Method for regulating and controlling carburizing efficiency of gear steel by using high-density grain boundary

Abstract

The invention discloses a method for regulating and controlling carburizing efficiency of gear steel by using high-density grain boundaries, which comprises the following steps of: S1, pretreating the gear steel to change the internal microstructure state of the gear steel, and generating the high-density grain boundaries on the surface layer and the subsurface layer; s2, the pretreated gear steel is subjected to carburizing treatment; and S3, the gear steel subjected to carburizing treatment is subjected to conventional heat treatment. A high-density large-angle grain boundary is introduced to the surface of the gear steel, carbon atoms can be attached more easily, the content of the carbon atoms on the surface layer of the gear steel is increased, and the surface layer hardness of the gear steel is improved; meanwhile, the depth of a carburizing layer of the gear steel can be increased, large-angle grain boundaries are more beneficial to diffusion of carbon atoms, along with carburizing, the surface layer of the pretreated gear steel contains a large number of large-angle grain boundaries, the diffusion efficiency of the carbon atoms in the gear steel is higher than that of unpretreated gear steel, and therefore the depth of the carburizing layer is increased, and the carburizing efficiency is improved; according to the invention, the time cost and the energy consumption are greatly optimized.

Description

Technical Field

[0001] The invention relates to the technical field of gear steel carburizing heat treatment, and in particular to a method for regulating the carburizing efficiency of gear steel by utilizing high-density grain boundaries. Background Art

[0002] 20CrMnTi gear steel has high toughness and excellent tensile properties, and is widely used in the construction of special-purpose vehicle gearbox gears. Gears wear out during the meshing process in a service environment, so the surface of 20CrMnTi is strengthened by carburizing heat treatment to improve its surface hardness and surface wear resistance and reduce the occurrence of wear. However, with the development of the automotive industry, special-purpose vehicles are developing rapidly in the direction of high speed and heavy load, and the gearbox gears are subject to greater loads. The original surface strengthening process can no longer meet the increasing requirements for the surface performance (tensile and wear resistance) of gear steel, so it is necessary to change and optimize its original process flow.

[0003] Carburizing heat treatment is a commonly used surface treatment strengthening method for gear steel. Its main purpose is to increase the carbon content of the surface layer of low-carbon steel and form a certain carbon concentration gradient. It is a chemical heat treatment process that heats and keeps low-carbon steel in a carburizing medium to allow carbon atoms to penetrate into the surface layer. Carburizing heat treatment can significantly improve the surface hardness and wear resistance of gear steel within a certain period of time. However, the temperature of carburizing heat treatment is too high, and the carburizing efficiency is low, which can easily lead to coarse metal grains and shallow carburized layers.

[0004] To meet the requirements of industry development, most of the current research focuses on improving the carburizing heat treatment process. The existing means to solve the insufficient surface performance of gear steel is to add a surface treatment process after the carburizing heat treatment for strengthening, such as Chinese patent CN 110951962 A, a high-performance gear heat treatment method for achieving fine and uniform structure. This can only improve the hardness and wear resistance of the outermost layer of gear steel, and has no effect on the depth and efficiency of the carburized layer of gear steel, and the strengthening effect is weak. Summary of the invention

[0005] The main purpose of the present invention is to propose a method for regulating the carburizing efficiency of gear steel by utilizing high-density grain boundaries. High-density grain boundaries and microstructures such as nanocrystals and ultrafine grains are introduced on the surface of the material through a pretreatment method, so that the number of large-angle grain boundaries in the surface and sub-surface areas of the gear steel is greatly increased, and the diffusion efficiency of carbon atoms at the large-angle grain boundaries is faster, making subsequent carburizing heat treatment easier to carry out, and also increasing the depth of the carburizing layer; moreover, the method of the present invention does not increase the time of the carburizing heat treatment while increasing the depth of the carburizing layer, thereby greatly reducing energy dissipation and being more environmentally friendly.

[0006] The technical solution adopted by the present invention is:

[0007] A method for regulating the carburizing efficiency of gear steel by using high-density grain boundaries comprises the following steps:

[0008] S1. Pre-treat the gear steel to change its internal microstructure and produce high-density grain boundaries in the surface and subsurface layers;

[0009] S2, carburizing the pretreated gear steel;

[0010] S3. Perform conventional heat treatment on the gear steel after carburizing treatment.

[0011] In the above scheme, in step S1, the gear steel is pretreated by ultrasonic shot peening or ultrasonic rolling.

[0012] In the above scheme, the parameters of the ultrasonic shot peening are set as follows: ultrasonic frequency is 15-25kHz, amplitude is 40-60μm, the distance between the shot and the material surface is 8-10mm, and the shot peening time is 4-10min.

