40Cr steel shaft sleeve surface strengthening method based on QPQ technology

Abstract

The invention discloses a 40Cr steel shaft sleeve surface strengthening method based on a QPQ technology. According to the method, after the 40Cr steel is quenched and tempered, the steps of soaking and cleaning, drying and preheating, salt bath nitriding, salt bath oxidation, follow-up polishing and the like are sequentially carried out, and QPQ composite treatment is achieved. A compact and uniform black oxidation film can be obtained on the surface of the shaft sleeve treated through the method, and the corrosion resistance of the shaft sleeve is remarkably improved. Test results show that the surface hardness of the shaft sleeve is improved by about 170%-210% compared with that of a base body, and the friction coefficient is reduced by about 40% under the dry friction condition. The technological process is relatively simple and convenient, workpiece deformation is small, and the method can serve as a favorable alternative scheme of a traditional chromium plating surface treatment mode.

Description

Technical Field

[0001] This invention belongs to the field of machining technology, specifically relating to a surface strengthening method for 40Cr steel bushings based on QPQ technology. Background Technology

[0002] Bushings are a widely used type of basic mechanical component, typically mounted on the outer surface of rotating shafts or smooth shafts. They serve two main functions: providing static support such as axial positioning or spacing, and acting as part of a sliding bearing to generate relative movement with the shaft, thereby reducing friction and wear. As a critical protective component in mechanical transmission systems, bushings are primarily used to protect high-cost spindles. Guide bushings (or wear-resistant bushings) are a typical form of bushing, often placed in the guiding and mating parts of molds or precision machinery. Because these bushings frequently endure repeated relative sliding friction and impact loads during service, high wear resistance is required of their materials, necessitating excellent wear resistance properties.

[0003] QPQ (Quench-Polish-Quench) salt bath composite process is a surface strengthening technology developed in recent years. Its core involves sequentially performing nitriding, oxidation, and subsequent surface treatment within the same process system. This creates a composite structure on the workpiece surface consisting of a high-hardness compound layer and a nitrogen-rich diffusion layer, thus simultaneously providing high hardness, excellent wear resistance, and good corrosion resistance. Under specific temperature and holding time conditions, active nitrogen, carbon, and oxygen elements diffuse into the surface and near-surface of the steel part, generating a dense composite compound layer and a nitrogen-rich diffusion layer, significantly improving the surface performance of the parts. Compared to traditional single nitriding, electroplating, or coating processes, QPQ technology offers significant advantages such as lower processing temperature, less workpiece deformation, lower surface friction coefficient, and superior wear resistance and anti-galling properties. Summary of the Invention

[0004] The purpose of this invention is to provide a surface strengthening method for 40Cr steel bushings with superior wear and corrosion resistance.

[0005] This invention is achieved through the following scheme: A surface strengthening method for 40Cr steel bushings based on QPQ technology, characterized by the following steps: Step 1: Before the quenching and tempering treatment, the 40Cr steel bushing is solvent cleaned, rinsed with a high-pressure water gun, and then promptly put into the drying process. Step 2: Place the cleaned bushing into the preheating furnace for preheating. After preheating to the specified temperature, transfer it to the quenching furnace, heat it to the specified quenching temperature and hold it at that temperature, then immediately immerse it in the cooling medium to cool to room temperature. Step 3: The quenched bushing is immediately tempered at the specified temperature, held at that temperature for a certain time, and then cooled to room temperature in the furnace to complete the base tempering treatment. Step 4: Before salt bath nitriding, perform final cleaning and preheating of the bushing. After rinsing, preheat it in an oven at a specified temperature for a certain period of time to ensure that the workpiece surface is clean, dry, and free of oil and rust. Step 5: Place the preheated 40Cr steel bushing into molten nitride salt for nitriding treatment; Step 6: Immediately after nitriding, transfer the bushing into an oxidation furnace and keep it at a temperature for a period of time to form a dense black oxide film on the surface; Step 7: Perform air cooling, desalination cleaning, and drying on the bushing again.

