A method for improving the fatigue life of a flexspline part of a harmonic reducer

Abstract

This invention provides a method for improving the fatigue life of flexure components in harmonic reducers, belonging to the field of metal microstructure control technology. This invention achieves grain refinement and homogenization, avoids carbides, greatly increases the stability of the retained austenite structure, prevents its decomposition under mechanical driving force during large strain processes, avoids microstructural changes that affect performance stability, eliminates residual stress, and improves the fatigue life of the flexure, enabling it to meet long-life service requirements.

Description

Technical Field

[0001] This invention relates to the field of metal microstructure control technology, and in particular to a method for improving the fatigue life of flexural components in a harmonic reducer. Background Technology

[0002] Harmonic reducers are a new type of mechanical transmission that relies on the controllable elastic deformation generated by flexible gears to transmit motion and force. They consist of four main components: a cam (wave generator), a flexible gear, an input shaft, and a rigid gear. Figure 1 When the wave generator rotates, it forces the flexure to undergo elastic deformation, causing its teeth to interact with the teeth of the rigid wheel, thereby achieving the purpose of transmission. The most unique component is the flexure, which undergoes two elliptical deformations per revolution during service. To meet this service characteristic, the flexure's wall thickness is generally designed to be relatively thin, around 0.5 mm. This thin-walled dimensional characteristic, along with the alternating stress generated by repeated deformation during service, makes the flexure highly susceptible to damage.

[0003] The service characteristics of a flexible gear include significant deformation, non-constant load, and additional tensile stress in the protruding areas where deformation occurs. Figure 2 As shown in Figure a, during deformation, the outer surface on the left bulges out to bear tensile stress, while the inner surface bears compressive stress, thus ensuring mechanical balance. Simultaneously, the rotating flexible wheel experiences alternating loads during service; a single local point undergoes continuous and repeated deformation, such as... Figure 2 When the flexible wheel a bends, its outer surface exhibits tensile stress, and it reaches a state of equilibrium after no longer deforming, where stress no longer exists (e.g., Figure 2 As shown in b), during the service process, in addition to the external load, the deformation of the flexible wheel itself constantly causes tensile stress to be generated on the outer surface and then disappears in a repeated state. This is equivalent to serving under alternating tensile load, that is, bearing fatigue load. These characteristics require the flexible wheel material to have high fatigue performance.

[0004] Improving service life is key to enhancing the quality of flexible gear products. Early imitations of foreign counterparts employed conventional gear microstructure control processes—tempering treatments. For example, the flexible gear used 40CrNiMo material, which typically undergoes a tempering process (quenching followed by high-temperature tempering to obtain tempered sorbite composed of ferrite and cementite). However, this process has significant performance defects. For instance, grain size was not effectively controlled, and the conventional quenching and tempering process resulted in localized grain boundary carbides, leading to stress concentration at the grain boundaries and instability in some retained austenite. This lack of targeted control for the specific service conditions of flexible gears resulted in a service life that could not be compared to foreign counterparts. Therefore, research is needed on the relationship between the microstructure and fatigue performance of flexible gears to explore the optimal microstructure and corresponding service mode that meets fatigue life requirements. Summary of the Invention

[0005] The purpose of this invention is to provide a method for improving the fatigue life of flexure components in a harmonic reducer, which can improve the fatigue resistance of the flexure and extend its service life.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for improving the fatigue life of flexspline components in a harmonic reducer, comprising the following steps:

[0008] The stiffened annealed flexible wheel blank is repeatedly subjected to solution treatment and quenching; the material of the flexural wheel blank includes 40CrNiMo or 35CrMnSi.

[0009] After the final solution treatment, the obtained billet is placed in the first salt melt for the first salt bath. The temperature of the first salt bath is 250-320℃ and the holding time is 5-50s.

[0010] After the first salt bath is completed, the obtained billet is transferred to the second salt melt for the second salt bath, the temperature of which is 350-450℃.

