Ultrasonic-assisted metal surface hardening heat treatment method

Abstract

An ultrasonic-assisted metal surface hardening heat treatment method is used for treating a metal workpiece and comprises the steps that S1, the surface of the metal workpiece is pretreated, specifically, cleaning treatment and activating treatment are executed in sequence; s2, the pretreated metal workpiece is placed in a heat treatment environment and heated to the surface hardening temperature under the protective atmosphere or vacuum condition; s3, multi-stage heat preservation treatment is conducted at the surface hardening temperature, wherein first-stage heat preservation and second-stage heat preservation are sequentially executed; s4, multi-section controlled cooling is conducted on the metal workpiece subjected to heat preservation treatment, and the metal workpiece is cooled to the room temperature; and S5, the metal workpiece cooled to the room temperature is subjected to tempering stabilizing treatment. The method has the advantage of improving the production efficiency.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for metallic materials, and in particular to an ultrasonic-assisted method for surface hardening heat treatment of metals. Background Technology

[0002] Metal surface hardening heat treatment is a key manufacturing process for improving the wear resistance, fatigue strength, and service life of mechanical parts, playing an indispensable role in equipment manufacturing, the automotive industry, and aerospace. Developing efficient and controllable surface hardening technologies is of great significance for improving the reliability and performance of high-end equipment.

[0003] Currently, traditional surface hardening techniques such as carburizing, nitriding, and induction hardening generally rely on thermal activation energy to drive diffusion or phase transformation processes. These methods have significant limitations: processing cycles are typically long and energy consumption is high; it is difficult to obtain a uniform hardened layer on complex-shaped workpieces, leading to soft spots or deformation; the process parameter window is narrow, and they are sensitive to material composition and early microstructure, resulting in significant challenges in batch stability control. Furthermore, while some advanced technologies, such as plasma nitriding or laser hardening, can partially improve these problems, they introduce new issues such as system complexity, high equipment investment, or poor integration with existing production lines. Overall, existing technologies struggle to achieve an ideal balance between processing efficiency, microstructure uniformity, and process economy, limiting their widespread application in precision and complex components.

[0004] Therefore, an ultrasonic-assisted heat treatment method for hardening metal surfaces is proposed to address the above problems. The core issue to be solved is how to overcome the defects of existing surface hardening technologies, such as long processing cycles, poor control of hardened layer uniformity (especially for complex-shaped workpieces), and insufficient process stability. Summary of the Invention

[0005] The purpose of this invention is to provide an ultrasonic-assisted heat treatment method for hardening metal surfaces, which has the advantage of improving production efficiency.

[0006] To achieve the above objectives, the present invention provides an ultrasonic-assisted heat treatment method for hardening metal surfaces, which processes metal workpieces. The method includes: S1, pre-treating the surface of the metal workpiece, the pre-treatment including cleaning and activation treatments performed sequentially; S2, placing the pre-treated metal workpiece in a heat treatment environment and heating it to the surface hardening temperature under a protective atmosphere or vacuum; S3, performing a multi-stage heat preservation treatment at the surface hardening temperature, including a first stage heat preservation and a second stage heat preservation performed sequentially; S4, performing multi-stage controlled cooling on the metal workpiece after the heat preservation treatment to cool it to room temperature; S5, performing tempering stabilization treatment on the metal workpiece cooled to room temperature; wherein, in the later stage of heating to the surface hardening temperature in step S2, during the first stage heat preservation and the second stage heat preservation in step S3, and in the early stage of each cooling stage in step S4, directionally adjustable ultrasonic vibrations are applied to the metal workpiece; the application of ultrasonic vibrations covers the austenitization, phase transformation, and early cooling process of the metal workpiece.

[0007] Preferably, step S1 includes: step S11, ultrasonically cleaning the surface of the metal workpiece with acetone for 10 minutes to remove oil stains; step S12, sandblasting the surface of the metal workpiece with 120-mesh alumina abrasive to increase surface roughness and activation area; step S13, immersing the metal workpiece in an 8% hydrochloric acid solution for 2 minutes to remove a very thin oxide film and further activate the surface; step S14, heating the surface of the metal workpiece to a preheating temperature of 200°C to 400°C and maintaining it for 15 to 45 minutes, and applying low-intensity ultrasonic waves with a power density of 0.05 W / cm² to 0.5 W / cm² during the activation treatment to obtain the pretreated metal workpiece.

