Stress shot blasting process and equipment for high-strength spring

Abstract

The invention relates to the technical field of metal material surface treatment and high-strength spring manufacturing, and discloses a stress shot blasting process and equipment for a high-strength spring, and the process comprises the following steps: carrying out hot shot blasting on a quenched spring, and forming initial dislocation by using cast steel shots; carrying out controllable water cooling on the spring, and inducing phase change to construct a tissue foundation; fine steel shots are used for secondary cold shot blasting, and grains are refined to construct middle-layer stress; carrying out stress shot blasting by using modified tungsten carbide to construct a deep gradient stress field; cleaning and tempering the spring, performing coating protection, and performing quality detection; the equipment comprises first hot shot blasting equipment, water cooling equipment, second cold shot blasting equipment, third stress shot blasting equipment and a control system. Through four-step cooperative treatment of hot shot blasting, controllable water cooling, cold shot blasting and final stress shot blasting, gradient residual compressive stress fields with depth and stability are constructed on the surface layer and the subsurface layer of the spring, cyclic load tensile stress borne by the spring in the service process is counteracted, and the fatigue performance of the finished spring is improved.

Description

Technical Field

[0001] This application relates to the field of metal material surface treatment and high-strength spring manufacturing technology, specifically a stress shot blasting process and equipment for high-strength springs. Background Technology

[0002] Springs, as a key basic component in mechanical systems, are widely used in various types of equipment. Their main functions are buffering, vibration damping, or energy storage. During their service life, springs must withstand high-frequency cyclic loads. Therefore, their fatigue life and operational reliability are core indicators for measuring their quality and determining the safety performance of the entire equipment.

[0003] In existing spring manufacturing practices, after the spring steel wire is wound into shape, it first undergoes a quenching and tempering treatment (i.e., quenching followed by medium-to-high temperature tempering). The purpose is to obtain a uniform tempered martensite or sorbite structure within the material, thereby providing the spring with basic strength and hardness. After heat treatment, the spring is sent to a shot peening machine for surface shot peening at room temperature. This step utilizes the impact of high-speed shot to form a layer of residual compressive stress on the spring surface, which counteracts the tensile stress generated during operation, thereby improving its fatigue resistance.

[0004] However, existing spring manufacturing technologies often sacrifice the toughness of materials by using heat treatment to create hard and brittle structures such as martensite, which are aimed at achieving high strength. This results in insufficient overall impact resistance of the spring. The residual compressive stress layer formed by shot peening at room temperature is usually shallow and has a steep stress gradient. Under high stress amplitude or in the presence of surface defects, its fatigue protection effect is limited. Therefore, this invention provides a stress shot peening process and equipment for high-strength springs to address the shortcomings of existing technologies. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this application is to provide a stress shot blasting process and equipment for high-strength springs, which solves the problem that existing spring strengthening processes often lead to a decrease in toughness when increasing strength, and it is difficult to construct a residual compressive stress field with optimized depth and gradient on the spring surface, thereby limiting its fatigue life.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] The first aspect of this invention provides a stress shot blasting process for high-strength springs, which uses a multi-stage, synergistic shot blasting and cooling process to construct a structure with specific microstructure and gradient residual compressive stress field on the spring surface, thereby improving the mechanical properties of the spring. The process includes the following steps:

[0008] S1. First-stage hot shot blasting: The quenched springs undergo a first-stage shot blasting treatment. This treatment is carried out within a temperature range of 240-290℃, using high-toughness cast steel shot with a particle size of 1.1-1.3mm and a hardness of 610-670HV. The blasting velocity is 50-70m / s, and the shot blasting intensity is controlled at an arc height of 0.35-0.55A on the Almen specimen. This step aims to form an initial dislocation structure and shallow residual compressive stress on the spring surface.

[0009] S2. Water Cooling and Microstructure Control: The springs that have undergone the first stage of shot blasting are then water-cooled. Cooling is implemented through multiple sets of high-pressure nozzles, with the cooling rate controlled at 100-200℃ / s. The springs are cooled to below 80℃. This step, by controlling the cooling process, increases the austenitization degree from 75% to 92% and induces bainitic phase transformation, thus building the microstructure basis for subsequent treatments.