[0006] Step 8: Polish using sandblasting.

[0007] Specifically, the 40Cr steel raw material used in step 1 has a carbon content of 0.37%-0.44%; a silicon content of 0.17%-0.37%; a manganese content of 0.5%-0.8%; a chromium content of 0.8%-1.1%; and sulfur and phosphorus, as impurity elements, must be controlled within the specified allowable range. The steel surface should be free of defects such as cracks, folds, and delamination.

[0008] Specifically, the cleaning described in step 1 above refers to immersing the bushing in a water tank containing a degreaser (the degreaser is diluted at a ratio of 1:10 and the pH value is not lower than 10) for 30-40 minutes, and finally rinsing it with clean water for 10 minutes.

[0009] Specifically, the drying process described in step 1 above involves drying at 80±5℃ for 20 minutes.

[0010] Specifically, the quenching temperature in step 2 above is strictly controlled within 840℃±10℃, the quenching time is 90min, the cooling medium is rapid bright quenching oil 821, and the cooling oil temperature is controlled between 40℃ and 110℃ to ensure that a uniform martensitic structure can be obtained. The performance is optimal when the cooling oil temperature is 60℃.

[0011] Specifically, the tempering temperature mentioned in step 3 above is controlled at 590℃ or above, the tempering time is 120 minutes, and after tempering, air cooling is performed to transform the microstructure into tempered sorbite and eliminate quenching stress.

[0012] Specifically, the cleaning in step 4 is the same as in step 1, and the subsequent preheating temperature is 380℃. After holding at this temperature for 60 minutes, the product is immediately transferred to the nitriding furnace.

[0013] Specifically, in step 5 above, the CNO- concentration in the molten salt is controlled at 32%-34%, the nitriding temperature is 580℃-620℃, and the nitriding time is 90-120min. During the heat preservation process, the product in the furnace needs to be lifted up and down 7-10 times every 50-60min, and the furnace cover should be closed immediately after completion.

[0014] Specifically, in step 6 above, the oxidation temperature is 380℃ and the oxidation time is 20-40 minutes; the salt should be slowly lowered into the oxidation furnace, not too quickly, to prevent the oxidized salt and nitride salt from reacting too quickly.

[0015] Specifically, the cleaning described in step 7 above refers to immersing the bushing in a soaking tank for 30 minutes, then rinsing it with tap water and drying it.

[0016] Specifically, in step 8 above, corundum sand is selected as the sandblasting medium, the sandblasting pressure is controlled at 0.2-0.6MPa and adjusted according to the bushing wall thickness and strength; the angle between the sandblasting gun and the inner wall surface is controlled at 45°-75° to ensure full coverage of the inner surface and avoid excessive local impact; the sandblasting time is controlled at 5-20min and can be dynamically adjusted according to the surface condition.

[0017] The beneficial effects of this invention are as follows: By optimizing the nitriding temperature, time, and oxidation stage temperature-time in the QPQ process, highly matched process parameters have been successfully developed. This allows a large number of active nitrogen atoms to be diffused into the bushing surface while a black oxide film forms on its surface. The bushing achieves high hardness, high wear resistance, and high corrosion resistance while significantly reducing processing deformation and residual stress. Unlike the hexavalent chromium contamination inherent in chromium plating, the surface modification process employed in this invention achieves performance enhancement without introducing toxic or harmful substances, thus better meeting the requirements of green manufacturing and sustainable development. Attached Figure Description

[0018] Figure 1 This is a cross-sectional microstructure of a 40Cr steel bushing based on QPQ technology according to the present invention; Figure 2 This is a comparison diagram of the surface hardness (left) and cross-sectional gradient hardness (right) of a 40Cr steel bushing based on QPQ technology according to the present invention. Figure 3 This is a comparison chart of the friction coefficients of a 40Cr steel bushing based on QPQ technology according to the present invention. Figure 4 This is a comparative diagram of electrochemical corrosion of a 40Cr steel bushing based on QPQ technology according to the present invention. Detailed Implementation

[0019] The present invention will be further illustrated below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field.

