A method for QPQ surface hardening of a flexspline and a rigid spline of a harmonic reducer

Abstract

This invention provides a QPQ surface strengthening method for flexible and rigid wheels in harmonic reducers, comprising the following steps: after machining the flexible or rigid wheel, it is sequentially degreased, cleaned, dried, and preheated; the preheated flexible or rigid wheel is nitrided to form a nitrided layer on its surface; the surface of the flexible or rigid wheel is treated mechanically or by tumbling; the polished flexible or rigid wheel is placed in molten oxide salt for oxidation treatment to form an oxide layer on the surface of the nitrided layer; after the oxidized flexible or rigid wheel is air-cooled, it is immediately rinsed with clean water and dried with hot air, and then coated with anti-rust oil. Compared with the prior art, this invention uses low-temperature salt bath nitriding and oxidation treatment, the QPQ process is short, has fewer process steps and is easy to control, and the salt bath can be recycled. The surface hardness of the workpiece reaches 800-850 HV, and the wear rate is reduced by more than 70% compared with the traditional tempering process, significantly extending the service life of the workpiece.

Description

Technical Field

[0001] This invention relates to the field of metal heat treatment, and more particularly to a QPQ surface strengthening method for flexible and rigid wheels in harmonic reducers. Background Technology

[0002] Harmonic reducers, as high-precision transmission mechanisms, are widely used in aerospace, robotics, and precision machine tools. Their core components, the flexible and rigid wheels, are subjected to long-term cyclic high stress, alternating contact, and rolling friction. During operation, the flexible wheel endures cyclic tensile and compressive stresses, while its meshing surface with the rigid wheel exhibits complex friction and wear behavior. Therefore, its surface wear resistance, fatigue resistance, and dimensional stability directly determine the reducer's lifespan and accuracy retention. Traditional heat treatment processes (such as tempering, surface quenching, or ion nitriding) can improve surface hardness to some extent, but they suffer from insufficient wear resistance, early failure of the friction pairs, and large heat treatment deformation, making it difficult to meet the stringent requirements of high precision and long lifespan for harmonic reducers.

[0003] QPQ (Quench-Polish-Quench) salt bath composite surface strengthening technology is a multi-stage processing technology that integrates salt bath nitriding, polishing, and oxidation. This technology uses low-temperature diffusion of active nitrogen atoms into the steel surface to form a diffusion layer and ε-Fe. 2-3 A nitrogen composite phase layer is formed, followed by polishing and oxidation to generate a dense Fe3O4 protective film, thereby significantly improving the surface hardness, wear resistance, and corrosion resistance of the material. Compared with traditional high-temperature nitriding or carburizing processes, the QPQ process has the following outstanding advantages:

[0004] 1) Significantly improved wear resistance: ε-Fe formed during the salt bath nitriding stage 2-3 The N and γ′-Fe4N phases have extremely high hardness, and when combined with an oxide film, they can significantly reduce the coefficient of friction, with the wear rate typically decreasing by more than 70% compared to the tempered state.

[0005] 2) Minimal heat treatment deformation: QPQ treatment temperature is lower than the phase transformation temperature of steel, and no austenitization or phase transformation expansion occurs. Therefore, the workpiece size changes little, making it particularly suitable for high-precision meshing parts such as flexible wheels and rigid wheels.

[0006] 3) Short process cycle and good repeatability: The salt bath heat transfer is uniform, the infiltration layer grows quickly, and the process is easy to control; after polishing, a uniform and dense composite layer can be obtained, reducing grinding and repair processes.

[0007] For the flexible and rigid wheel materials of harmonic reducers, the QPQ process can form a hard but not brittle composite reinforcement layer in the meshing contact area while maintaining the toughness and elastic modulus of the matrix, effectively suppressing adhesive wear, fatigue pitting, and spalling. At the same time, the low-temperature characteristics of the QPQ process avoid the out-of-roundness, expansion and contraction, and stress distortion problems that occur in the thin-walled structure of the flexible wheel during heat treatment, thus ensuring high-precision assembly requirements and long-term operational stability.