[0010] S3. Second-stage cold shot blasting: The cooled spring undergoes a second-stage cold shot blasting treatment. This treatment is carried out at room temperature, using fine steel shot with a particle size of 0.9-1.1 mm and a hardness of 610-670 HV. The blasting velocity is 60-80 m / s, and the shot blasting intensity is controlled at an arc height of 0.35-0.55A for the Almen specimen. This step refines the surface grains of the spring, introduces nanotwins, builds intermediate residual compressive stress on the microstructure, and achieves a dislocation density of 10. 15 m -2 .

[0011] S4. Stress Shot Blasting: Springs that have undergone the second-stage shot blasting process are then subjected to stress shot blasting. The blasting medium consists of surface-modified tungsten carbide microparticles with a particle size of 0.5-0.7 mm and a hardness of 610-670 HV. The blasting velocity is 60-90 m / s. This step, through high-intensity impact, constructs a deep layer of residual compressive stress on top of the intermediate layer of residual compressive stress on the spring surface, thereby forming a gradient-optimized stress field. The depth of the residual compressive stress layer reaches 0.6-0.8 mm, and the stress gradient is reduced by 40%.

[0012] S5. Post-processing and inspection: The springs are cleaned, decontaminated, subjected to low-temperature tempering, coated with anti-corrosion coating, and quality inspected. The temperature range for low-temperature tempering is 180-220℃.

[0013] A second aspect of the present invention provides a stress shot blasting device for high-strength springs, applied to the above-mentioned process, comprising:

[0014] The steel wire storage equipment is equipped with a steel wire straightening device, which is used to store or output steel wire and to straighten bent steel wire in conjunction with the steel wire straightening device.

[0015] Wire straightening device: Located at the output end of wire storage equipment, its structure includes at least two or more sets of staggered rollers. By adjusting the spacing and angle between the rollers, the wire undergoes plastic bending deformation as it passes through, thereby eliminating the initial bending of the wire.

[0016] First hot shot blasting equipment: This equipment is used to perform step S1. Its structure mainly includes a heating zone, a shot blasting zone, and a shot blasting media circulation system, with a material conveying system running through it.

[0017] Heating Zone: Located at the front of the equipment. It contains multiple sets of infrared or resistance heating elements, capable of uniformly heating the spring blanks on the conveyor belt to the target temperature of 240-290℃. The inner wall of the heating zone is covered with insulation material to reduce heat loss. Thermocouple temperature sensors are installed within the zone to monitor the furnace temperature in real time and feed it back to the central control system. The control system then achieves closed-loop control by adjusting the power of the heating elements.

[0018] Shot blasting zone: Immediately following the heating zone. At least one centrifugal shot blaster is symmetrically installed inside, either above, below, or around the conveyor belt. This shot blaster is driven by a high-speed motor, the motor speed of which is precisely controlled by a central control system, ensuring that the blasting speed of the shot medium (high-toughness cast steel shot) remains stable within the range of 50-70 m / s. The arrangement of the shot blasters ensures that all surfaces of the spring are uniformly blasted.

[0019] Media circulation system: Located at the bottom of the shot blasting zone, it is a complete closed-loop circulation system. Used high-toughness cast steel shot falls into the collection hopper at the bottom and is then conveyed to the separator above via a screw conveyor and bucket elevator. The separator removes dust and broken media through air separation and other methods; qualified media then re-enters the storage silo for reuse in the centrifugal shot blasting machine.

[0020] Water cooling equipment: This equipment is located immediately after the first hot shot blasting equipment and is used to perform step S2. Its structure is a closed cooling channel.

[0021] Cooling channel: Multiple high-pressure nozzle arrays are densely arranged along the material conveying path inside. These nozzles are arranged in a ring and can spray atomized or columnar industrial pure water onto the surface of the high-temperature spring from different angles.