[0011] After the second salt bath is completed, the resulting billet is subjected to cryogenic treatment at a temperature below -75°C for 5 to 12 hours.

[0012] Preferably, the number of solid solution and quenching processes is 2 to 5.

[0013] Preferably, the quenching method is oil quenching.

[0014] Preferably, the temperature of each solution treatment is independently 840–880°C, and the holding time is independently 40–120 min.

[0015] Preferably, the heat preservation time of the second salt bath is 5 to 10 minutes.

[0016] Preferably, the temperature of the cryogenic treatment is -80℃ to -75℃.

[0017] Preferably, the stress-relief annealing temperature is 880°C and the holding time is 2 hours.

[0018] This invention provides a method for improving the fatigue life of flexure components in a harmonic reducer, comprising the following steps: repeatedly performing solution treatment and quenching on a rough blank of the flexure component after stress-relief annealing; the material of the rough blank includes 40CrNiMo or 35CrMnSi; after the final solution treatment, the obtained rough blank is placed in a first salt melt for a first salt bath, the temperature of the first salt bath being 250-320℃ and the holding time being 5-50s; after the first salt bath is completed, the obtained rough blank is transferred to a second salt melt for a second salt bath, the temperature of the second salt bath being 350-450℃; after the second salt bath is completed, the obtained rough blank is subjected to deep cryogenic treatment, the temperature of the deep cryogenic treatment being below -75℃ and the time being 5-12h.

[0019] This invention first solutions-solutions the rough blank of a flexible wheel part to obtain a uniform solid solution, then quenches it to form austenite. Repeated solutions and quenchings yield uniform and fine austenite grains, achieving grain size control for high-performance materials. After the final solution treatment and holding, the rough blank is subjected to a first salt bath. Because martensite is a continuous transformation product, and the first salt bath temperature is low and the time is short, martensite can only nucleate in localized areas, resulting in a small amount of martensite within the austenite structure. This portion of martensite uniformly forms a portion of higher-hardness martensite within the supercooled austenite, serving as the backbone for subsequent phase transformation structures. The billet is then immediately subjected to a second salt bath after the first salt bath. This transforms the untransformed supercooled austenite from the first salt bath into lower bainite, thus completing the initial microstructure control and yielding a uniform mixture of a small amount of martensite, a large amount of bainite, and a trace amount of retained austenite. Finally, deep cryogenic treatment is performed to promote the transformation of unstable retained austenite into martensite, while the remaining material is stable retained austenite. The stable retained austenite can help with stress concentration, acting as a lubricating phase and preventing phase transformation of the retained austenite during subsequent service under mechanical stress.

[0020] Through the above processing technology, the present invention achieves grain size refinement and homogenization, avoids carbides, greatly increases the stability of the residual austenite structure, avoids its decomposition under mechanical driving force during large strain, prevents microstructure changes that affect performance stability, eliminates residual stress, improves the fatigue life of the flexible wheel, and enables it to meet the requirements of long service life. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the structure and operating principle of a harmonic reducer.

[0022] Figure 2 A schematic diagram of the stress distribution when a flexible wheel is subjected to strain (a) and a schematic diagram of the stress disappearing during strain recovery (b);

[0023] Figure 3Physical images of the flexible wheels prepared for the examples and comparative examples;

[0024] Figure 4 The grain size diagrams are for comparative examples 1 and 2.

[0025] Figure 5 The microstructure diagrams of Comparative Examples 1 and 2 in the modulation state are shown.

[0026] Figure 6 The grain size diagrams for Examples 1 and 2 after repeated solution treatment and quenching are shown.

[0027] Figure 7 Optical and SEM images of the microstructures of Examples 1-2 after the second salt bath;

[0028] Figure 8 for Figure 7 Enlarged view of the SEM image;

[0029] Figure 9 The stress-strain curves are for the examples and comparative examples. Detailed Implementation

[0030] This invention provides a method for improving the fatigue life of flexspline components in a harmonic reducer, comprising the following steps:

[0031] The stiffened annealed flexible wheel blank is repeatedly subjected to solution treatment and quenching; the material of the flexural wheel blank includes 40CrNiMo or 35CrMnSi.

