A composite strengthening treatment method for vehicle leaf spring based on residual stress attenuation regulation

Abstract

The application provides a kind of based on residual stress attenuation regulation and control composite strengthening treatment method of vehicle leaf spring, comprising: according to the service condition of target vehicle leaf spring, residual stress attenuation dynamics model under the coupling action of cyclic loading and service temperature rise is constructed, and the effective residual compressive stress threshold in the whole service period of leaf spring is determined;With the threshold as constraint condition, the initial residual compressive stress field required is solved reversely based on residual stress attenuation dynamics model;According to the characteristic parameter of initial residual compressive stress field, corresponding shot process parameter set is matched and determined;After the leaf spring completed quenching and tempering is heated and the bending pre-stress consistent with service stress direction is applied, high-temperature shot processing is executed;Deep cryogenic stabilization treatment is carried out to the leaf spring after high-temperature shot processing.The application takes residual stress attenuation model under the actual service condition of leaf spring as core, reversely designs composite process parameter, so that the residual stress of leaf spring is always kept above effective threshold in the whole service period.

Description

Technical Field

[0001] This invention relates to the field of surface strengthening technology for automotive leaf springs, specifically providing a composite strengthening treatment method for automotive leaf springs based on residual stress attenuation control. Background Technology

[0002] Leaf springs (or simply leaf springs) are the core load-bearing components of commercial vehicle suspension systems, responsible for load transfer, shock absorption, and guidance. Their fatigue life and service reliability directly determine the vehicle's driving safety. Shot peening is a core process in leaf spring manufacturing. By impacting the leaf spring surface with high-speed shot, plastic deformation of the surface material is caused, introducing a residual compressive stress field. This counteracts the alternating tensile stress experienced by the leaf spring during service, inhibiting the initiation and propagation of fatigue cracks, thereby significantly improving the fatigue life of the leaf spring.

[0003] Current technologies for shot peening automotive leaf springs mainly suffer from the following core defects: I. Existing processes generally prioritize initial stress over service stability. The vast majority of patents and technical solutions focus solely on increasing the initial residual compressive stress amplitude after shot peening, neglecting the significant attenuation of residual compressive stress in leaf springs under long-term cyclic loading and heavy temperature rise conditions. This rapid attenuation, especially within the first million cycles of service, is a core cause of early fatigue failure in leaf springs. Existing processes do not proactively design and control the attenuation process of residual stress, making it impossible to achieve stable control of residual stress throughout the entire service life.

[0004] Second, the current application of high-temperature shot peening is haphazard and lacks clear guidance on attenuation patterns. A few existing patents concerning high-temperature shot peening of leaf springs, such as CN103358234A's shot peening process for residual heat stress in springs and CN201210003857's method for thermal shot peening of steel leaf springs, only determine the temperature range through trial and error. Their core objective is merely to increase initial residual stress, without establishing a closed-loop logic of "spring and leaf service conditions - residual stress attenuation patterns - reverse design of process parameters." This makes it impossible to accurately match process parameters according to the actual service attenuation requirements of the leaf springs.

[0005] Third, existing composite processes lack synergy and specificity. Most existing composite processes involving shot peening and cryogenic treatment are designed for room-temperature shot peening and are primarily applied to bearing steel, mold steel, or automotive coil springs. Targeted research on automotive variable cross-section leaf springs is extremely limited. Without considering the service stress characteristics and residual stress decay properties of the leaf springs to achieve synergistic optimization of high-temperature shot peening and cryogenic treatment, it is impossible to simultaneously address the high initial residual compressive stress, long-cycle stress retention rate, and strength-toughness matching of the leaf springs, making it difficult to meet the extreme service requirements of heavy-duty commercial vehicle leaf springs. Summary of the Invention

[0006] In order to overcome the above-mentioned defects, the present invention is proposed to provide a solution or at least a partial solution to the above-mentioned problems.

[0007] This invention provides a composite strengthening treatment method for automotive leaf springs based on residual stress attenuation regulation, comprising the following steps: Based on the service conditions of the target automotive leaf spring, a dynamic model of residual stress decay under the coupled action of cyclic load and service temperature rise is constructed, and the effective residual compressive stress threshold during the entire service cycle of the leaf spring is determined. Using the effective residual compressive stress threshold during the entire service life of the leaf spring as a constraint, the initial residual compressive stress field required is solved in reverse based on the residual stress attenuation dynamic model. Based on the characteristic parameters of the initial residual compressive stress field, the corresponding shot peening process parameter set is determined. After the leaf springs that have been quenched and tempered are heated and subjected to bending prestress in the same direction as the service stress, high-temperature shot peening is performed according to the process parameter set. Deep cryogenic stabilization treatment is applied to leaf springs after high-temperature shot peening to suppress the attenuation rate of residual compressive stress during service.