[0013] In the above scheme, the parameters of the ultrasonic rolling are set as follows: ultrasonic frequency is 15-25kHz, static pressure is 0.2-0.4MPa, feed speed is 20-40mm / min, and the number of rolling passes is 16-20 times.

[0014] In the above scheme, in step S2, the method of carburizing treatment is: placing the gear steel in a carburizing furnace with a carburizing atmosphere concentration of 1%-1.2% at 900℃-920℃ for 180-240 minutes, so that carbon atoms diffuse to the pretreated surface of the gear steel and penetrate into it, and then tempering the gear steel after cooling, and then air cooling to room temperature after tempering.

[0015] In the above scheme, the tempering temperature is 600°C-650°C, and the time is 240-300 minutes.

[0016] In the above scheme, in step S3, the conventional heat treatment method is: placing the gear steel in a heat treatment furnace at 800-860°C for 60-100 minutes, and then quickly placing the gear steel in oil for quenching after the treatment, and then tempering it in the heat treatment furnace after quenching, and after tempering, placing the gear steel in the air to cool to room temperature.

[0017] In the above scheme, the tempering temperature is 160-200°C and the time is 120-180 minutes.

[0018] The beneficial effects produced by the present invention are:

[0019] 1. The method of the present invention introduces high-density and high-angle grain boundaries on the surface of the original gear steel. A large number of high-angle grain boundaries can make carbon atoms easier to adhere, increase the carbon atom content on the surface of the gear steel, and improve the surface hardness of the gear steel. At the same time, the high-density grain boundary method adopted by the present invention can increase the depth of the carburized layer of the gear steel. The high-angle grain boundaries are more conducive to the diffusion of carbon atoms. As the carburization proceeds, the surface of the pretreated gear steel contains a large number of high-angle grain boundaries, and the diffusion efficiency of carbon atoms inside it is higher than that of the unpretreated gear steel, thereby increasing the depth of the carburized layer and improving the carburizing efficiency. Therefore, this method can make the surface hardness value of the gear steel higher, increase the depth of the carburized layer, and improve the mechanical properties, which fully meets the current development requirements of the heavy-load direction of automobiles.

[0020] 2. The present invention achieves the optimization of the carburizing process. With the improvement of industry development requirements, the traditional carburizing heat treatment process requires a lot of time (increasing the carburizing time by 3-6 hours) and energy (increasing the carburizing temperature by 50°C-100°C) for treatment. In comparison, the new process proposed by the present invention has been greatly optimized in terms of time cost and energy dissipation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0022] Figure 1 is a process flow chart of Example 1 of the method of the present invention;

[0023] Figure 2 Schematic diagram of the grain boundary of the original gear steel in Example 1 of the method of the present invention;

[0024] Figure 3 Schematic diagram of the grain boundary of the original gear steel after ultrasonic shot peening pretreatment in Example 1 of the method of the present invention;

[0025] Figure 4 is a schematic diagram of the carburizing process of the gear steel after pretreatment in Example 1 of the method of the present invention;

[0026] Figure 5 It is a hardness distribution curve diagram of the samples treated by the method of the present invention and the samples treated by the original carburizing heat process. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0028] Embodiment 1:

[0029] The original material 20CrMnTi of the annealed automotive gearbox gear is selected as the original pattern, and the pattern size is 15mm-35mm in thickness × 35mm-65mm in length × 25mm-55mm in width. The method of using high-density grain boundaries to control the carburizing efficiency of gear steel proposed by the present invention is used to process gear steel. The overall process flow is as follows: Figure 1 As shown, including:

[0030] Step 1: Perform ultrasonic shot peening pretreatment on the gear steel, setting the ultrasonic frequency to 15-25kHz, the amplitude to 40-60μm, the shot peening distance to 8-10mm, and the shot peening time to 4-10min. The pretreatment process is completed when the surface roughness of the treated position of the sample is significantly improved.

[0031] The grain boundary inside a metal material refers to the boundary between two or more grains with different orientations, which has a great influence on the performance of the material itself. The more large-angle grain boundaries inside the material, the smaller the grain size inside the material. The high-density grain boundary area has higher hardness and higher fatigue resistance. In addition, during the carburizing process, the diffusion of carbon atoms mainly occurs at the grain boundaries of the material, and the grain boundaries promote the carburizing process.

