A method for eliminating residual stress of long axis large forging forming and quenching

Abstract

The application discloses a long-axis large forging forming and quenching residual stress elimination method, which comprises the following steps: step one, blank pretreatment and numerical simulation modeling; step two, isothermal precision forging forming; step three, segmented gradient quenching; step four, online constant-temperature stress relief; step five, low-frequency electromagnetic induction auxiliary stress relief; and step six, slow cooling to room temperature. The long-axis large forging forming and quenching residual stress elimination method has the beneficial effects that: an integrated process of isothermal precision forging, segmented gradient quenching, online constant-temperature stress relief and low-frequency electromagnetic induction auxiliary stress relief is adopted, and the whole process from forming to quenching and then to stress relief is covered, the generation of residual stress is inhibited from the source, and the stress control is more complete than that of a traditional single stress relief mode; near-net forming is realized through isothermal precision forging, and the deformation and cracking risk is effectively reduced by cooperating with segmented gradient cooling and partitioned speed control, so that the size precision and the microstructure uniformity of the long-axis large forging are greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of forging forming and heat treatment technology, specifically relating to a method for forming long shaft-type large forgings and eliminating residual stress after quenching. Background Technology

[0002] Long-shaft forgings are widely used in high-end equipment fields such as energy equipment, rail transportation, and aerospace. Their dimensional accuracy, microstructure uniformity, and residual stress level directly determine the reliability and service life of the equipment. The quenching process is a key step in improving the strength and toughness of forgings. However, due to their large cross-sectional dimensions, high length-to-diameter ratio, and differences in cross-section along the length and radial direction, large long-shaft forgings experience extremely uneven temperature field distribution during quenching. This easily leads to significant thermal stress and microstructure stress superposition, causing deformations such as bending, twisting, and ellipticization in the forgings, and even initiating quenching cracks, severely reducing product qualification rate and service safety. Therefore, how to efficiently and uniformly eliminate quenching residual stress while ensuring the forming accuracy of forgings has become an urgent technical problem to be solved in the manufacturing field of long-shaft forgings.

[0003] In existing technologies, the elimination of residual stress after quenching in large forgings often employs a combination of cold deformation and aging treatment. For example, patent publication number CN121406992A describes a method for reducing residual stress after quenching in large 7xxx aluminum alloy T-shaped cross-section long beam forgings. This method involves installing a cold deformation die on a rolling mill. The cold deformation die consists of an upper die and a lower die. The upper die is a cylindrical structure with a cavity similar to the cross-section of the forging. The lower die is a spindle-shaped structure that wraps around the cross-section of the forging after closing. The solution-quenched forging is placed on the rolling mill, and the die gap is adjusted according to the thickness and deformation amount to allow the forging to bite into and contact the ribs. The forging is rolled once in each direction along its length, controlling the total deformation of the ribs to be 0.5%-1.2%, so that the upper die cavity contacts the bottom plate. The forging is then rolled once in each direction along its length again, controlling the total deformation to be 1.2%-3.2%. Finally, aging treatment is performed to reduce residual stress.

[0004] The existing technology has the following problems: it cannot adapt to the structural characteristics of long-shaft large forgings, such as axisymmetry, large length-to-diameter ratio, and large cross-section, thus limiting its application scenarios; it relies on cold rolling plastic deformation to release stress, which easily introduces new processing stress and dimensional deviations, making it difficult to meet high dimensional accuracy requirements; it adopts a separate process of segmented cold deformation followed by subsequent aging, which is lengthy, energy-intensive, and has a long production cycle, and the deformation control depends on experience, resulting in poor uniformity of stress relief; it only optimizes the single stress relief stage after quenching, without covering the entire process of coordinated control from forming to quenching to stress relief, and cannot suppress the generation of residual stress from the source. Summary of the Invention