[0008] Preferably, based on the service conditions of the target automotive leaf spring, the dynamic model for residual stress decay under the coupled action of cyclic load and service temperature rise includes: Based on the service conditions, a preset alternating load range and temperature conditions are determined, and residual stress relaxation data of the target leaf spring under the alternating load and temperature conditions are collected. The residual stress relaxation data reflects the degradation law of residual compressive stress with the number of cycles under different load amplitudes and temperatures. Based on the residual stress relaxation data, a decay curve of residual stress as a function of cycle number is established, and the decay curve is fitted to obtain the mapping relationship between residual stress decay and cycle number, load amplitude, and service temperature.

[0009] Preferably, the residual stress relaxation data is obtained through finite element simulation combined with cyclic bending fatigue tests; The finite element simulation takes the material performance parameters of the target leaf spring, the service load spectrum of the vehicle suspension, and the service temperature rise range under extreme conditions as input conditions to simulate the residual stress evolution trend of the leaf spring under the coupled action of cyclic load and temperature rise, and outputs the predicted value of residual stress under different conditions. The cyclic bending fatigue test verifies the leaf spring specimen under the corresponding working conditions and obtains the measured attenuation data of residual stress, which is used to correct the prediction deviation of the finite element simulation. The finite element simulation and the cyclic bending fatigue test are iteratively verified until the deviation between the simulation prediction value and the test measurement value is within the preset tolerance range. At this time, the residual stress relaxation data obtained is used to establish the attenuation curve.

[0010] Preferably, the effective residual compressive stress threshold during the entire service life of the leaf spring is determined by the lower limit of the residual compressive stress under the target number of cycles during the entire service life of the leaf spring based on the mapping relationship; Using the effective residual compressive stress threshold within the entire service life of the leaf spring as a constraint, the initial residual compressive stress field required for inverse solution based on the residual stress attenuation dynamic model includes: substituting the target number of cycles within the entire service life of the leaf spring into the mapping relationship, using the effective residual compressive stress threshold within the entire service life of the leaf spring as the dependent variable value under the target number of cycles, using the load amplitude and the service temperature as known parameters in the mapping relationship, performing inverse operation on the mapping relationship, and inversely solving to obtain the peak value of residual compressive stress and the effective compressive stress layer depth when the number of cycles is zero; wherein, the peak value of residual compressive stress and the effective compressive stress layer depth jointly characterize the amplitude characteristics and depth distribution characteristics of the initial residual compressive stress field.

[0011] Preferably, determining the corresponding shot peening process parameter set based on the characteristic parameters of the initial residual compressive stress field includes: A mapping relationship between residual stress characteristic parameters and shot peening process parameters is established in advance. The mapping relationship takes the peak value of residual compressive stress and the effective compressive stress layer depth as inputs and shot peening intensity, shot kinetic energy and surface coverage as outputs. Substituting the residual compressive stress peak value and the effective compressive stress layer depth into the mapping relationship, the corresponding shot peening intensity value, shot kinetic energy value and surface coverage value are matched and output to form the shot peening process parameter set.

[0012] Preferably, the process further includes heating the quenched and tempered leaf spring and applying bending prestress, specifically including: The magnitude of the applied bending prestress is positively correlated with the peak value of the residual compressive stress. The higher the peak value of the residual compressive stress, the greater the applied bending prestress, so that the tension surface of the leaf spring is in a prestress state that matches the target residual compressive stress field during the shot peening process. The preset temperature range for heating is determined based on the contribution ratio of the service temperature to the attenuation rate coefficient.

[0013] Preferably, the cryogenic stabilization treatment of the leaf spring after high-temperature shot peening includes: After the leaf springs are shot-peened at high temperature, they are naturally cooled to room temperature, then slowly reduced to cryogenic temperature at a preset cooling rate, and held at cryogenic temperature for a preset time. Finally, they are uniformly heated back to room temperature at a preset heating rate.

[0014] Preferably, the reduction in the attenuation rate coefficient is controlled by adjusting the cooling rate, cryogenic temperature, and holding time of the cryogenic stabilization treatment.

[0015] Preferably, the high-temperature shot peening treatment employs a two-stage shot peening process, which includes coarse shot peening and fine shot peening, wherein: The coarse shot peening uses large-diameter shot to form a plastic deformation layer in the depth direction of the leaf spring, so that the distribution depth of residual compressive stress along the depth direction meets the effective compressive stress layer depth requirement obtained by the reverse solution. The shot peening uses small-diameter shot to intensively impact the surface of the leaf spring to smooth the surface micro-morphology, thereby reducing surface roughness and increasing the amplitude of surface residual compressive stress, so that the surface residual compressive stress reaches the peak value requirement of residual compressive stress obtained by the reverse solution. The deep residual compressive stress formed by the coarse shot peening and the surface residual compressive stress peak reached by the fine shot peening together constitute a residual compressive stress distribution consistent with the initial residual compressive stress field obtained by the inverse solution.

[0016] Preferably, after the cryogenic stabilization treatment, the leaf spring is further subjected to a low-temperature tempering treatment, wherein the temperature of the low-temperature tempering is lower than the tempering temperature during the quenching and tempering of the leaf spring.

[0017] The beneficial effects of this invention are as follows: 1. Based on the residual stress attenuation dynamic model under actual service conditions of leaf springs, composite process parameters are designed in reverse, breaking the limitation of the existing process of "emphasizing initial stress and neglecting stability", so that the residual stress of leaf springs is always kept above the effective threshold throughout the entire service cycle, and the residual compressive stress retention rate after 1 million cycles of load is increased by more than 30%.