[0022] Fluid control system: This system includes a high-pressure water pump, solenoid valves, and flow meters. The central control system precisely controls the water pump pressure and the opening of the solenoid valves based on the temperature of the inlet spring and the set cooling target, thereby strictly controlling the cooling rate within a process window of 100-200℃ / s.

[0023] Temperature monitoring and drainage: A non-contact temperature sensor (such as an infrared thermometer) is installed at the outlet of the cooling channel to confirm that the spring temperature has dropped below 80°C. A water collection tray and drainage system are provided at the bottom of the channel for recycling cooling water.

[0024] The second cold shot blasting equipment: This equipment is located downstream of the water cooling equipment and is used to perform step S3 at room temperature. Its overall structure includes a shot blasting chamber, a pneumatic shot feeding system, and a media circulation system.

[0025] Pneumatic shot delivery system: This is the core of the equipment. The system includes a pressurized tank for storing fine steel shot, a mixing valve that meterly delivers the shot blasting medium into the high-pressure airflow, and multiple blasting nozzles connected to a high-pressure air source. The central control system precisely controls the blasting speed and shot blasting intensity by adjusting the air supply pressure and the mixing valve. The nozzles are arranged to fully cover the springs.

[0026] Media circulation system: Similar to the first hot shot blasting equipment, its circulation system also includes collection, lifting and separation screening functions to ensure the cleanliness and reusability of fine steel shot.

[0027] The third stress shot blasting equipment: This equipment is the last processing unit in the production line, used to perform step S4. Its structure is similar to the second cold shot blasting equipment, but its core blasting components are different.

[0028] Hyperboloid Impeller Thruster: The core of this equipment is at least one hyperboloid impeller thru- ...

[0029] High-efficiency media circulation system: Considering the high cost of surface-modified tungsten carbide microparticles, the media circulation system of this equipment features enhanced sealing and separation efficiency to minimize media loss. The separator can accurately separate microparticles of suitable size and morphology for reuse.

[0030] In summary, this application includes at least one of the following beneficial technical effects:

[0031] 1. This invention employs a four-step synergistic process—hot shot blasting, controlled water cooling, cold shot blasting, and final stress shot blasting—to construct a gradient residual compressive stress field with significant depth and high stability on the surface and subsurface layers of the spring. This stress field effectively counteracts the cyclic tensile stress borne by the spring during service, fundamentally inhibiting the initiation and propagation of fatigue cracks, thereby improving the fatigue performance of the finished spring and significantly enhancing its reliability and service life.

[0032] 2. This invention, through precise control of the cooling rate, effectively induces favorable microstructure phase transformations based on the dislocation pre-formation through the first-stage hot shot blasting, and further refines the surface grains through the second-stage cold shot blasting. The resulting composite microstructure allows the spring to achieve high strength while maintaining excellent plasticity and toughness, avoiding the material embrittlement problem often caused by traditional strengthening processes.

[0033] 3. This invention, through a step-by-step approach, forms a deep and gently gradient residual compressive stress field, enhancing the stability of the stress layer. The preceding sequential processes establish an optimized microstructure for the spring surface. Building upon this, the final step utilizes a surface-modified shot blasting medium that more efficiently transfers impact kinetic energy to the material's subsurface layer. This allows for stress superposition and extension into deeper regions on the stress layer formed by the preceding processes, ensuring that the protective stress layer is less prone to failure under complex operating conditions. Attached Figure Description

[0034] Figure 1 This is a flowchart of the process steps in this application;

[0035] Figure 2 This is a diagram of the device architecture of this application;

[0036] Figure 3 This is a flowchart of the equipment operation process in this application.

[0037] Among them, 1. Steel wire storage equipment; 2. First hot shot blasting equipment; 3. Water cooling equipment; 4. Second cold shot blasting equipment; 5. Third stress shot blasting equipment. Detailed Implementation

[0038] The following is in conjunction with the appendix Figure 1 -Appendix Figure 3 This application will be described in further detail below.

[0039] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0040] Spring blank: Made of 55CrSi spring steel, with a specification of Φ10mm, and quenched to a hardness of 58-65HRC.

[0041] High-toughness cast steel shot: particle size 0.8-1.2mm, hardness 610-670HV.

