A laser processing method for controllable conversion of surface strengthening mechanism of U75V steel

Abstract

The present application relates to the technical field of metal material surface strengthening, and particularly relates to a laser processing method for controllable conversion of U75V steel surface strengthening mechanism. A plurality of groups of gradient laser process parameters are set to obtain peak surface temperatures of each group of machined surfaces, and the corresponding effective absorption surface energy density can be calculated for each group of parameters. According to the peak temperature, a normalized thermal response criterion is solved, and in combination with whether the cross section forms a continuously distributed remelted layer along the width direction of laser processing and machining, all process parameters are classified into first and second heat input intervals, and the two types of intervals correspond to quenching and remelted strengthening mechanisms respectively. The effective absorption surface energy density range matched with each heat input interval is determined. In actual processing, the corresponding heat input interval is selected according to the target strengthening mechanism, the process parameters are adjusted, and the controllable switching of the strengthening effect is realized.

Description

Technical Field

[0001] This invention relates to the field of surface strengthening technology for metallic materials, and in particular to a laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel. Background Technology

[0002] U75V steel is a pearlitic steel material widely used in rail transportation, possessing high strength, hardness, and load-bearing capacity. During long-term service, the surface of U75V steel is often subjected to complex friction and wear as well as alternating loads, which can easily lead to fatigue cracks, spalling, and failure, thereby affecting its service life and operational safety.

[0003] To address the need for improved wear resistance and fatigue resistance of U75V steel surface layers, laser surface strengthening employs two core mechanisms: The first is quenching-dominated strengthening, where the laser heats the surface to the complete austenitization temperature range, preventing melting and allowing for rapid cooling to generate a high-hardness martensite structure for hardening. The second is remelting-dominated strengthening, where further increased laser heat input causes the surface metal to melt, and the molten pool rapidly solidifies to form a remelted modified layer. The two strengthening mechanisms exhibit significant differences in surface microstructure, mechanical properties, and wear resistance, making them suitable for the strengthening requirements of rails on different railway lines.

[0004] Currently, methods for improving the surface properties of U75V steel fall into two categories: traditional heat treatment and laser surface heat treatment. Traditional heat treatment includes induction hardening, flame hardening, and surface carburizing, relying on large-area overall heating to achieve surface strengthening. However, these traditional processes generally suffer from problems such as large heating range, wide heat-affected zone, insufficient precision in microstructure control, and easy workpiece deformation, making it difficult to meet the needs of fine control and directional strengthening of the U75V steel surface microstructure. Laser surface heat treatment technology, due to its advantages such as high energy density, short heating time, strong local control capability, and small workpiece deformation, has become an important technical path for strengthening the surface of U75V steel. By adjusting process parameters such as laser power, scanning speed, spot size, and focal length, the heat input level of the workpiece surface can be changed, thereby affecting the surface temperature field distribution, phase transformation process, and solidification behavior, resulting in different types of strengthened microstructures on the U75V steel surface.

[0005] Most existing laser strengthening technologies passively alter the surface microstructure and properties of U75V steel by adjusting process parameters such as laser power and scanning speed. Parameter matching is highly dependent on experimental experience and repeated trial and error, lacking quantitative criteria for mechanism identification. This makes it difficult to accurately convert the laser strengthening mechanism of the steel surface, resulting in unstable strengthening effects, local overmelting, or insufficient hardening. Consequently, the surface properties of the rails are inconsistent, and the process repeatability is low, failing to meet the production requirements for precise industrial strengthening. Summary of the Invention

[0006] Therefore, the technical problem to be solved by the present invention is to overcome the shortcomings of the existing U75V steel laser surface strengthening process, which relies on empirical parameters and is difficult to accurately convert the laser strengthening mechanism of the steel surface, resulting in poor controllability of the strengthening effect of U75V steel and low process repeatability.

[0007] To address the aforementioned technical problems, this invention provides a laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel, comprising: The surface of the U75V steel workpiece is pretreated and clamped and positioned. The laser beam is adjusted so that it acts perpendicularly on the workpiece surface with a preset spot shape. Multiple sets of different laser process parameters were set to obtain the surface peak temperature of U75V steel workpieces during laser processing; each set of process parameters corresponds to an effective surface energy density. Based on the normalized thermal response criteria calculated from the peak surface temperature of each group, and considering whether the cross-section of the U75V steel workpiece processing layer forms a continuously distributed remelted layer along the width direction of the laser processing, the process parameters of each group are divided into two types of heat input ranges, and the two types of heat input ranges correspond to a U75V steel surface strengthening mechanism. Based on the effective absorption surface energy density corresponding to all process parameters within the same heat input range, determine the range of effective absorption surface energy density for each heat input range. Based on the heat input range of the target U75V steel surface strengthening mechanism, adjust the process parameters so that the effective surface energy density absorbed by the U75V steel workpiece is within the effective surface energy density range corresponding to that heat input range.