[0032] After the final solution treatment, the obtained billet is placed in the first salt melt for the first salt bath. The temperature of the first salt bath is 250-320℃ and the holding time is 5-50s.

[0033] After the first salt bath is completed, the obtained billet is transferred to the second salt melt for the second salt bath, the temperature of which is 350-450℃.

[0034] After the second salt bath is completed, the resulting billet is subjected to cryogenic treatment at a temperature below -75°C for 5 to 12 hours.

[0035] This invention involves repeatedly performing solution treatment and quenching on the rough blank of the flexible wheel part that has undergone stress-relief annealing.

[0036] In this invention, the material of the flex wheel part blank includes 40CrNiMo or 35CrMnSi. This invention does not have special requirements regarding the source of the stress-relief annealed flex wheel part blank; any source well-known in the art can be used. In an embodiment of this invention, the raw material bar is forged, cut into cylindrical blanks, then subjected to furnace cooling stress-relief annealing at 880℃ for 2 hours, and finally machined.

[0037] In this invention, the number of solution treatment and quenching cycles is preferably 2 to 5. Since no quenching is performed after the final solution treatment and holding, the final solution treatment cycle is not included in the calculation of the number of cycles.

[0038] In this invention, the temperature of each solid solution is preferably 840-880°C, more preferably 850-870°C, and most preferably 860°C; the holding time of each solid solution is preferably 40-120 min, more preferably 60-100 min, and even more preferably 70-80 min.

[0039] In this invention, the quenching method is preferably oil quenching. This invention forms austenite through solution treatment and quenching. Repeated solution treatment and quenching in this invention can obtain uniform and fine austenite grains, achieving grain size control of high-performance materials.

[0040] After the final solution treatment, the resulting billet is placed in the first salt melt for a first salt bath. The temperature of the first salt bath is 250-320°C, and the holding time is 5-50 seconds.

[0041] The present invention does not have any special requirements for the first salt melt; any salt melt known in the art for use in salt baths can be used. In the present invention, the temperature of the first salt bath is preferably 260–310°C, more preferably 270–300°C, and even more preferably 280–290°C; the holding time of the first salt bath is preferably 10–45 s, more preferably 20–40 s.

[0042] Since martensite is a continuous transformation product, and the first salt bath temperature is low and the time is short, martensite can only nucleate in local areas. Therefore, after the first salt bath, the present invention obtains a small amount of martensite in the austenitic structure, and this part of the martensite serves as the skeleton of the subsequent phase transformation structure.

[0043] After the first salt bath is completed, the resulting billet is transferred to the second salt melt for a second salt bath, the temperature of which is 350-450°C.

[0044] In this invention, the temperature of the second salt bath is preferably 370–430°C, more preferably 380–400°C. This invention does not have special requirements regarding the duration of the second salt bath; it can instantly transform untransformed supercooled austenite into lower bainite, typically within 5–10 minutes.

[0045] This invention transforms the untransformed supercooled austenite in the first salt bath into lower bainite using a second salt bath. This completes the initial microstructure control, resulting in a homogeneous mixture of a small amount of martensite, a large amount of bainite, and a trace amount of retained austenite. This microstructure combines the hardness of martensite with the strength and toughness of lower bainite. Through microstructure control, the yield strength is effectively improved without sacrificing plasticity and toughness, or significantly increasing hardness (excessive hardness leads to more machining defects).

[0046] After the second salt bath is completed, the present invention will subject the obtained billet to deep cryogenic treatment to transform the unstable residual austenite into martensite.

[0047] In this invention, the temperature of the cryogenic treatment is below -75°C, preferably -80°C to -75°C; the duration of the cryogenic treatment is 5 to 12 hours, preferably 7 to 10 hours, and more preferably 8 to 9 hours.

