Gear parts

Abstract

The purpose of the present invention is to provide a gear component in which the surface layer of the dedendum has a high hardness and the bending fatigue strength is excellent.　The gear component according to the present invention is characterized in that the surface layer of the dedendum includes a portion having a carbide area percentage of 10% or less and a carbide number density of 6 / 10 μm2 or less.

Description

[Technical Field]

[0001] The present invention relates to a gear component that has been subjected to high-concentration carburizing treatment. [Background technology]

[0002] Gears, which are components of power transmission devices such as transmissions and reducers, are generally manufactured by forming case-hardened steel into the part shape and then performing carburizing or carbo-nitriding treatments. In recent years, gears have become smaller and more compact in an effort to reduce costs, and there is a demand for the development of parts with even higher strength.

[0003] As gears become more compact, the load per gear increases. This increased load has led to increased demand for improved durability. Known durability-enhancing technologies include carburizing, which introduces carbon into heated steel under atmospheric pressure and quenches it to form a hard martensite structure, strengthening the component, and vacuum carburizing, which carburizes under reduced pressure to prevent the formation of brittle grain boundary oxides that can initiate fatigue fractures. In recent years, high-concentration carburizing has become popular, which introduces carbon above the eutectoid concentration into steel, precipitating hard carbides in the martensite matrix, strengthening the component.

[0004] It is generally known that in high-concentration carburizing, the precipitation form of carbides affects durability. For example, Patent Document 1 describes a method for treating a steel sheet in which the area ratio of carbides is 5% or more and the surface density of carbides with a diameter of 0.5 μm or less is 6.0 pieces / 10 μm. 2 The steel part for machine structural use described above is disclosed. With this part, it is possible to improve pitting resistance by dispersing and precipitating fine carbides. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-113168 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the component described in Patent Document 1, the surface area of ​​the interface between the large amount of precipitated carbides and the matrix increases. This interface is thermodynamically unstable and easily becomes the nucleus for pearlite transformation, bainite transformation, and other transformations during quenching. Furthermore, because alloying elements such as chromium in the matrix concentrate in the carbides, the concentration of alloying elements such as chromium in the matrix near the interface decreases, resulting in poor hardenability. These effects lead to the formation of structures with lower hardness than martensite, such as pearlite and bainite, during quenching. This phenomenon is likely to occur when applied to gears with thick rims and large heat capacities, and is particularly pronounced at the base of the teeth, where the cooling rate is slow. This reduces the hardness of the base surface and reduces the bending fatigue strength of the base of the teeth. In other words, when considering increasing the strength and extending the life of the entire gear, there is still room for improvement, especially at the base of the teeth.

[0007] In order to address the above-mentioned problems of the prior art, an object of the present invention is to provide a gear component having a high hardness at the surface layer of the tooth base and excellent bending fatigue strength. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention provides a gear component having a tooth portion having a tooth base, a tooth tip, and a tooth surface between the tooth base and the tooth tip, the gear component being made of a steel material that has been subjected to high-concentration carburizing treatment, wherein the tooth base has a carbide layer in which carbides are dispersed and precipitated on a surface layer, and the carbide area ratio in at least a part of the carbide layer at the tooth base is 10% or less, and the carbide number density is 6 particles / 10 μm 2 The present invention is characterized by the following: This specification includes the disclosure of Japanese Patent Application No. 2022-050297, from which the present application claims priority. [Effects of the Invention]

[0009] According to the gear component of the present invention, the area ratio of carbides is small, and the surface area of ​​the interface between the carbides and the matrix is ​​small. As a result, the number of nuclei of pearlite and bainite transformation during quenching is reduced, and the decrease in the concentration of alloying elements such as chromium in the matrix near the interface is small, thereby minimizing the decrease in hardenability. As a result, soft structures such as pearlite and bainite are less likely to form during quenching, improving the hardness of the tooth base surface layer and the bending fatigue strength of the tooth base. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a perspective view showing an embodiment of a gear component according to the present invention. FIG. [Figure 2] 1 is an axial cross-sectional view of a gear component according to an embodiment of the present invention. FIG. [Figure 3] FIG. 3 is an enlarged view of part A in FIG. 2. [Figure 4] 1 is a radial cross-sectional view of a gear component according to an embodiment of the present invention. FIG. [Figure 5] FIG. 5 is an enlarged view of part B in FIG. [Figure 6] 1A to 1C are diagrams illustrating a manufacturing process of a gear component according to an embodiment of the present invention. [Figure 7] FIG. 1 is a diagram showing an outline of a high-concentration carburizing treatment process. [Figure 8] FIG. 2 is a diagram showing an outline of a high-concentration carburizing treatment process in Examples and Comparative Examples. [Figure 9] FIG. 2 is a diagram showing the shape of a test piece for a tooth surface fatigue test in Examples and Comparative Examples. [Figure 10] 1 is a graph showing the relationship between material structure and properties in Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of a gear component of the present invention will be described with reference to the drawings. First, the composition of the gear component according to the present invention will be described. The gear component is made of a steel material containing various alloying elements, with the balance containing Fe and unavoidable impurities. Specifically, the gear component can be made of materials such as Cr steel (SCr material), Cr-Mo steel (SCM material), and Ni-Cr-Mo steel (SNCM material), as specified in JIS G4053.