[0020] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0021] Example 1

[0022] A surface strengthening method for 40Cr steel bushings based on QPQ technology, which is carried out according to the following process: The first step is to lower the fixture containing the 40Cr steel bushing into the degreasing agent tank. Dilute the degreasing agent with water at a 1:10 mass ratio, ensuring the liquid level is at least 50mm above the highest point of the fixture. The working solution temperature should be 40-60℃. Immerse for 30-40 minutes, gently raising or lowering the fixture or agitating it to remove surface oil. After degreasing, remove the fixture from the degreasing agent tank, allow it to drip dry for 1-2 minutes, and then lower it into a clean water tank for rinsing. Use tap water or deionized water at a temperature of 20-40℃, ensuring the liquid level is at least 50mm above the highest point of the fixture. Rinse for 8-12 minutes, gently shaking the fixture to help remove any remaining degreasing agent, ensuring minimal residue on the product surface.

[0023] The second step involves placing the cleaned 40Cr steel bushing from the first step into a heating furnace for quenching. The quenching temperature is controlled at 840℃±10℃, and the holding time is 90 minutes to ensure that the temperature inside and outside the bushing cross-section is sufficiently uniform. After heating, the bushing is immediately removed from the furnace and immersed in rapid bright quenching oil 821 for cooling, with the quenching oil temperature controlled within the range of 40-110℃.

[0024] The third step is to temper the quenched bushing promptly. The tempering temperature should not be lower than 590℃, preferably controlled within the range of 590-620℃, and the holding time should be 120 minutes. During the tempering process, the furnace temperature should be kept uniform and stable to ensure that the bushing is heated evenly. After tempering, the bushing should be removed from the furnace and air-cooled to allow the quenched martensite structure to transform into tempered sorbite, thus eliminating quenching stress.

[0025] The fourth step is to soak, clean, and dry the tempered bushing as in the first step. After confirming that the preheating furnace temperature has reached the preset 380℃, hoist the tooling with the bushing into the preheating furnace and keep it at that temperature for 60 minutes.

[0026] Fifth, confirm that the temperature inside the nitriding furnace has reached the preset 580℃; after inserting the shaft sleeve into the nitriding furnace, start timing and hold it at that temperature for 90 minutes. During the holding process, lift the shaft sleeve up and down 7-10 times every 50-60 minutes, even if it is briefly exposed to the liquid surface; at the same time, ensure that the CNO⁻ concentration inside the furnace is maintained at 32-34%.

[0027] Step 6: After confirming that the temperature inside the oxidation furnace has reached the preset 380℃, transfer the bushing from step 5 into the oxidation furnace. Lower it into the furnace slowly in sections: lower only a small section at a time, waiting for the reaction between the oxide salt and the residual nitride salt to stabilize and for the salt solution surface to stop churning before continuing to lower it until the bushing is completely submerged in the salt solution. Keep the bushing at this temperature inside the oxidation furnace for 30 minutes.

[0028] The seventh step is to clean, dry, and cool the oxidized bushing, just like in the first step.

[0029] Step 8: Select corundum sand as the sandblasting medium, control the sandblasting pressure at 0.2-0.6MPa, and adjust it according to the sleeve wall thickness and strength; control the angle between the sandblasting gun and the inner wall surface at 45°-75° to ensure full coverage of the inner surface and avoid excessive local impact; control the sandblasting time at 5-20 minutes, which can be dynamically adjusted according to the surface condition.

[0030] Example 2

[0031] This embodiment is based on Embodiment 1, and the difference between it and Embodiment 1 is as follows: (1) In the fifth step, the bushing is kept in the nitriding furnace for 120 minutes.

[0032] Example 3

[0033] This embodiment is based on Embodiment 1, and the difference between it and Embodiment 1 is as follows: (1) In the fifth step, the nitriding temperature of the bushing in the nitriding furnace is 620℃ and the holding time is 90min.