[0048] This invention promotes the transformation of unstable retained austenite into martensite through cryogenic treatment, while the remaining material is stable retained austenite. The stable retained austenite can help with stress concentration, acting as a lubricating phase, thus preventing the retained austenite from undergoing phase transformation during subsequent service under the influence of mechanical stress.

[0049] Through the above processing technology, the present invention achieves grain size refinement and homogenization, avoids carbides, greatly increases the stability of the residual austenite structure, avoids its decomposition under mechanical driving force during large strain, prevents microstructure changes that affect performance stability, eliminates residual stress, improves the fatigue life of the flexible wheel, and enables it to meet the requirements of long service life.

[0050] The following detailed description of the method for improving the fatigue life of flexural parts in a harmonic reducer, in conjunction with specific embodiments, should not be construed as limiting the scope of protection of this invention.

[0051] Comparative Example 1

[0052] (1) 40CrNiMo raw material bar is forged and cut into cylindrical billets, and then subjected to furnace cooling stress relief annealing at 880℃ for 2 hours.

[0053] (2) Machining into a rough blank, conditioning treatment: heat preservation at 880℃ for 2 hours, oil quenching, heat preservation at 650℃ for 4 to 6 hours, tempering and air cooling.

[0054] (3) Machining to the specified dimensions, machining the teeth, and completing.

[0055] Comparative Example 2

[0056] The only difference from Comparative Example 1 is that the raw material was replaced with 35CrMnSi.

[0057] Example 1

[0058] (1) 40CrNiMo raw material bar is forged and cut into cylindrical billets, and then subjected to furnace cooling stress relief annealing at 880℃ for 2 hours.

[0059] (2) The blank is machined into a rough blank, heated to 860°C and held for 90 minutes, and then oil quenched. The heating and quenching are repeated 4 times according to the above heating parameters. After the last holding at 860°C, the obtained blank is immersed in a salt melt at 280°C for the first salt bath and held for 6 seconds. It is then immediately transferred to a salt melt at 380°C for the second salt bath and held for 5 minutes. The obtained blank sample is then placed in a -75°C freezing facility for deep cryogenic treatment for 8 hours.

[0060] (3) Machining to the specified dimensions, machining the teeth, and completing (see actual picture). Figure 3 (Left image).

[0061] Example 2

[0062] (1) 35CrMnSi raw material bar is forged and cut into cylindrical billets, and then subjected to furnace cooling stress relief annealing at 880℃ for 2 hours.

[0063] (2) The blank is machined into a rough blank, heated to 860°C and held for 60 minutes, and then oil quenched. The heating and quenching are repeated 4 times according to the above heating parameters. After the last holding at 860°C, the obtained blank is immersed in a salt melt at 280°C for the first salt bath and held for 5 seconds. It is then immediately transferred to a salt melt at 380°C for the second salt bath and held for 5 minutes. The obtained blank sample is then placed in a -75°C freezing facility for deep cryogenic treatment for 8 hours.

[0064] (3) Machining to the specified dimensions, machining the teeth, and completing (see actual picture). Figure 3 (Right image).

[0065] Organizational characterization:

[0066] The grain size and microstructure of the flexible wheels prepared by conventional conditioning treatment (quenching + tempering) in Comparative Examples 1 and 2 were observed, and the results are as follows: Figure 4 and Figure 5 As shown.

[0067] As shown in the figure, the flexible wheel obtained by conventional modulation treatment clearly exhibits mixed crystals and has coarse grains (e.g., Figure 4 Simultaneously, a tempering treatment was applied, resulting in a microstructure of ferrite + cementite. When the tempering temperature was low, the hardness was high; increasing the tempering temperature to 650℃ resulted in carbide precipitation at the grain boundaries (e.g., Figure 5Because the interface between the carbide and the matrix is ​​incoherent, it leads to large micro-stress concentrations, which affect its fatigue performance.

[0068] The microstructures after repeated heating and quenching in Examples 1 and 2 were observed, and the results are as follows: Figure 6 As shown.