[0012] 1 is a perspective view of a gear component according to one embodiment of the present invention. This gear component 1 can be used as a gear for construction machinery such as a hydraulic excavator or a dump truck, but is not limited to this.

[0013] FIG. 2 is an axial cross-sectional view of the gear component 1. FIG. 3 is an enlarged view of portion A in FIG. 2, and schematically illustrates the axial cross-section of the gear component 1. The gear component 1 has an internal structure formed by high-concentration carburizing treatment, in which a matrix 11, a carburized layer 12, and a carbide layer 13 are formed in this order from the core to the surface 10. The carburized layer 12 is a region with a higher carbon concentration than the matrix 11. The carbide layer 13 is a layered region of the carburized layer 12 with an especially high carbon concentration, where carbides are dispersed and precipitated. The high-concentration carburizing treatment can also be referred to as a carburizing treatment that forms the carbide layer 13 (i.e., a carburizing treatment performed to the extent that carbides are precipitated in the region near the surface). The carburized layer 12 and the carbide layer 13 can also be collectively referred to as a layer (carburized layer) with a higher carbon concentration than the matrix 11.

[0014] 4 is a radial cross-sectional view (X-X' cross-sectional view in FIG. 2) of the gear component 1. The distance between the tooth portion 14 and the inner hole 15 is the rim thickness D.

[0015] 5 is an enlarged view of portion B in FIG. 4. Similar to the enlarged view of the axial cross section of the gear component 1 shown in FIG. 3, the tooth portion 14 is composed of, in order from the core of the gear component 1 toward the surface 10, a parent phase 11, a carburized layer 12, and a carbide layer 13. The area including the tip of the tooth portion 14 is the tooth tip 16, and the area including the intersection of the surface 10 with the pitch circle R is the tooth flank 17. Furthermore, the area including the point of contact between the surface 10 and a straight line that forms an angle of 30° with the center line P of the tooth portion 14 is the tooth base (tooth bottom) 18. That is, the tooth portion 14 includes the tooth base 18, the tooth tip 16, and the tooth flank 17 between the tooth base 18 and the tooth tip 16.

[0016] First, the surface layer of the tooth base 18, i.e., the carbide layer 13 at the tooth base 18, has a carbide area ratio of 10% or less in at least a part thereof, and the carbide number density is 6 particles / 10 μm 2 In the carbide layer 13 at the tooth base 18, if the carbide area ratio is greater than 10%, the concentration of alloy elements such as chromium in the matrix will decrease, resulting in a decrease in hardenability, a decrease in hardness, and a decrease in bending fatigue strength at the tooth base 18. 2 If it is larger, the surface area of ​​the interface between the carbide and the matrix increases, and a structure that is less hard than martensite is likely to form during quenching, thereby reducing the bending fatigue strength of the tooth base 18.

[0017] Next, it is preferable that the carbide area ratio of at least a portion of the surface layer of the tooth flank 17, i.e., the carbide layer 13 on the tooth flank 17, is 10% or less. If the carbide area ratio of the carbide layer 13 on the tooth flank 17 is greater than 10%, cracks are more likely to initiate and grow from coarse carbides during operation of the gear, which may shorten the pitting fatigue life of the tooth flank.

