JP2013213260A - Surface treatment method of gear - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treatment method of a gear, capable of improving the conformability of a tooth surface after being subjected to a treatment in which the tooth surface of a gear is highly hardened.SOLUTION: A surface treatment method of a gear includes: a first step (block B2) of forming a hardened layer at a surface of a gear; and then, a second step (block B3) of smoothing the hardened layer or causing a compressive residual stress to be generated at the surface. The surface treatment method of a gear further includes a third step (block B4) of forming a softened layer on the hardened layer by softening the surface of the gear on which the hardened layer has been formed, after the second step (block B3).

Description

  The present invention relates to a surface treatment method for gears that are in contact with each other by applying a load, and more particularly, to a surface treatment method for gears that have been subjected to a treatment for increasing the hardness of a surface such as carburizing and quenching used in a transmission or the like. It is.

  Gears used in vehicle transmissions and the like are generally subjected to treatments such as carburizing quenching, nitriding treatment, and induction hardening in order to increase the hardness of the gear teeth processed into a desired shape. Manufactured by shaving or grinding to smooth the tooth surface. In addition, by performing shot peening instead of the above-described shaving and grinding, a relatively soft incomplete quenching layer called a carburizing abnormal layer generated when carburizing and quenching is performed, and teeth are removed. Compressive residual stress is also generated on the surface. An example of such a gear manufacturing method is described in Patent Documents 1 to 3. For example, Patent Document 1 describes a method of manufacturing a gear configured to perform compressive residual stress on a tooth surface by performing shot peening after carburizing and quenching the gear.

  Patent Document 2 describes a gear manufacturing method in which a gear machined into a desired shape is subjected to shaving and grinding, and then the tooth surface is hardened by carburizing and quenching. According to the manufacturing method described in Patent Document 2, by manufacturing the gear in such an order, it is possible to prevent fine scratches called grinding marks generated during grinding from causing cracks and pitching. Or it can be suppressed. In addition, among the carburizing abnormal layers generated when carburizing and quenching is performed, the gears contact each other up to a depth of 10 μm from the tooth surface, that is, when the gears are in contact with each other. It is said that the contact fatigue strength of the tooth surface can be improved by being worn.

  Further, in Patent Document 3, shot peening is performed on a carburized and hardened gear to generate a compressive residual stress on the surface and the surface is made porous, and then shot peening is performed using relatively soft metal particles. A method of manufacturing a gear configured to form the metal coating on the surface by performing is described. According to the manufacturing method described in Patent Document 3, since the metal coating is formed on the surface of the gear, the adhesion between the other tooth surfaces with respect to one tooth surface of the gears meshing with each other (familiarity, In other words, the fatigue strength of the gear can be improved compared to the conventional case.

JP 2001-11572 A Japanese Unexamined Patent Publication No. 6-91441 JP-A-1-92069

  The tooth surfaces of gears that have undergone carburizing, nitriding, or shot peening are hardened, but their fatigue strength improves, but wear due to contact between the tooth surfaces is less likely to occur, so the tooth surfaces become familiar. It becomes difficult. Therefore, when a vehicle transmission is configured using the gears described in Patent Document 1 and Patent Document 2, the time required for so-called “run-in” becomes long, and in particular when running-in is performed. There is a possibility that the power transmission efficiency between the gears is lowered and the fuel consumption is deteriorated. In addition, in the gear described in Patent Document 3, the wear of the metal coating can improve the tooth surface fit, but when the metal coating is scraped off by wear, a metal coating is formed. Since the surface of the gear is made porous, there is a possibility that the familiarity may not be promoted depending on the surface roughness. It has been conventionally known that the carburized abnormal layer is a starting point for fatigue failure such as cracking and pitting, and contributes to a decrease in fatigue strength.

  The present invention has been made paying attention to the above technical problem, and provides a gear surface treatment method capable of improving the conformability of the tooth surface after performing the treatment for increasing the hardness of the tooth surface of the gear. It is intended to provide.

  In order to achieve the above object, the invention according to claim 1 is a first step of forming a hardened layer on the surface of the gear, and then a second step of smoothing the hardened layer or generating compressive residual stress on the surface. And a third step of forming a softened layer on the hardened layer by softening a surface of the gear on which the hardened layer is formed after the second step. Is a manufacturing method characterized by

  The invention of claim 2 is the invention of claim 1, wherein the third step is from the surface to a depth corresponding to the maximum roughness in the surface roughness of the hardened layer, or within 10 μm from the surface. It is a gear surface treatment method characterized by softening up to a depth of.