[0042] Fine steel shot: particle size 0.2-0.5mm, hardness 610-670HV.

[0043] Surface-modified tungsten carbide microparticles: These were prepared in-house in this example; detailed preparation steps can be found in the preparation example.

[0044] Preparation example:

[0045] Preparation steps of surface-modified tungsten carbide microparticles:

[0046] Raw material preparation: Select tungsten carbide (WC) microparticles with an average particle size of 0.5-0.7 mm and a matrix hardness of 92-95 HRA. Place the microparticles in an ultrasonic cleaner, clean with acetone solution for 15 min, then rinse with deionized water, and vacuum dry at 80℃ for 4 h to remove surface impurities and moisture.

[0047] Surface modification treatment: The cleaned and dried tungsten carbide particles are placed in a magnetron sputtering apparatus. The apparatus cavity is evacuated to 5.0 × 10⁻⁶. -4 Argon (Ar) and acetylene (C2H2) were introduced into the chamber as reactant gases. The argon flow rate was controlled at 200 sccm and the acetylene flow rate at 5 sccm. A radio frequency (RF) power supply was used, with the power set to 300W. Tungsten carbide microparticles were subjected to magnetron sputtering with high-purity graphite as the target material. The treatment time was set to 2 hours to uniformly deposit a nanocrystalline amorphous carbon film with a thickness of approximately 15-20 nm on the surface of the tungsten carbide microparticles.

[0048] Finished product collection and performance testing: After modification, the gas and power supply were stopped, and the microparticles were removed after the chamber cooled to room temperature. The obtained surface-modified tungsten carbide microparticles maintained a particle size of 0.5-0.7 mm and a hardness of 610-670 HV, with a dense amorphous carbon film on their surface.

[0049] Please see the appendix Figure 1-3 Example 1:

[0050] Raw material components (by mass parts):

[0051] High-toughness cast steel shot: 500 parts;

[0052] Fine steel shot: 300 parts;

[0053] Surface-modified tungsten carbide microparticles: 150 parts.

[0054] Process steps:

[0055] S1. First-stage hot shot blasting treatment: 55CrSi springs made of quenched and tempered steel wire are heated to 380℃ and held at this temperature for 40 minutes. Subsequently, when the springs reach 240℃, they are placed in a centrifugal shot blaster using high-toughness cast steel shot with a particle size of 1.2mm and a hardness of 640HV as the shot blasting medium, and shot blasting is performed on the springs at a projectile velocity of 50m / s. The shot blasting intensity is controlled such that the arc height value of the Almen specimen is ≥0.35A.

[0056] S2. Water Cooling and Structure Control: After the first stage of hot shot blasting, the spring is immediately transferred to water cooling equipment 3. The spring is then immersed in water at a cooling rate of 100℃ / s until its temperature drops below 80℃.

[0057] S3. Second-stage cold shot blasting: The cooled spring is transferred to the second cold shot blasting equipment 4. At room temperature, the pneumatic shot feeding system is started, using fine steel shot with a particle size of 1.0mm and a hardness of 640HV as the shot blasting medium, and the spring is shot blasted at a projectile speed of 60m / s. The shot blasting intensity is controlled at an arc height value of ≥0.35A for the Almen test specimen.

[0058] S4. Stress Shot Blasting: The springs after cold shot blasting are transferred to the third stress shot blasting equipment 5. The hyperboloid impeller blasting head is started, using surface-modified tungsten carbide microparticles with a particle size of 0.6mm and a hardness of 640HV as the shot blasting medium, and the springs are subjected to stress shot blasting at a projectile speed of 60m / s. The loading amount of surface-modified tungsten carbide microparticles in the third stress shot blasting equipment 5 is 150kg.

[0059] S5. Post-processing and Inspection: The springs that have undergone stress shot blasting are cleaned and decontaminated. They are then tempered in a low-temperature tempering furnace at 200℃ for 60 minutes. Finally, an anti-corrosion coating is applied, and the finished springs undergo fatigue limit testing and tensile testing.