[0018] Furthermore, the carbide area ratio of the carbide layer 13 on the tooth flank 17 is preferably larger than the carbide area ratio of the carbide layer 13 on the tooth flank 17. If the carbide area ratio of the carbide layer 13 on the tooth flank 17 is smaller, the effect of the carbides on the tooth flank 17 in improving the pitting fatigue life is reduced. On the other hand, if the carbide area ratio of the carbide layer 13 on the tooth flank 18 is larger, the concentration of alloying elements such as chromium in the matrix of the tooth flank 18 decreases, resulting in a decrease in hardenability, a decrease in hardness, and a decrease in bending fatigue strength of the tooth flank 18. Therefore, if the carbide area ratio of the surface layer of the tooth flank 17 decreases or if the carbide area ratio of the surface layer of the tooth flank 18 increases and becomes equal to or smaller than the carbide area ratio of the surface layer of the tooth flank 18, the durability of the entire gear may decrease.

[0019] Furthermore, the thickness of the carbide layer 13 at the tooth root 18 is preferably thicker than the thickness of the carbide layer 13 at the tooth flank 17. The thickness of the carbide layer 13 at the tooth flank 17 or the tooth root 18 refers to the thickness of the carbide layer 13 measured in a direction perpendicular to the surface of the tooth flank 17 or the tooth root 18. Internally initiated pitting fatigue damage at the tooth flank 17 occurs at the depth where the shear stress is maximum. Therefore, the carbide layer 13 does not need to be thick, as long as it is deeper than the depth where the shear stress is maximum. On the other hand, the thicker the carbide layer 13 at the tooth root 18, the greater the residual stress at the tooth root 18 and the improved tooth root bending fatigue strength. Therefore, if the thickness of the carbide layer 13 at the tooth root 18 is thicker than the thickness of the carbide layer 13 at the tooth flank 17, the durability of the entire gear will be improved.

[0020] Next, a method for manufacturing a gear component according to an embodiment of the present invention will be described with reference to the drawings. 6 is a diagram showing the manufacturing process for a gear component according to one embodiment of the present invention. First, a steel material having the above-described chemical composition is appropriately hot forged to be shaped to the dimensions required for the gear component. After that, it is normalized to remove the effects of the processed structure and processing distortion, and then cut into a gear shape. The surface is hardened by high-concentration carburizing and tempering. If necessary, a finish process may be performed after tempering to reduce heat treatment distortion.

[0021] Fig. 7 is a diagram showing an overview of the high-concentration carburizing treatment process. Hereinafter, the high-concentration carburizing treatment process used in manufacturing a gear component according to one embodiment of the present invention will be described with reference to Fig. 7. As shown in Fig. 7, the high-concentration carburizing treatment process includes a carburizing step and a reheating step.

[0022] In the carburizing process, after the gear component 1 is soaked, a hydrocarbon gas such as acetylene is intermittently introduced for a certain period of time to introduce and diffuse carbon from the surface to the interior of the material, forming a carburized layer 12 and a carbide layer 13. The carburizing temperature is set to the A1 transformation temperature or higher. If the carburizing temperature is lower than the A1 transformation temperature, the structure does not transform to austenite and carburization does not occur. A higher carburizing temperature shortens the carburizing time, but increases heat treatment distortion, so a temperature of approximately 980 to 1050°C is preferred. The carburizing temperature may be changed during the process as needed. The carburizing time is preferably set to approximately 1 to 10 hours to achieve the carburized layer 12 thickness required for the gear component. It is preferable to perform carburizing so that the surface carbon concentration at the end of the carburizing process is equal to or higher than the eutectoid carbon concentration and lower than the carbon concentration corresponding to the Acm line at the carburizing temperature. By maintaining the surface carbon concentration at or above the eutectoid carbon concentration, carbides in the carbide layer 13 are dispersed after the reheating process. These carbides improve bending fatigue strength and pitting fatigue life. If the surface carbon concentration is equal to or higher than the carbon concentration corresponding to the Acm line at the carburizing temperature, coarse carbides precipitate along the prior austenite grain boundaries during the carburizing process. The coarse carbides precipitated along the grain boundaries remain after the reheating process, resulting in reduced hardenability and reduced root hardness. A higher surface carbon concentration at the end of the carburizing process increases the carbide area ratio after the reheating process, reducing hardenability and root hardness. Therefore, a range of 1.0 to 2.0 wt% is preferable. The cooling rate in the carburizing process is not particularly limited as long as it maintains the above carbon concentration in a supersaturated state during cooling and does not form carbides that could become coarse carbides in the subsequent reheating process. The temperature after cooling in the carburizing process is set to a temperature lower than the A1 point. If the temperature after cooling in the carburizing step is equal to or higher than the A1 point, the austenite does not transform into pearlite, and the carbides in the carbide layer 13 do not disperse in the reheating step.

