A field deep cryogenic treatment method for improving dimensional stability of thin-wall bearing steel of harmonic reducer

Abstract

The present application relates to the technical field of bearing steel heat treatment processing, and discloses a field deep cryogenic treatment method for improving the dimensional stability of thin-wall bearing steel of harmonic reducer.The present application utilizes the transient field energy effect of directional pulse magnetic field to reduce the potential barrier of residual austenite to martensite phase transition under the deep cryogenic low-temperature harsh environment, which is different from the traditional random magnetic field modification, can directionally refine the residual austenite grains, accurately control the phase transition orientation, and eliminate the problems of microstructure anisotropy and local volume uneven deformation caused by random conversion of residual austenite; at the same time, the directional pulse magnetic field can orderly correct the internal distorted lattice of bearing steel, guide the global uniform release of locally concentrated micro residual stress, and completely eliminate the stress accumulation hidden danger. The temperature gradient stress of thin-wall bearing workpiece is greatly reduced, the problems of micro deformation and micro crack of thin-wall structure are avoided, and the microstructure is further stabilized and regularized through long-term natural aging, so that the problems of bearing long-term dimensional drift and precision attenuation are solved from the root.

Description

Technical Field

[0001] This invention relates to the field of heat treatment processing technology for bearing steel, specifically to a field-induced deep cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers. Background Technology

[0002] Harmonic reducers are core transmission components in robots, precision automation equipment, and aerospace precision transmission mechanisms, boasting significant advantages such as small size, large transmission ratio, high precision, and small backlash. Thin-walled bearings, as key load-bearing and rotational components of harmonic reducers, directly determine the transmission accuracy, service life, and operational stability of the reducer through their dimensional accuracy and geometric tolerance stability.

[0003] Currently, thin-walled bearings for harmonic reducers are mostly made of high-carbon chromium bearing steel. After quenching and low-temperature tempering, they possess high hardness and wear resistance. However, the bearing steel treated by traditional heat treatment processes contains a large amount of retained austenite and uneven microscopic residual stress. Retained austenite is an unstable metallographic structure. Under the long-term alternating load, periodic vibration, and temperature fluctuation conditions of the harmonic reducer, it will continuously undergo martensitic phase transformation, causing local volume expansion and dimensional drift of the bearing. At the same time, the gradual release of microscopic stress will lead to slight deformation of the bearing, ultimately causing failure problems such as reduced transmission accuracy, jamming, and excessive return clearance of the reducer.

[0004] Cryogenic treatment is a common process for improving the microstructure stability of bearing steel. Traditional cryogenic treatment often adopts a process mode of direct low-temperature rapid freezing, constant temperature holding, and natural warming. However, this process has obvious defects: First, sudden low-temperature changes can easily lead to thermal stress concentration in thin-walled bearings, causing micro-deformation and micro-crack defects in the thin-walled structure. Second, the conversion efficiency of residual austenite is low and incomplete under the action of low temperature alone, resulting in limited improvement in microstructure stability. Third, the micro-stress release is uneven, and the bearing still has dimensional decay problems after long-term use, which cannot meet the micron-level long-term dimensional stability requirements of high-precision harmonic reducers.

[0005] In existing technologies, conventional cryogenic processes do not optimize temperature change parameters for the thin-walled bearings' structural characteristics of thin wall thickness, high precision, and easy deformation, and lack auxiliary field-induced strengthening methods. This makes it impossible to simultaneously achieve residual structure improvement, uniform stress release, and precise deformation control, resulting in poor dimensional stability and short service life of thin-walled bearings, which restricts the precision upgrade and domestic substitution of high-end harmonic reducers. Summary of the Invention

[0006] This invention provides a field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers. It overcomes the shortcomings of existing traditional cryogenic treatment processes, such as incomplete transformation of residual austenite, uneven stress release, easy deformation of thin-walled workpieces, and poor long-term dimensional stability. The method utilizes a magnetic field-induced coupled gradient temperature change composite process to optimize the metallographic structure of the bearing steel, uniformly release residual stress, and suppress deformation defects, thereby significantly improving the dimensional accuracy and long-term stability of thin-walled bearing steel.

