Method for improving wear resistance of carburized gear steel surface based on laser shot peening technology

Abstract

The invention relates to the technical field of gear surface strengthening, in particular to a method for improving carburized gear steel surface wear resistance based on a laser peening technology, which comprises the following steps of: firstly, carburizing a gear steel sample to form a high-carbon surface layer by combining carburizing heat treatment with the laser peening technology; and then carrying out laser shot peening treatment by adopting high-power-density short-pulse laser. Plasma shock waves generated in the laser shot peening process induce the surface layer of the material to generate plastic deformation, unstable retained austenite is promoted to be converted into martensite, and the phenomenon of abnormal microhardness distribution is eliminated. According to the method, a high-amplitude residual compressive stress field can be excited in a carburized layer, a gradient nano structure is formed through induction, residual compressive stress distribution is improved, and the depth of an effective hardened layer is increased, so that the surface wear resistance and crack initiation resistance of the gear under the working conditions of high contact fatigue and heavy-load wear are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of gear surface strengthening, in particular to the composite strengthening direction of carburizing heat treatment and laser shock wave plastic deformation in the surface modification treatment of metal materials, and covers the material performance testing and analysis method. BACKGROUND

[0002] The existing carburizing heat treatment technology is a surface hardening method, aiming to solve the contradictory demand that the surface of steel parts is easy to wear and the core needs to maintain toughness during service; the process first places the parts in a carbon-rich atmosphere for heating, allowing carbon atoms to diffuse and penetrate into the surface layer, forming a high carbon concentration gradient; then quenching treatment is carried out, rapid cooling is used to promote the surface to undergo martensitic transformation, thereby obtaining high hardness; but quenching may introduce brittleness and internal stress, so the tempering step is used to adjust the structure, reduce brittleness and improve toughness; finally, the surface of the part has excellent wear resistance, and the core maintains good toughness, prolonging the service life under harsh working conditions.

[0003] The existing shot peening strengthening technology is a mechanical surface modification method, its core principle is to use high-speed projectile flow to impact the surface of metal parts, so that the surface layer material undergoes plastic deformation; the deformation layer cannot freely stretch due to the constraint of the lower layer material, thereby introducing a residual compressive stress field in the near-surface region. This residual compressive stress can effectively offset the tensile stress that the part bears under cyclic loading, significantly inhibiting the initiation and propagation of micro-cracks, thereby improving the fatigue resistance of the part. At the same time, the impact process also makes the surface layer structure work hardening, further improving the wear resistance. However, process control has an important influence on the effect, and the projectile parameters and spray angle need to be reasonably selected to obtain beneficial compressive stress while avoiding excessive increase in surface roughness or change in geometric size.

[0004] The existing carburizing heat treatment technology has the following technical pain points, specifically, during the carburizing and quenching process, a large number of unstable residual austenite is easily formed in the surface layer of gear steel, causing abnormal microhardness distribution, with surface hardness lower than subsurface hardness, and insufficient effective hardening layer depth, and the residual compressive stress distribution is not ideal, with the maximum value located in the subsurface rather than the outer surface, resulting in significant reduction in surface wear resistance and crack initiation resistance under high contact fatigue and heavy load wear conditions, such as automotive transmission gears or wind turbine gearbox application scenarios, which easily leads to early wear and fatigue failure; the fundamental reason is that the austenite transformation is not complete during the quenching cooling process after carburizing, and the residual phase has poor stability, which is easy to deform plastically or change phase under external load, thereby weakening the surface integrity and promoting crack initiation and propagation, and further affecting the reliability and life of the gear transmission system. SUMMARY

[0005] In view of the prior art deficiencies, the present application provides a method for improving the surface wear resistance of carburized gear steel based on laser peening technology, which solves the technical problems that a large number of unstable residual austenite exists in the surface layer of the gear steel after carburizing and quenching, causing the phenomenon of "lifting head" of the surface microhardness (i.e., the surface hardness is lower than the subsurface hardness), insufficient effective hardening layer depth, and non-ideal residual compressive stress distribution (the maximum value is located in the subsurface rather than the outer surface), thereby significantly reducing the surface wear resistance and crack initiation resistance of the gear under high contact fatigue and heavy load wear conditions.

[0006] To solve the above technical problems, the specific content of the present application is as follows: The present application provides a method for improving the surface wear resistance of carburized gear steel based on laser peening technology, comprising: Step 1: obtaining a gear steel sample, which includes predetermined geometric size and material grade data; Step 2: removing the surface scale of the gear steel sample by polishing to obtain a gear steel sample after surface pretreatment; Step 3: placing the gear steel sample after surface pretreatment in a carburizing atmosphere, using carrier gas and enriched gas for carburizing heat treatment, which includes strong carburizing treatment, diffusion treatment, quenching treatment and tempering treatment, thereby forming a high-carbon surface layer; Step 4: performing laser peening treatment on the gear steel sample after carburizing heat treatment, wherein the surface of the gear steel sample is irradiated by a high-power density short pulse laser, and a transparent confinement layer and an absorbing layer are used to induce plasma shock waves to cause plastic deformation of the surface of the gear steel sample; Step 5: performing microhardness testing on the gear steel sample after laser peening treatment, measuring the hardness value along the direction from the surface layer to the center, and obtaining hardness distribution data; Step 6: performing residual stress testing on the gear steel sample after laser peening treatment, using an X-ray stress tester to measure the residual stress, and obtaining residual stress distribution data; Step 7: determining the maximum compressive stress region based on the residual stress distribution data, cutting a sample from the corresponding position for microstructure analysis, and obtaining microstructure analysis results, which include observing the gradient nanostructure and analyzing the distribution characteristics of nanocrystalline and nanotwins; Step 8: based on the microstructure analysis results, preparing a friction and wear test pin from the region with gradient nanostructure, selecting a friction pair material for wear test, measuring the mass loss and calculating the wear resistance index to verify the wear resistance improvement effect of the gear steel sample.

[0007] Further, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology provided by the present application, wherein the step 3 comprises: The surface pretreated gear steel sample is placed in a carburizing atmosphere, and methanol is used as a carrier gas and kerosene is used as a rich gas; A strong carburizing treatment is performed by maintaining a strong carburizing potential for a strong carburizing time; A diffusion treatment is performed by maintaining a diffusion potential for a diffusion time; The diffusion-treated gear steel sample is cooled to a predetermined temperature in the furnace, and then the gear steel sample is taken out for quenching and tempering treatment under the protection of a carbon potential atmosphere.

[0008] Further, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology according to the present application, step 4 comprises: A YAG laser operating system is used to configure laser parameters for the gear steel sample after carburizing heat treatment, and the laser parameter configuration includes setting the repetition frequency, wavelength, pulse duration and spot diameter; Based on the laser parameter configuration, the overlap rate between two adjacent spots is adjusted; Flowing water is used as a transparent confinement layer and black tape is used as an absorbing layer to cover the surface of the gear steel sample; The surface of the gear steel sample is irradiated by high-energy laser pulses to induce plasma shock waves to cause plastic deformation of the surface of the gear steel sample.

[0009] Further, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology according to the present application, step 6 further comprises: Microhardness testing is performed on the gear steel sample after laser peening treatment, the hardness value is measured along the surface layer to the center direction, and hardness distribution data is obtained; Based on the hardness distribution data, the effective hardening layer depth is determined, and according to GB / T9450-2005 "Determination and verification of hardening layer depth of steel parts", the area with a hardness value higher than 550HV is regarded as the effective hardening layer depth; Residual stress testing is performed on the gear steel sample after laser peening treatment, and an X-ray stress tester is used to measure the residual stress at different depths to obtain residual stress distribution data; Based on the hardness distribution data and the residual stress distribution data, the surface wear resistance improvement effect is verified.

[0010] Further, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology according to the present application, step 7 comprises: Based on the residual stress distribution data, the maximum compressive stress region is determined, and a thin section sample is cut from the corresponding position on the surface of the gear steel sample after laser peening treatment; The cut thin section sample is ground with sandpaper and electrolytically polished; The gradient nanostructure of the polished thin section sample is observed using a transmission electron microscope; The distribution characteristics of nanocrystalline grains and nanotwins in the transmission electron microscope observation result are analyzed.

[0011] Further, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology comprises the following steps: Based on the microstructure analysis results, a friction and wear test pin is prepared from the region with gradient nanostructure; The bearing steel GCr15 is selected as the friction pair material; The prepared friction and wear test pin and the selected friction pair material are used to perform dry friction and wear test, and the mass loss of the test pin is measured; Based on the mass loss data, the average wear amount and the absolute wear resistance are calculated.

[0012] Further, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology comprises the following steps: Based on the mass loss data, the wear data difference of the gear steel sample before and after laser peening treatment is compared; The influence degree of the gradient nanostructure on the wear resistance is analyzed in combination with the wear data difference and the microstructure analysis results; According to the influence degree analysis results, the laser peening parameter configuration is optimized; Based on the optimized laser peening parameter configuration, the correlation between the residual compressive stress distribution and the wear resistance improvement effect is verified.

[0013] Further, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology comprises the following steps: Based on the optimized laser peening parameter configuration, the microhardness distribution and the residual stress distribution performance of the gear steel samples treated by different laser energies are compared, and the best laser energy range is determined based on the performance comparison results; Based on the best laser energy range, the laser peening parameters are configured, the laser peening treatment is integrated into the preset carburizing production line, and an integrated laser peening treatment carburizing production line is formed, and the integrated laser peening treatment carburizing production line is applied to the manufacturing of heavy load gear parts.

[0014] Further, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology comprises the following steps: Based on the integrated environment of the carburizing production line, the carbon potential and the temperature in the carburizing heat treatment process are monitored to obtain carbon potential temperature data; Based on the carbon potential temperature data and the gear surface geometric characteristics, the scanning strategy of the laser peening treatment is controlled; Based on the scanning strategy and the real-time processing feedback obtained from the laser power monitor and the surface topography sensor, the parameters of the transparent limiting layer and the absorbing layer are adjusted; By adjusting the laser pulse energy, spot overlap rate and limiting layer parameters, the surface roughness of the gear steel sample is controlled to meet the surface quality requirements of the preset roughness average value range.

[0015] Further, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology comprises the following steps: Based on the gear steel sample after laser peening treatment, metallographic structure analysis is carried out to obtain metallographic structure data; Based on the metallographic structure data, the content and morphological change of residual austenite are detected to obtain residual austenite data; Based on the metallographic structure data and the residual austenite data, the wear behavior under high contact fatigue working condition is simulated to obtain wear simulation data; Based on the metallographic structure data, the residual austenite data and the wear simulation data, a surface strengthening effect report is output.