[0023] In the reheating process, after the gear component 1 is soaked, a hydrocarbon gas such as acetylene is intermittently introduced for a certain period of time to introduce carbon from the surface to the interior of the material and diffuse it for quenching. This controls the carbide cores in the carbide layer 13 formed during the cooling process in the carburizing process and the heating and soaking processes in the reheating process so that the carbides are centered within the above-mentioned area ratio and number density ranges required for improving root hardness, resulting in a gear component 1 with high root hardness after quenching. Furthermore, if the carbides in the carbide layer 13 obtained by the supersaturated carbon concentration after the carburizing process can achieve the above-mentioned carbide area ratio and number density conditions, the introduction of acetylene or the like in the reheating process is not necessary. The temperature in the reheating process is preferably above the A1 transformation temperature and below the Acm temperature at the surface carbon concentration at the end of the reheating process. If the temperature in the reheating process is below the A1 transformation temperature, the structure will not transform to austenite, and the gear component 1 will not be hardened during the quenching process. If the temperature in the reheating process is equal to or higher than the Acm temperature at the surface carbon concentration of the component at the end of the carburizing process, the precipitated carbides in the carbide layer 13 will dissolve. The higher the temperature in the reheating process, the lower the number density of the carbides in the carbide layer 13. However, the amount of retained austenite after quenching increases, resulting in a decrease in hardness. Therefore, the temperature is preferably set to approximately 820 to 900°C. The reheating process time is longer than the general reheating time, specifically, approximately 3 to 24 hours, to allow the carbides in the carbide layer 13 to undergo Ostwald ripening and satisfy the above-mentioned conditions for the area ratio and number density of the carbides. Furthermore, to control the structure, the temperature may be appropriately changed during the process, or a cooling / heating process may be added as needed. The temperature after quenching in the reheating process is set to a temperature lower than the Ms point. If the temperature after quenching in the reheating process is equal to or higher than the Ms point, the austenite does not transform into martensite, resulting in no hardening.

[0024] In the tempering process after the high-concentration carburizing process, the temperature is appropriately set within a range of 150°C to 250°C, and the gear component 1 is soaked. In this case, the treatment time is not particularly limited as long as the gear component 1 is soaked, and the cooling method is also not particularly limited. [Example]

[0025] The present invention will be described in more detail below with reference to examples and comparative examples. Gear components 1 of Examples 1 to 3 and Comparative Examples 1 to 5 were produced by varying the high-concentration carburizing treatment conditions and rim thickness D as shown in Table 1. FIG. 8 is a diagram showing an overview of the high-concentration carburizing treatment process in each Example and Comparative Example. The carbide area ratio and carbide number density of the tooth flank 17 and tooth root 18 were calculated using image analysis software after etching a radial cross section of the gear component shown in FIG. 4 in sodium picrate heated to 80°C for 20 minutes to reveal only the carbides as black. The hardness ratios of the tooth tip 16, tooth flank 17, and tooth root 18 were calculated by dividing the value obtained by testing the radial cross section of the gear component shown in FIG. 4 using a Vickers hardness tester by the value obtained by testing a gear component that had undergone eutectoid carburization. The tooth surface fatigue life was evaluated by a tooth surface fatigue test, which is an element test that simulates tooth surface fatigue. FIG. 9 shows the shape of the test specimens used in the tooth surface fatigue test in each Example and Comparative Example. The pinion 20 was made by hot forging, normalizing, and cutting the steel material having the above-mentioned chemical composition, followed by high-concentration carburizing treatment shown in Table 1 and tempering. The gear 30 was made by hot forging, normalizing, and cutting the steel material having the above-mentioned chemical composition, followed by eutectoid carburizing treatment and tempering. The tooth surface fatigue test was carried out by meshing the pinion 20 and the gear 30 as a pair, using reducer lubricating oil at an oil temperature of 70°C, and applying a Hertzian surface pressure of 230 kgf / mm. 2 The test was conducted under the following conditions. The tooth surface fatigue life was defined as the number of cycles of the pinion 20 until pitting fatigue damage occurred on the tooth surface of the pinion 20. The tooth surface fatigue life ratio was calculated by dividing the tooth surface fatigue life of the pinion 20 that had undergone high-concentration carburizing treatment by the tooth surface fatigue life of the pinion 20 that had undergone eutectoid carburizing treatment. In Table 1, * indicates that the tooth root or the surface layer of the tooth surface contains a portion with a carbide area ratio exceeding 10%, or the carbide number density at the tooth root is 6 particles / 10 μm 2 This indicates cases where the part includes more than .