[0007] This invention provides the following technical solution: a field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers, comprising the following steps: S1. Pretreatment: The finely machined thin-walled bearing steel workpiece of the harmonic reducer is cleaned to remove oil and oxide scale, and then dried to remove impurities and moisture from the workpiece surface. Subsequently, the workpiece is placed in a closed heat treatment furnace and preheated to 80-100℃ and held for 20-30 minutes to eliminate the processing stress on the workpiece surface. S2. Gradient heating tempering: The preheated workpiece is heated to 180-220℃ at a gradient heating rate of 3-5℃ / min and tempered at a constant temperature for 40-60min to initially eliminate the residual quenching stress inside the bearing steel and stabilize the matrix structure. S3. Cryogenic and Pulsed Magnetic Field Coupled Isothermal Treatment: The tempered workpiece is quickly transferred to a closed-loop cryogenic treatment device equipped with a directional pulsed magnetic field system. It is first cooled to -120~-140℃ at a rate of 2-4℃ / min. After cooling to the preset low temperature, a pulsed magnetic field in a specific direction is activated. The parameters of the pulsed magnetic field are set as follows: magnetic field strength 80-120mT, pulse frequency 50-80Hz, single pulse duration 20-50μs. The magnetic field is arranged perpendicularly to the axial direction of the bearing steel workpiece. The isothermal coupling treatment lasts for 120-180min. Through the synergistic coupling effect of cryogenic low temperature and directional pulsed magnetic field, the residual austenite grains are refined in a directional manner, and the transformation orientation of residual austenite to martensite is regulated, so as to achieve an ordered phase transformation. S4. Gradient temperature recovery aging: After the cryogenic magnetic field coupling treatment is completed, the pulse magnetic field is turned off, and the temperature is raised to room temperature at a low gradient recovery rate of 1.5-2.5℃ / min. Then, the workpiece is placed in a constant temperature and dry environment for natural aging for 24-48 hours to completely release the microscopic residual stress and stabilize the metallographic structure. S5. Finishing and post-processing: After aging, the thin-walled bearing workpiece is lightly ground and precision corrected to remove minor deformation defects and complete the finished product processing.

[0008] In step S1, the cleaning process is performed using ultrasonic cleaning, with an anhydrous ethanol and deionized water mixture as the cleaning medium. The cleaning time is 10-15 minutes, the drying temperature is 100-120°C, and the drying time is 15-20 minutes.

[0009] In step S2, the gradient heating process is protected by high-purity nitrogen gas with a purity of ≥99.99% to prevent oxidation and decarburization of the bearing steel surface.

[0010] In step S3, the cryogenic treatment equipment adopts a liquid nitrogen closed-loop refrigeration method, and the temperature fluctuation is controlled within ±2℃ to ensure the uniformity of the temperature field.

[0011] In step S3, the pulsed magnetic field is a directional and uniform pulsed magnetic field. The magnetic field is arranged perpendicularly to the workpiece axis, without magnetic field deviation or random magnetic field interference. It can accurately control the residual austenite refinement effect and phase transformation orientation, ensuring that the overall microstructure modification of the workpiece is uniform and consistent.

[0012] In step S4, rapid heating and direct airflow from the outside are strictly prohibited during the gradient reheating process to avoid temperature difference deformation of the thin-walled structure.

[0013] The selected material for the thin-walled bearing steel of the harmonic reducer is GCr15 or GCr15SiMn high-carbon chromium bearing steel, and the workpiece wall thickness is 0.8-3mm.

[0014] The selected thin-walled bearing steel workpieces have a residual austenite content of ≤3%, a dimensional deformation of ≤0.5μm, and a dimensional drift of ≤0.2μm / 30d after long-term storage at room temperature.

[0015] The present invention has the following beneficial effects: 1. This invention utilizes a synergistic process of cryogenic cooling and pulsed magnetic field coupling. Under the harsh environment of cryogenic low temperature, it leverages the transient field energy effect of a directional pulsed magnetic field to reduce the potential barrier for the transformation of retained austenite to martensite. Unlike traditional random magnetic field modification, this method can directionally refine the retained austenite grains and precisely control the phase transformation orientation, eliminating the anisotropy and localized volumetric deformation problems caused by the disordered transformation of retained austenite. Simultaneously, the directional pulsed magnetic field can orderly correct the distorted lattice inside the bearing steel, guiding the uniform release of locally concentrated microscopic residual stress across the entire domain, completely eliminating the hidden danger of stress accumulation. Combined with a gradient heating and cooling process, replacing the traditional rapid cooling and heating mode, it greatly reduces the temperature gradient stress of thin-walled bearing workpieces, avoiding the problems of micro-deformation and micro-cracks in thin-walled structures. Furthermore, long-term natural aging further stabilizes and regularizes the metallographic structure, fundamentally solving the problems of long-term dimensional drift and precision degradation in bearings.