[0032] The internal grain boundary structure of the original gear steel is as follows Figure 2 As shown in Figure 1, the internal microstructure without pretreatment is mainly composed of sparse high-angle grain boundaries. The original gear steel is pretreated with ultrasonic shot peening to refine the surface and subsurface grains to produce nanocrystalline and ultrafine grains, and a large number of high-angle grain boundaries are prepared on its surface. Figure 2 As shown, it is prepared for the subsequent carburizing treatment process. After ultrasonic shot peening pretreatment, a large number of large-angle grain boundaries are generated in the overall area of ​​0-200μm from the surface of the gear steel, accompanied by grain refinement. Compared with the original internal grains of the gear steel, the overall number has increased significantly. The density of large-angle grain boundaries is the highest in the surface area of ​​0-10μm, and the density of large-angle grain boundaries in the sub-surface area of ​​10-40μm is also increased. The small-angle grain boundaries in the 40-200μm area have increased more. The overall grain boundary density growth ratio inside the gear steel after pretreatment is gradient distributed.

[0033] Step 2: Carburize the pretreated gear steel, place the gear steel in a carburizing furnace at 900℃-920℃ with a carburizing atmosphere concentration of 1%-1.2% for 180-240 minutes, diffuse the carbon atoms to the pretreated surface of the gear steel and infiltrate, and then perform tempering at 600℃-650℃ for 240-300 minutes after cooling, and air cool to room temperature after tempering. The main function of high-temperature tempering is to precipitate the carbon and alloy elements in the carburized layer in the form of carbides, which is convenient for reducing the residual austenite in the subsequent conventional heat treatment process.

[0034] In step 2, the gear steel with high-density grain boundaries in the surface and sub-surface regions is carburized. As the carburization proceeds, carbon atoms diffuse out in the atmosphere and adsorb onto the surface of the gear steel. Since the surface of the gear steel contains a large number of large-angle grain boundaries, the diffusion rate of carbon atoms at the large-angle grain boundaries is faster. As the carburization process proceeds, carbon atoms gradually diffuse from the surface of the gear steel through the large-angle grain boundaries to areas deeper from the surface. Although the carburizing process is accompanied by the growth of grains and the reduction of grain boundaries, the efficiency of carburizing promoted by grain boundaries is a dynamic process. As the temperature rises, carbon atoms inside the gear steel have already infiltrated through the large-angle grain boundaries. The schematic diagram of the carburization process at the large-angle grain boundaries is shown below. Figure 4 shown.

[0035] Step three, perform conventional heat treatment on the gear steel after carburizing treatment, place the gear steel in a heat treatment furnace at 800-860℃ for 60-100 minutes, and then quickly put the gear steel into oil for quenching. After quenching, perform tempering treatment in the heat treatment furnace at 160-200℃ for 120-180 minutes. After tempering, place the gear steel in air to cool to room temperature.

[0036] The main purpose of performing conventional heat treatment on the gear steel after carburizing in step three is to reduce the residual austenite content in the gear steel and further improve the overall hardness of the gear steel.

[0037] Comparative Example 1:

[0038] The annealed original material 20CrMnTi for automotive gearbox gears is selected as the original pattern. The pattern size is 15mm-35mm in thickness × 35mm-65mm in length × 25mm-55mm in width. The original process flow is used to process the gear steel. The specific steps are as follows:

[0039] Step 1: Carburize the processed gear steel by placing it in a carburizing furnace at 900°C-920°C with a carburizing atmosphere concentration of 1%-1.2% for 180-240 minutes to allow carbon atoms to diffuse into the pretreated surface of the gear steel and penetrate into it. After cooling, the gear steel is tempered at a temperature of 600°C-650°C for 240-300 minutes, and then air-cooled to room temperature.

[0040] Step 2: Carburize the gear steel and perform conventional heat treatment on it. Place the gear steel in a heat treatment furnace at 800-860°C for 60-100 minutes. After treatment, quickly put the gear steel into oil for quenching. After quenching, perform tempering treatment in a heat treatment furnace at 160-200°C for 120-180 minutes. After tempering, cool the gear steel in air to room temperature.

[0041] Figure 5 The hardness distribution curves of the sample treated by the method of the present invention (Example 1) and the sample treated by the original carburizing heat process (Comparative Example 1) are shown in the figure. The USP-Carburized curve represents the hardness distribution curve of Example 1, and the Carburized curve represents the hardness distribution curve of Comparative Example 1. By comparison, it can be seen that the method of the present invention has the following beneficial effects:

[0042] (1) The new process proposed by the present invention introduces high-density high-angle grain boundaries on the surface of the original gear steel. A large number of high-angle grain boundaries can make carbon atoms more easily attached, thereby increasing the carbon atom content on the surface of the gear steel and improving the surface hardness of the gear steel. Figure 5 As shown, the surface hardness value of the high-density grain boundary pattern at the surface 0-10μm is significantly higher than the hardness value of the original process pattern, about 50HV higher; the surface hardness value of the high-density grain boundary pattern in the sub-surface carburized layer is about 20HV higher.