[0008] Preferably, in step S2, the protective atmosphere is nitrogen, argon, hydrogen, or a mixture thereof, with a purity of not less than 99.9% and a flow rate of 0.5 L / min to 8 L / min; or the vacuum degree of the vacuum condition is not higher than 10⁻² Pa.

[0009] Preferably, step S2 includes: step S21, placing the pretreated metal workpiece in a heat treatment environment, and heating it from room temperature to an intermediate temperature of 350°C to 550°C at a first heating rate of 3°C / min to 15°C / min under a protective atmosphere or vacuum, and holding it at the intermediate temperature for 5 to 30 minutes; step S22, subsequently heating it to the surface hardening temperature at a second heating rate of 5°C / min to 25°C / min, wherein the second heating rate is greater than the first heating rate; wherein the surface hardening temperature refers to the surface hardening temperature of the metal workpiece.

[0010] Preferably, step S3 includes: step S31, determining the total heat preservation time based on the effective thickness of the metal workpiece, ranging from 20 minutes to 180 minutes; step S32, the first stage of heat preservation is carried out at a temperature 10°C to 30°C below the surface hardening temperature, and the heat preservation time is 1 / 4 to 1 / 2 of the total heat preservation time; step S33, the second stage of heat preservation is carried out at the surface hardening temperature, completing the remaining heat preservation time.

[0011] Preferably, step S3 further includes: ultrasonic vibration applied during the first stage of heat preservation, wherein the ultrasonic frequency is 20 kHz to 40 kHz and the power density is 0.5 W / cm² to 2 W / cm²; ultrasonic vibration applied during the second stage of heat preservation, wherein the ultrasonic frequency is 25 kHz to 100 kHz and the power density is 1 W / cm² to 5 W / cm²; and at least one parameter of the parameter combination of the ultrasonic vibration applied during the first stage of heat preservation is different from that of the parameter combination of the ultrasonic vibration applied during the second stage of heat preservation.

[0012] Preferably, step S4 includes: step S41, a first-stage cooling from the surface hardening temperature using a controlled gas at a cooling rate of 10°C / min to 50°C / min to a first intermediate temperature of 550°C to 650°C; the relatively slow cooling rate in this stage helps to reduce the temperature difference between the inside and outside of the workpiece; step S42, a second-stage cooling from the first intermediate temperature using rapid oil cooling at a cooling rate of 30°C / min to 150°C / min to a second intermediate temperature of 150°C to 250°C; this stage aims to allow the supercooled austenite to quickly pass through the pearlite transformation zone and enter the martensite transformation zone; step S43, a third-stage cooling from the second intermediate temperature using air cooling at a cooling rate of 5°C / min to 20°C / min to room temperature; the main purpose of this stage is to release some of the structural stress.

[0013] Preferably, step S4 further includes: continuously applying ultrasonic vibration during the first stage cooling and the second stage cooling, wherein the frequency of the ultrasonic vibration is 30 kHz to 80 kHz, the power density is gradually reduced from an initial 1.5 W / cm² to 0.2 W / cm², and the application is stopped when the temperature reaches the martensitic transformation initiation point.

[0014] Preferably, step S5 includes: step S51, heating the metal workpiece cooled to room temperature to a tempering temperature of 180°C to 500°C and holding it at that temperature for 60 minutes to 240 minutes; during the tempering process, applying ultrasonic vibration in a pulse mode, with a pulse width of 1 second to 5 seconds, a pulse interval of 3 seconds to 10 seconds, and a power density of 0.1 W / cm² to 1 W / cm²; step S52, after the metal workpiece is tempered, cooling the metal workpiece to room temperature at a cooling rate not exceeding 30°C / min.

[0015] Preferably, the applied ultrasonic vibration with adjustable direction means that, during the process, the angle between the ultrasonic propagation direction and the normal to the surface of the metal workpiece changes periodically within the range of 0 degrees to 60 degrees, and the frequency of the change is 0.1 Hz to 5 Hz.