[0060] Example 2:

[0061] Raw material components (by mass parts):

[0062] High-toughness cast steel shot: 500 parts;

[0063] Fine steel shot: 300 parts;

[0064] Surface-modified tungsten carbide microparticles: 150 parts.

[0065] Process steps:

[0066] S1. First-stage hot shot blasting treatment: 55CrSi springs made of quenched and tempered steel wire are heated to 380℃ and held at this temperature for 40 minutes. Subsequently, when the springs reach 290℃, they are placed in a centrifugal shot blaster using high-toughness cast steel shot with a particle size of 1.2mm and a hardness of 640HV as the shot blasting medium, and shot blasting is performed at a projection speed of 70m / s. The shot blasting intensity is controlled at an arc height value of 0.55A for the Almen specimen.

[0067] S2. Water Cooling and Structure Control: After the first stage of hot shot blasting, the spring is transferred to water cooling equipment 3. The spring is cooled by immersion at a cooling rate of 100℃ / s until its temperature drops below 80℃.

[0068] S3. Second-stage cold shot blasting: The cooled spring is transferred to the second cold shot blasting equipment 4. At room temperature, fine steel shot with a particle size of 1.0 mm and a hardness of 640 HV is used as the shot blasting medium, and shot blasting is performed at a projection speed of 80 m / s. The shot blasting intensity is controlled so that the arc height value of the Almen specimen is ≥0.35A.

[0069] S4. Stress shot blasting: The springs after cold shot blasting are transferred to the third stress shot blasting equipment 5. Surface-modified tungsten carbide microparticles with a particle size of 0.6 mm and a hardness of 640 HV are used for stress shot blasting at a projectile speed of 90 m / s.

[0070] S5. Post-processing and Inspection: The treated springs are cleaned and decontaminated. They are then tempered in a low-temperature tempering furnace at 200℃ for 45 minutes. Following this, an anti-corrosion coating is applied, and the finished springs undergo fatigue limit testing and tensile testing.

[0071] Comparative Example 1:

[0072] Compared with Example 1, the difference is that the second-stage cold shot blasting step is omitted after the first-stage hot shot blasting treatment; all other steps are the same.

[0073] Comparative Example 2:

[0074] The difference from Example 1 is that the first stage of shot blasting is performed at room temperature instead of at 380°C; all other aspects are the same.

[0075] Comparative Example 3:

[0076] The difference from Example 1 is that the stress shot blasting treatment uses conventional tungsten carbide microparticles without surface modification as the shot blasting medium, while the rest are the same.

[0077] Experiment 1:

[0078] Experimental objective: To test the performance of the spring samples blasted by shot blasting in Examples 1-2 and Comparative Examples 1-3, and to verify the tensile properties of each sample.

[0079] Test method: Five samples were randomly selected from the finished springs obtained in Examples 1-2 and Comparative Examples 1-3. The test was conducted according to the standard GB / T228.1-2010 "Metallic materials, tensile testing—Part 1: Test at room temperature". The test equipment was an MTSLandmark370.10 universal testing machine.

[0080] Sample preparation: Cut a specimen from the middle effective coil portion of each spring sample and process it into a circular cross-section specimen that meets the standard requirements. Use vernier calipers to measure the diameter of the original gauge length of each specimen at at least three different locations, and take the average value to calculate the original cross-sectional area.

[0081] Equipment setup: Clamp the sample onto the grips of the universal testing machine, ensuring alignment. Set the test program, with the loading rate set to a constant 2 mm / min.

[0082] Test execution: The test is started at room temperature (25℃). A uniaxial tensile load is applied to the specimen until it breaks. The equipment automatically records the load-displacement data throughout the process.

[0083] Data Processing: The maximum load before specimen fracture is read from the recorded data. The two parts of the fractured specimen are then joined together, the minimum cross-sectional diameter at the fracture surface is measured, and the reduction of area is calculated. The tensile strength (MPa) of the material is calculated based on the maximum load and the original cross-sectional area.

[0084] The experimental results are shown in Table 1.