[0008] Preferably, the process parameters include laser power, scanning speed, and effective spot width.

[0009] Preferably, the method for adjusting the process parameters to ensure that the effective surface energy density of the U75V steel workpiece is within the effective surface energy density range corresponding to the heat input range is as follows: According to the formula By adjusting the laser power, scanning speed, and effective spot width, the effective absorption surface energy density is controlled to fall within the effective absorption surface energy density range corresponding to the target heat input range; among which... The effective surface energy density absorbed by the U75V steel workpiece surface. The effective absorption coefficient of the laser. For laser power, For scanning speed, The effective spot width.

[0010] Preferably, the formula for calculating the normalized thermal response criterion based on the peak surface temperature of each group is as follows: , in, As a normalized thermal response criterion, The peak surface temperature. This is the temperature at which U75V steel fully austenitizes. The remelting threshold temperature for U75V steel surface.

[0011] Preferably, the complete austenitizing temperature of U75V steel ranges from 755.7 ℃ to 760 ℃.

[0012] Preferably, the method for dividing each group of process parameters into two categories of heat input ranges based on the normalized thermal response criterion calculated according to the peak surface temperature of each group, combined with whether the cross-section of the U75V steel workpiece processing layer forms a continuously distributed remelted layer along the width direction of the laser processing, includes: For each group of surface peak temperatures, determine whether the corresponding normalized thermal response criterion is greater than or equal to 0 and less than 1; If it is greater than or equal to 0 and less than 1, then determine whether the cross section of the U75V steel workpiece processing layer corresponding to the peak surface temperature of this group has not formed a continuously distributed remelted layer along the laser processing width direction. If so, then determine that the process parameters corresponding to the peak surface temperature of this group belong to the first heat input range. If the value is greater than or equal to 1, then determine whether the cross section of the U75V steel workpiece processing layer corresponding to the peak surface temperature of this group forms a continuously distributed remelted layer along the width direction of the laser processing. If so, then determine that the process parameters corresponding to the peak surface temperature of this group belong to the second heat input range.

[0013] Preferably, if the peak surface temperature is lower than the complete austenitization temperature of U75V steel, then the corresponding process parameters are determined to belong to the non-target region.

[0014] Preferably, the method for determining the effective absorption surface energy density range for each heat input range based on the effective absorption surface energy density corresponding to all process parameters within the same heat input range includes: Extract the effective absorption surface energy density values ​​corresponding to all process parameters under the same heat input range, and take the minimum and maximum values ​​of this set of values ​​as the lower limit and upper limit of the effective absorption surface energy density range matched by the corresponding heat input range, respectively.

[0015] Preferably, the preset spot shape is a flat-topped spot.

[0016] Preferably, each of the two types of heat input ranges is matched with an effective absorption surface energy density range, and the interval between the two effective absorption surface energy density ranges is defined as the enhancement mechanism transition boundary zone.

[0017] The above-mentioned technical solution of the invention has the following advantages over the prior art: This invention discloses a laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel. It constructs a joint quantitative judgment rule based on effective absorption surface energy density, normalized thermal response criteria, and microstructure morphology. First, multiple sets of laser process parameters are set to obtain the surface peak temperature. Based on the surface peak temperature and the cross-sectional remelting state, the process parameters are divided into two types of heat input intervals corresponding to quenching and remelting mechanisms. The effective absorption surface energy density range corresponding to each heat input interval is statistically obtained. During actual processing, the process parameters are adjusted directly according to the target strengthening mechanism, selecting the corresponding energy density range. This eliminates trial and error, achieves precise and controllable conversion of the strengthening mechanism, avoids unstable boundary intervals, improves process repeatability, and stabilizes the surface strengthening quality of the rail. Attached Figure Description

[0018] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein:

[0019] Figure 1 This is a schematic flowchart of a laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel according to the present invention.

