was predominantly tempered sorbite, with dispersed Nb(C,N), V4C3, and TiC carbide particles, and an average grain size of 8.5 μm.

[0035] Before machining, heat-treated forgings undergo dual non-destructive testing (NDT) using ultrasonic testing (UT) and magnetic particle testing (MT). Ultrasonic testing employs a 5 MHz dual-crystal probe, with sensitivity calibrated to the equivalent of a Φ2 mm flat-bottom hole. Magnetic particle testing is performed according to ASTM E1444 standards. After passing inspection, finishing is performed on a five-axis CNC machine tool. Toolpath planning avoids the area around the reinforcing rib roots, cutting only the flange sealing surface, threaded holes, and the inner surface of the flow channel. The inner surface of the flow channel is precision-bored using a diamond boring tool, with a surface roughness controlled below Ra 3.2 μm to reduce the risk of corrosion from high-speed, sand-containing fluids.

[0036] The internal reinforcing rib structure exhibits variable cross-sectional characteristics: in the core area where the main pipe and branch pipe intersect (defined as within 20 mm extending outward from the intersection corner), the reinforcing rib cross-sectional area is the largest, typically 120 mm²; along the branch pipe axis away from the intersection area, the cross-sectional area decreases linearly, reducing to 30 mm² at the branch pipe end; in the straight section of the main pipe, the cross-sectional area stabilizes at 40 mm². This variable cross-sectional shape is achieved through a gradual contour design of the mold core, strictly corresponding to the aforementioned topology optimization results. The reinforcing rib density is gradient-distributed: in the intersection corner area, a reinforcing rib is placed every 10 mm along the branch pipe axis; in the middle area of ​​the branch pipe, the spacing increases to 20 mm; and in the straight section of the main pipe, the spacing is 30 mm. This gradient reinforcement design ensures that the material distribution highly matches the actual stress field, significantly improving the local load-bearing capacity without increasing the external contour dimensions.

[0037] The mold cavity design also considers a compensation mechanism for metal cooling and shrinkage. In the area where the main pipe and branch pipes intersect, the cavity dimensions are enlarged by 1.0% in the reverse direction of the coefficient of thermal expansion (i.e., an enlargement factor of 1.010), while in the straight sections far from the intersection, the design follows a conventional linear shrinkage rate of 1.8% (i.e., an enlargement factor of 1.018). This differentiated compensation strategy ensures that the reinforcing ribs maintain their designed height in critical high-stress areas after molding, avoiding a decrease in reinforcement effect due to uneven shrinkage.

[0038] To verify the actual effect of the technical solution of the present invention, the following embodiments and comparative experiments were conducted.

[0039] Example 1: A DN100 integral four-way valve was manufactured using the complete process described above. The material was 30CrMoNiV alloy steel with a total microalloying element content of 0.22%. The reinforcing ribs were arranged according to the topology optimization results, with a density of 10 mm / rib in the intersection area and a cross-sectional area of ​​120 mm². The controlled cooling heat treatment parameters were as described previously.

[0040] Example 2: Except for adjusting the density of the reinforcing ribs to 15 mm / rib in the intersection area and keeping the cross-sectional area constant at 80 mm², the rest of the process is the same as in Example 1.

[0041] Example 3: The same materials and heat treatment process are used, but the reinforcing ribs are achieved by welding reinforcing plates. The reinforcing plates are made of forgings with the same composition and are treated with PWHT after welding.

[0042] Comparative Example 1: A four-way valve of the same specifications was manufactured using traditional casting technology. The material was ASTM A216 WCB cast steel, without internal reinforcing ribs.

[0043] Comparative Example 2: A split-welded structure is adopted, with the main pipe and branch pipes forged separately and then welded together. The weld is located at the root of the branch pipe without any reinforcement measures.

[0044] Comparative Example 3: The entire forging process was used, but without internal reinforcing ribs. The rest of the process was the same as in Example 1.