[0018] 2. Achieving strong synergistic optimization between high-temperature shot peening and cryogenic treatment, resulting in a significant improvement in overall performance. This invention utilizes the synergistic effect of high-amplitude deep residual compressive stress field constructed by high-temperature prestressed shot peening and the stable structure and stress of pinned dislocations achieved by cryogenic treatment. Without sacrificing the impact toughness of the leaf spring, it increases the bending fatigue life of the leaf spring by more than 50% and reduces the static stress relaxation rate by more than 40%, perfectly adapting to the extreme service conditions of heavy-duty commercial vehicles.

[0019] 3. The process is highly controllable and has extremely high engineering application value. The process steps of this invention are clear, and the parameter range is well-defined. The process parameters can be flexibly adjusted based on the leaf spring attenuation requirements of different vehicle models and operating conditions. It is suitable for large-scale industrial production on existing leaf spring production lines without the need for large-scale equipment modifications. The cost increase is controllable, and it has broad prospects for promotion and application.

[0020] 4. Possesses outstanding innovation and patent value. This invention establishes for the first time a complete closed-loop process of "residual stress attenuation law → reverse design of high-temperature shot peening process → stabilization and control of cryogenic treatment", which is clearly differentiated from the existing empirical high-temperature shot peening and room-temperature shot peening + cryogenic process, filling a technological gap in the industry. Attached Figure Description

[0021] The disclosure of this invention will become more readily understood with reference to the accompanying drawings. It will be readily understood by those skilled in the art that these drawings are for illustrative purposes only and are not intended to limit the scope of protection of this invention. Furthermore, similar numbers in the drawings are used to denote similar components, wherein: Figure 1 This is a schematic flowchart of a composite reinforcement treatment method for automotive leaf springs based on residual stress attenuation control, according to an embodiment of the present invention.

[0022] Figure 2 This is a comparison chart of the residual stress decay curves with the number of cycles in the embodiments and comparative examples of the present invention. Detailed Implementation

[0023] Some embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0024] like Figure 1-2 As shown, this invention provides a composite strengthening treatment method for automotive leaf springs based on residual stress attenuation regulation, comprising the following steps: Step S1: Based on the service conditions of the target automotive leaf spring, construct a dynamic model of residual stress attenuation under the coupled action of cyclic load and service temperature rise, and determine the effective residual compressive stress threshold during the entire service cycle of the leaf spring.

[0025] In this embodiment, the material of the vehicle leaf spring is any one of 60Si2Mn, 50CrVA, or SUP6 series spring steel, and the leaf spring is a commercial vehicle variable cross-section steel leaf spring or a constant cross-section steel leaf spring.

[0026] Service conditions: refers to the actual working state of automotive leaf springs, including repeated cyclic loads from vehicle movement (alternating forces from bumps, braking, and steering), and service temperature rise caused by driving friction / environment (high temperature will accelerate stress decay). Coupling effect: Cyclic load and temperature rise affect residual stress simultaneously. The superposition of the two will make the stress decay rate much faster than a single factor, so they must be included in the model at the same time; Effective residual compressive stress threshold: The lowest residual compressive stress threshold at which the leaf spring will not fail due to fatigue. If the stress is lower than this value in the later stage of service, the leaf spring is very likely to crack or break.

[0027] In one embodiment, step S1, constructing a dynamic model of residual stress decay under the coupled action of cyclic load and service temperature rise based on the service conditions of the target automotive leaf spring, includes: Step S11: Based on the service conditions, determine the preset alternating load range and temperature conditions, and collect the residual stress relaxation data of the target leaf spring under the alternating load and temperature conditions. The residual stress relaxation data reflects the degradation law of residual compressive stress with the number of cycles under different load amplitudes and temperatures.

[0028] In this embodiment, a preset alternating load range is defined: based on the actual service conditions of the leaf spring (vehicle load, braking impact, etc.), the repeated alternating force range that the leaf spring will bear during use is set to simulate the cyclic load when the vehicle is driving, covering different stress scenarios such as normal driving, heavy load, and harsh road conditions.

[0029] Temperature conditions are set to match the actual temperature rise environment of the leaf spring during service, including the temperature range caused by frictional heat generation during driving, changes in ambient temperature, and engine thermal radiation, to simulate the thermal relaxation state of the material at different temperatures.

[0030] Under the aforementioned combined conditions of different load amplitudes and temperatures, experimental tests were conducted to record the data of the entire process of the residual compressive stress inside the leaf spring gradually decreasing and relaxing with each load cycle.

[0031] Step S12: Based on the residual stress relaxation data, establish a decay curve of residual stress as a function of cycle number, and fit the decay curve to obtain the mapping relationship between residual stress decay and cycle number, load amplitude, and service temperature.

[0032] In this embodiment, the measured data from step S11 are plotted as a residual stress decay curve with the load cycle number as the horizontal axis and the residual compressive stress value as the vertical axis. Then, the discrete experimental points are transformed into a continuous and smooth function curve through mathematical fitting, eliminating experimental errors and obtaining a standardized decay law.

