A heat treatment process for cage carburizing, slow cooling, and secondary reheating quenching

Abstract

The application discloses a heat treatment process for carburizing and slow cooling secondary reheating quenching of a retainer, and the process comprises carburizing and slow cooling, air cooling circulation, secondary heating, oil bath quenching and low-temperature tempering, specifically, after high-temperature carburizing of a workpiece in a sealed box-type multi-purpose furnace, the workpiece is slowly cooled to 450-550 DEG C under high-pressure nitrogen protection in a front chamber to realize normalizing effect, then the workpiece is sent into a rear chamber again to be heated to 860 DEG C and kept, and finally, oil bath quenching and low-temperature tempering are carried out. The carburizing and normalizing process are continuously completed in the sealed box-type multi-purpose furnace, so that the workpiece surface decarburization is effectively avoided, the problems of local coarse structure of raw materials and coarse structure inheritance are solved, the structure is refined and uniform after secondary heating and quenching, the toughness and dimensional stability of the retainer are significantly improved, and the application is especially suitable for thin-walled parts with extremely small machining allowance.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology, and in particular to a heat treatment process for carburizing, slow cooling, and secondary reheat quenching of cages. Background Technology

[0002] Automotive CV joint cages are lightweight components, often using 20CrMnTi as the raw material. In conventional carburizing and quenching processes, defects in the raw material itself or insufficient pre-normalizing treatment frequently lead to problems such as localized coarse martensite and inherited coarse microstructure within the workpiece. After conventional carburizing and quenching, not only will the metallographic structure be substandard, but it can also cause serious quality issues such as excessive workpiece deformation and insufficient toughness.

[0003] In actual production, when the cage processing reaches the carburizing stage, it is basically the final process, and the machining allowance is usually no more than 0.05mm. If the conventional re-normalizing process is used to improve the microstructure, not only will a decarburized layer exceeding the machining allowance be generated on the workpiece surface, but it will also cause the cage to warp and deform out of control, resulting in a large number of scraps and huge economic losses. Summary of the Invention

[0004] This invention provides a heat treatment process for carburizing, slow cooling, and secondary reheating and quenching of cages, which solves the problems of unqualified metallographic structure, large deformation, and poor toughness after carburizing and quenching due to defects in the structure of raw materials in the prior art. It provides a heat treatment process for carburizing, slow cooling, and secondary reheating and quenching of cages to achieve microstructure refinement and homogenization, and avoid decarburization and deformation.

[0005] The technical solution of the present invention is as follows: A heat treatment process for carburizing, slow cooling, and secondary reheat quenching of a cage includes the following steps:

[0006] Step S1: Carburizing and slow cooling. The workpiece is placed in the rear chamber of a sealed box-type multi-purpose furnace and heated to 910°C for carburizing. Then, it is cooled to 870°C and kept at that temperature. After that, the workpiece is moved to the front chamber and slowly cooled to 450-550°C under high-pressure nitrogen protection and circulating air cooling conditions.

[0007] Step S2: Secondary heating. The workpiece, after slow cooling, is sent back into the rear chamber and heated to 860°C and held at that temperature.

[0008] Step S3: Oil bath quenching. After the workpiece is heated twice, it is moved to the front chamber and immersed in isothermal graded oil at 110-130℃ for cooling.

[0009] Step S4: Low-temperature tempering. Temper the quenched workpiece at 160-180℃.

[0010] Preferably, step S1, slow cooling of carburizing, specifically involves: placing the workpiece into the rear chamber, heating it to 780°C and holding it at that temperature for 30 minutes; carburizing at 910°C with a carbon potential CP of 1.15% for 120-150 minutes; then cooling it to 870°C with a carbon potential CP of 0.9% and isothermaling for 30 minutes; subsequently placing the workpiece in the upper layer of the front chamber, introducing high-pressure nitrogen gas at a flow rate of 6-10 m³ / h, and turning on the front chamber circulation fan to cool it down for 150 minutes, reducing the workpiece temperature to 450-550°C.

[0011] In the post-heating stage of the slow cooling carburizing process, the working atmosphere inside the furnace is composed of two active gases: methanol and propane. Methanol serves as the carrier gas, and after cracking, it forms a protective base atmosphere to prevent decarburization of the workpiece. Propane serves as the enrichment gas, which decomposes into active carbon atoms to precisely control and maintain the carbon potential inside the furnace. The methanol flow rate is 4.0-6.0 l / h, and the propane flow rate is 0.6-0.8 l / h.