[0016] The present application has the following advantages: The present application uses a composite strengthening method of carburizing heat treatment and laser peening technology, first forms a high-carbon surface layer on the surface of the gear steel by carburizing treatment to provide basic hardness, then makes the surface layer undergo ultra-high strain rate plastic deformation by the plasma shock wave induced by high-power density short pulse laser, effectively promotes the unstable residual austenite to transform into martensite, and eliminates the abnormal phenomenon of microhardness distribution; the high-amplitude residual compressive stress field excited in the carburized layer by laser peening treatment is spatially matched with the depth of the hardened layer, and the gradient nanostructure induced forms significantly improve the surface hardness and plastic deformation resistance, and the maximum value of the residual compressive stress distribution on the outermost surface can effectively inhibit the initiation and propagation of surface cracks; the synergistic effect of carburizing treatment and laser peening not only increases the effective hardened layer depth, but also significantly improves the surface wear resistance and crack initiation resistance of the gear under high contact fatigue and heavy load wear conditions by optimizing the stress distribution and microstructure, and the method can adapt to different modulus gear curves by adjusting the laser parameters, and has good engineering applicability. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings needed to be used in the embodiments, and obviously, other drawings can be obtained by the drawings without creative labor for those skilled in the art.

[0018] Figure 1 The flowchart of the method for improving the surface wear resistance of carburized gear steel based on laser peening technology; Figure 2 The microhardness comparison chart of carburized 20CrNiMo gear steel samples after laser peening treatment with different energy; Figure 3Residual stress comparison chart of carburized 20CrNiMo gear steel after different energy laser peening treatment; Figure 4 SEM image of gear steel along the carburizing direction at different depths after carburizing + 4J laser peening; Figure 5 Friction and wear test data chart (average wear and absolute wear resistance) of carburized 20CrNiMo gear steel after different energy laser peening treatment; Figure 6 Figure 4 Transmission electron microscope image of position A in the scanning electron microscope image; Figure 7 Figure 4 Transmission electron microscope image of position B in the scanning electron microscope image; Figure 8 Figure 4 Transmission electron microscope image of position C in the scanning electron microscope image. DETAILED DESCRIPTION

[0019] In order to make the technical solutions of the present application clearer, the present application will be described below in conjunction with specific embodiments of the present application and corresponding drawings. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application. The present application provided by each embodiment of the present application will be described in detail below in conjunction with the drawings. In order to better understand the purpose of the present application, the present application will be further described in detail below.

[0020] The strengthening method of the present application forms a high-hardness surface layer by carburizing the gear steel first, and then implements laser peening post-treatment: using laser shock waves to excite a high-amplitude residual compressive stress field in the carburized layer, and at the same time, by controlling the laser action area to cover the key stress surface of the gear, the residual compressive stress distribution and the depth of the carburized hardened layer form a spatial match, thereby synergistically improving the surface microhardness and the plastic deformation resistance, and finally significantly enhancing the wear resistance of the gear under frictional working conditions. Finally, the synergistic improvement of microhardness, residual stress and wear resistance is realized.

[0021] EMBODIMENT In this embodiment, gear steel 20CrNiMo (gear steel 8620 according to ASTM / SAE standard) of GB / T5216 standard is selected as the research object.

[0022] The method for improving the surface wear resistance of carburized gear steel based on laser peening technology in this embodiment includes the following steps: ​​​The 20CrNiMo gear steel was cut into 20mm x 20mm x 10mm cubic samples by wire cutting, and the surface of the samples was polished with sandpaper to remove the oxide skin. Then the samples were carburized. In the carburizing process, methanol and kerosene were selected as the carburizing medium. The carburizing temperature was 930℃, the strong carburizing potential was 1.15%, the strong carburizing time was 5h, the diffusion carbon potential was 0.7%, the diffusion time was 2h, and then the sample was cooled to 830℃ in the furnace, and then the sample was taken out and oil quenched, and then the sample was tempered at 180℃ for 4h.

[0023] The laser peening experiment used a laser operating system from Xi'an Tianruida Optoelectronic Technology Co., Ltd. The repetition frequency was 10Hz, the wavelength was 1064nm, the pulse duration was 20ns, the spot diameter was 3mm, and the overlap rate between two adjacent spots was 50%. A flowing water layer with a thickness of 2mm was used as a transparent confinement layer, and a black tape with a thickness of 0.1mm was used as an absorbing layer to protect the sample surface from thermal ablation. After the high-energy laser pulse was incident, the strip material was ablated, forming a high-intensity plasma shock wave confined by the water layer, which prevented the plasma from dissipating rapidly and absorbed more energy to diffuse into the metal. According to the relevant theoretical analysis, the optimal range of laser energy is 3-5J, so in this experiment, the laser energy values of 3J, 4J and 5J were used.

[0024] The obtained gear steel sample was cut vertically along the carburized surface, finely ground and mechanically polished, and then the hardness value of the section was measured using a WilsonWilbert401MVD type micro Vickers hardness tester. The hardness was measured every 100μm vertically from the surface to the center until the microhardness value was lower than 550HV (the area with a hardness value higher than 550HV was considered as the effective hardened layer, as shown by the black dashed line in Figure 2 The average microhardness value was loaded with a load of 200g for 15s, and the average microhardness value was averaged in 3 random tests to achieve accurate data. The microhardness of the gear steel under different conditions is shown in Figure 2 After carburizing treatment, the surface of the 20CrNiMo gear steel formed high-carbon acicular martensite; from the surface of the sample to the center, with the decrease of carbon content, the morphology of the martensite gradually transitioned from high-carbon acicular martensite on the surface to low-carbon lath martensite in the center. The hardness gradient curve after carburizing treatment showed a parabolic shape, and a "lifting" phenomenon occurred, because after carburizing and quenching, not only high-carbon acicular martensite structure was formed on the surface, but also a large amount of residual austenite was produced; the highest hardness value appeared in the subsurface, and the hardness gradually decreased with the decrease of carbon content from the surface to the center. As shown by the black dashed arrow in Figure 2 After high-temperature carburizing, the effective hardened layer of the 20CrNiMo gear steel sample was about 900μm thick.

[0025] The obtained gear steel sample is ultrasonically cleaned with alcohol, and the residual stress is measured by using a DST-17 X-ray stress tester by adopting the tilt fixed ψ method, and the cross-correlation method is used to determine the peak. The X-ray tube voltage is 20 KV, and the tube current is 5 mA. The test parameters are as follows: the target material is selected as Mn target, the X-ray wavelength λ is 1.544 mm, the beam diameter is 2 mm, the diffraction angle is ψ = 0°, 25°, 35° and 45°, the stress constant is -318 MPa / °, the 2θ scanning range is 151°-162°, and the scanning step is 0.1°. The electrolytic polishing machine is used for layer stripping, and the residual stress at different depths along the depth direction is measured. The electrolyte is selected as saturated sodium chloride solution with 3% glycerol, the electrolytic voltage is 18 V, the current is 3-5 A, the depth of electrolytic corrosion is controlled by controlling the polishing time, and the height difference before and after each layer stripping is measured by using the micrometer height gauge to obtain the layer stripping depth. The residual stress is measured after each layer stripping. In order to reduce the test error, the residual stress is measured at multiple points in each layer stripping area, and the values that are too large or too small are discarded, and finally the average value is taken. The comparison of the residual stresses of the gear steels under different conditions is shown in the following table. Figure 3 In addition, the effective hardening layer and residual compressive stress layer depth, surface microhardness and residual compressive stress value of the laser shot peening treatment are all greatly improved compared with those of the unshot peening treatment: the surface microhardness of the 5J laser shot peening treatment sample is as high as 847±4HV, which is increased by 30.3%; the effective hardening layer depth reaches 1130μm, which is increased by 25.6%, as shown in the following table. Figure 2 The surface residual compressive stress is increased by 147.6%, and the residual compressive stress layer depth is also significantly increased to 2500μm, with an increase of 56.25%, as shown in the following table. Figure 3 The increase of the effective hardening layer and residual compressive stress layer depth, surface microhardness and residual compressive stress value will improve the comprehensive performance of the material, so the 20CrNiMo gear steel after carburizing treatment can be selected for surface strengthening by laser shot peening treatment.

[0026] A Φ0.3mm×0.5mm round piece is cut from the surface layer after carburizing treatment by using an electric spark wire cutting machine, the round piece is pressed by a rubber, and the "8" is ground on the 400#-2000# sandpaper, and the front and back of the sample need to be ground; then electrolytic double spraying is carried out, and the prepared sample is observed by using a transmission electron microscope. The scanning electron microscope images of the gear steel after carburizing+4J laser shot peening at different depths along the carburizing direction are shown in the following table. Figure 4 As further approaching the top surface, the strain and strain rate of the top surface of the laser shot peening 4J sample reach the maximum value, a large number of nanocrystals are induced by laser shock waves, the corresponding ring-shaped diffraction spots show that there are nanocrystals, and the crystallographic orientation of these nanocrystals and nanotwins is randomly distributed, as shown in the following table. Figure 6 Figure 6 ​For the surface structure of the sample, it can be seen that the martensite lath structure is almost completely destroyed. Figure 7 For the near-surface structure of the sample, it can be seen that a large number of sub-boundaries subdivide the original coarse lath into many laths with a width of 220-300 nm, and there is a twin structure of martensite. Through analysis, it can be known that the selected area {112} <110> type twin, and there is no ridge in the morphology of the martensite twin, and the lengths of the thin sheets are not equal, and the distances between the twins are not the same, and the smallest sheet distance can reach 10 nm. Figure 8 For the subsurface structure of the sample, it can be clearly seen that there are coarse martensite laths (indicated by yellow dashed arrows), and the width ranges from 420 nm to 450 nm. This indicates that the surface of the carburized gear steel forms a gradient nanostructure after laser peening, and studies have shown that the nano-gradient structure can improve the wear resistance of the material.

[0027] Therefore, the obtained gear steel sample is made into a test pin with a diameter of Φ4.8 mm and a length of 10 mm, and the bearing steel GCr15 is selected as the material of the friction and wear disc as the friction pair. The surface roughness of the friction pair is about 0.8 μm, the hardness is about 60 HRC, the outer diameter is 44 mm, and the inner diameter is 28 mm. The surfaces of the sample and the grinding disc are polished to 2000# using sandpaper, and the sample is polished, washed with anhydrous alcohol and dried with a hair dryer before the experiment. At the beginning of the experiment, the disc is driven by a motor, and the speed can be adjusted within 10-1000 r / min. The experimental conditions are dry friction, and the environmental temperature is 25°C. The mass loss of the test pin before and after the experiment is measured by an electronic analytical balance with a precision of 0.1 mg, and the average wear volume is calculated, and the absolute wear resistance is calculated; the wear test data of the laser peening sample and the unpeened sample are shown in Table 1.