[0016] In summary, compared with the prior art, the ultrasonic-assisted metal surface hardening heat treatment method provided by the present invention has the following beneficial effects:

[0017] First, the present invention provides an ultrasonic-assisted heat treatment method for hardening metal surfaces. By designing a continuous ultrasonic application scheme that includes the later stage of heating, the entire holding stage, and the early stage of cooling, the ultrasonic energy can cover the key metallurgical processes from austenitization and diffusion of alloying elements to early phase transformation. This working method utilizes the acoustic flow and cavitation effects generated by ultrasound to significantly enhance atomic mobility and mass transfer processes at the interface, thereby promoting the homogenization of the microstructure at the microscale. It refines the initial austenite grains and provides more nucleation sites for subsequent phase transformations, resulting in a denser and more uniformly thick surface-hardened layer. Simultaneously, it reduces the total process time by approximately 15% to 25%, improving production efficiency.

[0018] Secondly, the ultrasonic-assisted heat treatment method for hardening metal surfaces provided by this invention employs a strategy combining multi-stage heat preservation with directionally adjustable ultrasonic vibration. In the first stage, at a slightly lower temperature, ultrasonic waves are used for pre-homogenization of the microstructure, preparing for full austenitization at the target temperature in the second stage. The directionally adjustable ultrasonic waves can periodically change the energy input angle or lock the optimal angle, ensuring that all surfaces of complex geometric workpieces, especially difficult-to-process areas such as grooves and internal holes, receive uniform ultrasonic energy. This working method weakens the energy shielding effect caused by the geometry, improves the uniformity of the overall hardened layer of complex parts, avoids the appearance of localized weak performance areas, broadens the applicability of this technology to complex components, and reduces the scrap rate caused by uneven hardening.

[0019] Third, the present invention provides an ultrasonic-assisted heat treatment method for hardening metal surfaces, which introduces pulsed ultrasonic vibration during the tempering stabilization stage. This design allows ultrasonic waves to intermittently apply micro-area energy to the material during tempering, a process aimed at releasing stress and stabilizing the microstructure. This promotes dislocation reorganization and dispersed precipitation of carbides while avoiding overheating. This dynamically assisted tempering process mechanism can more effectively eliminate microscopic internal stress and prevent the continuous distribution of brittle phases along grain boundaries. It can significantly improve the toughness and dimensional stability of the workpiece while maintaining its high hardness and wear resistance. The overall mechanical properties of the treated workpiece are optimized, and the tendency for deformation and the risk of early failure during service are reduced. Attached Figure Description

[0020] Figure 1 This is a flowchart of an ultrasonic-assisted metal surface hardening heat treatment method proposed in this invention. Detailed Implementation

[0021] The following will be combined with the appendix in the embodiments of the present invention. Figure 1 The technical solutions, structural features, objectives and effects achieved in the embodiments of the present invention will be described in detail.

[0022] It should be noted that the accompanying drawings are in a very simplified form and use non-precise proportions. They are only used to facilitate and clarify the purpose of illustrating the embodiments of the present invention, and are not intended to limit the implementation conditions of the present invention. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportional relationship, or adjustments to the size should still fall within the scope of the technical content disclosed in the present invention, provided that they do not affect the effects and objectives that the present invention can produce.

[0023] It should be noted that, in this invention, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only the expressly listed elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.

[0024] As attached Figure 1 As shown, this invention proposes an ultrasonic-assisted surface hardening heat treatment method for metal workpieces, including carbon steel, alloy steel, tool steel, stainless steel, or aluminum alloy. For iron-based materials, the surface hardening temperature is in the range of 50°C above the Ac1 point (eutectoid transformation start temperature) to 50°C above the Ac3 point (austenitization completion temperature). For aluminum alloy materials, the surface hardening temperature is in the range of 20°C to 80°C below the solidus temperature.

[0025] The method includes:

[0026] S1, Pre-treatment of the surface of the metal workpiece, the pre-treatment including: cleaning treatment and activation treatment performed in sequence;

[0027] S2, the pretreated metal workpiece is placed in a heat treatment environment and heated to the surface hardening temperature under a protective atmosphere or vacuum.

[0028] S3, a multi-stage heat preservation treatment is performed at the surface hardening temperature, including a first stage heat preservation and a second stage heat preservation performed sequentially.