[0085] Table 1: Tensile property test results

[0086] Test group Tensile strength (MPa) Example 1 1327 Example 2 1289 Comparative Example 1 1054 Comparative Example 2 1172 Comparative Example 3 1215

[0087] The test data in Table 1 show that after processing according to Examples 1 and 2, the spring samples achieved high tensile strength while maintaining a high level of reduction of area. This result is directly related to the synergistic effect of each step in the process flow. The complete four-step process, namely, the first-stage hot shot blasting, controlled water cooling, the second-stage cold shot blasting, and the final stress shot blasting, works together on the surface and subsurface layers of the spring to form a specific microstructure and stress distribution, thereby comprehensively improving the strength and plasticity of the material.

[0088] Comparing the results of Example 1 and Comparative Example 1, it can be seen that omitting the second-stage cold shot blasting process significantly reduced the tensile strength and reduction of area of ​​the spring. This is because the crucial microstructure control steps—namely, the rapid cooling-induced bainitic phase transformation and the surface grain refinement caused by cold shot blasting—were missing, failing to establish the necessary microstructure foundation for subsequent stress shot blasting. Consequently, an effective gradient stress field could not be formed, resulting in insufficient final mechanical properties.

[0089] Comparing the results of Example 1 with those of Comparative Examples 2 and 3 further confirms the necessity of specific process steps. In Comparative Example 2, the first stage of shot blasting was performed at room temperature, and its final performance was lower than that of Example 1, indicating that performing the first stage of shot blasting at high temperature has a specific effect on forming the initial dislocation structure. In Comparative Example 3, conventional tungsten carbide microparticles were used instead of surface-modified tungsten carbide microparticles, and its final performance was also lower than that of Example 1, indicating that using surface-modified shot blasting media is effective in constructing a deep residual compressive stress field on the basis of an optimized microstructure, thereby affecting the final mechanical properties.

[0090] Experiment 2:

[0091] Experimental objective: To test the spring fatigue performance of shot blasting in Examples 1-2 and Comparative Examples 1-3.

[0092] Experimental steps:

[0093] Fatigue performance testing was conducted according to standard GB / T4337-2015 "Metallic Materials Fatigue Testing - Rotational Bending Method". The testing equipment was a QBG-100 type rotational bending fatigue testing machine. Before testing, the samples were machined to the standard-specified dimensions and surface roughness. The test was conducted at room temperature (25℃), with a loading frequency of 50Hz and a stress ratio R=-1. The fatigue limit of each group of samples was determined using the lifting method, with a specified cycle number of 1×10⁻⁶ cycles. 7 The test is repeated 10 times. If the sample does not break within this cycle, it is considered to have passed the test. Record the results for each group of samples at 1×10⁻⁶. 7 Fatigue limit (MPa) under multiple cycles.

[0094] The experimental results are shown in Table 2.

[0095] Table 2: Fatigue Performance Test Results

[0096] Test group <![CDATA[Fatigue limit (MPa, R = -1, 10 7 cycles)]]> Example 1 585 Example 2 572 Comparative Example 1 355 Comparative Example 2 430 Comparative Example 3 495

[0097] The test data in Table 2 show that the fatigue limit values ​​of the spring samples treated in Examples 1 and 2 are significantly higher than those of all comparative examples. Fatigue performance is directly related to the residual stress state and microstructure of the material surface. This technical solution, through multi-level synergistic treatment, constructs a high-value, deep residual compressive stress field on the spring surface. This stress field can effectively counteract external tensile stress and inhibit the initiation and propagation of fatigue cracks, thus achieving a high fatigue limit.

[0098] Comparative Example 1, by omitting the intermediate water cooling and cold shot blasting steps, failed to form a refined surface microstructure and intermediate residual compressive stress, resulting in the lowest fatigue limit. This indicates that steps S2 and S3 are necessary for constructing a fatigue-resistant structure. Comparative Example 2, by replacing hot shot blasting with room temperature shot blasting, exhibited lower fatigue performance than Example 1. This demonstrates that initial plastic deformation at high temperatures plays a fundamental role in subsequent microstructure control and stress superposition. Although the fatigue limit of Comparative Example 3 was higher than that of Comparative Examples 1 and 2, it was still lower than that of Example 1. This indicates that the role of surface-modified tungsten carbide microparticles in introducing a deeper and more stable residual compressive stress field is irreplaceable by conventional tungsten carbide microparticles.

