Quenching method for improving surface compressive stress of carburizing steel bearing part

Abstract

The invention relates to a quenching method for improving surface compressive stress of a carburizing steel bearing part, and belongs to the technical field of bearing heat treatment. The quenching method comprises the step of carrying out graded cooling on a to-be-quenched carburizing steel bearing part, and the graded cooling comprises the following steps: carrying out primary cooling on the to-be-quenched carburizing steel bearing part until the core temperature of the part is reduced to an Ms point, and then carrying out secondary cooling until the surface temperature of the part is reduced to be below the Ms point; the cooling speed of the first-stage cooling is higher than that of the second-stage cooling. According to the quenching method for improving the surface compressive stress of the carburized steel bearing part, the surface residual compressive stress can be effectively improved, and the anti-fatigue life of the bearing part is prolonged.

Description

Technical Field

[0001] The invention relates to a quenching method for increasing the surface compressive stress of a carburized steel bearing part, belonging to the technical field of bearing heat treatment. Background Art

[0002] Bearings are important components that secure and reduce load friction during mechanical transmission. The trend toward more integrated bearings and the demand for higher performance are driving higher requirements for bearing performance. These requirements require not only high surface hardness and wear resistance, but also excellent high-temperature strength and structural toughness.

[0003] In the existing technology, the performance of bearing parts is often improved through the carburizing process. However, after carburizing, the carbon content of the bearing surface increases due to the infiltration of carbon atoms, the martensite transformation temperature Ms is lowered, and the martensite transformation incubation time is prolonged. In the subsequent heat treatment process, the residual compressive stress on the surface of the part is easily reduced or the properties change, thereby reducing the fatigue life of the bearing parts. Summary of the Invention

[0004] The object of the present invention is to provide a quenching method for increasing the surface compressive stress of a carburized steel bearing part, thereby solving the problem that the existing quenching method is difficult to effectively improve the residual compressive stress on the surface of the bearing part.

[0005] In order to achieve the above-mentioned object, the technical solution adopted by the quenching method for increasing the surface compressive stress of a carburized steel bearing part of the present invention is as follows: A quenching method for increasing the surface compressive stress of a carburized steel bearing part includes a step of performing graded cooling on the carburized steel bearing part to be quenched. The graded cooling includes first performing a primary cooling on the carburized steel bearing part to be quenched until the core temperature of the part drops to the Ms point, and then performing a secondary cooling on the part until the surface temperature of the part drops below the Ms point; the cooling rate of the primary cooling is greater than the cooling rate of the secondary cooling.

[0006] The quenching method for increasing the surface compressive stress of a carburized steel bearing part of the present invention utilizes the inconsistency between the Ms point of the core and the surface during the quenching process to adjust the cooling rate and realize a graded cooling process of first fast cooling and then slow cooling. During the quenching cooling process, a sufficiently large temperature difference can be ensured between the surface and the core of the part. After the part is quenched, a high residual compressive stress is formed on the surface. At the same time, a sufficient cooling rate is ensured between the surface and the core to form a martensitic phase transformation and a high hardness.

[0007] The quenching method for increasing the surface compressive stress of a carburized steel bearing part of the present invention can effectively increase the surface residual compressive stress and improve the fatigue life of the bearing part.

[0008] Preferably, both primary and secondary cooling are performed in an oil bath. During primary cooling, the bath is stirred and the parts are moved up and down. During secondary cooling, stirring is turned off and the parts are kept stationary. Further preferably, the oil temperature in the bath is 60±5°C. More preferably, the primary cooling duration is 2-3 minutes, and the secondary cooling process cools the parts to 90±5°C before they are removed from the bath.

[0009] Preferably, the secondary cooling is followed by water cooling in water at 5-10° C. More preferably, the water cooling time is 20-30 minutes.

[0010] Preferably, the holding temperature of the carburized steel bearing component to be quenched before quenching is 805-810°C.

[0011] Further preferably, the carburized steel bearing component is G20Cr2Ni4A steel, and the depth of the carburized layer is 4.0-5.0 mm.

[0012] The beneficial technical effect of the present invention is: by adjusting the cooling speed, a graded cooling process of first fast cooling and then slow cooling is realized, and the secondary water cooling method is added to effectively increase the surface residual compressive stress and improve the fatigue life of bearing parts. DETAILED DESCRIPTION

[0013] The quenching method for increasing the surface compressive stress of a carburized steel bearing part of the present invention is a pioneering invention.

[0014] After carburizing steel bearing parts, the carbon content in the surface increases due to the infiltration of carbon atoms, the martensitic transformation temperature, Ms, decreases, and the martensitic transformation incubation time is prolonged. During quenching, under normal circumstances, the core undergoes martensitic transformation first due to its high Ms point, and the volume expands. At this time, the surface is still in an austenite state with good plasticity, resulting in plastic deformation. When further cooled to below the Ms point of the surface, the surface undergoes martensitic transformation and volume expansion, but because the core has become martensite, it is difficult to produce plastic deformation, thus forming residual compressive stress on the surface. However, due to the influence of cooling rate and part size, during the cooling process, it is not possible to ensure that a sufficiently large temperature difference is formed between the surface and the core of the part, and it is impossible to ensure that the core of the part undergoes and completes the martensitic phase transformation earlier than the surface, which ultimately affects the nature and magnitude of the residual compressive stress on the part surface.