[0028] Table 1 Wear test data of laser peening sample and unpeened sample

[0029] The wear of the unpeened carburized sample is more serious. The wear resistance of the carburized sample treated by laser peening 5J is the best, with an average wear volume of 11.6 mg and an absolute wear resistance of 86.2 g -1 , which is more than one time lower than that of the unpeened sample; therefore, laser peening treatment can effectively reduce the wear volume of the 20CrNiMo carburized gear steel sample and improve the wear resistance of the material. This is due to the increase of the surface hardness of the treated sample and the strength enhancement caused by grain refinement. With the increase of the laser peening energy, a gradient nanostructure is formed on the surface of the sample after laser peening, which makes the wear volume show a decreasing trend, as shown in Figure 5 . This shows that optimizing the peening parameters can further improve the friction and wear performance of the material.

[0030] Please refer to Figure 1The application provides a method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology, comprising: Step 1, obtaining a gear steel sample, the gear steel sample comprising predetermined geometric size and material grade data; Step 1 involves obtaining a gear steel sample with predetermined geometric size and material grade data. Select gear steel grades that meet the requirements of carburizing treatment from the standard material library, such as low-carbon alloy gear steel, and prepare samples with specific shape and size by wire cutting technology. The predetermined geometric size is based on test standards or application scenario requirements, such as cubic or cylindrical shape, to adapt to the clamping and operation requirements of subsequent processing equipment. The material grade data provides the chemical composition and basic mechanical property information of the gear steel, providing a material basis for the carbon diffusion and phase transformation behavior of carburizing heat treatment.

[0031] The purpose of step 1 is to provide standardized and repeatable starting materials for the entire surface strengthening method, reducing the influence of material variation on the effectiveness of subsequent processes. The predetermined geometric size allows the sample to smoothly enter the carburizing furnace and laser shot peening system, while the material grade data realizes the carburizing compatibility and strengthening potential of the gear steel, laying a consistent condition for forming a high-carbon surface layer.

[0032] The logical relationship between step 1 and subsequent steps is reflected in the basic role of the material preparation stage. Step 2 of surface polishing treatment relies on the sample geometric size provided by step 1 to determine the treatment area and polishing range. Step 3 of carburizing heat treatment requires the carbon concentration gradient formation ability defined by the material grade data in step 1 to control the uniformity and depth of the carburized layer. Therefore, step 1 ensures the coherence and implementability of the entire method from material to performance verification by standardizing sample characteristics.

[0033] Step 2, removing the surface oxide scale of the gear steel sample by polishing treatment to obtain a gear steel sample after surface pretreatment; Step 2 involves surface pretreatment of the gear steel sample by removing the surface oxide scale through polishing treatment to obtain a gear steel sample after surface pretreatment. Polishing treatment uses sandpaper as the main tool to mechanically grind the surface of the gear steel sample to remove oxide scale and contaminants. This process is implemented by hand or mechanically, controlling the polishing force and direction to avoid surface damage while ensuring surface flatness and cleanliness. The purpose of removing the oxide scale is to expose the metal matrix to provide uniform and active surface conditions for subsequent carburizing heat treatment, as the presence of oxide scale will hinder the diffusion of carbon atoms and affect the uniformity and quality of the carburized layer formation.

[0034] The surface pretreated gear steel sample has a clean and non-oxidized surface feature, which facilitates the effective adsorption and penetration of carbon atoms in the carburizing atmosphere. The surface state after polishing is directly related to the effect of carburizing heat treatment. By eliminating the oxide scale and reducing surface defects, the gradient distribution of carbon concentration during carburizing is promoted. This step logically connects with the carburizing heat treatment of step 3. Surface pretreatment achieves full contact between the carburizing atmosphere and the metal matrix, laying the foundation for strong carburizing and diffusion treatment.

[0035] After the gear steel sample is polished, the surface roughness is controlled to avoid excessive roughness leading to uneven carburized layer or stress concentration. The pretreatment step also involves checking the surface quality to confirm that the oxide scale is completely removed and there is no residual impurity. Step 2 pretreatment is the initial link of the entire composite strengthening process, which ensures that the subsequent laser shot peening process can act on a clean surface, thereby optimizing the propagation of plasma shock waves and the plastic deformation effect. The quality of surface pretreatment directly affects the formation of gradient nanostructure and the stability of residual compressive stress distribution, ultimately contributing to the improvement of surface wear resistance.

[0036] Step 3, the surface pretreated gear steel sample is placed in a carburizing atmosphere, using carrier gas and enriched gas for carburizing heat treatment, which includes strong carburizing treatment, diffusion treatment, quenching treatment and tempering treatment, thereby forming a high-carbon surface layer; Step 3 involves placing the surface pretreated gear steel sample in a carburizing atmosphere, using carrier gas and enriched gas for carburizing heat treatment, which includes strong carburizing treatment, diffusion treatment, quenching treatment and tempering treatment, ultimately forming a high-carbon surface layer. The carburizing heat treatment process aims to introduce carbon elements on the surface of the gear steel through a controllable atmosphere environment, establishing a high-carbon concentration gradient to provide a hardness basis for subsequent laser shot peening treatment.

[0037] Methanol is used as the carrier gas and kerosene as the enriched gas for setting the carburizing atmosphere. Methanol as the carrier gas produces dilution gas, which plays a protective atmosphere and dilution role to prevent the workpiece surface from oxidizing; kerosene as the enriched gas provides a high-carbon potential environment to promote the diffusion of carbon atoms to the surface of the gear steel. The combination of carrier gas and enriched gas realizes drop-in controllable atmosphere carburizing, accurately regulating the carbon potential distribution during carburizing.

[0038] Strong carburizing treatment is performed in a strong carburizing potential environment, maintaining high carbon potential conditions for a period of time, allowing carbon atoms to rapidly penetrate the surface layer of the gear steel and form an initial high-carbon concentration region. This step accelerates carbon absorption through high carbon potential, laying the foundation for surface hardening. After strong carburizing treatment, the diffusion treatment stage is entered.

[0039] The diffusion treatment is performed in a diffusion carbon potential environment, maintaining a low carbon potential for a period of time to promote the uniform migration of surface carbon atoms to the center, eliminating the carbon concentration peak formed during the strong penetration stage, and optimizing the carbon distribution profile. The diffusion treatment achieves a smooth gradient of carbon elements between the surface and the center, avoiding tissue heterogeneity.

[0040] After the diffusion treatment is completed, the gear steel sample is cooled to a predetermined temperature in the furnace and is kept in a carbon potential atmosphere to prevent surface decarburization and stabilize the tissue state. Subsequently, quenching treatment is performed to promote martensitic transformation in the high-carbon region of the surface layer by rapid cooling to obtain high-hardness tissue. After quenching, tempering treatment is performed to heat at low temperature to adjust the martensite morphology, reduce internal stress and brittleness, and improve material toughness.

[0041] The carburizing heat treatment follows a sequential logic: the strong carburizing treatment establishes a high-carbon surface layer, the diffusion treatment balances the carbon distribution, the quenching treatment achieves tissue hardening, and the tempering treatment stabilizes the performance. The entire process is controlled by the atmosphere and temperature to form a uniform high-carbon layer on the surface of the gear steel, providing an ideal substrate for laser peening induced plastic deformation. The synergistic effect of carburizing and subsequent peening ultimately enhances the surface wear resistance and fatigue resistance.

[0042] Step 4, laser peening treatment is performed on the gear steel sample after carburizing heat treatment, in which a high-power density short pulse laser is used to irradiate the surface of the gear steel sample, and a transparent confinement layer and an absorbing layer are used to induce plasma shock waves to cause plastic deformation on the surface of the gear steel sample; Step 4 involves laser peening treatment on the gear steel sample after carburizing heat treatment, which aims to induce plasma shock waves by irradiating the sample surface with a high-power density short pulse laser to achieve surface plastic deformation. Laser peening treatment first requires configuring laser parameters, using a YAG laser operating system to set the repetition frequency, wavelength, pulse duration, and spot diameter. Parameter configuration provides the basic conditions for laser energy input, matches the laser characteristics with the material response, avoids thermal damage, and optimizes energy coupling efficiency.

[0043] After the laser parameters are configured, adjusting the overlap rate between two adjacent spots becomes a key step. The overlap rate controls the coverage of the spot scanning path, affecting the continuity and uniformity of the treated area. By reasonably setting the overlap rate, energy accumulation can be avoided or excessive, and the plastic deformation layer induced by the shock wave can completely cover the surface of the gear steel, laying the foundation for the spatial distribution of the residual compressive stress field.

[0044] Next, a transparent confinement layer and an absorbing layer are applied to cover the surface of the gear steel sample. The transparent confinement layer is usually a flowing water layer, and the absorbing layer is a black tape. The flowing water layer acts as a confinement medium, limiting the expansion of the plasma generated by laser ablation, extending the action time of the shock wave pressure; the black tape absorbs laser energy and converts it into mechanical impact, protecting the surface from thermal ablation. This step enhances the intensity of the shock wave by controlling the behavior of the plasma.

[0045] Finally, the surface of the gear steel sample is irradiated by high-energy laser pulses, and the laser energy is quickly absorbed by the absorbing layer, generating high-temperature and high-pressure plasma. The plasma forms a high-intensity shock wave under the confinement of the transparent confinement layer, propagating into the material interior. The shock wave pressure exceeds the dynamic yield strength of the material, inducing super-high strain rate plastic deformation of the surface layer, promoting the transformation of unstable residual austenite to martensite, and forming a gradient nanostructure. The plastic deformation process simultaneously introduces deep residual compressive stress, improving surface hardness and wear resistance. The steps are sequentially linked, the parameter configuration lays the energy foundation, the overlap rate optimizes the coverage effect, the confinement layer and the absorbing layer control the shock wave characteristics, and finally the laser irradiation achieves the strengthening goal, logically progressing layer by layer, and realizing reliable processing effect.

[0046] Step 5, microhardness testing of the gear steel sample after laser shot peening treatment, measuring the hardness value along the surface to the center direction, obtaining the hardness distribution data; Microhardness testing of the gear steel sample after laser shot peening treatment is a key step to evaluate the surface strengthening effect. This step aims to quantify the depth and hardening intensity of the plastic deformation layer induced by laser shot peening. Microhardness testing measures the hardness gradient from the surface to the center to obtain hardness distribution data, providing basic data support for subsequent analysis of effective hardening layer depth and residual stress distribution. Hardness distribution data directly reflects the modification degree of the laser shot peening process on the surface microstructure of the material, and is closely related to the gradient nanostructure formed in the previous laser shot peening treatment step.

[0047] Before microhardness testing, a representative sample needs to be cut from the laser shot peening treated gear steel sample, with the cutting direction perpendicular to the carburized surface to expose the complete cross section from the surface to the center. The cut sample is subjected to fine grinding and mechanical polishing to eliminate cutting damage and obtain a smooth observation surface, avoiding the influence of surface irregularities on hardness measurement accuracy. The polishing process uses multi-stage sandpaper for sequential grinding, finally achieving mirror finish, realizing clear and identifiable indentation morphology.

