A V-type carburizing and quenching process for 18CrNiMo7-6 heavy-duty gears

Abstract

This invention discloses a V-shaped carburizing and quenching process for 18CrNiMo7-6 heavy-duty gears. After the carburizing process, the workpiece is cooled to 400-500℃ and then directly enters the quenching process, creating a V-shaped temperature gradient from carburizing to quenching. This invention combines the advantages of conventional carburizing and cooling-down direct quenching processes with carburizing followed by reheating and quenching processes. It achieves grain refinement, reduces decarburization on the workpiece surface, ensures continuity between processes, improves furnace operating efficiency, reduces energy consumption, and is more environmentally friendly and energy-efficient.

Description

Technical Field

[0001] This invention relates to the carburizing and quenching process of gears, and more particularly to a V-type carburizing and quenching process for 18CrNiMo7-6 heavy-duty gears. Background Technology

[0002] For gears, carburizing and quenching offers the best overall mechanical properties compared to other heat treatment processes. Therefore, carburizing is the most widely used process in the production of high-parameter gears. The carburizing and quenching process varies depending on the gear material, structural dimensions, and carburized layer depth. Broadly speaking, it can be divided into direct quenching after carburizing and reheating after carburizing. The surface hardness, core hardness, surface microstructure, core microstructure, and grain size of gears obtained through different carburizing and quenching processes can be adjusted by parameters such as carbon potential, quenching temperature, cooling rate of the cooling medium, and cryogenic treatment. However, the biggest difference between the various process types lies in the length of the martensite lamellae. When the high-carbon austenite on the surface cools to slightly below Ms, the first martensite lamellar often penetrates the entire austenite grain, dividing it in two. This limits the size of subsequent martensite lamellae, resulting in smaller lamellae as they form. The size of the lamellar martensite formed within an austenite grain is highly non-uniform; its average size depends on the size of the austenite grain. According to standards such as ISO 6336-5 and GB3480.5-2021 "Calculation of load-carrying capacity of spur and helical gears - Part 5: Strength and mass of materials", the grain size of carburized and quenched materials should not be less than grade 5; otherwise, it is considered an unacceptable coarse grain.

[0003] The yield strength of steel is linearly related to the reciprocal of the square root of the grain diameter, i.e., σ s =σ0+, where σ0 is a constant, roughly equivalent to the yield strength of a single crystal, d is the average diameter of the polycrystalline material, and k is a constant characterizing the influence of grain boundaries on strength, which is related to the crystal structure. The atoms at grain boundaries are irregularly arranged, contain more impurities and defects, and have higher energy, hindering the passage of dislocations; that is, grain boundaries impede plastic deformation. The more grain boundaries there are, i.e., the finer the grains, the higher the strength of the material. This is the essence of grain refinement strengthening; therefore, grain refinement strengthening can improve the plasticity and toughness of materials while increasing their strength.

[0004] The 18CrNiMo7-6 pinion shaft used in the traction transmission of electric locomotives is characterized by high load, high speed, and high precision. At the same time, it must also withstand large impact loads and severe off-center loads, and must maintain its good performance under cold and high temperature conditions.

[0005] The existing carburizing and cooling direct quenching process for 18CrNiMo7-6 pinion shafts has fewer steps, less workpiece deformation, high efficiency and low cost. However, because there is no grain refinement process after carburizing, the grains are larger, there is more residual austenite and the surface hardness is lower.

[0006] The process of carburizing and then reheating and quenching the 18CrNiMo7-6 pinion shaft has the advantages of refining the grain size by air cooling after carburizing and reducing the carbon content in the matrix by precipitating carbides during the subsequent 650℃ high-temperature tempering. The amount of residual austenite after quenching is also relatively small, which is beneficial for refining the metallographic structure and grain size. However, air cooling after carburizing will cause severe decarburization on the surface of the workpiece. At the same time, the reheating will increase the production cycle, increase power consumption, increase cost, and cause large deformation during heat treatment.