[0020] Figure 2 This is a schematic diagram of the continuous cooling transformation curve of U75V steel;

[0021] Figure 3 This is a schematic diagram of the simulation results of the surface temperature field of U75V steel under different laser power conditions;

[0022] Figure 4 This is a schematic diagram of the surface microstructure obtained after laser treatment of U75V steel surface under a laser power of 800 W in an embodiment of the present invention. Figure 4 Image (a) is a tissue diagram under a 200x light microscope. Figure 4 (b) is a tissue diagram under a 500x light microscope;

[0023] Figure 5 This is a schematic diagram of the microhardness distribution curve of U75V steel along the surface towards the substrate after laser treatment under the condition of laser power of 800 W in an embodiment of the present invention.

[0024] Figure 6 This is a schematic diagram of the surface microstructure obtained after laser treatment of U75V steel surface under a laser power of 1200 W in an embodiment of the present invention. Figure 6 Image (a) is a tissue diagram under a 200x light microscope. Figure 6 (b) is a tissue diagram under a 500x light microscope;

[0025] Figure 7This is a schematic diagram of the microhardness distribution curve of U75V steel along the surface towards the substrate after laser treatment under the condition of laser power of 1200 W in an embodiment of the present invention.

[0026] Figure 8 This is a schematic diagram of the surface microstructure obtained after laser treatment of U75V steel surface under a laser power of 1600 W in an embodiment of the present invention. Figure 8 Image (a) is a tissue diagram under a 200x light microscope. Figure 8 (b) is a tissue diagram under a 500x light microscope;

[0027] Figure 9 This is a schematic diagram of the microhardness distribution curve of U75V steel along the surface towards the substrate after laser treatment under the condition of laser power of 1600 W in an embodiment of the present invention.

[0028] Figure 10 This is a schematic diagram of the surface microstructure obtained after laser treatment of U75V steel surface under a laser power of 2000 W in an embodiment of the present invention. Figure 10 Image (a) is a tissue diagram under a 200x light microscope. Figure 10 (b) is a tissue diagram under a 500x light microscope;

[0029] Figure 11 This is a schematic diagram of the microhardness distribution curve of U75V steel along the surface towards the substrate after laser treatment under the condition of laser power of 2000 W in an embodiment of the present invention. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0031] Reference Figure 1 As shown, this embodiment provides a laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel, including: Step S1: Pre-treat the surface of the U75V steel workpiece and complete the clamping and positioning. Adjust the laser beam so that it acts perpendicularly on the workpiece surface with a preset spot shape. In this embodiment, specifically, the preset light spot morphology is a flat-topped light spot, so as to reduce local overheating caused by uneven lateral energy distribution on the surface and improve the stability of the transition boundary between the first heat input interval and the second heat input interval.

[0032] In this embodiment, specifically, the surface of the U75V steel workpiece is pretreated to remove surface oxide scale, oil stains and impurities. The pretreated U75V steel workpiece is fixed on the laser processing platform, and the relative position, focal length and scanning path of the laser beam and the workpiece surface are adjusted so that the laser beam acts on the workpiece surface perpendicularly or approximately perpendicularly with a preset spot shape.

[0033] Step S2: Set multiple sets of different laser process parameters to obtain the surface peak temperature of the U75V steel workpiece during laser processing; each set of process parameters corresponds to an effective absorption surface energy density; In this embodiment, the process parameters specifically include laser power, scanning speed, and effective spot width.

[0034] Step S3: Based on the peak surface temperature of each group, calculate the normalized thermal response criterion. Combined with whether the cross section of the U75V steel workpiece processing layer forms a continuously distributed remelted layer along the width direction of the laser processing, divide each group of process parameters into two types of heat input intervals. The two types of heat input intervals correspond to a U75V steel surface strengthening mechanism. The continuously distributed remelted layer along the width direction of the laser processing refers to the surface tissue area where the remelted structure is continuously distributed along the width direction of the processing on the cross section of the processing layer and is not penetrated or separated by the unmelted matrix area.

[0035] In this embodiment, preferably, the formula for calculating the normalized thermal response criterion based on the peak surface temperature of each group is as follows: , in, As a normalized thermal response criterion, The peak surface temperature. This is the temperature at which U75V steel fully austenitizes. The remelting threshold temperature for U75V steel surface.

[0036] In this embodiment, the complete austenitization temperature of U75V steel is... The value of the complete austenitizing temperature of U75V steel is determined by a combination of the measured value of the continuous cooling transformation curve of U75V steel and the calculated value of the empirical formula for phase transformation. Preferably, the value of the complete austenitizing temperature of U75V steel is in the range of 755.7 ℃ to 760 ℃.