[0099] Experiment 3:

[0100] Experimental objective: To test the residual stress of springs after shot blasting in Examples 1-2 and Comparative Examples 1-3.

[0101] Experimental methods:

[0102] Three samples were randomly selected from the finished springs obtained in Examples 1-2 and Comparative Examples 1-3. Surface residual stress was measured using X-ray diffraction. The testing equipment was an XL-640 X-ray stress analyzer.

[0103] Experimental steps:

[0104] Sample preparation and positioning: Fix the spring sample on the test platform, ensuring that its test surface (outer surface of the spring wire) is stable and located at the center of the focal spot of the diffractometer.

[0105] Equipment parameter settings: A Cr-Kα radiation source was used as the X-ray source, the target tube operating voltage was set to 30kV, and the current was set to 8mA. The {211} crystal plane of ferrite was selected as the diffraction crystal plane. The scanning range of the angle (the angle between the sample surface normal and the normal of the diffraction crystal plane) was set to 0° to 45°, with a measurement point taken every 9°.

[0106] Surface stress measurement: Start the equipment and measure the diffraction angle of the {211} crystal plane at each preset angle.

[0107] Stress calculation and depth analysis: The interplanar spacing was calculated based on the values ​​at different angles. The residual compressive stress (MPa) on the surface was calculated by linear fitting. To obtain the distribution of residual stress along the depth, the sample surface was peeled layer by layer using electropolishing. Steps 3 and 4 were repeated after peeling to a certain depth until the stress value approached zero, thereby measuring the total depth of the residual compressive stress layer.

[0108] The experimental results are shown in Table 3.

[0109] Table 3: Residual Stress Test Results

[0110] Test group Surface residual compressive stress (MPa) Depth of residual compressive stress layer (mm) Example 1 -1148 0.182 Example 2 -1223 0.180 Comparative Example 1 -750 0.098 Comparative Example 2 -810 0.113 Comparative Example 3 -890 0.120

[0111] The test data in Table 3 show that both the samples from Example 1 and Example 2 obtained high values ​​of surface residual compressive stress and large stress layer depth. This is attributed to the complete four-step process: the first stage of hot shot blasting forms initial dislocations; rapid water cooling prevents the weakening of the residual compressive stress from the first stage of hot shot blasting and also restores the surface hardness of the spring; the second stage of cold shot blasting refines the surface grains and introduces intermediate compressive stress; and the final step of stress shot blasting utilizes the impact characteristics of surface-modified tungsten carbide particles to extend the compressive stress to the subsurface depth of the material based on the previous steps, ultimately forming a gradient-distributed, deep residual compressive stress layer.

[0112] Comparative Example 1 exhibits the lowest residual compressive stress and layer depth due to the absence of certain process steps; a single stress shot blasting operation is insufficient to establish a deep stress layer on an unoptimized microstructure. The results of Comparative Example 2 indicate that the temperature of the first-stage shot blasting is one of the factors affecting the final stress layer depth. A direct comparison of Comparative Example 3 with Example 1 clearly reveals the role of surface-modified tungsten carbide microparticles. Despite a higher surface stress value, its stress layer depth is significantly smaller than that of Example 1. This suggests that surface-modified tungsten carbide microparticles can more effectively transfer impact energy into the material interior, thereby forming a deeper protective stress field. This result directly corresponds to the differences in fatigue limits shown in Table 2.