[0033] In the mapping relationship, the load amplitude represents the contribution of cyclic load to residual stress decay. The larger the load amplitude, the higher the value of this factor, and the faster the corresponding stress decay rate. The service temperature characterizes the accelerating effect of service temperature rise on residual stress decay. The higher the temperature, the more easily the material undergoes thermal relaxation. The higher the value of this factor, the more obvious the effect of accelerating stress decay.

[0034] The load amplitude and the service temperature are coupled in the same attenuation rate coefficient in a product form, so that the coupling effect of load and temperature on residual stress attenuation is uniformly characterized by the attenuation rate coefficient.

[0035] In one embodiment, the residual stress relaxation data in step S12 is obtained through finite element simulation combined with cyclic bending fatigue testing, specifically including: Step S121: The finite element simulation takes the material performance parameters of the target leaf spring, the service load spectrum of the vehicle suspension matching, and the service temperature rise range under extreme conditions as input conditions to simulate the residual stress evolution trend of the leaf spring under the coupled action of cyclic load and temperature rise, and outputs the predicted value of residual stress under different conditions.

[0036] In this embodiment, the material performance parameters of the target leaf spring include: elastic modulus, Poisson's ratio, high-temperature mechanical properties, stress relaxation characteristics, and other material constitutive parameters of spring steel; the service load spectrum matched with the vehicle suspension: reproducing the actual cyclic alternating load magnitude, frequency, and loading method of the leaf spring during vehicle driving, braking, and heavy load; and the service temperature rise range under extreme conditions: simulating the actual working temperature range of the leaf spring under long-term driving and high-temperature environment.

[0037] Step S122: The cyclic bending fatigue test is performed on the leaf spring specimen under the corresponding working conditions to obtain the measured attenuation data of residual stress, which is used to correct the prediction deviation of the finite element simulation.

[0038] Step S123: The finite element simulation and the cyclic bending fatigue test are mutually iteratively verified until the deviation between the simulation prediction value and the test measurement value is within the preset tolerance range. At this time, the residual stress relaxation data obtained is used to establish the attenuation curve.

[0039] Step S2: Using the effective residual compressive stress threshold during the entire service life of the leaf spring as a constraint, solve the required initial residual compressive stress field in reverse based on the residual stress decay dynamic model.

[0040] In this embodiment, the effective residual compressive stress threshold during the entire service life of the leaf spring is determined by the lower limit of the residual compressive stress under the target number of cycles during the entire service life of the leaf spring, and the value is not lower than -300MPa.

[0041] In one embodiment, step S2, using the effective residual compressive stress threshold over the entire service life of the leaf spring as a constraint, and inversely solving the required initial residual compressive stress field based on the residual stress decay dynamic model, includes: Substituting the target number of cycles for the entire service life of the leaf spring into the mapping relationship, and using the effective residual compressive stress threshold within the entire service life of the leaf spring as the dependent variable value under the target number of cycles, and using the load amplitude and the service temperature as known parameters in the mapping relationship, the mapping relationship is inversely operated to obtain the peak value of residual compressive stress and the effective compressive stress layer depth when the number of cycles is zero; wherein, the peak value of residual compressive stress and the effective compressive stress layer depth jointly characterize the amplitude characteristics and depth distribution characteristics of the initial residual compressive stress field.

[0042] In this embodiment, the number of cycles is a design life indicator for vehicle operation. For example, if a leaf spring is designed to withstand 1 million alternating loads, then 1 million cycles is used as the number of cycles parameter in the function, representing the state at the end of the leaf spring's lifespan.

[0043] Step S3: Based on the characteristic parameters of the initial residual compressive stress field, match and determine the corresponding shot peening process parameter set.

[0044] In this embodiment, the initial residual compressive stress field characteristic parameters are: the required stress value, stress layer depth, stress uniformity, and other indicators.

[0045] Different shot peening parameters will introduce residual compressive stresses of different depths and magnitudes. By accurately matching the parameters, we can ensure that the initial residual compressive stress field of the design can be accurately reproduced after shot peening.

[0046] In one embodiment, step S3, determining the corresponding shot peening process parameter set based on the characteristic parameters of the initial residual compressive stress field, includes: Step S31: Establish a mapping relationship between residual stress characteristic parameters and shot peening process parameters in advance. The mapping relationship takes the residual compressive stress peak value and the effective compressive stress layer depth as inputs and shot peening intensity, shot kinetic energy and surface coverage rate as outputs.

[0047] In this embodiment, the residual compressive stress peak value is used to determine the required shot peening intensity level, the effective compressive stress layer depth is used to determine the required shot kinetic energy range, and the surface coverage is determined according to the requirements of the uniformity of the residual compressive stress field. Step S32: Substitute the residual compressive stress peak value and the effective compressive stress layer depth into the mapping relationship, and match and output the corresponding shot peening intensity value, shot kinetic energy value and surface coverage value to form the shot peening process parameter set.