[0037] like Figure 2 As shown, Figure 2 This is a schematic diagram of the continuous cooling transformation curve of U75V steel. In this embodiment, the U75V steel is fully austenitized. Used to determine whether the laser-treated surface has reached the condition for complete austenitization, and as a lower limit reference temperature for the normalized thermal response criterion; calculated based on the empirical formula for phase transformation of U75V steel chemical composition, the complete austenitization temperature is approximately , and the aforementioned The small difference indicates that the selected complete austenitizing temperature is reasonable.

[0038] The aforementioned critical phase transformation temperatures can serve as a basis for determining whether the surface of U75V steel has reached the conditions for complete austenitization and whether it has the foundation for forming a quenched phase transformation microstructure. For laser surface treatment, it is also necessary to combine the surface peak temperature, whether a continuous surface remelting layer is formed in the cross-section of the treated layer, and the microstructure morphology after treatment to determine the boundary between the quenching-dominated strengthening mechanism and the remelting-dominated strengthening mechanism.

[0039] U75V steel surface remelting threshold temperature The temperature field simulation results and the metallographic results of the treated layer section were used to jointly calibrate the temperature, which corresponds to the peak surface temperature when the first continuous surface remelting layer is formed on the treated layer section.

[0040] In this embodiment, preferably, the method of dividing each group of process parameters into two types of heat input ranges based on the normalized thermal response criterion calculated according to the peak surface temperature of each group, and in combination with whether the cross-section of the U75V steel workpiece processing layer forms a continuously distributed remelted layer along the width direction of the laser processing, includes:

[0041] For each group of surface peak temperatures, determine its corresponding normalized thermal response criterion. Is it greater than or equal to 0 and less than 1? If it is greater than or equal to 0 and less than 1, then determine whether the cross section of the U75V steel workpiece processing layer corresponding to the peak surface temperature of this group has not formed a continuously distributed remelted layer along the laser processing width direction. If so, then determine that the process parameters corresponding to the peak surface temperature of this group belong to the first heat input range. If the value is greater than or equal to 1, then determine whether the cross section of the U75V steel workpiece processing layer corresponding to the peak surface temperature of this group forms a continuously distributed remelted layer along the width direction of the laser processing. If so, then determine that the process parameters corresponding to the peak surface temperature of this group belong to the second heat input range.

[0042] The first heat input range and the second heat input range together constitute the process window for the continuous transformation of the surface strengthening mechanism of U75V steel. The first heat input range corresponds to the heat input range in which a continuous surface remelting layer is not formed on the surface of U75V steel and the surface structure is mainly composed of quenched phase transformation structure. The second heat input range corresponds to the heat input range in which a continuous surface remelting layer is formed on the surface of U75V steel and the surface structure is mainly composed of remelted rapid solidification structure.

[0043] This invention, combining the critical phase transformation temperature, surface peak temperature, and cross-sectional microstructure of the treated layer determined by the continuous cooling transformation (CCT) curve of U75V steel, determines the surface strengthening mechanism corresponding to different laser process parameters: when That is, the peak surface temperature Higher than the full austenitizing temperature of U75V steel And below the surface remelting threshold temperature Furthermore, when the treated layer cross-section does not form a continuous surface remelted layer and the surface structure is mainly composed of quenched phase transformation structure, the surface peak temperature is determined. The corresponding process parameters belong to the first heat input range; the surface structure is mainly quenched phase transformation structure, which means that after treatment, the U75V steel surface does not form a continuous surface remelting layer, and the surface structure is mainly quenched phase transformation structure with fine needle-like structure.

[0044] when When a continuous remelted surface layer is formed on the cross-section of the treated layer, and the surface structure is mainly composed of remelted and rapidly solidified structures, the peak surface temperature is determined. The corresponding process parameters belong to the second heat input range.

[0045] The above method can realize a continuous and controllable transformation of the surface strengthening mechanism of U75V steel from quenching-dominated to remelting-dominated, and establish the correspondence between the heat input range and the thickness and microstructure of the strengthening layer.

[0046] In this embodiment, preferably, if the surface peak temperature is lower than the complete austenitization temperature of U75V steel, then it is determined that the corresponding process parameters do not belong to either the first heat input range or the second heat input range, and belong to the non-target region.

[0047] In this embodiment, specifically, the method for determining the effective absorption surface energy density range matching each heat input interval based on the effective absorption surface energy density corresponding to all process parameters within the same heat input interval includes: Extract the effective absorption surface energy density values ​​corresponding to all process parameters under the same heat input range, and take the minimum and maximum values ​​of this set of values ​​as the lower limit and upper limit of the effective absorption surface energy density range matched by the corresponding heat input range, respectively.