[0007] With the increase in railway speed, locomotive traction transmission devices are also gradually developing towards high-speed and heavy-load directions, requiring gears to have high strength and good toughness. Therefore, designing a carburizing and quenching process that can meet the requirements of 18CrNiMo7-6 heavy-load gears such as the pinion shafts used in locomotive traction transmission devices, while also reducing production costs and meeting green and energy-saving requirements, is a technical problem that industry technicians urgently need to solve. Summary of the Invention

[0008] The purpose of this invention is to address the shortcomings of existing carburizing and quenching processes for treating 18CrNiMo7-6 heavy-duty gears by providing a V-type carburizing and quenching process for 18CrNiMo7-6 heavy-duty gears. The heavy-duty gears treated by this invention not only meet the usage requirements but also simplify the processing steps and reduce production costs.

[0009] To achieve the above objectives, the present invention can adopt the following technical solutions:

[0010] The V-shaped carburizing and quenching process for 18CrNiMo7-6 heavy-duty gears described in this invention involves cooling the workpiece to 400~500℃ after the carburizing process and then directly entering the quenching process, so that the temperature gradient from carburizing to quenching forms a V-shaped curve.

[0011] "V-shaped" refers to the continuous relationship between the lowest cooling temperature after carburizing (without needing to reach room temperature) and the temperature rise during quenching, forming a V-shaped curve.

[0012] Specifically, after the carburizing process is completed, the workpiece is first cooled to 830°C and then moved to the front chamber of the multi-purpose furnace. Under nitrogen protection, it is furnace cooled to 400~500°C and then enters the heating chamber of the multi-purpose furnace. After the temperature is raised to 650°C, it is held at that temperature and then heated to the quenching temperature for quenching treatment.

[0013] The advantages of this invention are:

[0014] By combining the advantages of conventional carburizing cooling direct quenching process and carburizing followed by reheating quenching process, it can refine grains, reduce decarburization on the workpiece surface, ensure the continuity between processes, improve the operating efficiency of the furnace, reduce energy consumption, and be more green and energy-saving.

[0015] The V-shaped carburizing and quenching process of this invention, on the one hand, allows carbides to precipitate during the cooling process of the workpiece, reducing the carbon content in the matrix and thus reducing the content of residual austenite after quenching; on the other hand, selecting a temperature of 400~500℃ also avoids the generation of non-equilibrium structures such as martensite and lower bainite (such non-equilibrium structures require sufficient high-temperature tempering to transform into tempered sorbite, reducing the tendency of structure inheritance during quenching); finally, the V-shaped carburizing and quenching process of this invention can also reduce the energy waste caused by surface decarburization of the workpiece during air cooling after carburizing and the reheating and quenching process after cooling. Attached Figure Description

[0016] Figure 1 This is a structural diagram of the 18CrNiMo7-6 pinion shaft.

[0017] Figure 2 This is a process diagram of the V-type carburizing and quenching process of the present invention.

[0018] Figure 3 This is a process diagram showing the carburizing process followed by reheating and quenching after the material is removed from the furnace.

[0019] Figure 4 This is a process diagram for carburizing, cooling, and direct quenching. Detailed Implementation

[0020] The process of the present invention will now be described in more detail with reference to the accompanying drawings to facilitate understanding by those skilled in the art.

[0021] In the following examples and comparative examples, the product is an 18CrNiMo7-6 electric locomotive pinion shaft, such as... Figure 1 As shown.

[0022] I. Example The V-type carburizing and quenching process of the 18CrNiMo7-6 heavy-duty gear of the present invention is as follows: Figure 2 As shown:

[0023] S1, carburization

[0024] The gear shaft is fed into the heating chamber of the multi-purpose furnace and heated in stages to 925℃. At this temperature, the carbon potential is 0.3% Cp for 1 hour. Carburizing begins: strong carburizing potential 1.15% Cp for 14 hours; diffusion 1 carbon potential 1.0% Cp for 5 hours; diffusion 2 carbon potential 0.80% Cp for 8 hours. Then, the furnace is cooled to 830℃ and homogenized for 2 hours, at which point the carburizing process is complete.