[0008] In summary, the QPQ composite surface strengthening process, with its comprehensive advantages of high hardness, high wear resistance, and low deformation, has become an ideal choice for surface strengthening of both the flexible and rigid wheels in harmonic reducers. However, existing QPQ processes still have room for improvement in terms of salt bath composition control, oxide layer uniformity, and dimensional accuracy. Therefore, developing an optimized QPQ surface strengthening process suitable for key components of harmonic reducers has significant engineering application value for improving the service life and reliability of transmission systems. Summary of the Invention

[0009] The purpose of this invention is to provide a QPQ (Quench-Polish-Quench) surface strengthening method for flexible and rigid wheels in harmonic reducers. By optimizing multiple process steps such as degreasing and cleaning, salt bath nitriding, polishing, and oxidation treatment, the method achieves a comprehensive improvement in the surface hardness, wear resistance, and low deformation of the parts. It is particularly suitable for applications in harmonic reducers that require high precision, long life, high load, and low deformation.

[0010] The present invention adopts the following technical solution to achieve the above objectives:

[0011] A QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer includes the following steps:

[0012] (1) Degreasing and cleaning: After the flexible wheel or rigid wheel is machined, it is degreased, cleaned and dried in sequence to ensure that the surface is clean and free of oil.

[0013] (2) Preheating: Preheat the cleaned flexible or rigid wheel;

[0014] (3) Salt bath nitriding: The preheated flexible or rigid wheel is nitrided to form a nitrided layer on its surface;

[0015] (4) Polishing: The surface of the flexible or rigid wheel is treated by mechanical or tumbling methods;

[0016] (5) Oxidation treatment: The polished flexible wheel or rigid wheel is placed in molten oxidized salt for oxidation treatment, so that an oxide layer is generated on the surface of the nitrided layer;

[0017] (6) Cleaning and rust prevention: After the oxidized flexible or rigid wheel is air-cooled, it should be rinsed with clean water and dried with hot air, and then coated with rust-preventive oil.

[0018] Further, in step (1), the flexible wheel is made of medium carbon alloy structural steel, selected from at least one of 40CrNiMoA, 40Cr, and 45 steel, and the rigid wheel is made of at least one of 40Cr, 45 steel, and 2Cr13 stainless steel.

[0019] Furthermore, in step (1), isopropanol is used as the cleaning agent for degreasing and cleaning, the cleaning temperature is 25~45℃, ultrasonic cleaning is performed for 8~10 minutes, and the product is air-dried at room temperature after cleaning.

[0020] Furthermore, in step (2), the preheating is carried out in a vacuum furnace at 300~350℃ for 5~15 minutes to avoid surface oxidation caused by sudden temperature rise during subsequent nitriding treatment.

[0021] Further, in step (3), the nitriding salt used in the salt bath nitriding consists of: 15-20% sodium cyanate, 10-15% potassium cyanate, 10-12% sodium carbonate, 10-12% potassium carbonate, 5-6% lithium carbonate, 10-15% urea, 2-3% cerium carbonate or lanthanum oxide, 0.5% MoO3, 0.3% Nb2O5, 3-5% sodium chloride, and 0.02-0.05% potassium sulfide. All of the above are mass percentages; the remainder consists of other alkali metal salt fillers and unavoidable impurities. The nitriding temperature is 500-530℃, and the nitriding time is 60-120 min. The main component of the nitrided layer formed by salt bath nitriding is ε-Fe. 2-3 N and γ′-Fe4N, in addition to small amounts of Mo2N and NbN, exist as precipitated phases.

[0022] Furthermore, in step (4), polishing is performed to make the surface roughness Ra≤0.4 μm, with the aim of removing the brittle white bright layer on the surface of the nitride layer and keeping the surface of the nitride layer intact.