[0016] 2. This invention has excellent microstructure modification effect and strong controllability. It adopts a deep cryogenic and directional pulsed magnetic field coupling process, which can directionally refine the residual austenite and precisely control the phase transformation orientation. The residual austenite is completely transformed and the phase transformation microstructure is regular and orderly. After treatment, the residual austenite content of bearing steel is ≤3%, which is far lower than the 8%-12% of the traditional deep cryogenic process. It completely solves the defects of disordered microstructure and anisotropy of the traditional process. The metallographic microstructure is uniform and dense, and the carbide grains are significantly refined, which improves the stability and consistency of the microstructure from the source. 3. The present invention significantly improves dimensional stability. The gradient temperature change process avoids the defects of cold and hot deformation of thin-walled workpieces. Combined with the uniform pressure release of the magnetic field, the dimensional deformation of the workpiece in a single process is ≤0.5μm. The dimensional drift is extremely low under long-term placement and alternating load conditions, which completely solves the problem of long-term accuracy decay of the bearings of harmonic reducers.

[0017] 4. The process of this invention has strong adaptability and high yield. It optimizes parameters for special structures with thin walls of 0.8-3mm, and features low temperature gradient temperature change, nitrogen protection, no sudden stress, no microcracks, no excessive deformation and other defects. The yield of workpieces can reach more than 99%, which is suitable for mass industrial production.

[0018] 5. The overall performance of this invention is comprehensively improved. The uniformity of the hardness of the treated bearing steel is improved, and the fatigue strength and wear resistance are significantly optimized. The overall transmission accuracy and service life of the harmonic reducer are increased by more than 30%, which can meet the high-precision transmission requirements of high-end industrial robots and precision aerospace equipment. Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention have been clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] A field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers includes the following steps: S1. Pretreatment: The finely machined thin-walled bearing steel workpiece of the harmonic reducer is cleaned to remove oil and oxide scale, and then dried to remove impurities and moisture from the workpiece surface. Subsequently, the workpiece is placed in a closed heat treatment furnace and preheated to 80-100℃ and held for 20-30 minutes to eliminate the processing stress on the workpiece surface. The cleaning process uses ultrasonic cleaning, with an anhydrous ethanol and deionized water mixture as the cleaning medium. The cleaning time is 10-15 minutes, and the drying temperature is 100-120℃ with a drying time of 15-20 minutes. The thin-walled bearing steel for harmonic reducers is made of GCr15 or GCr15SiMn high-carbon chromium bearing steel, with a workpiece wall thickness of 0.8-3mm; S2. Gradient heating tempering: The preheated workpiece is heated to 180-220℃ at a gradient heating rate of 3-5℃ / min and tempered at a constant temperature for 40-60min to initially eliminate the residual quenching stress inside the bearing steel and stabilize the matrix structure. The entire gradient heating process is protected by high-purity nitrogen gas with a purity of ≥99.99% to prevent oxidation and decarburization of the bearing steel surface. S3. Cryogenic and Pulsed Magnetic Field Coupled Isothermal Treatment: The tempered workpiece is quickly transferred to a closed-loop cryogenic treatment device equipped with a directional pulsed magnetic field system. It is first cooled to -120~-140℃ at a rate of 2-4℃ / min. After cooling to the preset low temperature, a pulsed magnetic field in a specific direction is activated. The pulsed magnetic field parameters are set as follows: magnetic field strength 80-120mT, pulse frequency 50-80Hz, single pulse duration 20-50μs. The magnetic field is arranged perpendicularly to the axial direction of the bearing steel workpiece. The isothermal coupling treatment lasts for 120-180min. Through the synergistic coupling effect of cryogenic low temperature and directional pulsed magnetic field, the residual austenite grains are refined in a directional manner, and the transformation orientation of residual austenite to martensite is regulated, thus achieving an ordered phase transformation. The pulsed magnetic field is a directional and uniform pulsed magnetic field, with the magnetic field arranged perpendicularly to the workpiece axis. There is no magnetic field offset or random magnetic field interference, which can precisely control the residual austenite refinement effect and phase transformation orientation, ensuring uniform and consistent overall microstructure modification of the workpiece. S4. Gradient temperature recovery aging: After the cryogenic magnetic field coupling treatment is completed, the pulse magnetic field is turned off, and the temperature is raised to room temperature at a low gradient recovery rate of 1.5-2.5℃ / min. Then, the workpiece is placed in a constant temperature and dry environment for natural aging for 24-48 hours to completely release the microscopic residual stress and stabilize the metallographic structure. During the gradient reheating process, rapid heating and direct airflow from the outside are strictly prohibited to avoid temperature-induced deformation of thin-walled structures. S5. Finishing after finishing: After aging, the thin-walled bearing workpiece is lightly ground and precision corrected to remove minor deformation defects, thus completing the finished product processing; The treated thin-walled bearing steel workpiece has a residual austenite content of ≤3%, a dimensional deformation of ≤0.5μm, and a dimensional drift of ≤0.2μm / 30d after long-term storage at room temperature. Example