[0005] The purpose of this invention is to provide a method for forming and eliminating residual stress in long shaft-type large forgings. By integrating isothermal precision forging, segmented gradient quenching, online isothermal stress relief and low-frequency electromagnetic induction-assisted stress relief into a synergistic process, the temperature field and stress field are controlled from the source to achieve stable production with high forming accuracy, high stress relief rate and low energy consumption.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a method for forming and eliminating residual stress during quenching of long-shaft large forgings, comprising the following steps: Step 1: Blank pretreatment and numerical simulation modeling: Surface cleaning and homogenization pretreatment are performed on the long shaft forging blank. A three-dimensional numerical model is established based on the forging material, size and structure. The temperature field and stress field distribution of the entire forging and quenching process are preset. Step 2: Isothermal Precision Forging: Using isothermal precision forging equipment, the forging rate and die temperature are controlled according to numerical simulation parameters to complete the near-net-shape forming of the billet; Step 3: Gradient quenching: After isothermal forging, the process is directly transferred to the step 3: gradient quenching process. The flow rate and cooling rate of the cooling medium are controlled according to the difference between the axial and radial sections of the forging to achieve gradient cooling. Step 4: Online isothermal stress relief: After quenching to the target temperature range, the forging is not transferred, but directly enters the online isothermal stress relief station to maintain the overall temperature stability of the forging and continuously release the quenching stress. Step 5: Low-frequency electromagnetic induction-assisted stress relief: Low-frequency electromagnetic induction is applied simultaneously during the online isothermal stress relief process to enhance stress release; Step 6: Slow cooling to room temperature: After stress relief, control the slow cooling rate to reduce to room temperature to obtain a low-stress, high-precision long shaft forging.

[0007] As a preferred technical solution of the present invention, in step one, the numerical simulation modeling adopts finite element simulation to calculate the temperature, strain and stress evolution of each node in forging and quenching in real time, and outputs closed-loop control parameters of forging rate, mold temperature and cooling flow rate.

[0008] As a preferred technical solution of the present invention, in step two, isothermal precision forging is carried out, and the difference between the die temperature and the final forging temperature of the billet is controlled between 0℃ and 30℃. The forging rate is set differently according to the axial and radial zones.

[0009] As a preferred technical solution of the present invention, in step three, the forging is divided into at least three independent cooling zones along its length. Each cooling zone is equipped with an independent medium circuit, and different flow rates are matched according to the cross-sectional size and heat dissipation conditions to achieve uniform axial cooling.

[0010] As a preferred technical solution of the present invention, the segmented gradient quenching adopts internal and external gradient cooling along the radial direction, and the surface cooling rate is higher than that of the core.

[0011] As a preferred technical solution of the present invention, in step four, the online isothermal stress relief is carried out, the isothermal temperature is set below the material phase transformation point and within the stress relief annealing temperature range, and the isothermal holding time is adaptively set according to the effective cross-sectional thickness of the forging.

[0012] As a preferred technical solution of the present invention, in step five, low-frequency electromagnetic induction is used to assist in stress relief. A low-frequency alternating magnetic field is uniformly covered along the axial direction of the forging and applied synchronously during the isothermal stage to promote dislocation slip and uniform stress release.

[0013] As a preferred technical solution of the present invention, the process from isothermal precision forging to segmented gradient quenching and then to online isothermal stress relief is a continuous online integrated process, and the forging does not need to be transferred or reheated.

[0014] As a preferred technical solution of the present invention, the cooling medium used for segmented gradient quenching is a water-soluble quenching liquid with stable temperature, and the injection pressure and flow rate are adjusted in real time through a closed-loop control system.

[0015] As a preferred technical solution of the present invention, in step six, the temperature is slowly cooled to room temperature, and the slow cooling rate is lower than a set threshold.

[0016] Compared with the prior art, the beneficial effects of the present invention are: It adopts an integrated process of isothermal precision forging, segmented gradient quenching, online isothermal stress relief, and low-frequency electromagnetic induction-assisted stress relief, covering the entire process from forming to quenching to stress relief. It suppresses the generation of residual stress from the source, which is different from the traditional single stress relief method and makes stress control more thorough. Near-net-shape forming is achieved through isothermal precision forging, combined with segmented gradient cooling and zoned speed control, effectively reducing the risk of deformation and cracking, and significantly improving the dimensional accuracy and microstructure uniformity of long shaft large forgings; Online isothermal stress relief combined with low-frequency electromagnetic induction assistance promotes dislocation slip and uniform stress release, resulting in a higher residual stress elimination rate and avoiding local stress concentration. Forging, quenching, and stress relief can be carried out continuously online, without transferring or reheating the forgings, which greatly shortens the production cycle, reduces energy consumption, and improves production efficiency and economy. It is adapted to the axisymmetric, high length-to-diameter ratio, and large cross-sectional structural features of long-shaft large forgings, breaking through the limitations of existing technologies that are only applicable to specific cross-sections and materials, and has greater versatility. Attached Figure Description