[0048] Step S4: Based on the effective absorption surface energy density corresponding to all process parameters within the same heat input range, determine the range of effective absorption surface energy density for each heat input range. Step S5: Adjust the process parameters according to the heat input range of the target U75V steel surface strengthening mechanism so that the effective surface energy density absorbed by the U75V steel workpiece is within the effective surface energy density range corresponding to the heat input range.

[0049] In actual processing, laser treatment is applied to the surface of U75V steel according to the determined first or second heat input range. The first heat input range corresponds to the processing condition where the U75V steel surface undergoes solid-state phase transformation quenching without forming a continuous surface remelted layer. The second heat input range corresponds to the processing condition where the U75V steel surface undergoes local melting and forms a continuous surface remelted layer. By controlling the first and second heat input ranges, a continuous and controllable transformation of the U75V steel surface strengthening mechanism from quenching-dominated to remelting-dominated is achieved, and a correspondence between the strengthening layer thickness, microstructure type, and heat input range is established.

[0050] In this embodiment, specifically, the method for adjusting the process parameters to ensure that the effective surface energy density of the U75V steel workpiece is within the effective surface energy density range corresponding to the heat input range is as follows: According to the formula By adjusting the laser power, scanning speed, and effective spot width, the effective absorption surface energy density is controlled to fall within the effective absorption surface energy density range corresponding to the target heat input range; among which... The effective absorbed surface energy density of the U75V steel workpiece, expressed in J / mm². 2 , The effective absorption coefficient of the laser. Laser power, in watts (W). The scanning speed is expressed in mm / s. The effective spot width in the vertical scanning direction is expressed in mm.

[0051] The effective absorption surface energy density This characterizes the cumulative energy input actually received per unit area of ​​material during laser scanning. For a rectangular flat-topped spot, if its effective length along the scanning direction is... The effective width in the vertical scanning direction is Then the instantaneous absorbed power density can be expressed as The duration of single-point heating can be expressed as Multiplying the two together yields the above formula. Therefore, this invention uses the effective spot width in the vertical scanning direction. Characterizes the cumulative surface energy input during the scanning process.

[0052] In this embodiment, the two types of heat input intervals each correspond to a specific effective absorption surface energy density range, and the interval between the two energy density ranges is defined as the strengthening mechanism transition boundary zone. This boundary zone lies between the upper limit of the first heat input interval and the lower limit of the second heat input interval. Within this energy range, only local melting traces will appear on the cross-section of the workpiece processing layer, and a stable and continuous remelted layer cannot be formed. Furthermore, the continuity of the remelted layer at different locations in the processing area fluctuates significantly. Therefore, when local melting traces begin to appear on the cross-section of the processing layer but a stable and continuous surface remelted layer has not yet been formed, it can be determined as the strengthening mechanism transition boundary zone.

[0053] In this embodiment, preferably, a controlled variable test scheme is adopted during calibration to conveniently and with a single variable calibrate the two types of heat input ranges and the boundary zone of the enhancement mechanism transition.

[0054] Under the condition that the scanning speed, spot size, focal length and scanning path are kept constant, gradient experiments are carried out by only adjusting the laser power in stages. This method can eliminate the interference of other process variables on the surface peak temperature and molten structure, and distinguish the two strengthening states of quenching and remelting only by the energy change brought about by the power gradient. Then, based on the normalized thermal response criterion and the cross-sectional remelting morphology, the process parameters are classified, and the effective absorption surface energy density range corresponding to the two types of heat input intervals is statistically obtained. The unstable region between the two energy density intervals is the boundary zone of the strengthening mechanism transition.

[0055] In this embodiment, preferably, after the laser treatment is completed, the workpiece is allowed to cool naturally or under a protective atmosphere to obtain a surface-strengthened structure after laser heat input regulation.

[0056] This invention targets the specific material system of U75V steel, relying on the known key phase transformation temperature, peak temperature of the processed surface, and surface microstructure characteristics of the steel to determine the range, making it more material-specific. The solution upgrades the laser treatment of U75V steel surface from simply controlling the heat input to a complete determination system for two types of heat input ranges and their boundaries, clearly defining the process window. It also uses the effective heat input characterization formula and normalized thermal response criteria to quantitatively distinguish between quenching and remelting strengthening mechanisms. At the same time, it combines multi-dimensional joint determination of key phase transformation temperature, peak temperature, and continuous remelted layer morphology from CCT curves, overcoming the limitations of relying solely on a single power parameter to divide the range, improving the accuracy of range division and process repeatability. It can also establish a correspondence between heat input ranges and strengthening layer thickness and microstructure type, providing a quantitative and clear design basis for optimizing the surface strengthening process of U75V steel.