[0021] This embodiment addresses the processing method for thin-walled bearing steel workpieces of harmonic reducers with material GCr15 and a wall thickness of 1.5mm, as follows: S1. Pretreatment: The workpiece is ultrasonically cleaned for 12 minutes with a 1:1 mixture of anhydrous ethanol and deionized water to remove surface oil and oxide scale. It is then dried at 110℃ for 18 minutes to remove moisture. The dried workpiece is then placed in a heat treatment furnace and preheated at 90℃ for 25 minutes to eliminate surface processing stress. S2. Gradient heating tempering: Heat to 200℃ at a rate of 4℃ / min, introduce 99.99% high-purity nitrogen for protection, and temper at a constant temperature for 50min to initially release internal quenching stress. S3. Cryogenic and pulsed magnetic field coupled isothermal treatment: The workpiece is transferred to a closed-loop cryogenic device equipped with a directional pulsed magnetic field and cooled to -130℃ at a rate of 3℃ / min. The axial vertical directional pulsed magnetic field is turned on with a magnetic field strength of 100mT, a pulse frequency of 65Hz, and a single pulse duration of 35μs. The isothermal coupling treatment is carried out for 150min, and the temperature fluctuation of the device is controlled within ±2℃. The residual austenite is directionally refined and the phase transformation orientation is regulated. S4. Gradient temperature recovery aging: Turn off the magnetic field and warm up to room temperature at a rate of 2℃ / min. Place the workpiece in a constant temperature and dry environment of 25℃ and let it age naturally for 36 hours. S5. Finishing process: Lightly grind the edges and corners of the workpiece, correct minor shape and position deviations, and complete the finished product processing.

[0022] Testing revealed that the thin-walled bearing steel treated in this embodiment had a residual austenite content of 2.1%, a workpiece dimensional deformation of 0.3 μm, and a dimensional drift of 0.12 μm after 30 days of room temperature storage, demonstrating excellent precision and stability. Example

[0023] This embodiment addresses the processing method for thin-walled bearing steel workpieces of harmonic reducers with material GCr15SiMn and a wall thickness of 2mm, as follows: S1. Pretreatment: Ultrasonic cleaning for 15 minutes, drying at 120℃ for 15 minutes, preheating and holding at 100℃ for 20 minutes; S2, Gradient heating tempering: Heat to 210℃ at a rate of 5℃ / min, under high-purity nitrogen protection, and temper at a constant temperature for 45min. S3. Cryogenic and pulsed magnetic field coupled isothermal treatment: Cool down to -135℃ at a rate of 4℃ / min, turn on the axial vertical directional pulsed magnetic field with a magnetic field strength of 110mT, a pulse frequency of 70Hz, a single pulse duration of 40μs, and maintain constant temperature coupling for 160min. S4, Gradient temperature recovery aging: Reheat to room temperature at a rate of 2.5℃ / min, and age naturally for 40 hours; S5. Finishing and post-processing: precision correction, grinding of minor defects, and completion of the process.

[0024] Testing revealed that the workpiece in this embodiment has a residual austenite content of 2.5%, a dimensional deformation of 0.4 μm, and a long-term dimensional drift of ≤0.15 μm, meeting the requirements for use in high-precision harmonic reducers.

[0025] Traditional cryogenic processing was used to process GCr15 thin-walled bearing workpieces of the same specifications: after quenching and tempering, the temperature was directly and rapidly reduced to -130℃ and held at that temperature for 150 minutes, followed by natural warming to room temperature. This process involved no pulsed magnetic field coupling, no directional microstructure control, no gradient temperature change, and no prolonged aging. Testing revealed that the workpiece contained 9.2% retained austenite, with disordered orientation and significant anisotropy in the microstructure. The dimensional deformation was 1.8 μm, and the 30-day dimensional drift was 0.8 μm. This process exhibited severe microstructure instability, large dimensional deviations, and poor consistency in accuracy, making it unsuitable for high-precision transmission applications.