[0029] S4, multi-stage controlled cooling is performed on the metal workpiece that has completed the heat preservation treatment to cool it to room temperature;

[0030] S5. Tempering and stabilizing treatment is performed on metal workpieces cooled to room temperature.

[0031] Specifically, during the later stage of heating to the surface hardening temperature in step S2, during the first and second stage heat preservation periods in step S3, and during the early stage of each cooling phase in step S4, directionally adjustable ultrasonic vibrations are applied to the metal workpiece; the application of the ultrasonic vibrations covers the austenitization, phase transformation, and early cooling processes of the metal workpiece.

[0032] Specifically, step S1 includes two stages: cleaning and activation. The cleaning process sequentially employs a combination of ultrasonic degreasing with organic solvents, sandblasting, and weak acid pickling; the activation process is carried out in a preheating furnace. This pretreatment stage utilizes the micro-zone cavitation and acoustic flow effects of ultrasound to effectively remove surface-adsorbed gases and impurities, and activate surface atoms, creating more favorable conditions for the subsequent diffusion and penetration of elements such as carbon or nitrogen.

[0033] Step S1 includes:

[0034] Step S11: Use acetone to perform ultrasonic cleaning on the surface of the metal workpiece for 10 minutes to remove oil stains.

[0035] Step S12: Use 120-mesh alumina abrasive to sandblast the surface of the metal workpiece to increase surface roughness and activation area.

[0036] Step S13: Immerse the metal workpiece in an 8% hydrochloric acid solution for 2 minutes to remove the extremely thin oxide film and further activate the surface.

[0037] Step S14: Heat the surface of the metal workpiece to a preheating temperature of 200°C to 400°C and hold for 15 to 45 minutes, and apply low-intensity ultrasonic waves with a power density of 0.05 W / cm² to 0.5 W / cm² during the activation treatment; to obtain the pretreated metal workpiece.

[0038] Specifically, the heating process in step S2 adopts a programmed temperature rise curve; wherein, the protective atmosphere is nitrogen, argon, hydrogen or a mixture thereof, with a purity of not less than 99.9% and a flow rate of 0.5 L / min to 8 L / min; or the vacuum degree of the vacuum condition is not higher than 10⁻² Pa.

[0039] Step S2 includes:

[0040] Step S21: Place the pretreated metal workpiece in a heat treatment environment and heat it from room temperature to an intermediate temperature of 350°C to 550°C at a first heating rate of 3°C / min to 15°C / min under a protective atmosphere or vacuum, and hold it at the intermediate temperature for 5 to 30 minutes.

[0041] Step S22, then heat to the surface hardening temperature at a second heating rate of 5°C / min to 25°C / min, and the second heating rate is greater than the first heating rate.

[0042] The surface hardening temperature refers to the surface hardening temperature of the metal workpiece; for example, for GCr15 bearing steel, this temperature is set to 850℃. This staged, rate-controlled heating strategy helps reduce thermal stress, avoids deformation of thin-walled or complex-shaped workpieces, and ensures a smooth transformation of the microstructure to austenite.

[0043] Specifically, step S3, the multi-stage heat preservation treatment, is divided into two consecutive stages and executed sequentially, including the first stage of heat preservation and the second stage of heat preservation.

[0044] Step S3 includes:

[0045] Step S31: Determine the total heat preservation time based on the effective thickness of the metal workpiece, ranging from 20 minutes to 180 minutes;

[0046] Step S32, the first stage of heat preservation is carried out at a temperature 10°C to 30°C below the surface hardening temperature, and the heat preservation time is 1 / 4 to 1 / 2 of the total heat preservation time;

[0047] Step S33: The second stage of heat preservation is carried out at the surface hardening temperature to complete the remaining heat preservation time;

[0048] This segmented heat preservation mechanism allows carbides or other second-phase particles to gradually dissolve in the first stage of heat preservation, achieving initial homogenization of the austenite composition. This provides a transition for the second stage of heat preservation to complete full austenitization at a higher temperature and control grain growth, which helps to obtain an initial austenite structure with more uniform composition and finer grains.

[0049] In a preferred embodiment, during the first and second stage of heat preservation, the applied ultrasonic vibration employs a differentiated combination of parameters.