[0026] [Table 1]

[0027] In Examples 1 to 3, gear components 1 having rim thicknesses D of 50 mm and 120 mm were treated under the high-concentration carburizing treatment conditions shown in FIG. 8A, and the reheating time was controlled within a range of 3 to 24 hours, whereby a carbide area ratio of 10% or less and a carbide number density of 6 particles / 10 μm were obtained on the surface of the tooth base. 2 The surface layer of the tooth surface was controlled to include a portion where the carbide area ratio was 10% or less.

[0028] In Comparative Examples 1, 2, 4, and 5, the gear components 1 having rim thicknesses D of 50 mm and 120 mm were treated under the high-concentration carburizing treatment conditions shown in FIG. 8B, and the reheating time was controlled within the range of 1 to 3 hours. As a result, the carbide area ratio exceeded 10% or the carbide number density was 6 particles / 10 μm on the surface of the tooth base. 2 In Comparative Example 4, the surface layer of the tooth surface was further controlled to include a portion where the carbide area ratio exceeded 10%.

[0029] In Comparative Example 3, a gear component 1 having a rim thickness D of 50 mm was treated under the high-concentration carburizing treatment conditions shown in FIG. 8C. The reheating process time was set to a range of 3 to 12 hours, and a hydrocarbon gas was introduced intermittently for a certain period of time during the reheating process, thereby controlling the surface layer of the tooth base and tooth surface to include areas with a carbide area ratio exceeding 10%.

[0030] Fig. 10 is a graph showing the relationship between the material structure and properties in each example and comparative example. Fig. 10A shows the relationship between the dedendum carbide number density and the dedendum hardness ratio, and plots those examples and comparative examples in Table 1 that have a dedendum carbide area ratio of 10% or less. The dedendum hardness ratio is calculated based on the dedendum carbide number density of 6 / 10µm. 2 FIG. 10B shows the relationship between the carbide area ratio at the root and the hardness ratio at the root. 2The following are plotted. The tooth root hardness ratio changes significantly when the tooth root carbide area ratio exceeds 10%. Figure 10C shows the relationship between the tooth surface carbide area ratio and the tooth surface fatigue life ratio. The tooth surface fatigue life ratio changes significantly when the tooth surface carbide area ratio exceeds 10%.

[0031] In addition, the present invention is not limited to the above-mentioned embodiment, and can be carried out in various modified forms without departing from the spirit of the present invention. For example, among the various configurations described above, at least the carbide area ratio in at least a part of the carbide layer 13 at the tooth base 18 is 10% or less, and the carbide number density is 6 / 10 μm 2 The following is fine. [Explanation of symbols]

[0032] 1 Gear parts 10 surface 11 Mother phase 12 Carburized layer 13 Carbide layer 14 Tooth 15 inner hole 16 Tooth tip 17 Tooth surface 18 Tooth dedendum 20 Pinion 30 Gear D Rim Thickness R Pitch circle W Face width P center line All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety.

Claims

1. A gear component having a tooth portion having a tooth base, a tooth tip, and a tooth surface between the tooth base and the tooth tip, the gear component being made of a steel material that has been subjected to high-concentration carburizing treatment, the tooth base, the tooth surface, and the tooth tip have a carbide layer in which carbides are dispersed and precipitated on the surface layer, The carbide area ratio in at least a part of the carbide layer at the tooth base is 10% or less, and the carbide number density is 6 particles / 10 μm ２ is as follows: A gear component, characterized in that the thickness of the carbide layer at the tooth base is greater than the thicknesses of the carbide layers at the tooth surface and the tooth tip.

2. 2. The gear component according to claim 1, wherein a carbide area ratio in at least a portion of the carbide layer on the tooth surface is 10% or less.

3. 2. The gear component according to claim 1, wherein a carbide area ratio of the carbide layer on the tooth surface is greater than a carbide area ratio of the carbide layer on the tooth base.

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