[0045] All specimens were tested for performance according to API 6A PR2 requirements, including room temperature tensile strength, -20°C Charpy impact test, 15000 psi pressure sealing test, and 10 5 Secondary pressure cyclic fatigue test. The test results are shown in the table below:

[0046] Sample number Yield strength (MPa) Tensile strength (MPa) Impact energy at -20℃ (J) Fatigue life (times) Pressure holding leakage rate (cm3 / min) Example 1 895 1020 112 >1.2×105 <0.1 Example 2 870 995 108 9.5×104 <0.1 Example 3 840 960 85 6.8×104 0.3 Comparative Example 1 380 620 42 3.2×104 1.8 Comparative Example 2 410 (Weld Zone) 590 (Weld Zone) 38 (Weld Zone) 2.5×104 2.5 Comparative Example 3 860 980 105 5.0×104 <0.1

[0047] As the data shows, Example 1 significantly outperforms the comparative example in all performance indicators, especially in fatigue life, where it is improved by more than 140%. Although Example 3 uses the same material, the presence of a welded interface significantly reduces its impact toughness and fatigue performance. Although Comparative Example 3 is integrally forged, it lacks directional reinforcing ribs, and its fatigue life is only 41.7% of that of Example 1.

[0048] Further metallographic analysis of the forging from Example 1 revealed that the metal flow lines in the reinforcing rib region were continuously distributed along the rib direction without any interruption or folding. EBSD (electron backscattering diffraction) results showed that the orientation difference angle at the interface between the reinforcing rib and the body was less than 5°, confirming a completely integral metallurgical bond. X-ray diffraction (XRD) analysis indicated that the final microstructure contained approximately 65% ​​lower bainite, approximately 12% retained austenite, approximately 23% tempered sorbite, and a carbide dispersion of 8.2 × 10⁻⁶. 4 particles / mm³.

[0049] In service simulation testing, the four-way connector of Example 1 underwent a 720-hour SSC (sulfide stress corrosion cracking) test in an acidic environment containing 15% H2S, 3% CO2, and 82% CH4, according to NACE TM0177 Method A, with a loading stress of 80% SMYS (specified minimum yield strength), and no cracks appeared. In contrast, the weld area of ​​Comparative Example 2 cracked within 240 hours.

[0050]

[0051] in, This represents the von Mises equivalent stress (unit: MPa). , , These are the three principal stress components of the principal stress tensor (unit: MPa). This formula is used in topology optimization to construct the objective function, identifying high-stress regions as the basis for stiffener placement.

[0052]

[0053] in, Indicates isothermal residence time (unit: minutes). The effective thickness of the forging (unit: mm). This is an empirical coefficient, with a value of 2.2 min / mm. This relationship is used to determine the holding time during the isothermal stage of controlled cooling heat treatment to ensure sufficient microstructure transformation.

[0054]

[0055] in, Indicates the cooling rate (unit: ℃ / s). The current temperature (in °C) is defined by this piecewise function, which defines cooling strategies for different temperature ranges in controlled-cooling heat treatment to suppress unfavorable phase transformations and promote lower bainite formation.

[0056] In summary, this invention achieves high reliability of the integral wellhead four-way joint under extreme operating conditions through integrated material-structure-process design. Its core lies in utilizing a multi-directional composite upsetting forming mechanism to generate gradient three-dimensional mesh reinforcing ribs that are completely integrated with the main body in situ during a single hot forming process. Combined with high-purity microalloyed steel and precise controlled cooling heat treatment, the final product is an integral forging that possesses high strength, high toughness, excellent corrosion resistance, and outstanding fatigue performance.