  According to a third aspect of the present invention, in the first aspect of the invention, the third step includes tempering the surface, relatively reducing the amount of carbon on the surface, and corroding the surface by etching. And a step of oxidizing the surface by oxygen by raising the temperature of the surface in an oxygen atmosphere.

  The invention of claim 4 is the invention of any one of claims 1 to 3, wherein the second step removes an abnormal carburization layer and a grain boundary oxide layer formed on the surface when the hardened layer is formed. Then, the gear surface treatment method includes any one of a tooth grinding process for smoothing the surface and shot peening for generating compressive residual stress on the surface.

  According to the first aspect of the present invention, since the softened layer is formed on the surfaces of the gears that are in contact with each other by applying a load to each other, the softened layer is worn quickly, and each softened layer is microscopically plastic. The surface roughness in each tooth surface can be reduced and the adhesion between the tooth surfaces can be improved by the fact that the deformation occurs quickly. The so-called familiarity can be promoted to improve the fatigue strength of the gear. In addition, the power transmission efficiency between the gears can be improved and heat generation between them can be avoided as compared with the case where the familiarity between the gears is not promoted. In addition, since the tooth surface is smoothened after the tooth surface is hardened, an incompletely hardened layer, which is referred to as a carburizing abnormal layer and can be a starting point of pitching, can be removed. As a result, according to the present invention, it becomes possible to improve the fuel efficiency of the vehicle using the gear configured as described above.

  According to the invention of claim 2, in addition to the same effect as that of the invention of claim 1, the softening layer has a depth corresponding to the maximum roughness of the tooth surface from the tooth surface or a depth of 13 μm from the tooth surface. Therefore, the softened layer consumed for the above-mentioned familiarity can be ensured sufficiently and sufficiently. Moreover, compared with the case where the carburizing abnormal layer is formed in a range deeper than 13 μm from the tooth surface, the strength against rotational bending fatigue of the gear can be ensured.

  According to the invention of claim 3, in addition to the effect similar to the effect of the invention of claim 1, the surface of the hardened layer can be effectively softened by any method.

  According to the invention of claim 4, in addition to the effect similar to the effect of any one of the inventions of claims 1 to 3, it is possible to smooth the surface by removing the carburizing abnormal layer and the grain boundary oxide layer. In addition, a compressive residual stress can be generated on the surface of the gear.

It is a block diagram which shows an example of the manufacturing process of the gear which concerns on this invention. It is a figure which shows typically the case where the maximum surface roughness of the gear blank surface before performing a softening process is 10 micrometers or less. It is a figure which shows typically the case where the maximum surface roughness of the gear blank surface before performing a softening process is 10 micrometers or more. The maximum surface roughness of the gear blank surface before the softening treatment is 10 μm or less, and the depth corresponding to the maximum surface roughness from the most concave portion on the surface of the gear blank is from the top of the maximum convex portion. It is a figure which shows typically the case where the depth to 10 micrometers is exceeded. It is a figure which shows typically the relationship between the maximum roughness of a tooth surface when the maximum roughness of the tooth surface in the use initial stage of a gearwheel is relatively small, and the maximum roughness of the tooth surface after performing a fitting process. It is a figure which shows typically the relationship between the maximum roughness of a tooth surface when the maximum roughness of the tooth surface in the use initial stage of a gear is relatively large, and the maximum roughness of the tooth surface after performing a conforming process. It is a figure which shows the measurement result of the power transmission efficiency in the gearwheel which is an Example which made surface hardness Hv550 or less, without removing a softening layer. It is a figure which shows the measurement result of the power transmission efficiency in the gear which is a comparative example which removed the softening layer and made surface hardness Hv700 or more. It is a figure which shows typically the measurement result of the Vickers hardness from the tooth surface in an Example.

  The present invention relates to a surface treatment of a member that contacts under load, and typical examples of the member include gears, bearings, and one-way clutches that transmit torque via rolling elements such as sprags. It is. In these members, the contact portions that are in contact with each other by applying a load are limited areas. In the present invention, it is only necessary that the surface treatment described below is performed on the surface of the contact portion which is at least the limited region.