[0025] S2, Quenching

[0026] After the carburizing process is completed, the workpiece is transferred to the front chamber of the multi-purpose furnace and cooled to 400~500℃ under nitrogen protection. Then the workpiece is transferred back to the heating chamber of the multi-purpose furnace, the temperature is raised to 650℃ and held for 4 hours, and then the temperature is raised to the quenching temperature of 830℃ and quenched for 2 hours.

[0027] In this invention, the quenching process is heated and held at 650°C, and the temperature uniformity time is extended. Firstly, during the sufficient holding period at 650°C, carbides are precipitated from the matrix to form dispersed carbides, which can hinder the growth of martensite needles during quenching. Furthermore, the precipitation of carbides can reduce the content of residual austenite after quenching. Secondly, it can reduce decarburization on the surface of the workpiece.

[0028] S3, Low-temperature tempering

[0029] After quenching, the workpiece is oil-cooled to room temperature, then transferred to the tempering chamber, heated to 180°C, and held for 15 hours for low-temperature tempering. Finally, the workpiece is taken out of the furnace and air-cooled to room temperature, thus completing the entire carburizing and quenching process.

[0030] The total process time of this invention is approximately 68 hours.

[0031] The conventional carburizing followed by reheating and quenching process and the direct quenching process after carburizing are used as comparative examples to compare with the process of the present invention.

[0032] II. Comparative Example 1: Reheating and quenching process after carburizing (as follows) Figure 3 As shown:

[0033] S1, carburization is the same as in Example 1;

[0034] S2, Cooling

[0035] After the carburizing process is completed, the workpiece is cooled to 830℃ for 2 hours and then air-cooled to room temperature. Then the workpiece is placed in the tempering chamber of the multi-purpose furnace and heated to 650℃ for 4 hours and then air-cooled to room temperature again.

[0036] S3, Quenching

[0037] The workpiece, cooled to room temperature, is placed in the heating chamber of the multi-purpose furnace. The temperature is raised to 650℃ and held for 2 hours. Finally, the temperature is raised to the quenching temperature of 830℃ and held for 2 hours before quenching.

[0038] S4, low-temperature tempering is the same as in Example 1.

[0039] Compared with the present invention, this process adds a second cooling step, and the total process time is about 87 hours.

[0040] III. Comparative Example 2: Carburizing and Cooling Direct Quenching Process Figure 4 As shown:

[0041] S1, same as in Example 1;

[0042] S2, Quenching

[0043] After the carburizing process is completed, the workpiece is cooled to 810℃ for 2 hours and then quenched.

[0044] S3, Low-temperature tempering

[0045] After quenching, the workpiece is oil-cooled to room temperature, then transferred to a tempering furnace and heated to 220°C. It is then held at that temperature for 15 hours for low-temperature tempering. Finally, the workpiece is taken out of the furnace and air-cooled to room temperature, thus completing the entire carburizing and quenching process.

[0046] This process involves direct quenching after carburizing, with fewer steps and a total processing time of approximately 57 hours.

[0047] IV. Comparison of the effects of three heat treatment processes:

[0048] The workpieces (18CrNiMo7-6 electric locomotive pinion shafts) treated with the three carburizing and quenching processes of the present invention and Comparative Examples 1 and 2 were tested, and the results are as follows:

[0049] 1. Surface microstructure of workpieces treated by three carburizing and quenching processes: The microstructure was tested according to GB / T 25744-2010 "Metallographic Inspection of Carburizing, Quenching and Tempering of Steel Parts". The surface martensite of the workpiece obtained in Comparative Example 2 was grade 5, the surface martensite of the workpiece obtained in this invention was grade 3, and the surface martensite of the workpiece obtained in Comparative Example 1 was grade 2.