[0050] Specifically, step S3 further includes: ultrasonic vibration applied during the first stage of heat preservation, wherein the ultrasonic frequency is 20 kHz to 40 kHz and the power density is 0.5 W / cm² to 2 W / cm²; ultrasonic vibration applied during the second stage of heat preservation, wherein the ultrasonic frequency is 25 kHz to 100 kHz and the power density is 1 W / cm² to 5 W / cm²; and at least one parameter of the parameter combination of the ultrasonic vibration applied during the first stage of heat preservation is different from that of the parameter combination of the ultrasonic vibration applied during the second stage of heat preservation.

[0051] The principle behind this strategy of adjusting parameters according to the heat preservation stage lies in the different ultrasonic energy requirements at different stages: the first stage heat preservation at lower temperatures requires a gentle energy input to avoid excessive fragmentation of undissolved carbides; while the second stage heat preservation at higher temperatures requires stronger acoustic flow and cavitation effects to promote atomic diffusion, break down solute gas masses, homogenize austenite composition, and increase nucleation sites, thereby providing an optimized microstructure for subsequent cooling transformation.

[0052] In a specific embodiment, step S4, which implements multi-stage controlled cooling, can be implemented using three-stage cooling.

[0053] Step S4 includes:

[0054] Step S41, the first stage of cooling starts from the surface hardening temperature and uses a controlled gas (such as forced convection nitrogen) to cool to a first intermediate temperature of 550°C to 650°C at a cooling rate of 10°C / min to 50°C / min; the relatively slow cooling rate in this stage helps to reduce the temperature difference between the inside and outside of the workpiece.

[0055] Step S42, the second stage of cooling is performed from the first intermediate temperature using rapid oil cooling at a cooling rate of 30°C / min to 150°C / min to a second intermediate temperature of 150°C to 250°C; this stage aims to allow the supercooled austenite to quickly pass through the pearlite transformation zone and enter the martensite transformation zone.

[0056] Step S43, the third stage of cooling starts from the second intermediate temperature and is carried out by air cooling at a cooling rate of 5°C / min to 20°C / min to room temperature; the main purpose of this stage is to release some of the tissue stress.

[0057] The principle of multi-stage cooling is to guide and control the phase transformation path and products by controlling the cooling rate in different temperature ranges. While ensuring the achievement of the target hardness (martensite), it effectively manages thermal stress and structural stress, and inhibits deformation and cracking.

[0058] Furthermore, in the first and second stages of controlled cooling, ultrasonic vibrations are continuously applied, but their power density gradually decreases as the temperature drops.

[0059] Specifically, step S4 further includes:

[0060] During both the first and second cooling stages, ultrasonic vibrations are continuously applied at a frequency of 30 kHz to 80 kHz, with the power density gradually decreasing from an initial 1.5 W / cm² to 0.2 W / cm², and ceasing when the temperature reaches the martensitic transformation initiation point. For example, the power density can be linearly reduced from 1.2 W / cm² at the start of the second cooling stage to 0.3 W / cm² at the end of the second cooling stage (180°C) before stopping.

[0061] The continuous action of ultrasound during the cooling process works by utilizing the instantaneous high pressure generated by cavitation as an additional driving force for martensite nucleation. Simultaneously, the acoustic flow effect helps break up the nascent acicular martensite and may influence the redistribution of carbon atoms. This intervention ultimately leads to a finer martensite structure, smaller lath sizes, and more dispersed carbide distribution, thereby improving toughness and residual stress state while maintaining high hardness.

[0062] Specifically, step S5 is the final tempering stabilization process.

[0063] Step S5 includes:

[0064] Step S51: Heat the metal workpiece cooled to room temperature to a tempering temperature of 180°C to 500°C and hold it for 60 minutes to 240 minutes; during the tempering process, apply ultrasonic vibration in pulse mode, with a pulse width of 1 second to 5 seconds, a pulse interval of 3 seconds to 10 seconds, and a power density of 0.1 W / cm² to 1 W / cm².

[0065] Step S52: After tempering, the metal workpiece is cooled to room temperature at a cooling rate not exceeding 30°C / min.