[0015] Among them, the Ms point is the critical temperature point where austenite begins to transform into martensite during the quenching and cooling process of steel, that is, the starting temperature point of transformation.

[0016] The technical concept of this invention is to use rapid cooling to reduce the core temperature of the part to below the Ms point during the initial quenching process, followed by slow cooling to induce martensitic transformation on the surface. During the quenching process, a sufficiently large temperature difference is maintained between the surface and the core of the part. After quenching, high residual compressive stress forms on the surface of the part. At the same time, sufficient cooling rates are ensured between the surface and the core, allowing for martensitic transformation and high hardness.

[0017] Generally speaking, the preparation of carburizing steel bearing parts includes carburizing, primary quenching, primary tempering, heating and insulation before quenching, secondary quenching treatment, and secondary tempering.

[0018] The primary quenching process is to rapidly cool the bearing components to obtain a martensitic structure and avoid the formation of network carbides. Generally, the bearing components are cooled to below 900°C and then subjected to oil or salt quenching. Preferably, the temperature is lowered to 880-900°C and held for 1-2 hours.

[0019] The primary tempering is mainly used to eliminate the retained austenite and refine the structure. The primary tempering can be kept at 650±10°C for 12h and then kept at 600±10°C for 12h.

[0020] For G20Cr2Ni4A steel, the heating and holding temperature before quenching can be controlled at 805~810℃ and the time is 2~3h.

[0021] The secondary quenching treatment is carried out using a staged cooling process of rapid cooling followed by slow cooling, followed by water cooling. Oil quenching can be used for both rapid and slow cooling. During rapid cooling, the oil tank is agitated and the parts are moved up and down. After 2-3 minutes, the oil tank is closed and allowed to stand for slow cooling until the temperature drops to 90±5°C. The water temperature for water cooling is 5-10°C, and the cooling time is 20-30 minutes.

[0022] The temperature of the secondary tempering can be controlled at 160±5℃ and the time is 12h±0.5h.

[0023] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0024] 1. Specific embodiment of the quenching method for increasing surface compressive stress of a carburized steel bearing component of the present invention Example 1 The quenching method for increasing the surface compressive stress of a carburized steel bearing part in this embodiment is a secondary quenching treatment for the carburized steel bearing part. The typical processing steps are described in detail as follows: Carburizing: The bearing jacket (made of G20Cr2Ni4A steel) is placed in a multi-purpose furnace for carburizing. When the carburized layer reaches a depth of 4.8mm, the bearing is removed from the furnace to obtain a carburized bearing part. The part dimensions are Φ580*Φ535*220mm.

[0025] Primary quenching treatment: After the carburizing of the bearing parts is completed, the bearing parts are cooled to 880℃ and kept warm for 1 hour. The bearing parts are then transferred to the salt tank for quenching. Before quenching, stirring is turned on at a speed of 1000rmp / min. After cooling for 40 minutes, the parts are taken out of the furnace for air cooling and cleaning.

[0026] Heating and insulation before primary tempering and quenching: Carburized bearing parts are subjected to primary tempering. Specifically, they are first kept at 650℃ for 12 hours, and then kept at 600℃ for 12 hours. After that, the parts after primary tempering are loaded into a pit furnace, heated to 810℃ and kept at this temperature for 2.5 hours, and then taken out of the furnace and put into the mold.

[0027] Secondary quenching treatment: transfer the workpiece into the oil tank with the oil temperature at 60℃, and start the oil tank stirring at the same time with a stirring speed of 1000rmp / min. At the same time, use the overhead crane to move the workpiece up and down to increase the cooling speed and realize rapid cooling of the workpiece. The cooling time is 3min. Then turn off the stirring of the oil tank and continue to let the workpiece stand in the oil tank for slow cooling. When the workpiece cools to 90℃, take it out of the oil tank. After the workpiece is removed from the oil tank, it is transferred to the water tank with a water temperature of 8℃. The workpiece is cooled in the water tank for 25min. After cooling is completed (basically cooled to the same temperature as the water, sometimes higher than the water temperature by less than 5℃), transfer the workpiece into the cleaning tank to clean the oil stains.

[0028] Secondary tempering: Transfer the cleaned workpiece to the tempering furnace, maintain it at 160℃ for 12 hours to complete stress relief tempering, and then take it out of the furnace.

[0029] In other implementation situations of this embodiment, during the quenching treatment stage, the rapid cooling treatment method may be to increase the stirring speed, increase the frequency of the movement, etc., and the slow cooling treatment method may be to reduce the stirring speed or stop stirring or stop the movement, etc.