[0023] Further, in step (5), the oxidizing salt used in the oxidation treatment consists of: sodium nitrate 14-16%, sodium nitrite 1-2%, sodium carbonate 30-32%, potassium carbonate 6-8%, lithium carbonate 3-5%, sodium hydroxide 2-3%, cerium nitrate or yttrium nitrate 1%, nickel nitrate 0.5%, sodium chloride 2-3%, and sodium sulfate 1-2%, all of which are mass percentages. The remainder consists of other alkali metal salt fillers and unavoidable impurities. The oxidation temperature is 360-380 ℃, and the oxidation time is 10-20 min. The main component of the oxide layer formed by the oxidation treatment is Fe3O4, and it also contains trace amounts of CeO2 / Y2O3 rare earth oxides.

[0024] Furthermore, in step (6), after drying, rust-preventive oil is applied to the surface of the flexible or rigid wheel to form a black glossy protective layer.

[0025] The present invention also provides a flexible or rigid wheel treated by the method described above, having a surface hardness of 800-850 HV, an average depth of the total reinforced layer of 30-40 μm, a total reinforced layer depth range of ≤5 μm, and a wear rate of ≤3.0 × 10⁻⁶. -7 mm 3 / (N·m).

[0026] This invention also provides the application of the aforementioned flexible or rigid wheel in the field of harmonic reducers. Compared with the prior art, this invention has significant advantages in improving the wear resistance and extending the service life of harmonic reducers.

[0027] The present invention has the following beneficial effects:

[0028] The QPQ surface strengthening method of this invention can effectively improve the surface hardness, wear resistance, and corrosion resistance of the flexible and rigid wheels in a harmonic reducer, while reducing deformation during heat treatment. Compared with existing surface treatment technologies, this method has the following significant advantages: Significantly improved wear resistance: The combination of the nitrided layer and oxide film achieves a workpiece surface hardness of 800-850 HV, reducing the wear rate by more than 70% compared to the traditional quenched and tempered state, significantly extending the service life of the workpiece. Reduced heat treatment deformation: Low-temperature salt bath nitriding and oxidation treatment ensures high precision for the flexible and rigid wheels. Simple process and low cost: The QPQ process has a short flow, few steps, and is easy to control. The salt bath is regenerable and recyclable, offering a significant cost advantage compared to other surface treatment methods.

[0029] In the QPQ process described in this invention, the nitriding agent is a multi-component composite system consisting of urea, soda ash, potassium carbonate, lithium carbonate, and sodium fluoride, with further introduction of cerium carbonate or lanthanum oxide. Since rare earth elements have a larger atomic radius than iron, their infiltration induces lattice distortion, causing nitrogen atoms to preferentially agglomerate in the distorted region and become nitride nuclei, thereby lowering the diffusion barrier and simultaneously increasing the thickness and hardness of the infiltrated layer. However, excessive rare earth elements can hinder the diffusion of nitrogen and carbon atoms, leading to a porous infiltrated layer. Therefore, this invention controls the amount of cerium carbonate or lanthanum oxide to 2%~3%. Based on this, the added MoO3 and Nb2O5 are reduced and further nitrided in a nitriding atmosphere to form MoN2 and NbN precipitates, which strengthen the nitrided layer.

[0030] In the oxidation process, compared with the traditional oxide salt composed of NaOH, KOH and NaNO3, the present invention introduces cerium nitrate or yttrium nitrate, which can reduce the activation energy of oxygen atoms, accelerate the nucleation rate of Fe3O4, and make the oxide film more dense; at the same time, the formation of Fe2O3 is inhibited by sodium nitrite, so there is no need for separate rust removal treatment afterwards.

[0031] Regarding process parameters, this invention controls the nitriding temperature at 500~530℃, which is lower than the traditional nitriding temperature. This effectively reduces heat treatment deformation and avoids the decrease in matrix hardness caused by tempering during high-temperature nitriding. The QPQ surface strengthening method of this invention can meet the requirements of high hardness, low friction, low deformation, and long-term stability for flexible and rigid wheels in harmonic reducers, and has significant application value in improving transmission accuracy and durability. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 The SEM microstructure of the 45 steel surface obtained in Example 1 of this invention;

[0034] Figure 2 The image shows the scanning results of the EDS surface of the 45 steel obtained in Example 1 of this invention. Detailed Implementation

[0035] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0036] Example 1

[0037] A QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer includes the following steps:

[0038] This embodiment uses a 45 steel flexible wheel;

[0039] The first step involves machining the flexible wheel and then degreasing it using isopropanol as the cleaning agent. During cleaning, the temperature is maintained at 45℃, and ultrasonic cleaning is performed for 10 minutes. After cleaning, the workpiece is air-dried at room temperature to ensure the surface is clean and free of oil.