[0066] The pulsed energy input works by utilizing the periodic stress field of ultrasound to promote dislocation slip and rearrangement, accelerate the maturation and spheroidization of precipitates, and potentially facilitate the stabilization transformation of retained austenite. This dynamically assisted tempering process, compared to traditional static tempering, more effectively eliminates microscopic internal stresses, promotes toughness recovery, and maintains necessary hardness and wear resistance by controlling precipitation behavior.

[0067] Furthermore, the direction of the ultrasonic vibration is adjustable. This can be achieved by mounting the ultrasonic transmitter onto a multi-degree-of-freedom robotic arm.

[0068] The aforementioned adjustable ultrasonic vibration refers to the fact that, during the process, the angle between the ultrasonic propagation direction and the normal to the surface of the metal workpiece changes periodically within the range of 0 to 60 degrees, with a frequency of 0.1 Hz to 5 Hz; or, based on the geometric characteristics of the metal workpiece, it is fixed at the optimal angle determined through previous process experiments within a specific time period.

[0069] For areas with special structures such as grooves or internal holes, the optimal incident angle that allows the sound wave energy to be most effectively transmitted to the interior of the area can be determined in advance through simulation or experimentation, and this angle can be locked during processing. The core function of this directionally adjustable design is to overcome the obstruction, reflection, and attenuation caused by the complex geometry of the workpiece on the propagation of ultrasonic waves, ensuring that three-dimensional curved surfaces and even difficult-to-process areas such as deep cavities and narrow slits can receive sufficient ultrasonic energy, thereby achieving a uniform distribution of the hardened layer in three-dimensional space.

[0070] This method is applicable to various types of metallic materials, including but not limited to medium carbon steel, low alloy steel, tool steel, and certain series of aluminum alloys. For specific materials, key parameters such as surface hardening temperature, holding time, and cooling medium need to be selected and set according to the material's phase diagram, TTT / CCT curve, and performance requirements. For example, when treating 38CrMoAl nitriding steel, the surface hardening temperature (referring to the nitriding temperature) can be selected in the range of 500℃ to 530℃, and ammonia decomposition gas is used as the protective and nitriding atmosphere. This parameter adaptability to different materials ensures that this method can be widely applied to various metal components in engineering practice.

[0071] The following describes an application example of H13 hot-work die steel parts with complex cavities. This die requires high surface hardness, high wear resistance, and good resistance to thermal fatigue, and the hardened layer needs to remain uniform across the complex geometry. The process is carried out in a dedicated heat treatment system integrating programmed heating, controlled cooling, and a six-axis robot carrying an ultrasonic transmitter.

[0072] The mold first undergoes a strict pretreatment process, especially the thorough cleaning and activation of the inner wall of the cavity.

[0073] For the surface of the metal workpiece, use acetone for 10 minutes of ultrasonic cleaning to remove oil stains; use 120-mesh alumina abrasive to sandblast the surface of the metal workpiece to increase surface roughness and activation area; immerse the metal workpiece in an 8% hydrochloric acid solution for 2 minutes to remove the extremely thin oxide film and further activate the surface.

[0074] The activation treatment includes maintaining a preheating temperature of 200°C to 400°C for 15 to 45 minutes, and applying low-intensity ultrasound with a power density of 0.05 W / cm² to 0.5 W / cm² during the activation treatment.

[0075] In a vacuum environment (vacuum maintained at 5 x 10⁻³ Pa), the workpiece is heated from room temperature to 450°C at a rate of 5°C / min; this intermediate temperature is held for 20 minutes to allow the temperature in the thickness direction of the workpiece to become uniform and to partially eliminate processing stress; then the heating rate is increased to 12°C / min and the workpiece is heated to the surface hardening temperature (austenitizing temperature of 1020°C).

[0076] The first stage of heat preservation is carried out at a temperature 15°C below the final surface hardening temperature, and lasts for approximately 40% of the preset total heat preservation time (e.g., 90 minutes). The second stage of heat preservation then raises the temperature to the final surface hardening temperature and completes the remaining heat preservation time.

[0077] During the first stage of heat preservation, ultrasonic waves with a frequency of 20kHz and a power density of 0.8W / cm² can be used.

[0078] After entering the second stage of heat preservation, the ultrasonic parameters are adjusted to a frequency of 35kHz and a power density of 2.0W / cm².