Claims

1. A stress shot blasting process for high-strength springs, characterized in that, Includes the following steps: S1. The first stage of shot blasting is performed on the 55CrSi spring made of quenched and tempered steel wire. High-toughness cast steel shot is used as the shot blasting medium to form an initial dislocation structure and shallow residual compressive stress on the spring surface. S2. Water cooling is applied to the spring that forms the initial dislocation structure and shallow residual compressive stress. The cooling rate is controlled and the microstructure is regulated to build the microstructure basis. S3. The springs that form the microstructure undergo a second-stage cold shot blasting process, using fine steel shot as the shot blasting medium to refine the spring surface, optimize the surface stress distribution, and build a middle layer of residual compressive stress on the microstructure basis. S4. The spring with intermediate residual compressive stress is subjected to stress shot blasting. Surface-modified tungsten carbide microparticles are used as shot blasting medium to induce deep bainitic phase transformation and build deep residual compressive stress on the basis of intermediate residual compressive stress, thereby forming a gradient-optimized stress field. S5. The spring is cleaned and decontaminated, subjected to low-temperature tempering, coated with anti-corrosion coating, and subjected to quality inspection to obtain a high-strength spring after stress shot blasting.

2. The stress shot blasting process for a high-strength spring according to claim 1, characterized in that, In step S1, the first-stage shot blasting treatment is carried out between 290-240℃. The high-toughness cast steel shot has a particle size range of 1.1-1.3mm, a hardness range of 610-670HV, a blasting speed of 50-70m / s, and the shot blasting intensity is controlled at an arc height value of 0.35-0.55A for the Almen specimen.

3. The stress shot blasting process for a high-strength spring according to claim 1, characterized in that, The shot blasting process uses a hot shot blasting equipment consisting of a heating furnace connected to a centrifugal shot blaster, and the water cooling system is equipped with multiple sets of high-pressure nozzles.

4. The stress shot blasting process for a high-strength spring according to claim 1, characterized in that, The cold shot blasting treatment uses a cold shot blasting equipment equipped with a pneumatic shot feeding system, and the stress shot blasting treatment uses a stress shot blasting equipment equipped with a hyperboloid impeller head.

5. The stress shot blasting process for a high-strength spring according to claim 3, characterized in that, In step S2, the cooling rate of the water cooling is controlled at 100-200℃ / s to cool the spring to below 80℃.

6. The stress shot blasting process for a high-strength spring according to claim 1, characterized in that, In step S3, the cold shot blasting is carried out at room temperature. The particle size of the fine steel shot is 0.9-1.1 mm, the hardness is 610-670 HV, the blasting speed is 60-80 m / s, and the shot blasting intensity is controlled at an arc height of 0.35-0.55 mmA on the Almen test piece.

7. The stress shot blasting process for a high-strength spring according to claim 1, characterized in that, In step S4, the shot blasting speed of the stress shot blasting treatment is 60-90 m / s, the particle size of the surface-modified tungsten carbide microparticles is in the range of 0.5-0.7 mm, and the hardness is in the range of 610-670 HV.

8. The stress shot blasting process for a high-strength spring according to claim 1, characterized in that, In step S5, the temperature range of the low-temperature tempering is 180-220℃, and the quality inspection steps include fatigue limit testing and tensile testing.

9. A stress shot blasting device for high-strength springs, applied to the stress shot blasting process of a high-strength spring as described in any one of claims 1-8, characterized in that, include: The steel wire storage equipment is equipped with a steel wire straightening device, which is used to store or output steel wire and to straighten bent steel wire in conjunction with the steel wire straightening device; The first hot shot blasting equipment, equipped with a heating furnace and a centrifugal shot blaster, is used to perform the first stage of shot blasting treatment on quenched springs. The water cooling equipment is equipped with multiple sets of high-pressure nozzles for water cooling of the springs after the first stage of shot blasting, and for cooling the springs to below 80°C at a cooling rate of 100-200°C / s. The second cold shot blasting equipment is equipped with a pneumatic shot feeding system, which is used to perform a second-stage cold shot blasting treatment on the cooled springs. The shot blasting treatment is carried out at room temperature. The third stress shot blasting equipment is equipped with a hyperboloid impeller blasting head, which is used to perform stress shot blasting on springs, with a blasting speed of 60-90m / s.

10. The stress shot blasting equipment for high-strength springs according to claim 9, characterized in that, The water cooling equipment includes a closed cooling channel, inside which multiple high-pressure nozzle arrays are densely arranged along the material conveying path.

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