[0048] In this embodiment, shot peening intensity is the core indicator characterizing the impact force of shot peening (usually measured using an Almen specimen). The higher the residual compressive stress value that needs to be introduced, the higher the corresponding shot peening intensity level, and the two are positively correlated.

[0049] The greater the kinetic energy of the projectile, the deeper its impact on the leaf spring, and the deeper the effective compressive stress layer can be formed inside the material; therefore, the deeper the designed effective compressive stress layer, the higher the required kinetic energy of the projectile (the kinetic energy of the projectile is determined by the projectile diameter, material, and ejection speed).

[0050] Surface coverage refers to the proportion of shot peening that covers the surface of the leaf spring. The higher the coverage, the more uniform the distribution of residual compressive stress. If a uniform initial residual compressive stress field with no local weak areas is required, a higher coverage is matched.

[0051] In one embodiment, the method further includes: heating the quenched and tempered leaf spring to a preset shot peening temperature, and applying a bending prestress consistent with the service force direction of the leaf spring through a tooling fixture, specifically including: The magnitude of the applied bending prestress is positively correlated with the peak value of the residual compressive stress. The higher the peak value of the residual compressive stress, the greater the applied bending prestress, so that the tension surface of the leaf spring is in a prestress state that matches the target residual compressive stress field during the shot peening process. The preset temperature range for heating is determined based on the contribution ratio of the service temperature to the attenuation rate coefficient.

[0052] In this embodiment, the greater the contribution of the service temperature to the decay rate coefficient, the higher the heating temperature is, so as to promote the dynamic recovery of dislocations through the thermal activation effect during the high-temperature shot peening process and form an initial dislocation configuration that is beneficial to subsequent cryogenic stabilization treatment.

[0053] The preset shot peening temperature is ~250℃, and the heating method is uniform heating by hot air circulation with a holding time of 15~30min to ensure uniform temperature of the leaf spring. The applied bending prestress is 30%~60% of the yield strength of the leaf spring material, and the prestress direction is consistent with the tensile direction of the leaf spring during service, so that the tensile surface of the leaf spring is in a tensile prestress state during the shot peening process.

[0054] The shot peening process parameters are determined based on the residual stress decay dynamic model, ensuring that the initial residual compressive stress peak value of the leaf spring surface after shot peening is ≥-800MPa and the effective compressive stress layer depth is ≥0.2mm, so that the residual stress remains higher than the effective residual compressive stress threshold after decay throughout the entire service cycle.

[0055] Step S4: After heating the quenched and tempered leaf spring and applying bending prestress consistent with the direction of service force, perform high-temperature shot peening treatment according to the process parameter set.

[0056] In this embodiment, the leaf spring first undergoes basic heat treatment to ensure the strength and toughness of the substrate; the leaf spring is then heated to a temperature suitable for high-temperature shot peening to improve the material's plasticity, prevent surface cracking caused by shot peening impact, and allow residual compressive stress to be introduced deeper and more stably; a bending preload is applied that is completely consistent with the actual service stress direction to simulate the actual stress state of the leaf spring; during shot peening, the preload and shot peening compressive stress are superimposed to ensure that the final distribution of residual compressive stress perfectly matches the actual stress scenario of the leaf spring, avoiding misalignment between stress distribution and service requirements, and maximizing the strengthening effect.

[0057] Step S5: Perform cryogenic stabilization treatment on the leaf spring after high-temperature shot peening to suppress the attenuation rate of residual compressive stress during service.

[0058] In this embodiment, cryogenic stabilization treatment involves placing the shot-peened leaf spring in an ultra-low temperature environment. This process refines the internal structure of the material, stabilizes the dislocation structure, and locks in the introduced residual compressive stress. After cryogenic stabilization, the rate of residual compressive stress decay is effectively controlled throughout the entire service life of the leaf spring. Even at the end of its service life, the remaining residual compressive stress remains no lower than the effective residual compressive stress threshold, ensuring fatigue resistance and preventing early fatigue failure.

[0059] In one embodiment, step S5, performing cryogenic stabilization treatment on the leaf spring after high-temperature shot peening, includes: After the leaf springs are shot-peened at high temperature, they are naturally cooled to room temperature, then slowly reduced to cryogenic temperature at a preset cooling rate, and held at cryogenic temperature for a preset time. Finally, they are uniformly heated back to room temperature at a preset heating rate.

[0060] In this embodiment, the values ​​of the cooling rate, cryogenic temperature, and holding time satisfy the following conditions: while avoiding thermal stress cracking of the leaf spring due to drastic temperature changes, the decay rate coefficient of the treated leaf spring in the residual stress decay kinetic model is lower than the corresponding value before cryogenic treatment; the cooling rate does not exceed 5℃ / min, the cryogenic temperature is in the range of -160℃ to -196℃, and the holding time is in the range of 1 to 4h, so as to promote the uniform precipitation of dispersed carbides and dislocation pinning in the leaf spring matrix, thereby suppressing the decay rate of residual compressive stress during subsequent service.