[0017] Figure 1 This is a flowchart of the method for forming and quenching residual stress relief of long shaft-type large forgings according to the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1 Please see Figure 1 This is the first embodiment of the present invention, which provides a method for forming and eliminating residual stress during quenching of large forgings of the long shaft type, including the following steps: Step 1: Billet Pretreatment and Numerical Simulation Modeling. First, the metal billets used for long-shaft forgings undergo comprehensive surface cleaning to remove oxide scale, oil, rust, burrs, and machining defects, preventing stress concentration and crack initiation during subsequent forging and quenching. Then, the billets undergo homogenization pretreatment by placing them in a heating furnace for heat preservation and diffusion treatment. This eliminates component segregation, microstructure inhomogeneity, and internal residual stress generated during casting or pre-machining, improving the billet's density and plastic deformation capacity. Based on the forging's material properties, geometric dimensions, aspect ratio, cross-sectional variation, and structural characteristics, professional finite element simulation software is used. A three-dimensional numerical model of the entire process of long-shaft forgings is established, and multi-physics field coupling calculations are performed on the entire process of forging, quenching, and stress relief. The distribution laws of temperature field, stress field, and strain field of each part of the forging under different processes are preset. Through numerical simulation, the temperature changes, plastic strain evolution and stress accumulation state of each node in the forging and quenching process are tracked in real time, and the optimal process parameters matching the forging are output, including target forging rate, die isothermal range, quenching cooling medium flow rate, spray pressure, segmented cooling sequence, etc., forming a closed-loop control parameter library for the entire process, providing accurate data support for subsequent processes, and realizing full-process digital precise control from forming to quenching to stress relief.

[0020] Step Two: Isothermal Precision Forging. The pre-treated billet is transferred to isothermal precision forging equipment. Based on the process parameters output by numerical simulation, the mold temperature and the final forging temperature of the billet are strictly controlled to keep the difference between the mold temperature and the final forging temperature of the billet stable at 2℃. This ensures that the entire forging process is in a near-isothermal state, avoiding problems such as uneven deformation resistance and local insufficient filling caused by temperature fluctuations. During the forging process, the forging rate is set differently for axial and radial zones according to the deformation requirements of different axial positions and different radial depths of the forging. The large cross-section area uses a matching low-speed forging to ensure full deformation, while the small cross-section and transition area use an appropriate high-speed forging to improve efficiency. This allows the billet to flow evenly and fill completely in the mold cavity, without local overload, folding, or incomplete filling. Near-net-shape forming is completed in one go, greatly reducing subsequent machining allowances and ensuring the overall dimensional accuracy and uniformity of the microstructure of long shaft forgings.

[0021] Step 3: Three-segment gradient quenching: After isothermal precision forging, the forging is directly transferred online to the segmented gradient quenching station without cooling, transfer, or reheating, achieving a seamless connection between forging and quenching. For long-shaft forgings with large lengths, axially varying cross-sections, and significant radial and internal thickness differences, a composite cooling method of axial segmentation and radial gradient is implemented: the forging is divided into at least three independent and controllable cooling zones along its length. Each cooling zone is equipped with an independent medium supply and return loop, and differentiated cooling medium flows are matched according to the cross-sectional size, heat dissipation conditions, and criticality of each segment. The amount and angle of the spray are adjusted to achieve synchronous and uniform cooling in the axial direction; the radial cooling mode adopts an internal and external gradient cooling mode, with the surface cooling rate being higher than the core cooling rate, and the temperature is controlled to drop steadily throughout the process without sudden cooling or heating fluctuations, effectively suppressing the large superposition of thermal stress and structural stress, and reducing the level of residual stress from the source; the segmented gradient quenching uses a water-soluble quenching liquid with stable and controllable temperature as the cooling medium, and the medium spray pressure, flow rate and coverage are adjusted in real time through a closed-loop flow and pressure control system to ensure that the cooling process is stable and repeatable, and the cooling accuracy meets the forming requirements of high-precision long shaft forgings.

[0022] Step 4: Online Isothermal Stress Relief. After the forging is quenched to the preset target temperature range, without transferring the forging or interrupting the process, the process is directly switched from the quenching station to the online isothermal stress relief station, entering a continuous integrated stress relief process. The overall temperature of the forging is kept stable below the phase transformation point of the material and within the conventional stress relief annealing temperature range to avoid phase transformation of the microstructure and the induction of new stress. The isothermal holding time is adaptively set according to the effective cross-sectional thickness of the forging, material characteristics, and stress level. The holding time is appropriately extended in thick cross-sectional areas to ensure that the residual stress inside the forging is fully and uniformly released under isothermal conditions. This process, together with isothermal precision forging and segmented gradient quenching, forms a continuous online integrated production line. The forging is transferred, reheated, and repeatedly heated and cooled throughout the process, significantly shortening the production cycle and reducing energy consumption, while avoiding collision deformation and temperature fluctuations during transportation.