[0026] As can be seen from the comparison of the embodiments and comparative examples, the field-induced gradient deep cryogenic process of the present invention can significantly optimize the metallographic structure of bearing steel, completely solve many defects of traditional processes, and greatly improve the dimensional stability and processing yield of thin-walled bearing steel.

[0027] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers, characterized in that: The process includes the following steps: S1. Pretreatment: The finely machined thin-walled bearing steel workpiece of the harmonic reducer is cleaned to remove oil and oxide scale, and then dried to remove impurities and moisture from the workpiece surface. Subsequently, the workpiece is placed in a closed heat treatment furnace and preheated to 80-100℃ and held for 20-30 minutes to eliminate the processing stress on the workpiece surface. S2. Gradient heating tempering: The preheated workpiece is heated to 180-220℃ at a gradient heating rate of 3-5℃ / min and tempered at a constant temperature for 40-60min to initially eliminate the residual quenching stress inside the bearing steel and stabilize the matrix structure. S3. Cryogenic and Pulsed Magnetic Field Coupled Isothermal Treatment: The tempered workpiece is quickly transferred to a closed-loop cryogenic treatment device equipped with a directional pulsed magnetic field system. It is first cooled to -120~-140℃ at a rate of 2-4℃ / min. After cooling to the preset low temperature, a pulsed magnetic field in a specific direction is activated. The parameters of the pulsed magnetic field are set as follows: magnetic field strength 80-120mT, pulse frequency 50-80Hz, single pulse duration 20-50μs. The magnetic field is arranged perpendicularly to the axial direction of the bearing steel workpiece. The isothermal coupling treatment lasts for 120-180min. Through the synergistic coupling effect of cryogenic low temperature and directional pulsed magnetic field, the residual austenite grains are refined in a directional manner, and the transformation orientation of residual austenite to martensite is regulated, so as to achieve an ordered phase transformation. S4. Gradient temperature recovery aging: After the cryogenic magnetic field coupling treatment is completed, the pulse magnetic field is turned off, and the temperature is raised to room temperature at a low gradient recovery rate of 1.5-2.5℃ / min. Then, the workpiece is placed in a constant temperature and dry environment for natural aging for 24-48 hours to completely release the microscopic residual stress and stabilize the metallographic structure. S5. Finishing and post-processing: After aging, the thin-walled bearing workpiece is lightly ground and precision corrected to remove minor deformation defects and complete the finished product processing.

2. The field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers according to claim 1, characterized in that: In step S1, the cleaning process uses ultrasonic cleaning, the cleaning medium is a mixture of anhydrous ethanol and deionized water, the cleaning time is 10-15 min, the drying temperature is 100-120℃, and the drying time is 15-20 min.

3. The field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers according to claim 1, characterized in that: In step S2, high-purity nitrogen gas is introduced throughout the gradient heating process for protection, with a nitrogen purity of ≥99.99%, to prevent oxidation and decarburization of the bearing steel surface.

4. The field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers according to claim 1, characterized in that: In step S3, the cryogenic treatment equipment adopts a liquid nitrogen closed-loop refrigeration method, and the temperature fluctuation is controlled within ±2℃ to ensure the uniformity of the temperature field.

5. The field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers according to claim 1, characterized in that: In step S3, the pulsed magnetic field is a directional and uniform pulsed magnetic field. The magnetic field is arranged vertically along the workpiece axis, without magnetic field offset or random magnetic field interference. It can accurately control the refinement effect of residual austenite and the orientation of phase transformation, ensuring that the overall microstructure modification of the workpiece is uniform and consistent.

6. The field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers according to claim 1, characterized in that: In step S4, rapid heating and direct airflow from the outside are strictly prohibited during the gradient reheating process to avoid temperature difference deformation of the thin-walled structure.

7. The field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers according to claim 1, characterized in that: The thin-walled bearing steel of the harmonic reducer is made of GCr15 or GCr15SiMn high-carbon chromium bearing steel, and the workpiece wall thickness is 0.8-3mm.

8. The field-induced cryogenic treatment method for improving the dimensional stability of thin-walled bearing steel in harmonic reducers according to claim 1, characterized in that: The treated thin-walled bearing steel workpiece has a residual austenite content of ≤3%, a dimensional deformation of ≤0.5μm, and a dimensional drift of ≤0.2μm / 30d after long-term storage at room temperature.