[0048] Microhardness testing is performed using a micro Vickers hardness tester, which measures the hardness value by pressing a diamond indenter into the sample surface under a specific load, holding the load for a specific time, and then unloading, measuring the diagonal length of the indentation, and calculating the hardness value. The measurement path is set with constant intervals along the surface layer to the core direction, and multiple indentation tests are performed at each measurement point to reduce random errors. The hardness values are recorded from the surface, gradually extending to the interior, until the hardness values are below a specific threshold and tend to be stable, covering the entire modified region.

[0049] The hardness distribution data is obtained based on the average calculation of multiple measurement values, and the data processing process includes taking the arithmetic mean after removing outliers, and generating a distribution curve of hardness with depth. The distribution curve shows the trend of hardness gradually transitioning from high values on the surface layer to the core matrix hardness, and the curve shape reveals the uniformity and depth of the hardening layer introduced by laser peening. The hardness distribution data is related to laser peening parameters such as laser energy and spot overlap ratio, and is used to optimize process conditions.

[0050] The hardness distribution data is further used to determine the effective hardening layer depth, which is defined as the thickness of the region where the hardness value is higher than a specific standard, which is set with reference to the matrix hardness. The effective hardening layer depth characterizes the penetration range of surface strengthening and is an important indicator for evaluating the wear resistance and fatigue resistance of gear steel. The hardness test results are mutually verified with the degree of residual austenite transformation and the formation of gradient nanostructure, providing a basis for subsequent residual stress testing and microstructure analysis.

[0051] The microhardness testing step is causally related to the laser peening treatment step, and the plastic deformation induced by laser peening leads to grain refinement and increased dislocation density, thereby improving surface hardness. The hardness distribution data is coordinated with the subsequent residual stress testing step to jointly verify the surface wear resistance improvement effect and fully reveal the synergistic mechanism of the composite strengthening method. The entire process provides quantitative basis for engineering application through hardness gradient analysis, supporting the performance prediction of gear steel under high load conditions.

[0052] Step 6, residual stress testing is performed on the laser peening treated gear steel sample, using an X-ray stress tester to measure residual stress, and obtaining residual stress distribution data; Step 6 involves residual stress testing on the laser peening treated gear steel sample, using an X-ray stress tester to measure residual stress, and obtaining residual stress distribution data. This step aims to quantify the characteristics of the residual compressive stress field introduced by laser peening, providing basic data for subsequent gradient nanostructure analysis and wear resistance evaluation. Before residual stress testing, the gear steel sample needs to be pre-treated on the surface, using alcohol for ultrasonic cleaning to remove surface contaminants and grease, ensuring the cleanliness of the test area and avoiding measurement errors.

[0053] X-ray stress measurement adopts the tilt fixed psi method for residual stress measurement. By adjusting the multi-angle setting of the diffraction angle ψ, the X-ray diffraction peak displacement is measured, and the residual stress value is calculated combined with the stress constant. During the test process, the beam diameter and scanning range need to be adapted to the geometry of the sample surface to cover the key strengthening area. Instrument parameters such as target material selection and wavelength need to match the material characteristics to improve measurement accuracy.

[0054] To obtain the residual stress distribution along the depth direction, electrolytic polishing machine is used for layer stripping. The electrolyte is composed of specific components, and the material surface is removed layer by layer by controlling the polishing time, and the depth of layer stripping is monitored using a dial indicator. After each layer stripping, X-ray stress measurement is performed, and the residual stress values at different depths are recorded. To improve the reliability of the data, multiple measurements are performed on each layer stripping area, and the average value is calculated after excluding the values that are too large or too small, and a continuous residual stress distribution curve is generated.

[0055] The residual stress distribution data is used to identify the maximum compressive stress area, and the sample cutting position in step 7 is guided. The test results can intuitively show the influence of laser peening parameters on stress distribution, as shown in Figure 3 , Figure 3 The residual stress distribution of carburized gear steel samples after different energy laser peening treatment is shown.

[0056] Step 7, based on the residual stress distribution data, determine the maximum compressive stress area, cut the sample from the corresponding position for microstructure analysis, and obtain the microstructure analysis results, including observing gradient nanostructure, and analyzing the distribution characteristics of nanocrystalline and nanotwins; Based on the residual stress distribution data, the maximum compressive stress area is determined, which depends on the residual stress test results of the gear steel sample after laser peening treatment. The maximum compressive stress area is usually located near the surface of the sample, representing the core action area of the laser shock wave induced plastic deformation. Selecting this area for microstructure analysis is because the peak value area of residual compressive stress corresponds to the most severe plastic deformation part, and the microstructure changes most significantly, which helps to reveal the strengthening mechanism.

[0057] Thin sample is cut from the corresponding position on the surface of the gear steel sample after laser peening treatment, and the cutting process is carried out using an electric spark wire cutting machine to ensure the integrity of the sample and avoid introducing additional stress. The size of the thin sample needs to be adapted to the observation requirements of the transmission electron microscope to accurately obtain the representative area and provide a basis for subsequent microanalysis.

[0058] The sliced samples are subjected to sandpaper grinding and electrolytic polishing. Sandpaper grinding uses multiple levels of sandpaper to sequentially grind the sample surface, removing the damage layer generated during cutting and obtaining a smooth observation surface. Electrolytic polishing uses specific electrolyte and voltage parameters to eliminate surface stress concentration and prepare ultra-thin regions to meet the electron beam penetration requirements of the transmission electron microscope. The sample preparation steps aim to obtain a defect-free and uniform thickness observation surface to create conditions for high-resolution imaging.

[0059] The gradient nanostructure of the polished sliced sample is observed using a transmission electron microscope. The transmission electron microscope uses a high-energy electron beam to penetrate the sample and capture the changes in the organization morphology from the surface layer to the core through the imaging system. The gradient nanostructure exhibits a continuous distribution of gradually increasing grain size, reflecting the gradient of plastic deformation induced by laser shock waves. The observation results display the organizational features at different depths, such as the arrangement patterns of nanocrystalline grains and nanotwins.

[0060] The distribution characteristics of nanocrystalline grains and nanotwins in the transmission electron microscope observation results are analyzed. Nanocrystalline grains refer to crystalline particles with sizes in the nanometer range, and the distribution characteristics include grain size uniformity and orientation distribution; nanotwins are intracrystalline twin boundaries, and the analysis involves twin density and morphological changes. The distribution characteristics reveal the grain refinement and twin strengthening mechanisms caused by laser peening, with nanocrystalline grains improving surface hardness and nanotwins enhancing material toughness. The analysis results are correlated with hardness distribution data and residual stress distribution data to verify the contribution of gradient nanostructure to surface wear resistance and provide a theoretical basis for optimizing laser peening parameters.

[0061] Step 8, based on the microstructure analysis results, a friction and wear test pin is prepared from the region with gradient nanostructure, a friction pair material is selected for wear testing, and by measuring the mass loss and calculating the wear resistance index, the wear resistance improvement effect of the gear steel sample is verified.

[0062] Based on the microstructure analysis results, a friction and wear test pin is prepared from the region with gradient nanostructure, this step relies on the gradient nanostructure characteristics obtained from transmission electron microscope observation to determine the core area of plastic deformation induced by laser peening. Microstructure analysis reveals the distribution patterns of surface nanocrystalline grains and nanotwins, providing a basis for selecting sample regions with significant strengthening effects, thereby ensuring the representativeness of the wear test sample.

[0063] When preparing the friction and wear test pin, a wire electrical discharge machine is used to cut a cylindrical sample from the surface of the gear steel sample, and the sample is derived from the maximum compressive stress region in the residual stress distribution data. The cut sample is subjected to fine grinding and polishing to form a standard diameter and length test pin, and the surface finish and geometric accuracy are controlled during the machining process to eliminate the influence of edge effects on the wear test. The preparation of the test pin ensures that the wear test sample has a uniform surface state, reducing experimental errors.

[0064] The bearing steel GCr15 is selected as the friction pair material, based on the high hardness and good wear resistance of the material, the pairing contact conditions in the actual working condition of the gear are simulated. The hardness matching of the friction pair material and the gear steel sample helps to truly reflect the wear behavior under high contact fatigue, so that the experimental results have engineering reference value. The geometric size and surface roughness of the friction pair need to meet the standard specifications to maintain the consistency of the experimental conditions.

[0065] When performing dry friction wear test, the experimental setup includes controlling the rotation speed range and environmental temperature, simulating the sliding contact conditions under the state of no lubrication. The test pin and the friction disc move relative to each other under a certain load, and the cyclic sliding is achieved by motor driving, which reproduces the high stress working condition in gear transmission. The friction coefficient and temperature change are monitored during the experiment to evaluate the surface stability.

[0066] The mass loss of the test pin is measured by a precision electronic balance, and the mass of the sample is weighed before and after the experiment, and the mass difference is calculated as the wear loss. The mass loss data is averaged by multiple repeated measurements to improve the reliability of the data. The measurement process avoids environmental vibration and humidity interference to ensure the weighing accuracy.

[0067] Based on the mass loss data, the average wear loss and the absolute wear resistance are calculated, the average wear loss is defined as the mass loss per unit sliding distance or per unit time, and the absolute wear resistance is the inverse of the wear loss, which represents the ability of the material to resist wear. The calculation process uses standard statistical methods to achieve comparability of the indicators. The wear resistance index quantifies the surface modification effect introduced by laser peening.

[0068] When verifying the wear resistance improvement effect of the gear steel sample, the difference in wear data between before and after laser peening is compared, and the contribution of gradient nanostructure to wear resistance is evaluated in combination with the microstructure analysis results. The wear test data shows the trend of change of the average wear loss and the absolute wear resistance, as shown in Figure 5 , the significance of surface strengthening effect is confirmed by comparison. The verification step closes the logical chain of the technical scheme, and confirms the synergistic effect of carburizing and laser peening composite strengthening.

[0069] Step 8 inherits the microstructure analysis of step 7, and the organizational structure characteristics are associated with the macro performance through wear test, forming a complete process from treatment to verification. The order of each sub-step of preparing test pin, selecting friction pair, experimental setup, measurement and calculation is connected, which gradually transforms micro data into wear resistance index, and finally supports the conclusion of surface strengthening effect.

[0070] Specifically, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology provided by the present application comprises the following steps: The surface pretreated gear steel sample is placed in a carburizing atmosphere, using methanol as the carrier gas and kerosene as the enrichment gas; A strong carburizing treatment is performed under a strong carburizing potential environment for a strong carburizing time; A diffusion treatment is performed under a diffusion carburizing potential environment for a diffusion time; The diffusion treated gear steel sample is cooled to a predetermined temperature in the furnace, and then is taken out for quenching and tempering treatment.

[0071] Step 3 involves a carburizing heat treatment process, aiming to form a high-carbon surface layer on the gear steel sample to provide a foundation hardness support for subsequent laser peening treatment. First, the surface pretreated gear steel sample is placed in a controllable carburizing atmosphere, using methanol as the carrier gas and kerosene as the enrichment gas. Methanol plays a dilution and protection role, and kerosene is responsible for providing a high-carbon potential environment to promote the diffusion of carbon atoms to the sample surface. This step regulates the carbon potential of the gas atmosphere through gas combination to establish initial carburizing conditions.