[0079] During the process, directional ultrasonic vibrations are applied, and the robotic arm can be programmed to make the angle between the propagation direction of the ultrasonic beam and the normal of the surface of the workpiece being processed be between 0 and 50 degrees, and to oscillate periodically at a frequency of 0.5 Hz.

[0080] The first stage of cooling begins at the surface hardening temperature and is performed using methods such as forced convection nitrogen, cooling at a rate of approximately 20°C / min to approximately 550°C. This relatively slow cooling rate helps to reduce the temperature difference between the inside and outside of the workpiece.

[0081] The second stage of cooling switches to rapid oil cooling, increasing the cooling rate to approximately 60°C / minute until the workpiece temperature drops to approximately 150°C. This stage aims to allow the supercooled austenite to quickly pass through the pearlite transformation zone and enter the martensite transformation zone.

[0082] The third stage of cooling uses air cooling, which reduces the temperature to room temperature at a slow cooling rate of less than 10°C / minute. The main purpose of this stage is to release some of the tissue stress.

[0083] During the process, ultrasound is continuously applied but the power decreases linearly.

[0084] The workpiece was heated to the selected tempering temperature in an air-circulating furnace, twice at 560℃, and held for 150 minutes each time. Simultaneously, ultrasonic pulse-assisted tempering was performed for 2 hours each time, with the following parameters: pulse width 2 seconds, pulse interval 8 seconds, and power density 0.4 W / cm².

[0085] Tests showed that the mold had a hardened layer with a thickness of 0.8 mm to 1.0 mm, a hardness of 52-54 HRC, and a uniform and dense structure on all working surfaces, including deep cavities and corners, which fully met its stringent service requirements.

[0086] This demonstrates how combining programmed thermal cycling, multi-stage ultrasonic energy injection, and adaptive application processes for complex geometries can systematically achieve the precise fabrication of high-performance, uniform surface-hardened layers for complex components.

[0087] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. An ultrasonic-assisted heat treatment method for hardening metal surfaces, used to process metal workpieces, characterized in that, The method includes: S1, Pre-treatment of the surface of the metal workpiece, the pre-treatment including: cleaning treatment and activation treatment performed in sequence; S2, the pretreated metal workpiece is placed in a heat treatment environment and heated to the surface hardening temperature under a protective atmosphere or vacuum. S3, a multi-stage heat preservation treatment is performed at the surface hardening temperature, including a first stage heat preservation and a second stage heat preservation performed sequentially. S4, multi-stage controlled cooling is performed on the metal workpiece that has completed the heat preservation treatment to cool it to room temperature; S5, tempering and stabilization treatment for metal workpieces cooled to room temperature; Specifically, during the later stage of heating to the surface hardening temperature in step S2, during the first and second stage heat preservation periods in step S3, and during the early stage of each cooling phase in step S4, directionally adjustable ultrasonic vibrations are applied to the metal workpiece; the application of the ultrasonic vibrations covers the austenitization, phase transformation, and early cooling processes of the metal workpiece.

2. The ultrasonic-assisted heat treatment method for hardening metal surfaces according to claim 1, characterized in that, Step S1 includes: Step S11: Use acetone to perform ultrasonic cleaning on the surface of the metal workpiece for 10 minutes to remove oil stains. Step S12: Use 120-mesh alumina abrasive to sandblast the surface of the metal workpiece to increase surface roughness and activation area. Step S13: Immerse the metal workpiece in an 8% hydrochloric acid solution for 2 minutes to remove the extremely thin oxide film and further activate the surface. Step S14: Heat the surface of the metal workpiece to a preheating temperature of 200°C to 400°C and maintain it for 15 minutes to 45 minutes. During the activation treatment, apply low-intensity ultrasonic waves with a power density of 0.05 W / cm² to 0.5 W / cm² to obtain the pretreated metal workpiece.

3. The ultrasonic-assisted heat treatment method for hardening metal surfaces according to claim 2, characterized in that, In step S2, the protective atmosphere is nitrogen, argon, hydrogen, or a mixture thereof, with a purity of not less than 99.9% and a flow rate of 0.5 L / min to 8 L / min; or the vacuum condition has a vacuum degree of not more than 10⁻² Pa.