[0061] The reduction in the attenuation rate coefficient is controlled by adjusting the cooling rate, cryogenic temperature, and holding time of the cryogenic stabilization treatment. The relationship between the reduction magnitude and the cooling rate, cryogenic temperature, and holding time is pre-calibrated through process experiments, and is used to select specific process parameters for cryogenic treatment based on the target reduction magnitude.

[0062] In one embodiment, the high-temperature shot peening treatment employs a two-stage shot peening process, which includes coarse shot peening and fine shot peening, wherein: The coarse shot peening uses large-diameter shot to form a plastic deformation layer in the depth direction of the leaf spring, so that the distribution depth of residual compressive stress along the depth direction meets the effective compressive stress layer depth requirement obtained by the reverse solution. The shot peening uses small-diameter shot to intensively impact the surface of the leaf spring to smooth the surface micro-morphology, thereby reducing surface roughness and increasing the amplitude of surface residual compressive stress, so that the surface residual compressive stress reaches the peak value requirement of residual compressive stress obtained by the reverse solution. The deep residual compressive stress formed by the coarse shot peening and the surface residual compressive stress peak reached by the fine shot peening together constitute a residual compressive stress distribution consistent with the initial residual compressive stress field obtained by the inverse solution.

[0063] Specifically, rough shot peening is first performed using large-diameter shot with a diameter of 0.8-2.0 mm. The high impact kinetic energy forms a plastic deformation layer with a depth of not less than 0.5 mm to construct a deep residual compressive stress field. Then, fine shot peening is performed using small-diameter shot with a diameter of 0.2-0.6 mm. The dense shallow impact smooths the micro-protrusions on the cold-forged surface to optimize the surface residual stress, increase the amplitude of the surface residual compressive stress, and reduce the surface roughness to Ra≤0.8μm.

[0064] In one embodiment, after the cryogenic stabilization treatment, the leaf spring is further subjected to a low-temperature tempering treatment to eliminate the brittleness and micro-stress generated by the newly formed martensite during the cryogenic treatment. The temperature of the low-temperature tempering is lower than the tempering temperature during the quenching and tempering of the leaf spring.

[0065] Based on steps S1-S5, the present invention firstly constructs a dynamic model of residual stress decay under actual service conditions of leaf springs, breaking the misconception of blindly pursuing high initial stress in existing processes. With the core objective of ensuring that the residual stress is always higher than the effective threshold throughout the entire service cycle, the process parameters are designed in reverse to achieve active control of residual stress. Secondly, high-temperature prestressed shot peening and cryogenic treatment form a strong synergistic effect: high-temperature shot peening, through plastic deformation in a hot state, obtains an initial residual compressive stress field with a higher amplitude, greater depth, and more uniform distribution than that of room-temperature shot peening. At the same time, through dynamic recovery, it forms a more stable dislocation cellular structure, providing a high-quality microstructure and stress substrate for subsequent cryogenic treatment. Subsequent cryogenic treatment promotes the uniform precipitation of nanoscale carbides in the matrix, pins the dislocation configuration introduced by high-temperature shot peening, significantly increases the resistance to dislocation movement, and fundamentally suppresses the relaxation and attenuation of residual stress under cyclic loading and temperature rise. At the same time, it completely transforms the small amount of residual austenite induced by shot peening strain into stable martensite, avoiding stress abrupt changes caused by phase transformation during service, and further stabilizing the residual stress field. Finally, low-temperature tempering eliminates the brittleness and internal stress of the newly formed martensite after cryogenic treatment, ensuring high residual compressive stress while taking into account the impact toughness of the leaf spring, thus achieving comprehensive optimization of the leaf spring's strength, toughness, fatigue life, and stress stability.