[0057] Based on Example 1, Example 2 uses U75V steel as the substrate material. Its surface is mechanically ground and cleaned to remove surface oxide scale and oil. The treated sample is fixed on a laser processing platform and laser scanning is performed using a flat-top spot mode. The effective size of the flat-top spot is 10 mm × 2 mm, and the scanning speed is 2 mm / s. The effective spot width perpendicular to the scanning direction is taken as... The effective absorption coefficient of the laser is taken as The effective absorption surface energy densities at four laser powers of 800 W, 1200 W, 1600 W, and 2000 W were calculated to be as follows: , , and .

[0058] The temperature field on the surface of U75V steel under different laser powers was simulated and analyzed. For example... Figure 3 As shown, Figure 3 This is a schematic diagram of the simulated temperature field results for the surface of U75V steel under different laser powers. According to the simulation results, the peak surface temperatures under the four laser power conditions of 800 W, 1200 W, 1600 W, and 2000 W are approximately 861.6 ℃, 1177 ℃, 1606 ℃, and 1918 ℃, respectively. Among these, the peak temperatures under the 800 W and 1200 W conditions are significantly higher than [the required temperature range]. This indicates that the surface layer of U75V steel can reach the conditions of complete austenitization; the peak temperature under the conditions of 1600 W and 2000 W is further increased, and combined with the observation of the cross-sectional microstructure of the treated layer, it can be determined that it has entered the thermal response range corresponding to surface remelting.

[0059] like Figure 4 , Figure 5 As shown, Figure 4 This is a schematic diagram of the surface microstructure obtained after laser treatment of U75V steel surface under a laser power of 800 W in an embodiment of the present invention. Figure 4 Image (a) is a tissue diagram under a 200x light microscope. Figure 4 (b) is a tissue diagram under a 500x light microscope; Figure 5 This is a schematic diagram of the microhardness distribution curve of U75V steel along the surface towards the substrate after laser treatment under the condition of laser power of 800 W in an embodiment of the present invention.

[0060] After laser treatment at 800 W, the sample was allowed to cool naturally in air. Metallographic preparation and microstructural observation of the treated sample revealed no obvious melting characteristics on the surface, instead forming a quenched phase transformation structure dominated by fine needle-like structures. Microhardness testing was performed on the sample from the surface towards the matrix. The results showed that the surface hardness was higher than the matrix hardness, forming a continuous hardness gradient distribution, with a reinforcing layer depth of approximately 200 μm. These results indicate that... Under the condition of effectively absorbing surface energy density, the peak surface temperature of U75V steel is higher than the complete austenitization temperature, and no continuous surface remelting layer is formed on the cross section of the treated layer. Therefore, this process parameter belongs to the low heat input condition in the first heat input range.

[0061] like Figure 6 , Figure 7 As shown, Figure 6 This is a schematic diagram of the surface microstructure obtained after laser treatment of U75V steel surface under a laser power of 1200 W in an embodiment of the present invention. Figure 6 Image (a) is a tissue diagram under a 200x light microscope. Figure 6 (b) is a tissue diagram under a 500x light microscope; Figure 7 This is a schematic diagram of the microhardness distribution curve of U75V steel along the surface towards the substrate after laser treatment under the condition of laser power of 1200 W in an embodiment of the present invention.

[0062] Microstructural observation of the treated samples under 1200 W laser power revealed a significant increase in the number of acicular structures on the sample surface compared to the 800 W condition, resulting in a denser microstructure. Furthermore, no continuous surface remelting layer was formed on the treated layer cross-section, indicating that the process parameters fall within the higher heat input range of the first heat input interval. Corresponding microhardness testing showed further improvement in sample surface hardness, a significantly extended hardness plateau region, and an increased reinforced layer depth of approximately 600 μm. These results demonstrate that within the first heat input range, increasing the effective absorbed surface energy density is beneficial for increasing the thickness of the quenching-dominated reinforced layer.