  FIG. 1 is a block diagram showing an example of a manufacturing process of a gear according to the present invention. First, a gear material called a gear blank is prepared, and this is formed into a desired gear shape by, for example, a lathe, a milling machine, or a hobbing machine. So-called gear cutting is performed (block B1). The gear blank mentioned above is comprised by metals, such as steel and cast iron as an example. Next, the surface of the gear blank that has been subjected to gear cutting as described above is hardened to perform a surface hardening process that improves wear resistance and fatigue resistance (block B2). Examples of the treatment include carburizing and quenching treatment in which the surface of the steel is hardened by carbon intrusion, nitriding treatment in which nitrogen is infiltrated into the steel surface instead of carbon, or these treatments simultaneously or continuously. This is a conventionally known process such as a carburizing and nitriding process that is configured to be performed. Note that in place of these treatments, induction hardening, flame hardening treatment, or boride treatment may be performed. In this block B2, the main point is that it is configured to improve the mechanical properties of the gear blank subjected to gear cutting.

  When the surface hardening treatment is performed in the block B2, the surface of the gear blank is inevitably referred to as oxide (grain boundary oxide) such as silicon (Si), manganese (Mn), and chromium (Cr). ) Or a layer of the grain boundary oxide is formed, and a diffusion type transformation product such as pearlite or bainite, which is called an incompletely quenched layer, is formed around it. This incompletely hardened layer may be referred to as a carburized abnormal layer when carburizing and quenching is performed. When such an incompletely hardened layer is present on the tooth surface of the gear to some extent, it is known that the incompletely hardened layer causes a reduction in the fatigue strength of the gear, or a starting point for cracks and pitching. . Therefore, in the present invention, the grain boundary oxide layer and the incompletely quenched layer are removed from the surface of the gear blank by post-quenching processing such as grinding and polishing after the processing in the block B2. Shot peening is performed to leave compressive stress on the surface of the gear blank for the purpose of processing and increasing the fatigue strength of the gear (block B3). By the processing in these blocks B3, the carburizing abnormal layer and the grain boundary oxidation layer formed by the processing in the block B2 are removed. In addition, since the carburizing abnormal layer and the grain boundary oxide layer are softer than the hardened layer, the conformability is good, and the adaptability deteriorates when the carburizing abnormal layer and the grain boundary oxide layer are removed. The above-mentioned tooth grinding process is a conventionally known process such as shaving for smoothing the tooth surface by meshing the above gear, that is, a gear blank and a shaving cutter, or a honing process for grinding the tooth surface with a grindstone. It is. Also, shot peening is a conventionally known technique that generates a compressive residual stress on the tooth surface by projecting a large amount of steel called a shot onto a target object, in this invention a gear blank that has been ground. It is processing.

  Subsequently, the process which softens from the tooth surface to the predetermined thickness or depth among the hardened layer in the tooth surface of the gear blank formed as mentioned above is performed (block B4). This softening process may be a conventionally known process called electron beam processing. Specifically, in the electron beam processing, the surface of the gear blank is accelerated by a bundle of electrons, that is, the temperature in the range of 10 μm from the surface of the gear blank, for example, is set when performing so-called tempering. This is a process of softening the surface by raising the temperature to a temperature. If the energy density and irradiation time of the electron beam described above are adjusted so that the temperature at a depth of 10 μm from the surface of the gear blank described above is about 200 ° C. at which tempering is started, the above range is softened. be able to. Although it is clear that the surface temperature of the gear blank becomes higher than the temperature at 10 μm by irradiating the electron beam, the surface temperature is set to a so-called quenching temperature of approximately 700 ° C. or lower. . Further, instead of electron beam processing, laser hardening processing may be performed. That is, the type of laser irradiating the surface of the gear blank, the energy density, the focus, and the like so that the temperature at a depth of 10 μm from the surface of the gear blank is about 200 ° C. and the surface temperature is about 700 ° C. or less. What is necessary is just to adjust irradiation time and a pulse. That is, this softening treatment may be a conventionally known treatment such as tempering that reheats the surface that has been increased in hardness as described above to reduce the hardness of the surface and restore toughness, Furthermore, the process called decarburization which softens by removing carbon from the surface of steel, for example may be sufficient. When the surface of the gear blank is softened by the heat treatment as described above, for example, if the temperature at 10 μm from the surface of the gear blank is about 200 ° C. and the surface temperature is about 700 ° C. or less as described above. Good. Further, instead of these treatments, a solution having corrosiveness to the steel or cast iron forming the gear blank, that is, an etching treatment for softening the surface by corroding the surface with an etching solution, or the above-described gear blank in an oxygen atmosphere. It may be a treatment in which grain boundary oxides are positively formed on the surface by being exposed to a high temperature. That is, when the surface of the gear blank is softened by such chemical treatment, the kind of the etching solution, the concentration of the etching solution, and the oxygen are used so that the range from the surface of the gear blank to 10 μm is corroded or oxidized. The concentration, exposure time and temperature can be adjusted. In short, the processing in this block B4 only needs to be configured so as to soften the predetermined thickness and depth from the tooth surface in the hardened layer. In addition, the predetermined depth is a range from the tooth surface to a depth corresponding to the maximum roughness in the surface roughness, or from the above-described tooth surface to a depth of 10 μm, as will be described later.