[0066] Example 1 This embodiment focuses on 60Si2Mn variable cross-section steel leaf springs for commercial vehicles. The leaf springs are oil-quenched at 870℃ and tempered at 430℃, resulting in a base hardness of HRC48~50 and a yield strength ≥1200MPa. The specific processing method includes the following steps: S1. Construction of Residual Stress Residual Stress Attenuation Dynamic Model and Determination of Target Threshold: The material performance parameters of the leaf spring, the service load spectrum of the matching heavy truck model (maximum alternating bending stress 750MPa), and the service temperature rise under extreme working conditions (maximum 80℃) were collected. Through ANSYS finite element simulation combined with indoor three-point bending cyclic fatigue test, a dynamic model of residual stress attenuation under cyclic load and temperature rise was established, and the effective residual compressive stress threshold of the leaf spring in the whole service cycle was determined to be -350MPa. S2. High-temperature prestressed shot peening based on residual stress attenuation kinetic model: The tempered leaf spring is heated to 350℃ in a hot air circulating furnace and held for 20 minutes to ensure uniform temperature throughout the leaf spring; a bending prestress consistent with the service force direction is applied to the leaf spring using a special tooling fixture, with a prestress magnitude of 480MPa (40% of the material yield strength); the shot peening process parameters are matched based on the residual stress attenuation kinetic model in step S1: HRC60~62 steel wire shot is used, and secondary grade shot peening is adopted. The coarse shot peening uses 1.0mm diameter shot, with a shot peening intensity of 0.6 Amm and a coverage of 200%; the fine shot peening uses 0.4mm diameter shot, with a shot peening intensity of 0.20 Amm and a coverage of 100%; high-temperature shot peening is performed, and the initial residual compressive stress peak value on the surface of the leaf spring after shot peening is -806MPa, and the effective compressive stress layer depth is 0.25mm; S3. Cryogenic Stabilization Treatment: After high-temperature shot peening, the leaf spring is naturally cooled to room temperature, placed in a programmable cryogenic chamber, and cooled to -180℃ at a rate of 3℃ / min. After holding at this temperature for 2 hours, it is uniformly heated back to room temperature at a rate of 3℃ / min. Example 2 This embodiment focuses on processing 50CrVA high-stress steel leaf springs used in high-end buses. The leaf springs undergo oil quenching at 860℃ followed by tempering at 450℃, resulting in a base hardness of HRC49~51 and a yield strength ≥1300MPa. The specific processing method includes the following steps: S1. Residual stress: Construction of the dynamic model for residual stress attenuation and determination of the target threshold: The material performance parameters of the leaf spring, the service load spectrum of the matching bus model (maximum alternating bending stress 800MPa), and the service temperature rise under extreme conditions (maximum 70℃) were collected. Through finite element simulation combined with cyclic fatigue test, a dynamic model for residual stress attenuation was established, and the effective residual compressive stress threshold of the leaf spring during the entire service cycle was determined to be -380MPa. S2. High-temperature prestressed shot peening based on residual stress attenuation dynamic model: The tempered leaf spring is heated to 380℃ and held for 15 min, and a bending prestress of 650 MPa (50% of the material yield strength) is applied; the shot peening process parameters are matched: steel wire cut shot with HRC60~63 is used, and two-stage shot peening is used. The coarse shot peening uses 0.8 mm diameter shot, shot peening intensity of 0.5 A mm, and coverage of 200%; the fine shot peening uses 0.3 mm diameter shot, shot peening intensity of 0.18 A mm, and coverage of 100%; the initial residual compressive stress peak value of the leaf spring surface after shot peening is -824 MPa, and the effective compressive stress layer depth is 0.3 mm; S3. Cryogenic Stabilization Treatment: After high-temperature shot peening, the leaf spring is cooled to room temperature, then lowered to -190℃ at a rate of 4℃ / min, held at that temperature for 3 hours, and then raised back to room temperature at a rate of 4℃ / min. Comparative Example 1 This comparative example uses existing conventional processes and treats the same 60Si2Mn variable cross-section steel leaf spring as Example 1. It adopts the industry-leading room temperature stress shot peening process with the following parameters: applying a bending prestress of 480MPa at room temperature, using 0.6mm steel wire shot, shot peening intensity of 0.45Amm, surface coverage of 200%, and no cryogenic treatment or low-temperature tempering is performed after shot peening.

[0067] Comparative Example 2 This comparative example uses the existing conventional high-temperature shot peening process. The object treated is the same 60Si2Mn variable cross-section steel leaf spring as in Example 1. High-temperature prestressed shot peening at 350℃ is used, and the parameters are completely consistent with step S2 of Example 1. No cryogenic treatment or low-temperature tempering is performed after shot peening.

[0068] Performance Testing and Comparison The leaf spring samples treated in Examples 1, 2, 1, and 2 were subjected to performance tests. The test items included: initial residual compressive stress, residual compressive stress and retention rate after 1 million cycles of bending fatigue, bending fatigue life (50% survival rate), and static stress relaxation rate. The test results are shown in the table below. Table 1

[0069] The test results show that, compared with the existing conventional process, the leaf spring of the present invention has a residual compressive stress retention rate of more than 30%, a bending fatigue life of more than 50%, and a significant reduction in static stress relaxation rate. It perfectly achieves stable control of residual stress throughout the entire service life, and the overall performance has been greatly improved, verifying the significant beneficial effects of the present invention.

[0070] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the original technical features, and the technical solutions resulting from these changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A composite strengthening treatment method for automotive leaf springs based on residual stress attenuation regulation, characterized in that, Includes the following steps: Based on the service conditions of the target automotive leaf spring, a dynamic model of residual stress decay under the coupled action of cyclic load and service temperature rise is constructed, and the effective residual compressive stress threshold during the entire service cycle of the leaf spring is determined. Using the effective residual compressive stress threshold during the entire service life of the leaf spring as a constraint, the initial residual compressive stress field required is solved in reverse based on the residual stress attenuation dynamic model. Based on the characteristic parameters of the initial residual compressive stress field, the corresponding shot peening process parameter set is determined. After the leaf springs that have been quenched and tempered are heated and subjected to bending prestress in the same direction as the service stress, high-temperature shot peening is performed according to the process parameter set. Deep cryogenic stabilization treatment is applied to leaf springs after high-temperature shot peening to suppress the attenuation rate of residual compressive stress during service.