[0072] Next, a strong carburizing treatment is performed under a strong carburizing potential environment for a specific time. The strong carburizing stage accelerates the penetration of carbon atoms through a high-carbon potential atmosphere, allowing the gear steel surface layer to quickly absorb carbon elements and form a high-carbon concentration gradient. After the strong carburizing treatment is completed, a diffusion treatment is performed under a diffusion carburizing potential environment for a corresponding time. The diffusion stage aims to reduce the peak surface carbon concentration, allowing carbon atoms to migrate uniformly towards the center, eliminating the organizational heterogeneity caused by the concentration gradient, and optimizing the carbon distribution profile.

[0073] After the diffusion treatment is completed, the gear steel sample is cooled to a predetermined temperature in the furnace, and then is taken out for quenching and tempering treatment. Quenching promotes the formation of a high-carbon region in the surface layer through rapid cooling, resulting in a high-hardness structure. Subsequently, tempering treatment is performed to adjust the martensite morphology through low-temperature heating, reducing internal stress and brittleness, and improving material toughness.

[0074] The sub-steps of the carburizing heat treatment follow a sequential logic: the strong carburizing treatment establishes a high-carbon surface layer foundation, the diffusion treatment balances carbon distribution, the quenching treatment achieves organizational hardening, and the tempering treatment stabilizes performance. The entire process is achieved through atmosphere control, temperature management, and time coordination, allowing the gear steel surface layer to form a uniform high-carbon layer, providing an ideal substrate for laser peening induced plastic deformation. The synergistic effect of carburizing and subsequent peening ultimately enhances surface wear resistance and fatigue resistance.

[0075] Specifically, the method for improving the wear resistance of carburized gear steel surface based on laser peening technology according to the present application comprises the following steps: The YAG laser operating system is used to configure laser parameters for the carburized and heat-treated gear steel sample. The laser parameter configuration includes setting the repetition frequency, wavelength, pulse duration, and spot diameter. Based on the laser parameter configuration, the overlap rate between two adjacent spots is adjusted. Flowing water is used as a transparent confinement layer and black tape is used as an absorbing layer to cover the surface of the gear steel sample. The surface of the gear steel sample is irradiated by high-energy laser pulses to induce plasma shock waves, causing plastic deformation of the surface of the gear steel sample.

[0076] Step 4 involves a laser peening process aimed at plastic deformation strengthening of the surface of the carburized and heat-treated gear steel sample through high-energy laser shock waves. First, the YAG laser operating system is used to configure laser parameters for the gear steel sample, setting key parameters including repetition frequency, wavelength, pulse duration, and spot diameter. The repetition frequency determines the emission rate of laser pulses, affecting the processing efficiency; the wavelength needs to match the material's absorption characteristics to optimize energy coupling; the pulse duration controls the laser action time to avoid thermal damage; and the spot diameter defines the size of the laser action area, affecting the processing uniformity. Parameter configuration provides a basis for subsequent laser impact, achieving coordination between energy input and material response.

[0077] Based on the laser parameter configuration, the overlap rate between two adjacent spots is adjusted to optimize the coverage effect of the laser scanning path. The overlap rate controls the degree of overlap between spots, avoiding missed processing areas or excessive energy accumulation, thereby ensuring the continuity and uniformity of the plastic deformation layer induced by shock waves. The overlap rate adjustment works in conjunction with laser parameters to achieve complete coverage of the surface treatment area, laying the foundation for the spatial distribution of residual compressive stress fields.

[0078] Flowing water is used as a transparent confinement layer and black tape is used as an absorbing layer to cover the surface of the gear steel sample. The flowing water layer acts as a confinement medium, limiting the expansion of plasma generated by laser ablation, prolonging the action time of shock wave pressure; the black tape as an energy absorbing layer protects the sample surface from direct thermal ablation, while promoting the conversion of laser energy into mechanical impact. The combination of transparent confinement layer and absorbing layer forms a closed environment, enhancing the intensity of plasma shock waves and controlling the range of surface heat-affected zone.

[0079] By irradiating the surface of the gear steel sample with high-energy laser pulses, laser energy is quickly absorbed by the absorbing layer and vaporized, generating high-temperature and high-pressure plasma. The plasma expansion is limited by the transparent confinement layer, forming a high-intensity shock wave propagating into the material interior. The shock wave pressure exceeds the dynamic yield strength of the material, inducing super-high strain rate plastic deformation of the surface layer, leading to grain refinement and increased dislocation density.

[0080] The plastic deformation process promotes the transformation of unstable residual austenite to martensite, eliminates microhardness anomalies, and introduces gradient nanostructure and deep residual stress field in the carburized layer. The laser parameter configuration, overlap rate adjustment, application of limiting layer and absorbing layer, and laser irradiation steps are sequentially connected to form a complete process chain of laser peening treatment, which ultimately improves the wear resistance and fatigue resistance of the gear steel surface.

[0081] Specifically, the method for improving the wear resistance of carburized gear steel surface based on laser peening technology according to the present application further comprises the following steps: Microhardness testing is performed on the gear steel sample after laser peening treatment, and the hardness value is measured along the surface to the center direction to obtain hardness distribution data; Based on the hardness distribution data, the effective hardening layer depth is determined, and according to GB / T9450-2005 "Determination and verification of hardening layer depth of steel parts", the area with hardness value higher than 550HV is regarded as the effective hardening layer depth; Residual stress testing is performed on the gear steel sample after laser peening treatment, and X-ray stress tester is used to measure the residual stress at different depths to obtain residual stress distribution data; Based on the hardness distribution data and residual stress distribution data, the improvement effect of surface wear resistance is verified.

[0082] Step 6 involves performance testing and effect verification of the gear steel sample after laser peening treatment, aiming to obtain key data through microhardness testing and residual stress testing, and evaluate the improvement effect of surface wear resistance based on these data. First, microhardness testing is performed on the gear steel sample after laser peening treatment, and micro Vickers hardness tester is used to measure the hardness value along the surface to the center direction of the sample at constant intervals, and a certain load is applied during the measurement process and the loading time is kept, and the hardness distribution curve from the surface to the interior is obtained. Hardness distribution data reflects the depth and hardening strength of the plastic deformation layer induced by laser peening, providing a basis for evaluating the effective hardening layer. The microhardness test results show the hardness comparison after different laser energy treatment.

[0083] Based on the hardness distribution data, the effective hardening layer depth is determined, which is defined as the thickness of the area with hardness value higher than a certain threshold value, and the threshold value is set according to the hardness of the material matrix. The hardness distribution curve shows a trend of gradually decreasing hardness from the surface to the center, and the effective hardening layer depth represents the penetration range of laser peening strengthening effect, and the increase of depth indicates that the surface hardening effect is significant, which helps to resist plastic deformation and wear. The effective hardening layer depth is directly related to the hardness distribution data, which provides a basis for comparison for subsequent residual stress analysis.

[0084] Residual stress testing is performed on the laser peening treated gear steel samples. X-ray stress measurement instrument is used to measure residual stress at different depths using the tilt-fixing method. Before testing, the sample surface is electrolytically polished to remove layers, and the depth of layer is controlled to obtain stress distribution data along the depth direction. The X-ray stress measurement instrument uses a specific target and diffraction angle to calculate residual stress values by measuring diffraction peak displacement, obtaining residual stress distribution curves from the surface to the core. Residual stress distribution data reveals the characteristics of the compressive stress field introduced by laser peening, including surface compressive stress values and compressive stress layer depth. The residual stress test results show the stress comparison after different laser energy treatments.

[0085] The hardness distribution data and residual stress distribution data are used to verify the improvement of surface wear resistance. The hardness distribution data reflects the material surface plastic deformation resistance, and the residual stress distribution data represents the crack initiation resistance performance. The increase of effective hardening layer depth and the increase of surface residual compressive stress work together to optimize the matching degree of stress distribution and hardening layer, thereby enhancing the wear resistance. The verification process compares the hardness and stress data before and after laser peening, analyzes the influence of gradient nanostructure and residual austenite transformation on wear resistance, and confirms the strengthening effect of laser peening treatment in the carburized layer. Hardness and stress test data provide support for wear resistance indicators, and finally realize the quantitative evaluation of surface wear resistance.

[0086] Specifically, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology according to the present application comprises the following steps: Based on the residual stress distribution data, the maximum compressive stress region is determined, and a thin sample is cut from the corresponding position on the surface of the laser peening treated gear steel sample; The cut thin sample is ground with sandpaper and electrolytically polished; The gradient nanostructure of the polished thin sample is observed using a transmission electron microscope; The nanocrystalline and nanotwin distribution characteristics in the transmission electron microscope observation results are analyzed.

[0087] Step 7 involves microstructure analysis of the laser peening treated gear steel sample, aiming to observe the gradient nanostructure by transmission electron microscope and analyze the nanocrystalline and nanotwin distribution characteristics, so as to reveal the laser peening strengthening mechanism. First, based on the residual stress distribution data, the maximum compressive stress region is determined, which is usually located near the surface layer of the sample. The residual stress distribution data comes from the aforementioned residual stress test results. The maximum compressive stress region represents the core action area of laser shock wave induced plastic deformation, providing accurate positioning for microanalysis. A thin sample is cut from the corresponding position on the surface of the laser peening treated gear steel sample. The cutting process uses an electric spark wire cutting machine to ensure sample integrity, and the size of the thin sample needs to be adapted to the requirements of transmission electron microscope observation.

[0088] The cut-out thin slice sample is subjected to sandpaper grinding and electrolytic polishing treatment. The sandpaper grinding uses multi-stage sandpaper to sequentially grind the sample surface, remove the cutting damage layer and obtain a smooth observation surface. The electrolytic polishing treatment uses a specific electrolyte and voltage parameters to eliminate surface stress concentration and prepare an ultra-thin region to meet the electron beam penetration requirements of the transmission electron microscope. The grinding and polishing treatment together ensures that the sample surface is defect-free and uniform in thickness, laying the foundation for high-resolution observation.

[0089] The gradient nanostructure of the polished thin slice sample is observed using a transmission electron microscope. The transmission electron microscope uses a high-energy electron beam to penetrate the sample and capture the internal organizational morphology through an imaging system. The gradient nanostructure exhibits a continuous change in grain size from the surface layer to the core, reflecting the gradient of plastic deformation induced by laser shock waves. The observation results display the organizational characteristics at different depths, such as the arrangement patterns of nanocrystalline grains and nanotwins. The scanning electron microscope and transmission electron microscope images of the carburized + 4J laser shot gear steel at different depths along the carburization direction exhibit typical characteristics of the gradient nanostructure, as shown in Figs. Figure 4 , 6, 7 and 8.