[0034] Example 4

[0035] This embodiment is based on Embodiment 1, and the difference between it and Embodiment 1 is as follows: (1) In the fifth step, the nitriding temperature of the bushing in the nitriding furnace is 620℃ and the holding time is 120min.

[0036] Comparative Example 1 This comparative example is a bushing that has not undergone QPQ treatment.

[0037] 1. The microstructure of the bushing cross-section after treatment in Examples 1-4 is shown in the figure. Figure 1 . Figure 1The results show that the process of this invention forms a gradient strengthening layer structure on the surface of the 40Cr bushing, consisting of a dense oxide film layer, a composite compound layer, a diffusion layer, and a tempered sorbite core, from the outside in. The transitions between layers are smooth, and the interface bonding is good. In Examples 1-4, the thickness of the surface composite compound layer is generally controlled between 10 and 30 μm, and the thickness of the diffusion layer is approximately 0.25 to 0.35 mm, which meets relevant standards. While ensuring the toughness and dimensional stability of the core, the surface hardness and wear resistance are significantly improved. In Examples 3 and 4, the compound layer thickness is relatively thick because the higher temperature promotes the nitrogen diffusion rate; at the same time, the extended holding time also provides more nitrogen to diffuse into the matrix.

[0038] 2. The bushings of the examples and comparative examples were subjected to Vickers hardness tests, and the results are shown in the figure. Figure 2 The results show that the surface hardness of the QPQ-treated bushing can reach up to 666 HV0.2, which is significantly higher than that of the untreated bushing. The QPQ-treated bushing exhibits a gradient hardness distribution from the surface to the core: the surface hardness remains at 500–650 HV0.2 within the 0–0.1 mm range, the hardness in the 0.1–0.3 mm diffusion layer is approximately 350–550 HV0.2, and then gradually transitions to the tempered sorbite hardness of approximately 250–320 HV0.2 in the core. These results demonstrate that the QPQ composite treatment process of this invention forms a high-hardness gradient reinforcement layer on the bushing surface without significantly reducing the core toughness, thus greatly improving the surface wear resistance and fatigue resistance of the bushing.

[0039] 3. Dry friction tests were conducted on the bushings of the examples and comparative examples. The results are shown in [the table below]. Figure 3 The results showed that the friction coefficients of the QPQ-treated bushings in all embodiments were significantly lower than those of the untreated comparative bushings throughout the dry friction test, with smaller curve fluctuations and better tribological stability. The friction coefficient of the untreated bushings was approximately 0.55–0.60 in the stable phase, while the stable friction coefficients of Examples 1–4 were controlled at 0.35–0.45 under the same operating conditions, a reduction of approximately 30%–40% compared to the comparative examples. This indicates that the composite compound layer and dense oxide film layer formed on the bushing surface can effectively reduce the friction coefficient, suppress adhesive wear, and improve the stability of the friction pair during operation, thereby significantly improving the wear resistance and service reliability of the bushings.

[0040] 4. A portion of the bushing treated in the examples and comparative examples was subjected to electrochemical corrosion testing. The test was conducted on a Chenhua electrochemical workstation using a three-electrode system with a 3.5% NaCl solution. A Pt electrode was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode. The results are shown in […]. Figure 4 Table 1. From Figure 4The corresponding electrochemical parameters show that, compared to the untreated bushing (corrosion potential Ecorr is -0.9426V, corrosion current density Icorr is 4.51×10⁻⁻⁻⁶), the corrosion resistance is significantly lower. 4 Compared to A, where the polarization resistance Rp is only 179Ω, the corrosion potential of the bushings in each embodiment has shifted significantly to the positive, the corrosion current has decreased by about two orders of magnitude, the polarization resistance has increased by about 75 to 137 times, and the corrosion resistance has been significantly improved.