[0063] like Figure 8 , Figure 9 As shown, Figure 8 This is a schematic diagram of the surface microstructure obtained after laser treatment of U75V steel surface under a laser power of 1600 W in an embodiment of the present invention. Figure 8 Image (a) is a tissue diagram under a 200x light microscope. Figure 8 (b) is a tissue diagram under a 500x light microscope; Figure 9 This is a schematic diagram of the microhardness distribution curve of U75V steel along the surface towards the substrate after laser treatment under the condition of laser power of 1600 W in an embodiment of the present invention.

[0064] Under a laser power of 1600 W, compared to 1200 W, the surface peak temperature increased significantly, entering the thermal response range corresponding to the formation of surface remelting structures. Combined with the obvious remelting characteristics and rapid solidification structures observed in the microstructure of the treated layer cross-section, it can be determined that this process parameter belongs to the second heat input range. Microstructural observation of the treated sample shows obvious remelting characteristics on the sample surface, forming a dense, rapidly solidified structure, beneath which are the heat-affected zone and the matrix region, exhibiting a clear layered structure. Corresponding microhardness test results show that the sample surface maintains a high hardness level, while the depth of the reinforcing layer increases significantly, reaching approximately 1200 μm. These results indicate that when the effective absorption surface energy density increases to... At that time, the surface strengthening mechanism of U75V steel had changed from being dominated by quenching to being dominated by remelting.

[0065] like Figure 10 , Figure 11 As shown, Figure 10 This is a schematic diagram of the surface microstructure obtained after laser treatment of U75V steel surface under a laser power of 2000 W in an embodiment of the present invention. Figure 10 Image (a) is a tissue diagram under a 200x light microscope. Figure 10 (b) is a tissue diagram under a 500x light microscope; Figure 11 This is a schematic diagram of the microhardness distribution curve of U75V steel along the surface towards the substrate after laser treatment under the condition of laser power of 2000 W in an embodiment of the present invention.

[0066] Temperature field simulation results under 2000 W laser power indicate that the peak surface temperature of U75V steel is approximately 1918 ℃, higher than that under 1600 W laser power, suggesting a more complete surface remelting and rapid solidification process under this condition. Combined with the formation of a continuous remelted microstructure in the treated layer cross-section, this process parameter can be determined to be a relatively high heat input condition within the second heat input range. Microstructural observation of the treated sample shows a continuous remelted microstructure on the sample surface, which is fine and dense, with the heat-affected zone and matrix region below it. Corresponding microhardness distribution results indicate that the sample surface maintains a high hardness level, and the depth of the reinforced layer further increases, reaching approximately 1500 μm. These results demonstrate that a thicker remelted-dominated surface reinforced layer can be obtained under conditions of higher effective absorbed surface energy density.

[0067] In summary, under the condition that the scanning speed, spot size, focal length, and scanning path remain constant, as the laser power increases from 800 W to 2000 W, the effective surface energy density absorbed by the U75V steel surface increases from... Increase to The corresponding peak surface temperature increased from about 861.6 ℃ to about 1918 ℃, and the surface strengthening mechanism of the material gradually changed from quenching to remelting.

[0068] Among them, under 800 W conditions The peak surface temperature is approximately 861.6 ℃, which is higher than... The processed layer did not form a continuous surface remelted layer, corresponding to the first heat input range; under 1200 W conditions. The surface peak temperature is approximately 1177 ℃, and the treated layer has not yet formed a continuous surface remelted layer, corresponding to the first heat input range; under 1600 W conditions The surface peak temperature is approximately 1606 ℃, and the treated layer forms a distinct remelted structure, corresponding to the second heat input range; under 2000 W conditions The surface peak temperature is about 1918 ℃, and the treated layer forms a continuous remelted structure, corresponding to the second heat input range under a higher heat input level.

[0069] The results of this embodiment show that no continuous surface remelted layer was formed at 1200 W, while a significant remelted structure was formed at 1600 W. Therefore, the transformation boundary zone of the surface strengthening mechanism of U75V steel is located between the heat input ranges corresponding to 1200 W and 1600 W, i.e., it is located in... and Between. Correspondingly, the surface remelting threshold temperature The corresponding peak temperature range is between 1177 ℃ and 1606 ℃.

[0070] The boundary zone of the surface strengthening mechanism of U75V steel is not a single fixed point, but rather the range of heat input transitioning from the first heat input range to the second heat input range. Within this range, local melting marks or unstable continuity of the remelted layer may appear on the treated layer cross-section. By adding an intermediate power point and combining temperature field simulation and metallographic results of the treated layer cross-section, the range of this boundary zone can be further refined. Through the joint determination of effective absorption surface energy density, normalized thermal response criteria, temperature field simulation results, and microstructure morphology of the treated layer cross-section, a continuous and controllable transition of the surface strengthening mechanism of U75V steel from quenching-dominated to remelting-dominated can be achieved, and a correspondence between the heat input range and the thickness and microstructure type of the strengthened layer can be established.