[0043] (2) The high-density grain boundary method adopted by the present invention can increase the depth of the carburized layer of the gear steel. The large-angle grain boundaries are more conducive to the diffusion of carbon atoms. As the carburization progresses, the surface of the pretreated gear steel contains a large number of large-angle grain boundaries. The diffusion efficiency of carbon atoms in it is higher than that of the unpretreated gear steel, thereby increasing the depth of the carburized layer. Figure 5 As shown in the figure, the thickness of the carburized layer of the sample treated by the original process is about 950μm, and the thickness of the carburized layer of the sample treated by the high-density grain boundary method is about 1000μm, which is an increase of about 50μm. It should be noted that the hardness distribution inside the material is composed of the hardness of the carburized layer and the hardness of the matrix layer. Figure 5 The hardness of the matrix is ​​500HV, and the part with a hardness distribution greater than 500HV from the surface to the depth direction belongs to the carburized layer.

[0044] In addition, the present invention also achieves the optimization of the carburizing process. With the improvement of industry development requirements, the traditional carburizing heat treatment process requires a lot of time (increasing the carburizing time by 3-6 hours) and energy (increasing the carburizing temperature by 50°C-100°C) for processing, while the method of the present invention does not need to increase the carburizing time and temperature in the carburizing process, and the parameters of the carburizing process do not need to be changed to achieve the effect of increasing hardness and increasing the carburized layer. Therefore, compared with this, the new process proposed by the present invention has been greatly optimized in terms of time cost and energy dissipation.

[0045] It should be pointed out that, according to the needs of implementation, the various steps / components described in this application can be split into more steps / components, and two or more steps / components or partial operations of steps / components can be combined into new steps / components to achieve the purpose of the present invention.

[0046] The order of execution of each step in the above embodiment does not mean the order of execution. The order of execution of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiment of the present application.

[0047] It should be understood that those skilled in the art can make improvements or changes based on the above description, and all these improvements and changes should fall within the scope of protection of the appended claims of the present invention.

Claims

1. A method for regulating the carburizing efficiency of gear steel by using high-density grain boundaries, characterized in that: The following steps are involved: S1. Pre-treat the gear steel to change its internal microstructure and produce high-density grain boundaries in the surface and subsurface layers; S2, carburizing the pretreated gear steel; S3. Perform conventional heat treatment on the gear steel after carburizing treatment.

2. The method for regulating the carburizing efficiency of gear steel by utilizing high-density grain boundaries according to claim 1, characterized in that: In step S1, the gear steel is pretreated by ultrasonic shot peening or ultrasonic rolling.

3. The method for regulating the carburizing efficiency of gear steel by utilizing high-density grain boundaries according to claim 2, characterized in that: The parameters of the ultrasonic shot peening are set as follows: ultrasonic frequency is 15-25 kHz, amplitude is 40-60 μm, the distance between the shot and the material surface is 8-10 mm, and the shot peening time is 4-10 min.

4. The method for regulating the carburizing efficiency of gear steel by utilizing high-density grain boundaries according to claim 2, characterized in that: The parameters of the ultrasonic rolling are set as follows: ultrasonic frequency is 15-25kHz, static pressure is 0.2-0.4MPa, feed speed is 20-40mm / min, and rolling times is 16-20 times.

5. The method for regulating the carburizing efficiency of gear steel by utilizing high-density grain boundaries according to claim 1, characterized in that: In step S2, the method of carburizing treatment is: placing the gear steel in a carburizing furnace with a carburizing atmosphere concentration of 1%-1.2% at 900℃-920℃ for 180-240 minutes, so that carbon atoms diffuse into the pretreated surface of the gear steel and penetrate into it, and then tempering the gear steel after cooling, and then air cooling to room temperature after tempering.

6. The method for regulating the carburizing efficiency of gear steel by utilizing high-density grain boundaries according to claim 5, characterized in that: The tempering temperature is 600℃-650℃, and the time is 240-300 minutes.

7. The method for regulating the carburizing efficiency of gear steel by utilizing high-density grain boundaries according to claim 1, characterized in that: In step S3, the conventional heat treatment method is: placing the gear steel in a heat treatment furnace at 800-860°C for 60-100 minutes, and then quickly placing the gear steel in oil for quenching after treatment, and then tempering it in the heat treatment furnace after quenching, and after tempering, placing the gear steel in air to cool to room temperature.

8. The method for regulating the carburizing efficiency of gear steel by utilizing high-density grain boundaries according to claim 7, characterized in that: The tempering temperature is 160-200°C and the time is 120-180 minutes.

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