[0041] 1. The microstructure of the bushing cross-section after treatment in Examples 1-4 is shown in the figure. Figure 1 . Figure 1 The results show that the process of this invention forms a gradient strengthening layer structure on the surface of the 40Cr bushing, consisting of a dense oxide film layer, a composite compound layer, a diffusion layer, and a tempered sorbite core, from the outside in. The transitions between layers are smooth, and the interface bonding is good. In Examples 1-4, the thickness of the surface composite compound layer is generally controlled between 10 and 30 μm, and the thickness of the diffusion layer is approximately 0.25 to 0.35 mm, which meets relevant standards. While ensuring the toughness and dimensional stability of the core, the surface hardness and wear resistance are significantly improved. In Examples 3 and 4, the compound layer thickness is relatively thick because the higher temperature promotes the nitrogen diffusion rate; at the same time, the extended holding time also provides more nitrogen to diffuse into the matrix.

[0042] 2. The bushings of the examples and comparative examples were subjected to Vickers hardness tests, and the results are shown in the figure. Figure 2 The results show that the surface hardness of the QPQ-treated bushing can reach up to 666 HV0.2, which is significantly higher than that of the untreated bushing. The QPQ-treated bushing exhibits a gradient hardness distribution from the surface to the core: the surface hardness remains at 500–650 HV0.2 within the 0–0.1 mm range, the hardness in the 0.1–0.3 mm diffusion layer is approximately 350–550 HV0.2, and then gradually transitions to the tempered sorbite hardness of approximately 250–320 HV0.2 in the core. These results demonstrate that the QPQ composite treatment process of this invention forms a high-hardness gradient reinforcement layer on the bushing surface without significantly reducing the core toughness, thus greatly improving the surface wear resistance and fatigue resistance of the bushing.

[0043] 3. Dry friction tests were conducted on the bushings of the examples and comparative examples. The results are shown in [the table below]. Figure 3The results showed that the friction coefficients of the QPQ-treated bushings in all embodiments were significantly lower than those of the untreated comparative bushings throughout the dry friction test, with smaller curve fluctuations and better tribological stability. The friction coefficient of the untreated bushings was approximately 0.55–0.60 in the stable phase, while the stable friction coefficients of Examples 1–4 were controlled at 0.35–0.45 under the same operating conditions, a reduction of approximately 30%–40% compared to the comparative examples. This indicates that the composite compound layer and dense oxide film layer formed on the bushing surface can effectively reduce the friction coefficient, suppress adhesive wear, and improve the stability of the friction pair during operation, thereby significantly improving the wear resistance and service reliability of the bushings.

[0044] 4. A portion of the bushing treated in the examples and comparative examples was subjected to electrochemical corrosion testing. The test was conducted on a Chenhua electrochemical workstation using a three-electrode system with a 3.5% NaCl solution. A Pt electrode was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode. The results are shown in […]. Figure 4 Table 1. From Figure 4 The corresponding electrochemical parameters show that, compared to the untreated bushing (corrosion potential Ecorr is -0.9426V, corrosion current density Icorr is 4.51×10⁻⁻⁻⁶), the corrosion resistance is significantly lower. 4 Compared to A, where the polarization resistance Rp is only 179Ω, the corrosion potential of the bushings in each embodiment has shifted significantly to the positive, the corrosion current has decreased by about two orders of magnitude, the polarization resistance has increased by about 75 to 137 times, and the corrosion resistance has been significantly improved.

[0045] Table 1 Polarization curve test data

[0046] In summary, the surface strengthening method for 40Cr steel bushings based on QPQ technology proposed in this invention constructs a composite functional layer structure with a gradual transition from the outside to the inside on the bushing surface, resulting in strong interlayer bonding and smooth transitions. Verification through examples shows that this invention's process can significantly improve surface hardness and load-bearing capacity while maintaining the bushing's core toughness and dimensional stability, significantly reducing the coefficient of friction, improving stability during friction, and effectively suppressing adhesive wear. Furthermore, electrochemical corrosion testing demonstrates that the compound layer and dense oxide film formed by this invention effectively block corrosive media from eroding the substrate, significantly improving the bushing's reliability and service life in corrosive environments.