[0012] Preferably, the secondary heating in step S2 is as follows: the workpiece is sent back into the rear chamber, heated to 780°C and held for 30 minutes; then heated to 860°C with a carbon potential CP of 0.8% and held for 70-120 minutes; then cooled to 830°C with a carbon potential CP of 0.75% and held at the same temperature for 30 minutes.

[0013] In this secondary heating step, the working atmosphere inside the furnace is also composed of two active gases, methanol and propane, to maintain a protective atmosphere and prevent decarburization or slight carbon replenishment of the workpiece. The methanol flow rate is 3-5 l / h and the propane flow rate is 0.5-0.7 l / h.

[0014] Preferably, in step S3, the workpiece is kept in the oil for 30 minutes and the stirring frequency is 20-30Hz.

[0015] Preferably, the low-temperature tempering time in step S4 is 150-180 minutes.

[0016] Preferably, the workpiece is an automotive ball cage retainer made of 20CrMnTi material.

[0017] The beneficial effects of this invention are as follows: by continuously completing the carburizing and normalizing processes in a sealed box-type multi-purpose furnace, the entire process is carried out under a protective atmosphere, which completely eliminates the surface decarburization problem that is difficult to avoid in conventional re-normalizing processes, and protects the workpiece's only small machining allowance.

[0018] The controlled slow cooling process after carburizing effectively breaks the genetic chain of coarse structures and eliminates the problems of localized martensite coarsening and mixed crystals left over from raw materials or previous processes.

[0019] Slow cooling and normalizing followed by secondary heating and quenching can re-nucleate and refine austenite grains, ultimately resulting in a uniform and fine tempered martensite structure, which significantly improves the toughness of the cage.

[0020] The entire process employs isothermal graded oil quenching and precise temperature and carbon potential control, which minimizes heat treatment deformation of the thin-walled cage and improves the product qualification rate. Attached Figure Description

[0021] Figure 1 Metallographic images of uncarburized and quenched raw materials with locally coarse and abnormal microstructures are shown using a special etchant to reveal the grains.

[0022] Figure 2 To obtain the metallographic image of the microstructure with coarse martensite after conventional carburizing and quenching process, 4% nitric acid alcohol solution was used for etching.

[0023] Figure 3 The metallographic image, which has a uniform structure and excellent martensite needles after being processed by the process of the embodiment of the present invention, is etched with 4% nitric acid alcohol solution. Detailed Implementation

[0024] The accompanying drawings provide a further description of a heat treatment process for carburizing, slow cooling, and secondary reheating quenching of a cage according to the present invention.

[0025] A heat treatment process for carburizing, slow cooling, and secondary reheating and quenching of cages, performed in a sealed box-type multi-purpose furnace, specifically includes the following steps:

[0026] Carburizing and slow cooling: Place the workpiece into the rear chamber of a sealed box-type multi-purpose furnace, heat it to 780℃ and hold it for 30 minutes;

[0027] The temperature was raised to 910℃, the carbon potential CP was set to 1.15%, and carburizing was carried out for 135 minutes.

[0028] Cool down to 870℃, set the carbon potential CP to 0.9%, and hold at this temperature for 30 minutes.

[0029] After carburizing, the workpiece is moved to the upper part of the pre-chamber, and high-pressure nitrogen gas is introduced at a flow rate of 8 m³ / h. The pre-chamber circulating fan is then turned on for forced air cooling, and the temperature is lowered for 150 minutes, reducing the workpiece temperature to approximately 500°C. This step achieves the normalizing effect under a protective atmosphere, eliminating coarse microstructures.

[0030] The working atmosphere inside the furnace during the post-carburizing and heat preservation stages consists of two active gases: methanol and propane. Methanol serves as the carrier gas, and after cracking, it forms a protective base atmosphere to prevent decarburization of the workpiece. Propane serves as the enrichment gas, decomposing to release active carbon atoms for precise control and maintenance of the carbon potential inside the furnace. The specific flow rates are: methanol 5.0 l / h, propane 0.7 l / h.

[0031] Secondary heating: The workpiece, which has been slowly cooled to 500°C, is sent back into the rear chamber, heated to 780°C, and held for 30 minutes.

[0032] Heat to 860℃, set the carbon potential CP to 0.8%, and hold for 95 minutes;

[0033] The temperature was lowered to 830℃, the carbon potential CP was set to 0.75%, and the workpiece was isothermaled for 30 minutes to achieve uniform austenitization and grain refinement.

[0034] The furnace working atmosphere at this stage also consists of two active gases: methanol and propane. These gases are used to maintain a protective atmosphere and prevent decarburization or slight recarburization of the workpiece. The specific flow rates are: methanol 4.0 l / h and propane 0.6 l / h.