[0040] The second step involves placing the cleaned flexible wheel into a vacuum furnace for preheating. The preheating temperature is set at 350℃, and the holding time is 12 minutes. Preheating in a vacuum environment prevents oxidation of the workpiece surface and ensures the uniformity of the subsequent nitriding treatment.

[0041] The third step involves subjecting the preheated flexible wheel to salt bath nitriding. The nitriding salt composition is as follows: sodium cyanate 20%, potassium cyanate 15%, sodium carbonate 12%, potassium carbonate 12%, lithium carbonate 6%, urea 15%, cerium carbonate 3%, MoO3 0.5%, Nb2O5 0.3%, sodium chloride 4%, and potassium sulfide 0.02%. All percentages are by mass; the remainder consists of other alkali metal salt fillers and unavoidable impurities. The salt bath nitriding temperature is set at 530℃, and the nitriding time is 120 minutes. This precise control of the salt bath composition allows for the formation of an ε-Fe layer with a thickness of 20–35 μm on the workpiece surface. 2-3 The N and γ′-Fe4N composite nitriding layer effectively improves surface hardness and wear resistance.

[0042] The fourth step is to remove the brittle white layer on the workpiece surface by mechanical polishing after salt bath nitriding, ensuring that the surface roughness Ra≤0.4 μm, maintaining the integrity of the composite nitriding layer and not damaging the surface structure.

[0043] The fifth step involves immersing the polished flexible wheel in molten oxidizing salt for oxidation treatment. The oxidizing salt composition includes: 16% sodium nitrate, 1.8% sodium nitrite, 32% sodium carbonate, 8% potassium carbonate, 5% lithium carbonate, 3% sodium hydroxide, 1% cerium nitrate, 0.5% nickel nitrate, 3% sodium chloride, and 1.7% sodium sulfate. All percentages are by weight; the remainder consists of other alkali metal salt fillers and unavoidable impurities. The oxidation treatment temperature is controlled at 380℃, and the oxidation time is 20 minutes. During the oxidation process, the dense Fe3O4 oxide film formed by the oxidizing salt significantly improves the surface's corrosion resistance and wear resistance.

[0044] The sixth step involves immediately air-cooling the flexible wheel after oxidation treatment and rinsing with clean water to remove any residual salt or chemicals from the surface. The workpiece is then dried with hot air and finally coated with a thin layer of anti-rust oil to ensure it is unaffected by environmental factors during storage and transportation.

[0045] Figure 1 The SEM microstructure of the 45 steel surface obtained in Example 1 shows that the substrate surface has a dense and uniformly thick nitride layer and an oxide layer, with the nitride layer being approximately twice the thickness of the oxide layer. Figure 2 The surface microstructure is shown in the EDS surface scan results. It can be seen that the nitrided layer is mainly enriched with Fe and N elements, the oxide layer is mainly enriched with Fe and O elements, and the matrix is ​​mainly composed of Fe, Cr, Si and Mn.

[0046] Example 2

[0047] This embodiment uses a 45 steel rigid wheel;

[0048] The first step involves machining the steel wheel and then degreasing it using isopropanol as the cleaning agent. During cleaning, the temperature is maintained at 25℃, and ultrasonic cleaning is performed for 8 minutes. After cleaning, the workpiece is air-dried at room temperature to ensure the surface is clean and free of oil.

[0049] The second step involves placing the cleaned flexible wheel into a vacuum furnace for preheating. The preheating temperature is set at 300℃, and the holding time is 5 minutes. Preheating in a vacuum environment prevents oxidation of the workpiece surface and ensures the uniformity of the subsequent nitriding treatment.