Claims

1. A surface strengthening method for 40Cr steel bushings based on QPQ technology, characterized in that: The following steps are included: Step 1: Before the quenching and tempering treatment, the 40Cr steel bushing is solvent cleaned, rinsed with a high-pressure water gun, and then promptly put into the drying process. Step 2: Place the cleaned bushing into the preheating furnace for preheating. After preheating to the specified temperature, transfer it to the quenching furnace, heat it to the specified quenching temperature and hold it at that temperature, then immediately immerse it in the cooling medium to cool. Step 3: The quenched bushing is immediately tempered at the specified temperature, held at that temperature for a certain time, and then cooled to room temperature in the furnace to complete the base tempering treatment. Step 4: Before salt bath nitriding, perform final cleaning and preheating of the bushing. After rinsing, preheat it in an oven at a specified temperature for a certain period of time to ensure that the workpiece surface is clean, dry, and free of oil and rust. Step 5: Place the preheated 40Cr steel bushing into molten nitride salt for nitriding treatment; Step 6: Immediately after nitriding, transfer the bushing into an oxidation furnace and keep it at a temperature for a period of time to form a dense black oxide film on the surface; Step 7: Perform air cooling, desalination cleaning, and drying on the bushing again; Step 8: Polish using sandblasting.

2. The surface strengthening method for 40Cr steel bushings based on QPQ technology according to claim 1, characterized in that: The 40Cr steel raw materials used in step 1 have a carbon content of 0.37%-0.44%, a silicon content of 0.17%-0.37%, a manganese content of 0.5%-0.8%, a chromium content of 0.8%-1.1%, and sulfur and phosphorus, as impurity elements, must be controlled within the specified allowable range.

3. The surface strengthening method for 40Cr steel bushings based on QPQ technology according to claim 1, characterized in that: The cleaning described in step 1 refers to immersing the bushing in a water tank containing a degreaser (the dilution ratio of the degreaser is 1:10) for 30-40 minutes, and finally rinsing it with clean water for 10 minutes.

4. The surface strengthening method for 40Cr steel bushings based on QPQ technology according to claim 1, characterized in that: The drying process described in step 1 involves drying at 80±5℃ for 20 minutes.

5. The surface strengthening method for 40Cr steel bushings based on QPQ technology according to claim 1, characterized in that: The quenching temperature in step 2 is strictly controlled within 840℃±10℃, the quenching time is 90min, and the cooling medium is rapid bright quenching oil 821.

6. The surface strengthening method for 40Cr steel bushings based on QPQ technology according to claim 1, characterized in that: The tempering temperature mentioned in step 3 is controlled at 590℃ or above, the tempering time is 120 minutes, and air cooling is performed after tempering.

7. The surface strengthening method for 40Cr steel bushings based on QPQ technology according to claim 1, characterized in that: The cleaning process in step 4 is the same as in step 1, and the preheating temperature is 380℃.

8. The surface strengthening method for 40Cr steel bushings based on QPQ technology according to claim 1, characterized in that: In step 5, the CNO- concentration in the molten salt is controlled at 32%-34%, the nitriding temperature is 580℃-620℃, and the nitriding time is 90-120min.

9. The surface strengthening method for 40Cr steel bushings based on QPQ technology according to claim 1, characterized in that: In step 6, the oxidation temperature is 380℃ and the oxidation time is 20-40 min.

10. The surface strengthening method for 40Cr steel bushings based on QPQ technology according to claim 1, characterized in that: The cleaning described in step 7 refers to immersing the bushing in a soaking tank for 30 minutes, then rinsing it with tap water and drying it. The polishing in step 8 is done by sandblasting, using corundum sand as the sandblasting medium. The sandblasting pressure is controlled at 0.2 to 0.6 MPa, the angle between the sandblasting gun and the inner wall surface is controlled at 45° to 75°, and the sandblasting time is controlled at 5 to 20 minutes.

Images