[0057] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for manufacturing an integral oil well trunk with forged reinforcing ribs, characterized in that, Includes the following steps: Based on the load distribution characteristics of the target four-way under the set working pressure, the spatial layout, cross-sectional shape and the fusion transition relationship between the internal reinforcing ribs and the walls of the main pipe and branch pipes are determined by the finite element inverse topology optimization method, and a near-net-shape three-dimensional model of the blank with the pre-set reinforcing rib prototype is generated. Based on the model, a cylindrical initial billet made of high-purity alloy steel was prepared. The alloy steel underwent dual refining treatment of vacuum melting and electroslag remelting, with an oxygen content of less than 20 ppm and a sulfur content of less than 5 ppm. The initial billet was heated to the temperature range of the austenitic single-phase region and held at that temperature for homogenization to obtain a uniform and fine equiaxed grain structure. After homogenization, the billet is placed in a multi-station die forging press and undergoes four consecutive hot forming processes: pre-upsetting, radial splitting upsetting, axial reverse extrusion forming, and final forging shaping. During this process, the die cavity is equipped with a raised core array that precisely matches the spatial trajectory of the reinforcing rib. This core array applies local constraint and guidance to the billet during the metal flow, forcing the metal to fill the reinforcing rib groove along the preset flow line direction, thereby generating a reinforcing rib structure that is completely integrated with the body in situ on the inner wall of the four-way cavity. The formed forgings are subjected to controlled cooling heat treatment, including first passing through the pearlite transformation zone at an air cooling rate, then passing through the bainite transformation nose temperature zone by forced air cooling, and finally completing isothermal residence at a temperature higher than the martensite initiation transformation temperature, in order to obtain a multiphase structure mainly composed of tempered sorbite with dispersed carbides.

2. The manufacturing method of an integral wellhead four-way valve with forged reinforcing ribs according to claim 1, characterized in that, The construction process of the near-net-shape blank 3D model includes: based on the real load spectrum of the four-way connector under a working pressure of 15000 psi, establishing a multi-physics field coupled simulation model containing the axial, radial, and circumferential stress components of the main pipe; defining a topology optimization objective function driven by the von Mises equivalent stress gradient in the model, and applying volume constraints so that the optimization results retain material only in areas where the stress concentration factor is higher than a preset safety threshold; the resulting optimized configuration is a three-dimensional mesh structure that extends along the principal tensile stress direction of the junction area of ​​the main pipe and the branch pipe, and forms an arc-shaped transition connection at the junction corner; after geometric smoothing, the configuration is embedded in the outer contour of the initial cylindrical blank to form a near-net-shape blank model with internal cavity pre-formation features.

3. The manufacturing method of an integral wellhead four-way valve with forged reinforcing ribs according to claim 1, characterized in that, The multi-station forging press is equipped with four independently servo-controlled punch systems, corresponding to the upper die main punch, the lower die anti-top punch, the left radial punch, and the right radial punch, respectively. In the pre-upsetting process, the upper die main punch and the lower die anti-top punch move down synchronously, compressing the initial billet to a height reduction of 30% to 40%. In the radial splitting upsetting process, the left and right radial punches advance towards the center at the same rate, forcing the metal to split horizontally and enter the pre-forming cavities of the left and right branch pipes, while the upper die main punch maintains a constant back pressure. In the axial reverse extrusion forming process, the lower die anti-top punch pushes the bottom of the billet upward, causing the metal to flow in the reverse direction along the main pipe axis and fill the upper section cavity of the main pipe, while the left and right radial punches maintain their locked positions. In the final forging and shaping process, the four punches work together to close the die cavity in a micro-incremental manner, ensuring that the metal completely conforms to all detailed features, including the reinforcing rib grooves.

4. The manufacturing method of an integral wellhead trunk with forged reinforcing ribs according to claim 1, characterized in that, The array of raised cores inside the mold cavity is made of heat-resistant alloy and its surface is treated with laser cladding tungsten carbide coating. The core array is embedded in the corresponding positions of the upper mold and the side mold through a dovetail groove structure, and is provided with cooling channels for forced heat dissipation by introducing compressed air during the forming interval. The cross-sectional profile of each core is strictly consistent with the cross-section of the reinforcing rib it forms, and has a slight taper along the length direction. A gap of 0.5 mm to 1.5 mm is reserved between the end of the core and the inner wall of the main pipe or branch pipe. This gap is completely filled by metal in the final forging stage to achieve seamless fusion between the reinforcing rib and the body wall.