[0050] The third step involves subjecting the preheated flexible wheel to salt bath nitriding. The nitriding salt composition is as follows: sodium cyanate 15%, potassium cyanate 10%, sodium carbonate 10%, potassium carbonate 10%, lithium carbonate 5%, urea 10%, lanthanum oxide 2%, MoO3 0.5%, Nb2O5 0.3%, sodium chloride 3%, and potassium sulfide 0.02%. All percentages are by mass; the remainder consists of other alkali metal salt fillers and unavoidable impurities. The salt bath nitriding temperature is set to 500℃, and the nitriding time is 60 minutes. This precise control of the salt bath composition allows for the formation of an ε-Fe layer with a thickness of 20–35 μm on the workpiece surface. 2-3 The N and γ′-Fe4N composite nitriding layer effectively improves surface hardness and wear resistance.

[0051] The fourth step is to remove the brittle white layer on the workpiece surface by mechanical polishing after salt bath nitriding, ensuring that the surface roughness Ra≤0.4 μm, maintaining the integrity of the composite nitriding layer and not damaging the surface structure.

[0052] The fifth step involves immersing the polished flexible wheel in molten oxidizing salt for oxidation treatment. The oxidizing salt composition includes: 14% sodium nitrate, 1% sodium nitrite, 30% sodium carbonate, 6% potassium carbonate, 3% lithium carbonate, 2% sodium hydroxide, 1% cerium nitrate, 0.5% nickel nitrate, 2% sodium chloride, and 2% sodium sulfate. All percentages are by weight; the remainder consists of other alkali metal salt fillers and unavoidable impurities. The oxidation treatment temperature is controlled at 360℃, and the oxidation time is 10 minutes. During the oxidation process, the dense Fe3O4 oxide film formed by the oxidizing salt significantly improves the surface's corrosion resistance and wear resistance.

[0053] The sixth step involves immediately air-cooling the flexible wheel after oxidation treatment and rinsing with clean water to remove any residual salt or chemicals from the surface. The workpiece is then dried with hot air and finally coated with a thin layer of anti-rust oil to ensure it is unaffected by environmental factors during storage and transportation.

[0054] Example 3

[0055] In this embodiment, a 40CrNiMoA steel flexible wheel is selected;

[0056] The first step involves machining the steel wheel and then degreasing it using isopropanol as the cleaning agent. During cleaning, the temperature is maintained at 35℃, and ultrasonic cleaning is performed for 9 minutes. After cleaning, the workpiece is air-dried at room temperature to ensure the surface is clean and free of oil.

[0057] The second step involves placing the cleaned flexible wheel into a vacuum furnace for preheating. The preheating temperature is set at 325℃, and the holding time is 10 minutes. Preheating in a vacuum environment prevents oxidation of the workpiece surface and ensures the uniformity of the subsequent nitriding treatment.

[0058] The third step involves subjecting the preheated flexible wheel to salt bath nitriding. The nitriding salt composition is as follows: sodium cyanate 17%, potassium cyanate 13%, sodium carbonate 11%, potassium carbonate 11%, lithium carbonate 6%, urea 13%, cerium carbonate 2%, MoO3 0.5%, Nb2O5 0.3%, sodium chloride 4%, and potassium sulfide 0.02%. All percentages are by mass; the remainder consists of other alkali metal salt fillers and unavoidable impurities. The salt bath nitriding temperature is set at 520℃, and the nitriding time is 90 minutes.

[0059] The fourth step is to remove the brittle white layer on the workpiece surface by mechanical polishing after salt bath nitriding, ensuring that the surface roughness Ra≤0.4 μm, maintaining the integrity of the composite nitriding layer and not damaging the surface structure.

[0060] The fifth step involves immersing the polished flexible wheel in molten oxidizing salt for oxidation treatment. The oxidizing salt composition includes: 15% sodium nitrate, 1.5% sodium nitrite, 31% sodium carbonate, 7% potassium carbonate, 4% lithium carbonate, 2% sodium hydroxide, 1% cerium nitrate, 0.5% nickel nitrate, 3% sodium chloride, and 1.5% sodium sulfate. All percentages are by weight; the remainder consists of other alkali metal salt fillers and unavoidable impurities. The oxidation treatment temperature is controlled at 370℃, and the oxidation time is 15 minutes. During the oxidation process, the dense Fe3O4 oxide film formed by the oxidizing salt significantly improves the surface's corrosion resistance and wear resistance.