[0030] Example 2 The quenching method for increasing the surface compressive stress of a carburized steel bearing part in this embodiment is a secondary quenching treatment for the carburized steel bearing part. The typical processing steps are described in detail as follows: Carburizing: The bearing jacket (made of G20Cr2Ni4A steel) is placed in a multi-purpose furnace for carburizing. The carburized layer is removed from the furnace to a depth of 4.0 mm, resulting in a carburized bearing component. The component dimensions are Φ620*Φ582*243 mm.

[0031] Primary quenching treatment: same as in Example 1.

[0032] Primary tempering and heating and insulation before quenching: same as in Example 1.

[0033] Secondary quenching treatment: same as in Example 1.

[0034] Secondary tempering: same as in Example 1.

[0035] 2. Comparative Example Comparative Example 1 The quenching method for increasing the surface compressive stress of carburized steel bearing parts in this comparative example is a secondary quenching treatment for carburized steel bearing parts. The typical processing steps are described as follows: Carburizing: Same as Example 1.

[0036] Primary quenching treatment: same as in Example 1.

[0037] Primary tempering and heating and insulation before quenching: same as in Example 1.

[0038] Secondary quenching treatment: transfer the workpiece into the oil tank (the oil temperature in the oil tank is constant at 60℃), turn on the oil tank stirring at the same time, the stirring speed is 500r / min, no movement, cool for 3 minutes, then turn off the oil tank stirring, continue to let the workpiece stand in the oil tank for slow cooling treatment, when the workpiece cools to 90℃, remove from the oil tank; after removing from the oil tank, remove the mold and clean the oil stains.

[0039] Secondary tempering: same as in Example 1.

[0040] Comparative Example 2 The quenching method for increasing the surface compressive stress of carburized steel bearing parts in this comparative example is a secondary quenching treatment for carburized steel bearing parts. The typical processing steps are described as follows: Carburizing: Same as Example 2.

[0041] Primary quenching treatment: same as in Example 1.

[0042] Primary tempering and heating and insulation before quenching: same as in Example 1.

[0043] Secondary quenching treatment: same as Comparative Example 1.

[0044] Secondary tempering: same as in Example 1.

[0045] 3. Experimental Examples This example evaluates the treatment effect of each embodiment and comparative example on bearing parts, and tests the surface hardness and surface residual compressive stress of the bearing sleeves obtained by the treatment of each embodiment and comparative example respectively. The surface hardness is tested in accordance with JB / T 7361-2023 "Test Method for Hardness of Rolling Bearing Parts", and the surface residual stress is tested in accordance with GB / T 7704-2017 "Non-destructive Testing X-ray Stress Determination Method". The results are shown in Table 1.

[0046] Table 1 Surface hardness and surface residual compressive stress test results of various embodiments and comparative examples It can be seen from the experimental results in Table 1 that the carburized steel bearing parts quenched according to the method of the present invention have higher surface hardness and surface residual compressive stress, which can better improve the fatigue life of the bearing parts.

Claims

1. A quenching method for increasing surface compressive stress of a carburized steel bearing part, characterized in that: The method comprises the steps of performing graded cooling on the carburized steel bearing parts to be quenched, wherein the graded cooling comprises firstly performing primary cooling on the carburized steel bearing parts to be quenched until the core temperature of the parts drops to the Ms point, and then performing secondary cooling on the bearing parts until the surface temperature of the parts drops below the Ms point; the cooling rate of the primary cooling is greater than the cooling rate of the secondary cooling.

2. The quenching method for increasing surface compressive stress of a carburized steel bearing component according to claim 1, wherein: The primary cooling and the secondary cooling are both carried out in an oil tank. During the primary cooling, the oil tank is turned on for stirring and the parts are moved up and down. During the secondary cooling, the oil tank is turned off for stirring and the parts are kept still.

3. The quenching method for increasing surface compressive stress of a carburized steel bearing component according to claim 2, wherein: The oil temperature of the oil tank is 60±5°C.

4. The quenching method for increasing surface compressive stress of a carburized steel bearing component according to claim 3, wherein: The first-stage cooling time is 2 to 3 minutes, and the second-stage cooling cools the parts to 90±5°C before exiting the oil tank.

5. The quenching method for increasing surface compressive stress of a carburized steel bearing component according to claim 1, 2, 3 or 4, characterized in that: After the secondary cooling, water cooling is performed in water at 5-10°C.

6. The quenching method for increasing surface compressive stress of a carburized steel bearing component according to claim 5, characterized in that: The water cooling time is 20 to 30 minutes.

7. The quenching method for increasing surface compressive stress of a carburized steel bearing component according to claim 1, wherein: The holding temperature of the carburized steel bearing component to be quenched before quenching is 805-810°C.

8. The quenching method for increasing surface compressive stress of a carburized steel bearing component according to claim 7, wherein: The carburized steel bearing parts are made of G20Cr2Ni4A steel, and the depth of the carburized layer is 4.0-5.0 mm.

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