2. The method according to claim 1, characterized in that, Based on the service conditions of the target automotive leaf spring, a dynamic model for residual stress decay under the coupled action of cyclic load and service temperature rise is constructed, including: Based on the service conditions, a preset alternating load range and temperature conditions are determined, and residual stress relaxation data of the target leaf spring under the alternating load and temperature conditions are collected. The residual stress relaxation data reflects the degradation law of residual compressive stress with the number of cycles under different load amplitudes and temperatures. Based on the residual stress relaxation data, a decay curve of residual stress as a function of cycle number is established, and the decay curve is fitted to obtain the mapping relationship between residual stress decay and cycle number, load amplitude, and service temperature.

3. The method according to claim 2, characterized in that, The residual stress relaxation data were obtained through finite element simulation combined with cyclic bending fatigue tests. The finite element simulation takes the material performance parameters of the target leaf spring, the service load spectrum of the vehicle suspension, and the service temperature rise range under extreme conditions as input conditions to simulate the residual stress evolution trend of the leaf spring under the coupled action of cyclic load and temperature rise, and outputs the predicted value of residual stress under different conditions. The cyclic bending fatigue test verifies the leaf spring specimen under the corresponding working conditions and obtains the measured attenuation data of residual stress, which is used to correct the prediction deviation of the finite element simulation. The finite element simulation and the cyclic bending fatigue test are iteratively verified until the deviation between the simulation prediction value and the test measurement value is within the preset tolerance range. At this time, the residual stress relaxation data obtained is used to establish the attenuation curve.

4. The method according to claim 2, characterized in that, The effective residual compressive stress threshold during the entire service life of the leaf spring is determined by the lower limit of the residual compressive stress under the target number of cycles during the entire service life of the leaf spring based on the mapping relationship. Using the effective residual compressive stress threshold within the entire service life of the leaf spring as a constraint, the initial residual compressive stress field required for inverse solution based on the residual stress attenuation dynamic model includes: substituting the target number of cycles within the entire service life of the leaf spring into the mapping relationship, using the effective residual compressive stress threshold within the entire service life of the leaf spring as the dependent variable value under the target number of cycles, using the load amplitude and the service temperature as known parameters in the mapping relationship, performing inverse operation on the mapping relationship, and inversely solving to obtain the peak value of residual compressive stress and the effective compressive stress layer depth when the number of cycles is zero; wherein, the peak value of residual compressive stress and the effective compressive stress layer depth jointly characterize the amplitude characteristics and depth distribution characteristics of the initial residual compressive stress field.

5. The method according to claim 4, characterized in that, Based on the characteristic parameters of the initial residual compressive stress field, the corresponding shot peening process parameter set is determined by matching and determining the following: A mapping relationship between residual stress characteristic parameters and shot peening process parameters is established in advance. The mapping relationship takes the peak value of residual compressive stress and the effective compressive stress layer depth as inputs and shot peening intensity, shot kinetic energy and surface coverage as outputs. Substituting the residual compressive stress peak value and the effective compressive stress layer depth into the mapping relationship, the corresponding shot peening intensity value, shot kinetic energy value and surface coverage value are matched and output to form the shot peening process parameter set.

6. The method according to claim 5, characterized in that, This also includes heating the quenched and tempered leaf springs and applying bending prestress, specifically including: The magnitude of the applied bending prestress is positively correlated with the peak value of the residual compressive stress. The higher the peak value of the residual compressive stress, the greater the applied bending prestress, so that the tension surface of the leaf spring is in a prestress state that matches the target residual compressive stress field during the shot peening process. The preset temperature range for heating is determined based on the contribution ratio of the service temperature to the attenuation rate coefficient.

7. The method according to claim 6, characterized in that, Cryogenic stabilization treatment for leaf springs after high-temperature shot peening includes: After the leaf springs are shot-peened at high temperature, they are naturally cooled to room temperature, then slowly reduced to cryogenic temperature at a preset cooling rate, and held at cryogenic temperature for a preset time. Finally, they are uniformly heated back to room temperature at a preset heating rate.

8. The method according to claim 7, characterized in that, The reduction in the attenuation rate coefficient is controlled by adjusting the cooling rate, cryogenic temperature, and holding time of the cryogenic stabilization treatment.

9. The method according to claim 6, characterized in that, The high-temperature shot peening treatment employs a two-stage shot peening process, which includes coarse shot peening and fine shot peening, wherein: The coarse shot peening uses large-diameter shot to form a plastic deformation layer in the depth direction of the leaf spring, so that the distribution depth of residual compressive stress along the depth direction meets the effective compressive stress layer depth requirement obtained by the reverse solution. The shot peening uses small-diameter shot to intensively impact the surface of the leaf spring to smooth the surface micro-morphology, thereby reducing surface roughness and increasing the amplitude of surface residual compressive stress, so that the surface residual compressive stress reaches the peak value requirement of residual compressive stress obtained by the reverse solution. The deep residual compressive stress formed by the coarse shot peening and the surface residual compressive stress peak reached by the fine shot peening together constitute a residual compressive stress distribution consistent with the initial residual compressive stress field obtained by the inverse solution.

10. The method according to claim 7, characterized in that, Following the cryogenic stabilization treatment, the leaf spring is further subjected to a low-temperature tempering treatment, wherein the low-temperature tempering temperature is lower than the tempering temperature during the quenching and tempering of the leaf spring.

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