[0023] Step 5: Low-frequency electromagnetic induction-assisted stress relief: Throughout the online isothermal stress relief process, low-frequency electromagnetic induction is applied simultaneously to create a synergistic strengthening effect with isothermal stress relief. A low-frequency electromagnetic induction device is uniformly arranged along the forging axis to generate a low-frequency alternating magnetic field with full axial coverage and uniform intensity. Without changing the temperature and microstructure of the forging, this promotes dislocation slip, grain boundary stress relaxation, and uniform stress diffusion within the forging, compensating for the shortcomings of insufficient stress release in local areas with simple thermal stress relief. This significantly improves the residual stress elimination rate and uniformity, and avoids local stress concentration in long-shaft forgings.

[0024] Step Six: Slow Cooling to Room Temperature: After low-frequency electromagnetic induction-assisted stress relief, the forging enters a controllable slow cooling stage. The forging is slowly cooled to room temperature at a steady cooling rate of 1℃ / min. The cooling rate is strictly controlled to avoid exceeding the limit, thus preventing secondary residual stress from being generated again due to the temperature gradient during the slow cooling process. After slow cooling, a large long shaft forging with high dimensional accuracy, small deformation, low residual stress level, and uniform and stable microstructure is finally obtained. This can effectively avoid problems such as deformation, cracking, and dimensional deviations during subsequent processing and service, and significantly improve the product qualification rate and service life.

[0025] Example 2 Please see Figure 1 This is the second embodiment of the present invention, which is based on the previous embodiment, but differs in that: Step Two: Isothermal Precision Forging: The pre-treated billet is transferred to isothermal precision forging equipment. Based on the process parameters output by numerical simulation, the mold temperature and the final forging temperature of the billet are strictly controlled to ensure that the difference between the mold temperature and the final forging temperature of the billet is stably controlled within 15℃. This ensures that the entire forging process is in a near-isothermal state, avoiding problems such as uneven deformation resistance and local insufficient filling caused by temperature fluctuations. During the forging process, the forging rate is set differently for axial and radial zones according to the deformation requirements of different axial positions and different radial depths of the forging. The large cross-section area uses a matching low-speed forging to ensure full deformation, while the small cross-section and transition area use an appropriate high-speed forging to improve efficiency. This allows the billet to flow evenly and fill completely in the mold cavity, without local overload, folding, or incomplete filling. Near-net-shape forming is completed in one go, greatly reducing subsequent machining allowances and ensuring the overall dimensional accuracy and uniformity of the microstructure of long shaft forgings.

[0026] Step Six: Slow Cooling to Room Temperature: After low-frequency electromagnetic induction-assisted stress relief, the forging enters a controlled slow cooling stage. The forging is slowly cooled to room temperature at a steady cooling rate of 3℃ / min. The cooling rate is strictly controlled to avoid exceeding the limit, so as to avoid secondary residual stress caused by temperature gradient during the slow cooling process. After slow cooling, a large long shaft forging with high dimensional accuracy, small deformation, low residual stress level, and uniform and stable microstructure is finally obtained. This can effectively avoid problems such as deformation, cracking, and dimensional deviation during subsequent processing and service, and significantly improve the product qualification rate and service life.

[0027] Example 3 Please see Figure 1 This is the third embodiment of the present invention, which is based on the previous embodiment, but differs in that: Step Two: Isothermal Precision Forging. The pre-treated billet is transferred to isothermal precision forging equipment. Based on the process parameters output by numerical simulation, the mold temperature and the final forging temperature of the billet are strictly controlled to ensure that the difference between the mold temperature and the final forging temperature of the billet is stably controlled within 30℃. This ensures that the entire forging process is in a near-isothermal state, avoiding problems such as uneven deformation resistance and local insufficient filling caused by temperature fluctuations. During the forging process, the forging rate is set differently for axial and radial zones according to the deformation requirements of different axial positions and different radial depths of the forging. The large cross-section area uses a matching low-speed forging to ensure full deformation, while the small cross-section and transition area use an appropriate high-speed forging to improve efficiency. This allows the billet to flow evenly and fill completely in the mold cavity, without local overload, folding, or incomplete filling. Near-net-shape forming is completed in one go, greatly reducing subsequent machining allowances and ensuring the overall dimensional accuracy and uniformity of the microstructure of long shaft forgings.