[0061] The sixth step involves immediately air-cooling the flexible wheel after oxidation treatment and rinsing with clean water to remove any residual salt or chemicals from the surface. The workpiece is then dried with hot air and finally coated with a thin layer of anti-rust oil to ensure it is unaffected by environmental factors during storage and transportation.

[0062] Comparative Example 1

[0063] This comparative example uses a 45 steel flexible wheel. Compared with Example 1, the third step of the salt bath nitriding treatment did not add cerium carbonate. The composition of the nitriding salt used is: sodium cyanate 17%, potassium cyanate 13%, sodium carbonate 11%, potassium carbonate 11%, lithium carbonate 6%, urea 13%, MoO3 0.5%, Nb2O5 0.3%, sodium chloride 4%, potassium sulfide 0.02%, and the remainder being unavoidable impurities. Other steps are the same as in Example 1.

[0064] Comparative Example 2

[0065] This comparative example uses a 45 steel flexible wheel. Compared with Example 1, the temperature for the third step of salt bath nitriding is set to 580°C, and the nitriding time is 120 minutes. Other steps are the same as in Example 1.

[0066] Comparative Example 3

[0067] This comparative example uses a 45 steel flexible wheel. Compared with Example 1, the temperature for the third step of salt bath nitriding is set to 480°C, and the nitriding time is 120 minutes. Other steps are the same as in Example 1.

[0068] Comparative Example 4

[0069] This comparative example uses a 45 steel flexible wheel. Compared to Example 1, the third step of the salt bath nitriding treatment did not include MoO3 and Nb2O5. The nitriding salt composition used was: sodium cyanate 20%, potassium cyanate 13%, sodium carbonate 11%, potassium carbonate 11%, lithium carbonate 6%, urea 13%, cerium carbonate 2%, sodium chloride 4%, and potassium sulfide 0.02%. All of the above are mass percentages, and the remainder are unavoidable impurities. Other steps are the same as in Example 1.

[0070] Comparative Example 5

[0071] This comparative example uses a 45 steel flexible wheel. Compared to Example 1, sodium nitrite was not added in the fifth oxidation treatment step. The oxidizing salt composition used includes: 14% sodium nitrate, 30% sodium carbonate, 6% potassium carbonate, 3% lithium carbonate, 2% sodium hydroxide, 1% cerium nitrate, 0.5% nickel nitrate, 2% sodium chloride, and 2% sodium sulfate. All of the above are mass percentages, and the remainder are unavoidable impurities. Other steps are the same as in Example 1.

[0072] Comparative Example 6

[0073] This comparative example uses a 45 steel flexible wheel. Compared to Example 1, cerium nitrate was not added in the fifth oxidation treatment step. The oxidizing salt composition used includes: 14% sodium nitrate, 1% sodium nitrite, 30% sodium carbonate, 6% potassium carbonate, 3% lithium carbonate, 2% sodium hydroxide, 0.5% nickel nitrate, 2% sodium chloride, and 2% sodium sulfate. All of the above are mass percentages, and the remainder are unavoidable impurities. Other steps are the same as in Example 1.

[0074] Comparative Example 7

[0075] This comparative example uses 45 steel rigid wheels and does not employ the QPQ surface strengthening process.

[0076] Samples were taken from the surfaces of the flexible or rigid wheels obtained in Examples 1-3 and Comparative Examples 1-7 to obtain corresponding samples. Tensile tests were conducted on the samples according to national standard GB / T 2975, impact tests according to GB / T 229, Vickers hardness tests according to GB / T 4340.1-2024, fatigue performance tests according to GB / T 12718-2009, and wear performance tests according to GB / T 12444-2006. The wear load was 100N and the sliding distance was 10mm. The wear rate (W) of the flexible and rigid wheels of the harmonic reducer was calculated using the following formula: Wear loss weight (Δm) / (Load (F) × Sliding distance (S)). The test results are shown in Table 1.