  The gears are meshed with each other to wear the softening layer described above, thereby smoothing each tooth surface and performing a conforming process for improving the adhesion between the tooth surfaces (block B5). The familiar process in the block B5 is basically a conventionally known process called “run-in”. This familiarization process can be omitted.

  Here, the relationship between the thickness or depth of the softened layer and the fatigue strength of the gear will be briefly described. Although the details are not illustrated, the softened layer is formed from the gear surface to the inside of the gear beyond 13 μm. It is generally known that the fatigue strength of the gear in this case is smaller than the fatigue strength when the softening layer is formed to a depth of less than 13 μm from the gear surface. Therefore, it is preferable that the above-mentioned softening layer is within a range not exceeding 13 μm from the gear surface, and even if it has a softening layer, in order to ensure sufficient fatigue strength, the depth should be 10 μm from the tooth surface. Is more preferable. The fatigue test is, for example, a so-called rotational bending fatigue test in which a predetermined bending moment is applied to an object and the object is rotated to repeatedly apply stress until the object breaks. The softened layer is a softened layer called a carburized abnormal layer that is inevitably formed on the surface of the gear when the gear is carburized and quenched as an example.

  The thickness or depth for forming the softening layer will be briefly described. FIG. 2 schematically shows a case where the maximum roughness R in the surface roughness of the gear blank before the softening process in the block B4 is 10 μm or less. As shown in FIG. 2, when the maximum surface roughness R of the gear blank is 10 μm or less, at least the depth corresponding to the maximum surface roughness R is softened from any position on the surface of the gear blank. It only has to be. In FIG. 2, it is shown as a region with cross hatching. When the maximum surface roughness R is 10 μm or more, as shown in FIG. 3, at least the top of the most convex portion on the surface of the gear blank (hereinafter sometimes referred to as the maximum convex portion). It is sufficient that the range corresponding to the depth of 10 to 10 μm is softened. FIG. 3 shows the region with cross-hatching. Furthermore, the maximum surface roughness R is 10 μm or less, and the depth corresponding to the maximum surface roughness R from the most concave portion on the surface of the gear blank is 10 μm deep from the top of the maximum convex portion. When exceeding, as shown in FIG. 4, the range corresponding to the depth from the top part of the largest convex part to 10 micrometers should just be softened. In FIG. 4, it is shown as a region with cross hatching.