4. The ultrasonic-assisted heat treatment method for hardening metal surfaces according to claim 3, characterized in that, Step S2 includes: Step S21: Place the pretreated metal workpiece in a heat treatment environment and heat it from room temperature to an intermediate temperature of 350°C to 550°C at a first heating rate of 3°C / min to 15°C / min under a protective atmosphere or vacuum, and hold it at the intermediate temperature for 5 to 30 minutes. Step S22, then heat to the surface hardening temperature at a second heating rate of 5°C / min to 25°C / min, and the second heating rate is greater than the first heating rate; Among them, surface hardening temperature refers to the surface hardening temperature of a metal workpiece.

5. The ultrasonic-assisted heat treatment method for hardening metal surfaces according to claim 4, characterized in that, Step S3 includes: Step S31: Determine the total heat preservation time based on the effective thickness of the metal workpiece, ranging from 20 minutes to 180 minutes; Step S32, the first stage of heat preservation is carried out at a temperature 10°C to 30°C below the surface hardening temperature, and the heat preservation time is 1 / 4 to 1 / 2 of the total heat preservation time; Step S33, the second stage of heat preservation is carried out at the surface hardening temperature to complete the remaining heat preservation time.

6. The ultrasonic-assisted heat treatment method for hardening metal surfaces according to claim 5, characterized in that, Step S3 further includes: The ultrasonic vibration applied during the first stage of heat preservation has a frequency of 20 kHz to 40 kHz and a power density of 0.5 W / cm² to 2 W / cm². The ultrasonic vibration applied during the second stage of heat preservation has a frequency of 25 kHz to 100 kHz and a power density of 1 W / cm² to 5 W / cm². Furthermore, the combination of ultrasonic vibration parameters applied during the first stage of heat preservation differs from the combination of ultrasonic vibration parameters applied during the second stage of heat preservation by at least one parameter.

7. The ultrasonic-assisted heat treatment method for hardening metal surfaces according to claim 6, characterized in that, Step S4 includes: Step S41, the first stage of cooling starts from the surface hardening temperature and uses a controlled gas (such as forced convection nitrogen) to cool to a first intermediate temperature of 550°C to 650°C at a cooling rate of 10°C / min to 50°C / min; the relatively slow cooling rate in this stage helps to reduce the temperature difference between the inside and outside of the workpiece. Step S42, the second stage of cooling is performed from the first intermediate temperature using rapid oil cooling at a cooling rate of 30°C / min to 150°C / min to a second intermediate temperature of 150°C to 250°C; this stage aims to allow the supercooled austenite to quickly pass through the pearlite transformation zone and enter the martensite transformation zone. Step S43, the third stage of cooling starts from the second intermediate temperature and is carried out by air cooling at a cooling rate of 5°C / min to 20°C / min to room temperature; the main purpose of this stage is to release some of the tissue stress.

8. The ultrasonic-assisted heat treatment method for hardening metal surfaces according to claim 7, characterized in that, Step S4 further includes: During the first and second stage cooling, ultrasonic vibrations are continuously applied at a frequency of 30 kHz to 80 kHz, with the power density gradually decreasing from an initial 1.5 W / cm² to 0.2 W / cm², and the application is stopped when the temperature reaches the martensitic transformation initiation point.

9. The ultrasonic-assisted heat treatment method for hardening metal surfaces according to claim 7, characterized in that, Step S5 includes: Step S51: Heat the metal workpiece cooled to room temperature to a tempering temperature of 180°C to 500°C and hold it for 60 minutes to 240 minutes; during the tempering process, apply ultrasonic vibration in pulse mode, with a pulse width of 1 second to 5 seconds, a pulse interval of 3 seconds to 10 seconds, and a power density of 0.1 W / cm² to 1 W / cm². Step S52: After tempering, the metal workpiece is cooled to room temperature at a cooling rate not exceeding 30°C / min.

10. The ultrasonic-assisted heat treatment method for hardening metal surfaces according to claim 9, characterized in that, The applied direction-adjustable ultrasonic vibration refers to the process in which the angle between the ultrasonic propagation direction and the normal to the surface of the metal workpiece changes periodically within the range of 0 to 60 degrees, with a frequency of 0.1 Hz to 5 Hz.

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