[0077] Table 1 Comparison of surface hardness and wear rate between the examples and comparative examples

[0078]

[0079] Table 1 shows that the surface hardness of Examples 1, 2, and 3 are 832 HV, 846 HV, and 821 HV, respectively, while the hardness of the comparative examples is generally lower. The hardness of Comparative Example 1 is 721 HV, that of Comparative Example 2 is 632 HV, that of Comparative Example 3 is 641 HV, and that of Comparative Example 7 is even lower, at only 421 HV. The hardness of the examples is generally higher than that of the comparative examples, indicating that the nitriding and oxidation treatments in the examples can significantly improve the surface hardness of the workpiece and enhance its wear resistance.

[0080] The differences in wear-resistant layer depth, hardness, and wear resistance between the examples and comparative examples can be explained by the following scientific principles. In the examples, by using a reasonable ratio of nitride and oxide salt components and precisely controlling the nitriding and oxidation temperatures, a deep and dense nitride layer and oxide film were successfully formed on the workpiece surface, which significantly improved the surface hardness, wear resistance, and corrosion resistance of the workpiece. In particular, in Examples 1 and 2, the nitride layer depth and hardness reached high values, with Example 2 achieving a hardness of 846 HV and a nitride layer depth of 34 μm, exhibiting excellent wear resistance with a wear rate of only 2.4 × 10⁻⁶. -7 mm 3 / (N·m). This phenomenon occurs because, during the nitriding process, the nitrogen source diffuses to the substrate surface through a chemical reaction in a high-temperature salt bath, forming very hard nitrides (such as ε-Fe) with the iron-based material. 2-3Nitrides (N and γ′-Fe4N) significantly improve surface hardness. Furthermore, the Fe3O4 oxide film formed by the oxidation treatment further enhances the surface's corrosion resistance, and this oxide film also possesses excellent tribological properties, effectively reducing friction and wear.

[0081] In contrast, Comparative Example 1 and Comparative Example 2 showed higher wear rates, at 5.7 × 10⁻⁶ respectively. -7 mm 3 / (N·m) and 6.4×10 -7 mm 3 / (N·m), Comparative Example 1 did not add the rare earth compound cerium carbonate during the nitriding process, indicating that the nitriding treatment failed to penetrate to a sufficient depth, resulting in relatively low surface hardness and consequently significantly reduced wear resistance. Comparative Example 2 used a higher nitriding temperature and softened the substrate by tempering, obtaining a relatively thick nitrided layer of 36 μm, but the wear resistance was still poor. Comparative Example 3 used a lower nitriding temperature, resulting in weak nitrogen atom diffusion and insufficient penetration depth, thus failing to form a sufficiently thick nitrided layer. Comparative Example 4 did not add MoO3 and Nb2O5 during nitriding, resulting in insufficient nitrided layer depth and poor uniformity of the reinforcing layer. Comparative Example 5 did not add sodium nitrite during the oxidation process, and Comparative Example 6 did not add cerium nitrate during the oxidation process, both resulting in insufficient oxide layer depth and poor uniformity of the reinforcing layer.

[0082] Comparative Example 7, which underwent no nitriding or oxidation strengthening treatment, had a hardness of 421 HV and a strengthening layer depth of 0 μm. This resulted in extremely poor surface hardness and wear resistance, with a wear rate as high as 13.7 × 10⁻⁶. -7 mm 3 / (N·m), which is much higher than that of all examples and comparative samples. This is because the materials that have not undergone nitriding and oxidation treatment lack a reinforcing layer and a protective film, making their surfaces prone to wear under frictional conditions.

[0083] In scientific terms, nitriding treatment forms a hardened layer through the diffusion of nitrogen atoms, and the nitrided phase (such as ε-Fe) 2-3 Nitrides (N and γ′-Fe4N) provide high hardness, thereby improving wear resistance. Too low a nitriding temperature leads to incomplete nitriding, a thin nitrided layer, insufficient hardness, and poor wear resistance; while too high a nitriding temperature softens the matrix, affecting the overall performance of the material, reducing hardness, and increasing wear. Therefore, a reasonable nitriding temperature, appropriate nitriding time, and the formation of an oxide film are crucial factors in ensuring the surface hardness and wear resistance of the material.