[0090] The distribution characteristics of nanocrystalline grains and nanotwins in the transmission electron microscope observation results are analyzed. Nanocrystalline grains refer to crystalline particles with a size in the nanometer range, and the distribution characteristics include grain size uniformity and orientation distribution; nanotwins are intracrystalline twinning interfaces, and the analysis involves twinning density and morphological changes. The distribution characteristics reveal the grain refinement and twinning strengthening mechanisms caused by laser shot peening, with nanocrystalline grains improving hardness and nanotwins enhancing toughness. The analysis results are correlated with hardness distribution data and residual stress distribution data to verify the contribution of the gradient nanostructure to surface wear resistance. The microstructure analysis provides a theoretical basis for optimizing laser shot peening parameters and supports the overall evaluation of surface strengthening effects.

[0091] Specifically, the method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to the present application, step 8 comprises: Based on the microstructure analysis results, a friction and wear test pin is prepared from the region with a gradient nanostructure; Select bearing steel GCr15 as the friction pair material; Perform dry friction and wear tests using the prepared friction and wear test pin and the selected friction pair material, and measure the mass loss of the test pin; Based on the mass loss data, calculate the average wear amount and absolute wear resistance.

[0092] Step 8 involves a friction and wear test verification process aimed at evaluating the improvement effect of laser peening on the surface wear resistance of carburized gear steel by quantifying wear data. First, based on the microstructure analysis results, friction and wear test pins are prepared from the region with gradient nanostructure, the preparation process includes using an electric spark wire cutting machine to cut a cylindrical sample from the surface of the laser peening treated gear steel sample, and then processing it into a standard diameter and length test pin through precision grinding and polishing to ensure surface finish and geometric accuracy. The preparation of the test pin realizes that the sample for wear test is derived from the gradient nanostructure region with the most significant strengthening effect, providing a representative sample for accurate evaluation of wear resistance.

[0093] Next, bearing steel GCr15 is selected as the friction pair material, which has high hardness and good wear resistance, simulating the characteristics of the paired material in the actual working condition of the gear. The selection of the friction pair material is based on its compatibility with the gear steel and the typical application scenario, making the wear test results more valuable for engineering reference. After pairing the test pin with the friction pair material, dry friction and wear test is carried out, the experiment is carried out in a controlled environment, with specific speed and load conditions set to simulate high contact fatigue working conditions. Dry friction conditions exclude the influence of lubricants, highlighting the wear resistance of the surface material itself. During the experiment, the motor drives the friction disc to rotate, the test pin contacts the friction disc and moves relative to it, causing wear.

[0094] The mass loss of the test pin before and after the experiment is measured using a precision electronic balance, and the mass loss data reflects the consumption of the material during the wear process. After measurement, the average wear amount and the absolute wear resistance are calculated based on the mass loss data, the average wear amount represents the mass loss per unit time or unit distance, and the absolute wear resistance is the inverse of the wear amount, representing the material's ability to resist wear. The calculated wear resistance indicators are used to quantitatively compare the performance differences before and after laser peening. The friction and wear test data of carburized 20CrNiMo gear steel samples after laser peening with different energies shows the comparison results of average wear amount and absolute wear resistance.

[0095] The wear test data is consistent with the microstructure analysis, hardness distribution and residual stress distribution data, which together verify the improvement effect of laser peening technology on surface wear resistance, providing experimental basis for optimizing process parameters.

[0096] Specifically, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology according to the present application further comprises: Comparing the wear data of the gear steel samples before and after laser peening based on the mass loss data; Analyzing the influence degree of gradient nanostructure on wear resistance in combination with wear data difference and microstructure analysis results; Optimizing laser peening parameter configuration according to the influence degree analysis results; The correlation between the distribution of residual compressive stress and the improvement of wear resistance is verified based on the optimized laser peening parameters.

[0097] Comparing the wear data difference of gear steel samples before and after laser peening treatment based on mass loss data, first, the friction and wear test data of gear steel samples before and after laser peening treatment need to be collected systematically, including the mass loss values of untreated samples and different laser energy treated samples. The mass loss of the test pin before and after the dry friction and wear test is measured by a precision electronic balance, and the average wear amount and absolute wear resistance indicators are calculated. Statistical analysis methods and visual charts are used for data comparison to quantify the wear resistance improvement amplitude introduced by laser peening. The friction and wear test data of carburized 20CrNiMo gear steel samples after different energy laser peening treatment are shown in FIGS. 8A and 8B, which show the comparison results of average wear amount and absolute wear resistance. Figure 5 The difference comparison of wear data provides quantitative basis for subsequent gradient nanostructure influence analysis, and establishes a preliminary correlation between surface modification effect and wear performance.

[0098] The influence degree of gradient nanostructure on wear resistance is analyzed based on the wear data difference and microstructure analysis results. The microstructure analysis results come from the gradient nanostructure characteristics obtained by transmission electron microscope observation, including nano-grain size distribution and nano-twin density. The wear data difference shows the wear resistance improvement trend of the samples after laser peening treatment, and is correlated with the formation depth and uniformity of gradient nanostructure. The influence degree analysis establishes the corresponding relationship between wear rate and nanostructure parameters to evaluate the contribution mechanism of gradient nanostructure in improving surface hardness and toughness. The analysis results reveal the degree of grain refinement and twinning strengthening in inhibiting crack initiation and reducing wear debris, providing theoretical support for parameter optimization.

[0099] According to the influence degree analysis results, the laser peening parameter configuration is optimized. The optimization process adjusts key parameters such as laser energy, spot overlap rate and pulse duration. The influence degree analysis results indicate the dominant factors of gradient nanostructure on wear resistance, such as the positive correlation between nano-twin density and wear resistance, then increase the laser energy to enhance the shock wave intensity; if the nano-grain uniformity is insufficient, adjust the spot overlap rate to improve the plastic deformation gradient. Parameter optimization is based on regression analysis of experimental data to determine the optimal combination range of laser parameters, so that the gradient nanostructure and the distribution of residual compressive stress achieve synergistic strengthening effect.

[0100] The correlation between the residual compressive stress distribution and the wear resistance improvement effect is verified based on the optimized laser peening parameter configuration. In the verification process, a new round of gear steel samples are treated by laser peening using the optimized parameters, and the residual stress distribution and wear resistance are measured. The residual stress distribution data is obtained by an X-ray stress tester, and the wear resistance improvement effect is quantified by wear test data. The correlation analysis confirms the contribution weight of the residual compressive stress distribution to wear inhibition by comparing the matching degree of the residual compressive stress layer depth, surface compressive stress value and wear resistance index before and after optimization. The verification results are used to correct the laser peening process window, realizing the reliability and repeatability of surface wear resistance improvement.

[0101] Specifically, the method for improving the surface wear resistance of carburized gear steel based on laser peening technology according to the present application further comprises: Based on the optimized laser peening parameter configuration, the microhardness distribution and residual stress distribution performance of gear steel samples treated by different laser energies are compared, and the best laser energy range is determined based on the performance comparison results; Based on the best laser energy range, laser peening parameters are configured, laser peening treatment is integrated into the preset carburizing production line, forming a carburizing production line integrated with laser peening treatment, and the carburizing production line integrated with laser peening treatment is applied to the manufacturing of heavy load gear parts.

[0102] Based on the optimized laser peening parameter configuration, the microhardness distribution and residual stress distribution performance of gear steel samples treated by different laser energies are compared. First, a series of gear steel samples with varying laser energy parameters are prepared. The samples are subjected to carburizing heat treatment and laser peening treatment, and the laser energy parameters are set in a gradient within the optimized range. After preparation, the hardness values are measured along the surface layer to the center direction of the sample using a micro Vickers hardness tester to obtain the hardness distribution curve; at the same time, the residual stress at different depths is measured using an X-ray stress tester to obtain the residual stress distribution curve. The performance comparison process includes superimposed analysis of the hardness distribution curve and the residual stress distribution curve of different laser energy samples to evaluate the change trend of the surface hardness, effective hardening layer depth, residual compressive stress value and compressive stress layer depth. The microhardness comparison chart of carburized 20CrNiMo gear steel samples after different energy laser peening treatment shows the change law of hardness distribution with laser energy, as shown in Figure 2 The residual stress comparison chart of carburized 20CrNiMo gear steel samples after different energy laser peening treatment shows the correlation between the residual stress distribution and the laser energy, as shown in Figure 3 The comparative analysis focuses on the plastic deformation strength induced by laser energy and the synergistic effect of the carburized layer to provide a data basis for determining the best energy range.

[0103] The optimal laser energy range was determined based on performance comparison results, which revealed the quantitative relationship between laser energy and microhardness distribution and residual stress distribution. The determination process involved analyzing the surface hardness peak and the maximum effective hardened layer depth in the hardness distribution curve, combined with the extreme values ​​of surface compressive stress and the saturation point of the compressive stress layer depth in the residual stress distribution curve, to identify the laser energy parameter window. The determination principles included uniformity of hardness distribution, stability of residual compressive stress, and avoiding excessive energy input that could lead to surface damage. The optimal laser energy range should ensure the complete formation of the gradient nanostructure, sufficient transformation of residual austenite, and matching of the residual compressive stress distribution with the carburized layer depth. The determination results were based on regression analysis of experimental data, establishing a mapping relationship between laser energy and performance indicators to guide the setting of integrated parameters for the production line.

[0104] Based on the optimal laser energy range, laser peening is integrated into a pre-designed carburizing production line. The integration process involves adding a laser peening workstation downstream of the carburizing heat treatment line. This workstation is equipped with a YAG laser operating system and a supply system for the transparent confinement layer and absorption layer. The integrated design sets laser parameters, such as repetition frequency, pulse duration, and spot diameter, based on the optimal laser energy range, and adjusts the spot overlap rate to adapt to the geometric features of the gear surface. The integrated production line enables continuous processing of carburizing and laser peening, with workpieces transferred via conveyor belts or robotic arms, synchronizing the processing rhythm with the carburizing cycle. The integrated control system monitors carbon potential and temperature data, providing real-time feedback to adjust laser peening parameters and maintain processing consistency. The integration goal is to create a highly efficient composite strengthening production line and improve surface modification efficiency.

[0105] An integrated laser shot peening carburizing production line was applied to the manufacturing of heavy-duty gear parts. The application process optimized the laser shot peening scanning strategy and parameter configuration to address the high contact fatigue and heavy wear conditions of heavy-duty gears. Implementation included adjusting the laser spot scanning path based on the gear module and tooth surface curvature, and controlling the shock wave coverage of key stress areas such as the tooth root and tooth surface. The output gear parts from the production line undergo composite strengthening through carburizing and laser shot peening, resulting in a gradient nanostructure and deep residual compressive stress on the surface, significantly improving wear resistance and crack initiation resistance. Application validation involved bench testing to simulate actual working conditions, detecting gear life and failure modes, and confirming that the strengthening effect meets the requirements of heavy-duty applications such as wind turbine gearboxes or engineering machinery transmission systems. Application promotion is based on production line standardization and parameter databases, adapting to diverse gear specifications.