[0035] Oil bath quenching: After secondary heating, the workpiece is pulled to the front chamber and immersed in isothermal graded oil at 120°C for 30 minutes, while stirring is started at a frequency of 25Hz.

[0036] Cleaning: After quenching, the workpiece is taken out of the furnace and cleaned routinely to remove surface oil and dirt.

[0037] Low-temperature tempering: The cleaned workpiece is tempered at 170℃ for 165 minutes to eliminate quenching stress and stabilize the microstructure.

[0038] Processing results

[0039] After the above-mentioned process, the 20CrMnTi automotive CV joint cage was subjected to metallographic examination using a 4% nitric acid alcohol solution for etching. Figure 3 As shown, the internal microstructure of the workpiece is uniform and fine tempered martensite, without coarse martensite or microstructure inheritance, and is rated as qualified and excellent. Meanwhile, the workpiece surface shows no decarburization, the deformation is controlled within the process requirements, and the product's toughness and dimensional accuracy meet high-performance requirements.

[0040] The above description is only a preferred embodiment of the present invention. All other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

Claims

1. A heat treatment process for carburizing, slow cooling, and secondary reheating and quenching of a cage, characterized in that, It includes the following steps: Step S1: Carburizing and slow cooling. The workpiece is placed in the rear chamber of a sealed box-type multi-purpose furnace and heated to 910°C for carburizing. Then, it is cooled to 870°C and kept at that temperature. After that, the workpiece is moved to the front chamber and slowly cooled to 450-550°C under high-pressure nitrogen protection and circulating air cooling conditions. Step S2: Secondary heating. The workpiece, after slow cooling, is sent back into the rear chamber and heated to 860°C and held at that temperature. Step S3: Oil bath quenching. After the workpiece is heated twice, it is moved to the front chamber and immersed in isothermal graded oil at 110-130℃ for cooling. Step S4: Low-temperature tempering. Temper the quenched workpiece at 160-180℃.

2. The heat treatment process for carburizing, slow cooling, and secondary reheat quenching of a cage according to claim 1, characterized in that, The specific steps of S1, carburizing and slow cooling, are as follows: the workpiece is placed in the rear chamber, heated to 780°C and held for 30 minutes; the carburizing temperature is 910°C, the carbon potential CP is 1.15%, and the time is 120-150 minutes; then the temperature is lowered to 870°C, the carbon potential CP is 0.9%, and isothermal for 30 minutes; subsequently, the workpiece is placed in the upper layer of the front chamber, high-pressure nitrogen gas with a flow rate of 6-10 m³ / h is introduced, and the front chamber circulation fan is turned on to cool down for 150 minutes, so that the workpiece temperature drops to 450-550°C.

3. The heat treatment process for carburizing, slow cooling, and secondary reheat quenching of a cage according to claim 1, characterized in that, The secondary heating in step S2 specifically involves: sending the workpiece back into the rear chamber, heating it to 780°C and holding it for 30 minutes; then heating it to 860°C with a carbon potential CP of 0.8% and holding it for 70-120 minutes; then cooling it down to 830°C with a carbon potential CP of 0.75% and holding it at that temperature for 30 minutes.

4. The heat treatment process for carburizing, slow cooling, and secondary reheat quenching of a cage according to claim 1, characterized in that, In step S3, the workpiece is kept in the oil for 30 minutes, and the stirring frequency is 20-30Hz.

5. The heat treatment process for carburizing, slow cooling, and secondary reheat quenching of a cage according to claim 1, characterized in that, The low-temperature tempering time in step S4 is 150-180 minutes.

6. The heat treatment process for carburizing, slow cooling, and secondary reheat quenching of a cage according to claim 1, characterized in that, In step S1, the working atmosphere inside the furnace includes methanol and propane. Methanol is used as a carrier gas and forms a protective base atmosphere after cracking to prevent decarburization of the workpiece. Propane is used as an enrichment gas to decompose and release active carbon atoms, which are used to precisely control and maintain the carbon potential inside the furnace. The methanol flow rate is 4.0-6.0 l / h and the propane flow rate is 0.6-0.8 l / h.

7. The heat treatment process for carburizing, slow cooling, and secondary reheat quenching of a cage according to claim 1, characterized in that, In step S2, the working atmosphere inside the furnace includes methanol and propane, which are used to maintain a protective atmosphere and prevent decarburization or slight carbon replenishment of the workpiece. The methanol flow rate is 3-5 L / h and the propane flow rate is 0.5-0.7 L / h.

8. The heat treatment process for carburizing, slow cooling, and secondary reheat quenching of a cage according to claim 1, characterized in that, The workpiece is an automotive ball cage cage made of 20CrMnTi material.

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