[0050] Conclusion: The martensite level of the workpiece surface obtained by the V-shaped carburizing and quenching process (the present invention) is close to that of the carburizing and reheating quenching process (Comparative Example 1), and is significantly better than the direct quenching process after carburizing (Comparative Example 2).

[0051] 2. Surface hardness of workpieces treated by three carburizing and quenching processes: The surface hardness of the workpiece obtained by Comparative Example 2 is 58.0~58.5 HRC; the surface hardness of the workpiece obtained by the present invention is 60.0~60.5 HRC; the surface hardness of the workpiece obtained by Comparative Example 1 is 60.0~61.0 HRC.

[0052] Conclusion: The surface hardness of the workpiece obtained by the V-shaped carburizing and quenching process (the present invention) is close to that of the carburizing and reheating quenching process (Comparative Example 1), and is significantly better than the direct quenching process after carburizing (Comparative Example 2).

[0053] 3. Grain size at different locations on workpieces treated by three carburizing and quenching processes:

[0054] The actual grain size of the 18CrNiMo7-6 electric locomotive pinion shaft sample before carburizing was grade 10.2. The carburizing temperature was 925℃ and the carburizing time was 28h. The actual grain size results of the sample at three positions obtained by three different carburizing and quenching processes are shown in the table below.

[0055] Grain size at different locations under different carburizing and quenching processes

[0056]

[0057] As can be seen from the data in the table, the biggest difference between the three processes lies in the difference in surface grain size. The grain size gradually decreases from the surface to the core of the entire diffusion layer. The sample treated by the process of this invention has a surface grain size of 9.4, which is better than Comparative Example 2 (6.1) and slightly lower than the surface grain size of 9.9 of the sample treated by Comparative Example 1. This fully meets the requirements for the use of 18CrNiMo7-6 pinion shafts in locomotive traction transmission devices.

[0058] 4. Decarbonization status:

[0059] In the V-type carburizing and quenching process of the present invention, the surface decarburization of the 18CrNiMo7-6 electric locomotive pinion shaft after carburizing and cooling in a nitrogen-protected furnace is 22~36μm. In the carburizing and reheating quenching process (Comparative Example 1), the decarburization after air cooling is 43~57μm. Compared with the two, the surface decarburization of the present invention is reduced.

[0060] 5. Total time for the entire process:

[0061] The total time for the V-type carburizing and quenching process of this invention is 68 hours, while the total time for the reheating and quenching process after carburizing in Comparative Example 1 is 87 hours. Compared with the latter, the time used in this invention is reduced by 19 hours. The carburizing and quenching process is less and more continuous, which improves the furnace operating efficiency, shortens the production cycle, reduces energy consumption, and lowers production costs.

[0062] In summary, the V-type carburizing and quenching process of this invention significantly outperforms the direct quenching process after carburizing in terms of surface martensite level, surface hardness, and grain size. It can achieve surface martensite level, surface hardness, and grain size similar to the reheating and quenching process after carburizing. However, the carburizing and quenching process has fewer and more continuous steps, reduces decarburization after carburizing, shortens the production cycle by about 19 hours, and reduces energy consumption and heat treatment costs.

Claims

1. A V-type carburizing and quenching process for 18CrNiMo7-6 heavy-duty gears, characterized in that: After the carburizing process is completed, the workpiece is cooled to 400 ~ 500℃ and then directly enters the quenching process, so that the temperature gradient from carburizing to quenching forms a V-shaped curve. Specifically, after the carburizing process is completed, the workpiece is first cooled to 830°C and then moved to the front chamber of the multi-purpose furnace. Under nitrogen protection, it is furnace cooled to 400-500°C and then enters the heating chamber of the multi-purpose furnace. After the temperature is raised to 650°C, it is held for 4 hours and then the temperature is raised to the quenching temperature for quenching treatment.

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