[0106] Specifically, the method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to the present invention further includes: Based on the integrated environment of the carburizing production line, the carbon potential and temperature during the carburizing heat treatment process are monitored to obtain carbon potential and temperature data. Based on carbon potential temperature data and gear surface geometry, a scanning strategy for controlling laser shot peening is developed. Based on the scanning strategy and real-time processing feedback obtained from the laser power monitor and surface topography sensor, the parameters of the transparent confinement layer and the absorption layer are adjusted. By adjusting the laser pulse energy, spot overlap rate, and confinement layer parameters, the surface roughness of the gear steel sample is controlled to meet the surface quality requirements within the preset average roughness range.

[0107] Based on the integrated environment of the carburizing production line, the carbon potential and temperature during the carburizing heat treatment process are monitored to obtain carbon potential and temperature data. This process involves real-time data acquisition through carbon potential sensors and thermocouples installed inside the carburizing furnace. The carbon potential sensors monitor the carbon concentration in the atmosphere, and the thermocouples measure the furnace temperature. The data acquisition system records the changes in carbon potential and temperature over time. The carbon potential and temperature data reflect the stability and consistency of the carburizing process, providing a process basis for subsequent laser shot peening. The monitoring data is used to determine the quality of the carburized layer formation and to prevent carbon potential fluctuations or temperature anomalies from affecting the uniformity of the high-carbon surface layer.

[0108] Based on carbon potential temperature data and gear surface geometry, a scanning strategy for laser shot peening is controlled. The strategy involves designing the laser beam scanning path according to the gear tooth surface curvature, module, and root fillet characteristics. The beam path planning uses a CAD model imported into the laser control system to generate a multi-axis linkage trajectory. Carbon potential temperature data is used to correlate the carburized layer depth with laser energy matching; for example, a higher carbon potential results in a thicker carburized layer, so the scanning density is adjusted to cover the deeper strengthening area. The gear surface geometry determines the beam overlap rate and incident angle, ensuring the shock wave is uniformly applied to key areas of the tooth surface.

[0109] Based on the scanning strategy and real-time processing feedback obtained from the laser power monitor and surface morphology sensor, the parameters of the transparent confinement layer and the absorption layer are adjusted. Real-time processing feedback, such as laser energy fluctuations or changes in the surface heat-affected zone, is obtained through the laser power monitor and surface morphology sensor. The adjustment of transparent confinement layer parameters includes controlling the thickness and flow rate of the flowing water layer to maintain plasma confinement strength; the adjustment of absorption layer parameters involves changing the thickness or material of the black adhesive tape to optimize energy absorption efficiency. Parameter adjustments are based on feedback data during the execution of the scanning strategy, dynamically balancing shock wave intensity and thermal damage risk.

[0110] By adjusting the laser pulse energy, spot overlap rate, and confinement layer parameters, the surface roughness of the gear steel sample is controlled to meet the preset average roughness range and surface quality requirements. Surface roughness control is achieved by adjusting the laser pulse energy, spot overlap rate, and confinement layer parameters; for example, reducing the laser energy or increasing the overlap rate can reduce surface waviness. The surface quality requirements are set based on gear application standards, such as the average roughness range. Real-time data measured by an online roughness meter is fed back to the parameter adjustment system to form a closed-loop control, ensuring that the surface finish after laser shot peening meets the service requirements under high contact fatigue conditions.

[0111] Specifically, the method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to the present invention further includes: Metallographic analysis was performed on gear steel samples after laser shot peening to obtain metallographic data. Based on metallographic data, the content and morphological changes of retained austenite were detected to obtain data on retained austenite. Based on metallographic data and retained austenite data, wear behavior under high contact fatigue conditions was simulated to obtain wear simulation data. Based on metallographic data, retained austenite data, and wear simulation data, a surface strengthening effect report is generated.

[0112] Metallographic analysis was performed on gear steel samples after laser shot peening to obtain metallographic data. The process began with cutting metallographic samples from the surface of the gear steel samples, followed by mounting, grinding, and polishing to prepare standard metallographic samples. Then, a chemical etchant was used to expose the microstructure. Metallographic analysis employed optical microscopy or scanning electron microscopy to observe the microstructure from the surface to the core, including martensite morphology, carbide distribution, and grain boundary characteristics. During the analysis, microstructure homogeneity, grain size, and phase composition were recorded, obtaining metallographic data such as microstructure type, grain size grade, and defect distribution, providing a basis for subsequent retained austenite analysis.

[0113] Based on metallographic data, the content and morphological changes of retained austenite were detected to obtain retained austenite data. The detection methods employed were X-ray diffraction analysis or electron backscatter diffraction. X-ray diffraction analysis used specific targets and diffraction angles, and the volume fraction of retained austenite was calculated from the diffraction peak intensities. Morphological change analysis included assessment of retained austenite grain size, distribution uniformity, and stability. The detection process combined with phase distribution information from the metallographic data to quantify the degree of transformation of retained austenite in the carburized layer, obtaining retained austenite data such as content percentage and morphological parameters.

[0114] Based on metallographic and retained austenite data, wear behavior under high contact fatigue conditions is simulated to obtain wear simulation data. The simulation process uses finite element analysis software or a dedicated wear simulation platform. Simulation input parameters include hardness distribution from the metallographic data, phase transformation sensitivity from the retained austenite data, and high contact fatigue conditions such as cyclic loading, sliding speed, and contact stress. Simulation outputs wear depth, crack initiation location, and life prediction, obtaining wear simulation data such as wear rate, stress distribution, and failure threshold.

[0115] Based on metallographic data, retained austenite data, and wear simulation data, a surface strengthening effect report is generated. The report generation process integrates all analysis results, performing data correlation and trend analysis. The report content covers metallographic characteristics, retained austenite transformation effect, simulated wear performance comparison, and laser shot peening parameter optimization suggestions. The output report is presented in a standardized format, including tables, curves, and conclusions, to verify the surface wear resistance improvement effect and guide process improvements.

[0116] This invention addresses the technical problem of a large amount of structurally unstable retained austenite on the surface of gear steel after carburizing and quenching through a composite strengthening method combining carburizing heat treatment and laser shot peening. Carburizing heat treatment forms a high-carbon surface layer on the gear steel, but the quenching process may lead to incomplete transformation of the retained austenite, resulting in abnormal microhardness distribution. This manifests as a "rising" phenomenon where the surface hardness is lower than the subsurface hardness. Simultaneously, the effective hardened layer depth is insufficient, and the residual compressive stress distribution is unsatisfactory, with the maximum value located in the subsurface rather than the outer surface. These problems reduce the surface wear resistance and crack initiation resistance of gears under high contact fatigue and heavy-load wear conditions.

[0117] Laser shot peening, as a post-treatment step, involves irradiating the carburized gear steel surface with a high-power-density short-pulse laser. A plasma shock wave is induced using a transparent confinement layer and an absorption layer. The shock wave causes ultra-high strain rate plastic deformation in the surface layer, promoting the transformation of unstable residual austenite into martensite, thereby eliminating microhardness anomalies and improving the uniformity of hardness distribution. The plastic deformation process excites a high-amplitude residual compressive stress field in the carburized layer. By controlling laser parameters such as energy and spot overlap rate, the residual compressive stress distribution is spatially matched to the depth of the carburized hardened layer, adjusting the maximum compressive stress value to the outermost surface.

[0118] Simultaneously, the laser shock wave induces the formation of gradient nanostructures, including nanocrystals and nanotwins, improving surface hardness and resistance to plastic deformation. The gradient nanostructure increases the effective hardened layer depth, while the optimized residual compressive stress distribution effectively inhibits the initiation and propagation of surface cracks. Carburizing provides a high-hardness matrix, and laser peening strengthens the surface properties. The synergistic effect of both significantly improves the surface wear resistance and crack initiation resistance of gears under high contact fatigue and heavy-load wear conditions by improving the microstructure and stress distribution.

[0119] This invention addresses the technical problem caused by the presence of a large amount of structurally unstable retained austenite on the surface of gear steel after carburizing and quenching. It achieves this through a composite strengthening method combining carburizing heat treatment and laser shot peening. Carburizing heat treatment first forms a high-carbon surface layer on the gear steel, providing a basic hardness for subsequent strengthening. However, the carburizing and quenching process may lead to incomplete transformation of the retained austenite, causing abnormal microhardness distribution, manifested as a lower surface hardness than the subsurface layer. Simultaneously, the effective hardened layer depth is insufficient, and the residual compressive stress distribution is unsatisfactory, with the maximum value located in the subsurface layer rather than the outer surface. Laser shot peening, as a post-treatment step, uses a high-power-density short-pulse laser to irradiate the surface of the carburized gear steel sample, and utilizes a transparent confinement layer such as flowing water and an absorption layer such as black tape to induce plasma shock waves.

[0120] Plasma shock waves induce ultra-high strain rate plastic deformation in the material surface, which promotes the transformation of unstable residual austenite into martensite, thereby eliminating microhardness anomalies and making the hardness distribution more uniform. Laser shot peening excites a high-amplitude residual compressive stress field in the carburized layer. By controlling laser parameters such as energy and spot overlap rate, the residual compressive stress distribution is spatially matched with the depth of the carburized hardened layer, adjusting the maximum compressive stress value to the outermost surface and improving stress distribution. Simultaneously, laser shock waves induce the formation of gradient nanostructures, including the distribution of nanocrystals and nanotwins. This structure improves surface hardness and resistance to plastic deformation, and increases the effective hardened layer depth. Carburizing provides a high-hardness matrix, and laser shot peening further enhances surface properties. The synergistic effect of both significantly improves the surface wear resistance and crack initiation resistance of gears under high contact fatigue and heavy-load wear conditions by optimizing the microstructure and stress distribution. Microhardness and residual stress test data verified the improvement in hardness and stress distribution, while transmission electron microscopy confirmed the formation of gradient nanostructures. Tribological tests quantified the improvement in wear resistance, ultimately achieving enhanced surface integrity.

[0121] The technical features of this invention are explained below: Gradient nanostructures refer to nanoscale structures in which the grain size changes continuously from the surface to the core of a material. Laser shot peening induces ultra-high strain rate plastic deformation, which refines the surface grains to the nanoscale, while the core retains a coarse-grained structure. This structural gradient significantly improves the surface hardness and toughness of the material through grain boundary strengthening and dislocation strengthening mechanisms, while also improving the uniformity of stress distribution.

[0122] Plasma shock wave induced plastic deformation is achieved by irradiating the metal surface with a high-power-density short-pulse laser. The absorption layer vaporizes instantaneously to form a high-temperature and high-pressure plasma. Under the constraint of a transparent confinement layer, the plasma generates a high-intensity shock wave. The shock wave pressure exceeds the dynamic yield strength of the material, causing the surface layer to undergo ultra-high strain rate plastic deformation, thereby introducing defect structures such as high-density dislocations and twins.