[0028] Step Six: Slow Cooling to Room Temperature: After low-frequency electromagnetic induction-assisted stress relief, the forging enters a controlled slow cooling stage. The forging is slowly cooled to room temperature at a steady cooling rate of 5℃ / min. The cooling rate is strictly controlled to avoid exceeding the limit, thus preventing secondary residual stress from being generated again due to the temperature gradient during the slow cooling process. After slow cooling, a large long shaft forging with high dimensional accuracy, small deformation, low residual stress level, and uniform and stable microstructure is finally obtained. This can effectively avoid problems such as deformation, cracking, and dimensional deviations during subsequent processing and service, and significantly improve the product qualification rate and service life.

[0029] Although embodiments of the invention have been shown and described (see the detailed description above), it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for forming and quenching residual stress of large forgings with long shafts, characterized in that: Includes the following steps: Step 1: Blank pretreatment and numerical simulation modeling: Surface cleaning and homogenization pretreatment are performed on the long shaft forging blank. A three-dimensional numerical model is established based on the forging material, size and structure. The temperature field and stress field distribution of the entire forging and quenching process are preset. Step 2: Isothermal Precision Forging: Using isothermal precision forging equipment, the forging rate and die temperature are controlled according to numerical simulation parameters to complete the near-net-shape forming of the billet; Step 3: Gradient quenching: After isothermal forging, the process is directly transferred to the step 3: gradient quenching process. The flow rate and cooling rate of the cooling medium are controlled according to the difference between the axial and radial sections of the forging to achieve gradient cooling. Step 4: Online isothermal stress relief: After quenching to the target temperature range, the forging is not transferred, but directly enters the online isothermal stress relief station to maintain the overall temperature stability of the forging and continuously release the quenching stress. Step 5: Low-frequency electromagnetic induction-assisted stress relief: Low-frequency electromagnetic induction is applied simultaneously during the online isothermal stress relief process to enhance stress release; Step 6: Slow cooling to room temperature: After stress relief, control the slow cooling rate to reduce to room temperature to obtain a low-stress, high-precision long shaft forging.

2. The method for forming and quenching residual stress relief of long shaft-type large forgings according to claim 1, characterized in that: In step one, the numerical simulation modeling uses finite element simulation to calculate the temperature, strain and stress evolution at each node of forging and quenching in real time, and outputs closed-loop control parameters for forging rate, die temperature and cooling flow rate.

3. The method for forming and quenching residual stress relief of long shaft-type large forgings according to claim 1, characterized in that: In step two, isothermal precision forging is carried out, with the temperature difference between the die and the final forging temperature of the billet controlled between 0℃ and 30℃, and the forging rate set differently according to the axial and radial zones.

4. The method for forming and quenching residual stress relief of long shaft-type large forgings according to claim 1, characterized in that: In step three, segmented gradient quenching is performed, dividing the forging length into at least three independent cooling zones. Each cooling zone is equipped with an independent medium circuit, with different flow rates matched according to the cross-sectional size and heat dissipation conditions to achieve uniform axial cooling.

5. The method for forming and quenching residual stress relief of large forgings of long shaft type according to claim 4, characterized in that: Segmented gradient quenching employs internal and external gradient cooling along the radial direction, with the surface layer cooling rate higher than that of the core.

6. The method for forming and quenching residual stress relief of long shaft-type large forgings according to claim 1, characterized in that: In step four, online isothermal stress relief is performed. The isothermal temperature is set below the material's phase transformation point and within the stress relief annealing temperature range. The isothermal holding time is adaptively set according to the effective cross-sectional thickness of the forging.

7. The method for forming and quenching residual stress relief of long shaft-type large forgings according to claim 1, characterized in that: In step five, low-frequency electromagnetic induction is used to assist in stress relief. A low-frequency alternating magnetic field is uniformly applied along the axial direction of the forging and applied synchronously during the isothermal stage to promote dislocation slip and uniform stress release.

8. The method for forming and quenching residual stress relief of long shaft-type large forgings according to claim 1, characterized in that: From isothermal precision forging to segmented gradient quenching and then to online isothermal stress relief, it is a continuous online integrated process, and the forgings are not transferred or reheated.

9. The method for forming and quenching residual stress relief of long shaft-type large forgings according to claim 1, characterized in that: The cooling medium used in segmented gradient quenching is a water-soluble quenching liquid with stable temperature, and the injection pressure and flow rate are adjusted in real time through a closed-loop control system.

10. The method for forming and quenching residual stress relief of long shaft-type large forgings according to claim 1, characterized in that: In step six, the temperature is slowly cooled to room temperature, with the cooling rate being lower than the set threshold.

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