5. The manufacturing method of an integral wellhead trunk with forged reinforcing ribs according to claim 1, characterized in that, The reinforcing rib structure has variable cross-sectional characteristics: the cross-sectional area of ​​the reinforcing rib is the largest in the core area where the main pipe and branch pipe meet, and gradually decreases in the direction away from the meeting area until it smoothly transitions with the inner wall of the main pipe or branch pipe; the arrangement density of the reinforcing rib is the highest in the corner area of ​​the meeting, with one rib every 10 mm along the axial direction of the branch pipe, while it is sparse to one rib every 30 mm in the straight section area of ​​the main pipe, forming a gradient reinforcement distribution.

6. The manufacturing method of an integral wellhead four-way valve with forged reinforcing ribs according to claim 1, characterized in that, The isothermal residence stage in the controlled cooling heat treatment process is carried out in a dedicated isothermal quenching furnace. The furnace atmosphere is a nitrogen-hydrogen mixed protective gas, and the dew point is controlled below -40℃. The isothermal temperature is set in the intermediate range between the bainite transformation completion temperature and the martensite transformation initiation temperature. The residence time is based on the effective thickness of the forging according to an empirical formula. Determined, where k is 2.2 min / mm. The effective thickness of the forging is then determined; subsequently, a high-temperature tempering treatment is performed, with the tempering temperature within 50°C below the Ac1 critical point, and the holding time is calculated based on the effective thickness of the forging at one hour per 25 mm.

7. A method for manufacturing an integral wellhead trunk with forged reinforcing ribs according to claim 1, characterized in that, The high-purity alloy steel contains trace amounts of niobium, vanadium, and titanium composite additives in its chemical composition, with the total amount controlled between 0.10% and 0.30%. These additives are used to precipitate fine carbonitride particles during hot forming, pin grain boundaries, inhibit austenite grain growth, promote bainitic phase transformation during controlled cooling, refine the final microstructure, and improve the material's fracture toughness and resistance to sulfide stress corrosion cracking.

8. A method for manufacturing an integral wellhead trunk with forged reinforcing ribs according to claim 1, characterized in that, The mold cavity design takes into account the compensation mechanism for metal cooling and shrinkage: in the area where the main pipe and branch pipe meet, the cavity size is enlarged by 0.8% to 1.2% in the opposite direction according to the coefficient of thermal expansion, while in the straight section area far from the meeting area, it is designed according to the conventional shrinkage rate, so as to ensure that the reinforcing ribs still maintain the design height in the critical area after forming.

9. A method for manufacturing an integral wellhead trunk with forged reinforcing ribs according to claim 1, characterized in that, The diameter-to-height ratio of the initial blank is strictly calculated so that it exactly fills the projected area of ​​the mold preforming cavity after pre-upsetting. This ratio is determined by the high-temperature flow stress curve of the material and the volume of the mold cavity, and is solidified into process parameters after trial molding verification.

10. A method for manufacturing an integral wellhead trunk with forged reinforcing ribs according to claim 1, characterized in that, Before machining, the formed forgings must undergo dual non-destructive testing, namely ultrasonic testing and magnetic particle testing. The machining process uses a five-axis CNC machine tool. The tool path planning avoids the area at the root of the reinforcing rib. Only the flange sealing surface, threaded hole and inner surface of the flow channel are finished. The surface roughness of the inner surface of the flow channel is controlled below Ra 3.2 μm. At the same time, the multi-station die forging press communicates with the central control system in real time via industrial Ethernet. The central control system has an embedded forming process monitoring module based on digital twin technology, which is used to dynamically adjust the punch speed and form a traceable quality file.

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