[0123] The transformation of retained austenite into martensite is caused by the mechanical energy provided by the laser shock wave, which induces strain-induced martensitic transformation in the unstable retained austenite in the carburized layer. The transformation process is accompanied by volume expansion and shear strain, which not only eliminates the negative impact of soft-phase retained austenite on hardness, but also further improves surface hardness and dimensional stability through phase transformation strengthening.

[0124] The matching of residual compressive stress distribution with hardened layer depth refers to the spatial distribution of the residual compressive stress field introduced by laser shot peening in the carburized hardened layer being coordinated with the hardened layer depth. The maximum compressive stress value is located on the outermost surface and gradually decreases towards the center. This matching relationship can effectively suppress the initiation and propagation of contact fatigue cracks in the transition zone of the hardened layer.

[0125] Laser parameter configuration optimization involves adjusting key parameters in laser processing, such as repetition frequency, wavelength, pulse duration, and spot diameter, to match material properties and processing objectives. The optimization process is based on performance comparison data of microhardness distribution and residual stress distribution to determine the optimal laser energy range, thereby enhancing the intensity of plasma shock waves and controlling plastic deformation, and improving the consistency and efficiency of surface strengthening.

[0126] In microstructure analysis, the distribution of nanotwins refers to the density and morphological characteristics of nanoscale twin interfaces in the surface of materials observed and quantified using transmission electron microscopy. As crystal defects, the uniformity of distribution and density variation of nanotwins reflect the degree of plastic deformation induced by laser shot peening. The twin strengthening mechanism improves the hardness and toughness of materials by hindering dislocation movement, thereby contributing to the improvement of surface wear resistance.

[0127] In the calculation of wear resistance index, the absolute wear resistance is defined as the reciprocal of the mass loss data obtained from the wear test, with the unit being grams to the power of negative 1. It characterizes the material's ability to resist wear per unit mass loss. The calculation process is based on the mass loss value of the test pin measured by a precision electronic balance. The higher the absolute wear resistance value, the better the material's wear resistance performance. It is used to quantify the surface wear resistance improvement effect after laser shot peening.

[0128] The integrated laser shot peening carburizing production line application refers to embedding a laser shot peening workstation into an existing carburizing heat treatment production line. Based on the optimal laser energy range, parameters such as spot overlap rate and pulse energy are configured to achieve continuous carburizing and shot peening processes. The application goal is to mass-produce heavy-duty gear parts, obtain gradient nanostructures and deep residual compressive stress through composite strengthening, and improve the wear resistance and service life of parts under high contact fatigue conditions.

[0129] Friction and wear testing is a method for evaluating the surface properties of materials. It simulates actual wear conditions by preparing a standard sample pin and subjecting it to relative motion with the friction pair material under controlled conditions, and measures the mass loss of the sample to quantify the material's wear resistance. This experiment is used to verify the effect of laser shot peening on improving the wear resistance of gear steel surfaces, and provides wear data support based on dry friction conditions and high contact fatigue simulation.

[0130] Metallographic analysis is a microscopic structure observation technique that examines the material's microstructure characteristics, including grain size, phase distribution, and defect morphology, after cutting, grinding, polishing, and chemical etching. The analysis aims to reveal the gradient nanostructure induced by laser shot peening, such as nanocrystals and twin arrangements, providing microscopic evidence for the strengthening mechanism.

[0131] The residual austenite was detected using X-ray diffraction or electron backscatter diffraction to quantitatively measure the volume fraction and morphological characteristics of the residual austenite in the carburized layer, and to assess its stability and transformation degree. The detection results were correlated with the martensitic phase transformation induced by laser shot peening to explain the reasons for the improved hardness distribution and enhanced wear resistance.

[0132] Microhardness testing uses a Vickers hardness tester to measure hardness values ​​point by point along the material surface towards the center, obtaining a hardness distribution curve. The test data is used to determine the effective hardened layer depth, characterize the surface hardening effect after laser shot peening, and verify the uniformity of the plastic deformation layer.

[0133] The residual stress test uses an X-ray stress meter to measure the stress values ​​at different depths using the tilting fixation method to obtain residual compressive stress distribution data; the test reveals the stress field characteristics introduced by the laser shock wave, including the magnitude and depth of surface compressive stress, and evaluates the resistance to crack initiation.

[0134] Wear simulation employs finite element analysis or dedicated software platforms, inputting material microstructure and operating parameters to predict wear behavior under high contact fatigue, such as wear depth and crack initiation location. Simulation results are compared with experimental data to verify the effect of laser shot peening on improving wear resistance and to optimize process parameters.

Claims

1. A method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology, characterized in that, include: Step 1: Obtain a gear steel sample, wherein the gear steel sample includes predetermined geometric dimensions and material grade data; Step 2: Remove the oxide scale from the surface of the gear steel sample by grinding to obtain the gear steel sample after surface pretreatment. Step 3: Place the pre-treated gear steel sample in a carburizing atmosphere and perform carburizing heat treatment using carrier gas and enriched gas. The carburizing heat treatment includes strong carburizing treatment, diffusion treatment, quenching treatment and tempering treatment, thereby forming a high carbon surface layer. Step 4: The gear steel sample after carburizing heat treatment is subjected to laser shot peening treatment. The surface of the gear steel sample is irradiated with a high power density short pulse laser, and plasma shock waves are induced by a transparent confinement layer and an absorption layer to cause plastic deformation of the surface of the gear steel sample. Step 5: Perform microhardness testing on the gear steel sample after laser shot peening, measure the hardness value from the surface to the center, and obtain hardness distribution data. Step 6: Perform residual stress testing on the gear steel sample after laser shot peening. Use an X-ray stress meter to measure the residual stress and obtain residual stress distribution data. Step 7: Determine the region of maximum compressive stress based on residual stress distribution data, cut samples from the corresponding locations for microstructure analysis, and obtain microstructure analysis results. The microstructure analysis includes observing gradient nanostructures and analyzing the distribution characteristics of nanocrystals and nanotwins. Step 8: Based on the microstructure analysis results, prepare friction and wear test pins from regions with gradient nanostructures, select friction pair materials for wear experiments, and verify the wear resistance improvement effect of the gear steel sample by measuring mass loss and calculating wear resistance index.

2. The method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to claim 1, characterized in that, Step 3 includes: The pretreated gear steel sample was placed in a carburizing atmosphere, with methanol as the carrier gas and kerosene as the enrichment gas. Maintain strong carburizing time under strong carburizing potential environment and perform strong carburizing treatment; The diffusion process is performed while maintaining the diffusion time within the diffusion carbon potential environment. The diffusion-treated gear steel sample was cooled to a predetermined temperature in the furnace, held at that temperature under a carbon potential atmosphere, and then removed for quenching and tempering.

3. The method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to claim 2, characterized in that, Step 4 includes: The YAG laser operating system was used to configure the laser parameters of the gear steel sample after carburizing heat treatment. The laser parameter configuration included setting the repetition frequency, wavelength, pulse duration and spot diameter. Based on the laser parameter configuration, adjust the overlap rate between two adjacent laser spots; Flowing water was used as a transparent confinement layer and black tape was used as an absorption layer to cover the surface of the gear steel sample. High-energy laser pulses are used to irradiate the surface of gear steel samples, inducing plasma shock waves to cause plastic deformation of the surface of the gear steel samples.

4. The method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to claim 3, characterized in that, Step 6 also includes: Microhardness tests were performed on the gear steel samples after laser shot peening. Hardness values ​​were measured along the direction from the surface to the center to obtain hardness distribution data. The effective hardened layer depth is determined based on hardness distribution data, and the area with a hardness value higher than 550 HV is considered the effective hardened layer depth. Residual stress was tested on the gear steel samples after laser shot peening. The residual stress at different depths was measured using an X-ray stress meter to obtain residual stress distribution data. The surface wear resistance improvement effect was verified based on hardness distribution data and residual stress distribution data.

5. The method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to claim 4, characterized in that, Step 7 includes: Based on the residual stress distribution data, the region of maximum compressive stress was determined, and thin sheet samples were cut from the corresponding positions on the surface of the gear steel sample after laser shot peening. The cut thin-slice samples were subjected to sandpaper grinding and electrolytic polishing. The gradient nanostructure of the polished thin-film sample was observed using a transmission electron microscope. The distribution characteristics of nanocrystals and nanotwins observed by transmission electron microscopy were analyzed.

6. The method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to claim 5, characterized in that, Step 8 includes: Based on the microstructure analysis results, friction and wear test pins were prepared from regions with gradient nanostructures. GCr15 bearing steel was selected as the friction pair material. Dry friction and wear tests were conducted using the prepared friction and wear test pins and the selected friction pair materials, and the mass loss of the test pins was measured. The average wear and absolute wear resistance are calculated based on mass loss data.

7. The method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to claim 6, characterized in that, Also includes: The wear data of gear steel samples before and after laser shot peening were compared based on mass loss data. The influence of gradient nanostructures on wear resistance was analyzed by combining wear data differences and microstructure analysis results. Optimize laser shot peening parameter configuration based on the impact analysis results; The correlation between residual compressive stress distribution and wear resistance improvement effect was verified based on the optimized laser shot peening parameter configuration.

8. The method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to claim 7, characterized in that, Also includes: Based on the optimized laser shot peening parameter configuration, the microhardness distribution and residual stress distribution performance of gear steel samples treated with different laser energies were compared, and the optimal laser energy range was determined based on the performance comparison results. Based on the optimal laser energy range, the laser peening parameters are configured, and the laser peening process is integrated into a pre-designed carburizing production line to form an integrated laser peening carburizing production line. This integrated laser peening carburizing production line is then applied to the manufacturing of heavy-duty gear parts.

9. The method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to claim 8, characterized in that, Also includes: Based on the integrated environment of the carburizing production line, the carbon potential and temperature during the carburizing heat treatment process are monitored to obtain carbon potential and temperature data. Based on carbon potential temperature data and gear surface geometry, a scanning strategy for controlling laser shot peening is developed. Based on the scanning strategy and real-time processing feedback obtained from the laser power monitor and surface topography sensor, the parameters of the transparent confinement layer and the absorption layer are adjusted. By adjusting the laser pulse energy, spot overlap rate, and confinement layer parameters, the surface roughness of the gear steel sample is controlled to meet the surface quality requirements within the preset average roughness range.

10. The method for improving the surface wear resistance of carburized gear steel based on laser shot peening technology according to claim 9, characterized in that, Also includes: Metallographic analysis was performed on gear steel samples after laser shot peening to obtain metallographic data. Based on metallographic data, the content and morphological changes of retained austenite were detected to obtain data on retained austenite. Based on metallographic data and retained austenite data, wear behavior under high contact fatigue conditions was simulated to obtain wear simulation data. Based on metallographic data, retained austenite data, and wear simulation data, a surface strengthening effect report is generated.

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