[0071] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0072] This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart... Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0073] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0074] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0075] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel, characterized in that, include: The surface of the U75V steel workpiece is pretreated and clamped and positioned. The laser beam is adjusted so that it acts perpendicularly on the workpiece surface with a preset spot shape. Multiple sets of different laser process parameters were set to obtain the surface peak temperature of U75V steel workpieces during laser processing; each set of process parameters corresponds to an effective surface energy density. Based on the normalized thermal response criteria calculated from the peak surface temperature of each group, and considering whether the cross-section of the U75V steel workpiece processing layer forms a continuously distributed remelted layer along the width direction of the laser processing, the process parameters of each group are divided into two types of heat input ranges, and the two types of heat input ranges correspond to a U75V steel surface strengthening mechanism. Based on the effective absorption surface energy density corresponding to all process parameters within the same heat input range, determine the range of effective absorption surface energy density for each heat input range. Based on the heat input range of the target U75V steel surface strengthening mechanism, adjust the process parameters so that the effective surface energy density absorbed by the U75V steel workpiece is within the effective surface energy density range corresponding to that heat input range.

2. The laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel according to claim 1, characterized in that, The process parameters include laser power, scanning speed, and effective spot width.

3. The laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel according to claim 2, characterized in that, The method for adjusting process parameters to ensure that the effective absorbed surface energy density of the U75V steel workpiece is within the range of effective absorbed surface energy density corresponding to the heat input range is as follows: According to the formula By adjusting the laser power, scanning speed, and effective spot width, the effective absorption surface energy density is controlled to fall within the effective absorption surface energy density range corresponding to the target heat input range; among which... The effective surface energy density absorbed by the U75V steel workpiece surface. The effective absorption coefficient of the laser. For laser power, For scanning speed, The effective spot width.

4. The laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel according to claim 1, characterized in that, The formula for calculating the normalized thermal response criterion based on the peak surface temperature of each group is as follows: ， in, As a normalized thermal response criterion, The peak surface temperature. This is the temperature at which U75V steel fully austenitizes. The remelting threshold temperature for U75V steel surface.

5. The laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel according to claim 4, characterized in that, The range of the complete austenitizing temperature for U75V steel is 755.7 ℃ to 760 ℃.

6. The laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel according to claim 1, characterized in that, Based on the normalized thermal response criteria calculated from the peak surface temperature of each group, and considering whether a continuous remelted layer is formed along the width of the laser processing of the U75V steel workpiece, the method for classifying each group of process parameters into two categories of heat input ranges includes: For each group of surface peak temperatures, determine whether the corresponding normalized thermal response criterion is greater than or equal to 0 and less than 1; If it is greater than or equal to 0 and less than 1, then determine whether the cross section of the U75V steel workpiece processing layer corresponding to the peak surface temperature of this group has not formed a continuously distributed remelted layer along the laser processing width direction. If so, then determine that the process parameters corresponding to the peak surface temperature of this group belong to the first heat input range. If the value is greater than or equal to 1, then determine whether the cross section of the U75V steel workpiece processing layer corresponding to the peak surface temperature of this group forms a continuously distributed remelted layer along the width direction of the laser processing. If so, then determine that the process parameters corresponding to the peak surface temperature of this group belong to the second heat input range.

7. The laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel according to claim 6, characterized in that, If the peak surface temperature is lower than the complete austenitization temperature of U75V steel, then the corresponding process parameters are determined to be in the non-target region.

8. The laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel according to claim 1, characterized in that, Methods for determining the effective absorption surface energy density range for each heat input interval based on the effective absorption surface energy density corresponding to all process parameters within the same heat input interval include: Extract the effective absorption surface energy density values ​​corresponding to all process parameters under the same heat input range, and take the minimum and maximum values ​​of this set of values ​​as the lower limit and upper limit of the effective absorption surface energy density range matched by the corresponding heat input range, respectively.

9. The laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel according to claim 1, characterized in that, The preset spot shape is a flat-topped spot.

10. The laser processing method for controllable conversion of the surface strengthening mechanism of U75V steel according to claim 1, characterized in that, Each of the two types of heat input ranges is matched with an effective absorption surface energy density range. The interval between the two effective absorption surface energy density ranges is defined as the enhancement mechanism transition boundary zone.

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