[0069] Depend on Figure 6 It can be seen that after repeated heating and quenching treatment according to the present invention, the average grain size of 40CrNiMo decreased from 8-10 μm after the original forging to 3.45 μm, and the average grain size of 35CrMnSi decreased from 10-15 μm after the original forging to 2.74 μm, thus achieving significant refinement of the microstructure.

[0070] The tissues of Examples 1 and 2 were observed after the second salt bath, and the results are shown in the figure. Figure 7 The top two images are optical images, and the bottom two are SEM images. Figure 7 It can be seen that, through treatment, microstructures with significant lath structure were obtained in the materials. These microstructures have a combination of high hardness (martensite) and high toughness (lower bainite), which are high-quality microstructures that can meet fatigue performance requirements.

[0071] right Figure 7 The SEM images were magnified for observation, and the results are shown in the figure. Figure 8 . Figure 8 This more clearly demonstrates that the present invention obtained a stable martensite + lower bainite microstructure after the second salt bath. Furthermore, the present invention also observed the microstructures after cryogenic treatment in Examples 1 and 2. The results showed that the microstructure did not change after cryogenic treatment; it merely promoted the transformation of unstable retained austenite, effectively reducing the content of unstable retained austenite.

[0072] Mechanical property testing:

[0073] The flexible wheels prepared in Comparative Examples 1-2 and Examples 1-2 were subjected to mechanical property tests, and the obtained stress-strain curves are shown in the figure. Figure 9 , Figure 9 The specific data is shown in Table 1.

[0074] As can be clearly seen from the tensile stress-strain curve, the mechanical properties, especially the yield strength, after treatment by the present invention are significantly improved compared with those of conventional modulation treatment, while the plasticity index is basically not significantly reduced. This indicates that under the microstructure control of the present invention, the metal material has been significantly improved to meet the service conditions of the flexible wheel.

[0075] Table 1 Mechanical properties of the embodiments and comparative examples

[0076]

[0077]

[0078] The applicant used a dedicated testing device for the life quality of flexible gears in harmonic reducers (currently preparing for patent application, so the specific structure cannot be disclosed) to test the life of the flexible gears prepared in Examples 1-2 and Comparative Examples 1-2. The results showed that the flexible gears in Examples 1-2 broke last and had a longer service life.

[0079] Table 2 shows the mechanical properties of a Japanese product reported in a certain literature. As can be seen from Table 2, the present invention significantly improves the strength and plasticity compared to the Japanese product by minimizing the microstructural factors affecting fatigue, such as eliminating coarse grains, carbides, and unstable residual carbides.

[0080] Table 2 Mechanical properties of a certain flexible wheel in Japan

[0081]

[0082] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for improving the fatigue life of flexspline components in a harmonic reducer, characterized in that, Includes the following steps: The stiffened annealed flexible wheel blank is repeatedly subjected to solution treatment and quenching; the material of the flexural wheel blank includes 40CrNiMo or 35CrMnSi. After the final solution treatment, the obtained billet is placed in the first salt melt for the first salt bath. The temperature of the first salt bath is 250~320℃ and the holding time is 10~45s. After the first salt bath is completed, the obtained billet is transferred to the second salt melt for the second salt bath, the temperature of which is 350~450℃. After the second salt bath is completed, the resulting blank is directly subjected to deep cryogenic treatment at a temperature below -75°C for 5 to 12 hours.

2. The method according to claim 1, characterized in that, The number of solution treatments and quenchings is 2 to 5.

3. The method according to claim 1 or 2, characterized in that, The quenching method is oil quenching.

4. The method according to claim 1 or 2, characterized in that, Each solution treatment was performed at a temperature of 840-880°C and for a holding time of 40-120 minutes.

5. The method according to claim 1, characterized in that, The second salt bath should be kept warm for 5 to 10 minutes.

6. The method according to claim 1, characterized in that, The cryogenic treatment temperature is -80℃ to -75℃.

7. The method according to claim 1, characterized in that, The stress-relief annealing temperature is 880℃, and the holding time is 2 hours.

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