  On the other hand, FIG. 5 schematically shows the relationship between the roughness of the tooth surface when the tooth surface roughness is relatively small in the initial use of the gear and the surface roughness of the tooth surface after the blending process. FIG. 6 shows the relationship between the surface roughness of the tooth surface when the gear surface roughness is relatively large in the initial use of the gear and the surface roughness of the tooth surface after the fitting process. It is shown schematically. 5 and 6, the vertical axis indicates the surface roughness of the gear, and the horizontal axis indicates the tooth width, that is, the length in the tooth width direction in which the surface roughness is measured as an example. As shown in FIG. 5 and FIG. 6, when the familiarity processing is performed, the familiarity processing is performed because, for example, a portion protruding upward in FIG. 5 and FIG. 6 wears against the average line of the roughness curve. It is recognized that the surface roughness of the subsequent tooth surface is smoother than that at the beginning of use. Further, as shown in FIGS. 5 and 6, the thickness or depth of the tooth surface worn by the conforming treatment is parallel to the average line of the tooth surface roughness curve at the initial use of the gear, and FIGS. The distance between the line segment that passes through the top of the most convex part in FIG. 6 and the top part of the most convex part in FIG. 5 and FIG. Or less than the maximum roughness. This is presumably because an oil film is formed between the tooth surfaces as wear progresses, and the gears receive loads from each other through the oil film. In addition to this, it is considered that the contact state in which fine unevenness on each tooth surface is plastically deformed gradually decreases, while the contact state in which the unevenness is elastically deformed is gradually increased. In other words, it can be said that the gears are brought into contact with each other mainly due to the oil film and the elastic deformation of the tooth surfaces, so that wear does not easily progress, and as a result, familiarity is terminated. .

  Next, the result of having measured the power transmission efficiency for every gear with different surface hardness is shown. FIG. 7 shows a measurement result of power transmission efficiency in a gear which is an example in which the surface hardness is Hv 550 or less without removing the softened layer, and FIG. 8 shows the surface hardness of Hv 700 or more by removing the softened layer. The measurement result of the power transmission efficiency in the gear which is a comparative example is shown. As shown in FIGS. 7 and 8, it was recognized that the power transmission efficiency was improved as the familiarity progressed. In addition, in the gear having a low surface hardness due to the softening layer, it was recognized that the power transmission efficiency due to running-in increases, that is, the difference in power transmission efficiency before and after running-in is large. These results indicate that the so-called conformability is improved by reducing the hardness of the tooth surface, and that the conformability is decreased when the surface hardness is increased.

  Moreover, the measurement result of the Vickers hardness from the tooth surface in an Example is typically shown in FIG. As shown in FIG. 9, in the state where the processing up to the block B3 is completed, the Vickers hardness is substantially constant regardless of the depth from the surface in the range of several tens of μm from the tooth surface. On the other hand, in this example, it is recognized that the Vickers hardness on the side closer to the surface than 10 μm from the surface is smaller than the Vickers hardness in the portion deeper than 10 μm from the surface, that is, the vicinity of the surface is softened.

  As described above, according to the surface treatment method for a gear according to the present invention, even if the tooth surface is hardened by removing the carburizing abnormal layer inevitably generated by carburizing and quenching, the surface of the hardened layer is obtained. Since the softened surface is used for the conforming treatment, the conformability of the gear can be improved while maintaining the conventional tooth surface strength. By improving the familiarity between the gears, the power transmission efficiency between them can be improved. In addition to these, since the softened layer is formed on the surface of the gear, it is possible to reduce the time required for familiarization. As a result, it is possible to improve the fuel efficiency of a vehicle equipped with a transmission having a gear having such a configuration.

  Here, the relationship between the above-described example and the present invention will be briefly described. The processing in the block B2 corresponds to the first step in the present invention, and the processing in the block B3 corresponds to the second step in the present invention. The processing in block B4 corresponds to the third step in the present invention.

  Block B2 ... 1st process, Block B3 ... 2nd process, Block B4 ... 3rd process.

Claims (4)

In the gear surface treatment method comprising a first step of forming a hardened layer on the surface of the gear and a second step of subsequently smoothing the hardened layer or generating compressive residual stress on the surface,
A gear surface treatment method comprising: a third step of forming a softened layer on the hardened layer by softening a surface of the gear having the hardened layer formed thereon after the second step. The said 3rd process softens from the said surface to the depth equivalent to the maximum roughness in the surface roughness of the said hardened layer, or to the depth within 10 micrometers from the said surface. Gear surface treatment method. The third step includes a step of tempering the surface, a step of relatively reducing the amount of carbon on the surface, a step of corroding the surface by etching, and increasing the temperature of the surface in an oxygen atmosphere. The method of claim 1, further comprising oxidizing the surface with oxygen. In the second step, when the hardened layer is formed, a grinding treatment for smoothing the surface by removing the carburizing abnormal layer and the grain boundary oxide layer formed on the surface, and compressive residual stress on the surface 4. The gear surface treatment method according to claim 1, further comprising any one of shot peening and the like.

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