[0084] In summary, the embodiments, by precisely controlling the composition of nitride and oxide salts and rationally setting the nitriding and oxidation temperatures, formed a deep and dense nitride layer and oxide film on the workpiece surface, significantly improving the material's hardness and wear resistance, and ensuring its excellent performance in high-load, long-life applications. In contrast, the comparative samples, due to insufficient nitride layer depth, incomplete oxide film, or softening of the base metal, exhibited poor hardness and wear resistance, failing to meet high-performance requirements.

[0085] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0086] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer, characterized in that, Includes the following steps: (1) Degreasing and cleaning: After the flexible wheel or rigid wheel is machined, it is degreased, cleaned and dried in sequence; (2) Preheating: Preheat the cleaned flexible or rigid wheel; (3) Salt bath nitriding: The preheated flexible or rigid wheel is nitrided to form a nitrided layer on its surface; (4) Polishing: The surface of the flexible or rigid wheel is treated by mechanical or tumbling methods; (5) Oxidation treatment: The polished flexible wheel or rigid wheel is placed in molten oxidized salt for oxidation treatment, so that an oxide layer is generated on the surface of the nitrided layer; (6) Cleaning and rust prevention: After the oxidized flexible or rigid wheel is air-cooled, it should be rinsed with clean water and dried immediately, and then coated with rust-preventive oil.

2. The QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer according to claim 1, characterized in that, In step (1), the flexible wheel is made of medium carbon alloy structural steel, selected from at least one of 40CrNiMoA, 40Cr, and 45 steel, and the rigid wheel is made of at least one of 40Cr, 45 steel, and 2Cr13 stainless steel.

3. The QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer according to claim 1, characterized in that, In step (1), isopropanol is used as the cleaning agent for degreasing and cleaning. The cleaning temperature is 25~45℃, ultrasonic cleaning is performed for 8~10 minutes, and the product is air-dried at room temperature after cleaning.

4. The QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer according to claim 1, characterized in that, In step (2), preheating is performed in a vacuum furnace at 300~350℃ for 5~15 minutes.

5. The QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer according to claim 1, characterized in that, In step (3), the nitride salts used in the salt bath nitriding are: sodium cyanate 15-20%, potassium cyanate 10-15%, sodium carbonate 10-12%, potassium carbonate 10-12%, lithium carbonate 5-6%, urea 10-15%, cerium carbonate or lanthanum oxide 2-3%, MoO3 0.5%, Nb2O5 0.3%, sodium chloride 3-5%, and potassium sulfide 0.02-0.05%, all of which are mass percentages.

6. The QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer according to claim 5, characterized in that, The nitriding temperature is 500~530℃, and the nitriding time is 60~120 min.

7. The QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer according to claim 1, characterized in that, In step (4), polishing is performed to make the surface roughness Ra ≤ 0.4 μm.

8. The QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer according to claim 1, characterized in that, In step (5), the oxidizing salts used in the oxidation treatment consist of: sodium nitrate 14-16%, sodium nitrite 1-2%, sodium carbonate 30-32%, potassium carbonate 6-8%, lithium carbonate 3-5%, sodium hydroxide 2-3%, cerium nitrate or yttrium nitrate 1%, nickel nitrate 0.5%, sodium chloride 2-3%, and sodium sulfate 1-2%, all of which are mass percentages.

9. The QPQ surface strengthening method for flexible and rigid wheels in a harmonic reducer according to claim 8, characterized in that, The oxidation temperature is 360–380 °C, and the oxidation time is 10–20 min.

10. The flexible or rigid wheel processed by the method according to any one of claims 1 to 9, characterized in that, Its surface hardness is 800~850 HV, the average depth of the total reinforced layer is 30~40μm, the range of total reinforced layer depth is ≤5μm, and the wear rate is ≤3.0×10. -7 mm 3 / (N·m).

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