JP2007307678A - Shot peening method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem wherein a value itself of compressive residual stress of a generated surface layer is low, and fatigue strength is not sufficient since hardness of a shot grain is low or a grain size of the shot grain is large in a conventional shot peening. <P>SOLUTION: In this shot peening method to be performed after performing carburizing treatment for a surface of a steel member, first shot peening is performed by the first shot grain having hardness ≥HRC60 in Rockwell hardness and the grain size ≥200μm and ≤400μm, and second shot peening is performed by the small second shot grain having hardness ≥HRC60 in Rockwell hardness and a grain size ratio ≥0.1 and ≤0.3 to the first shot grain. More preferably, both of the hardness of the first shot grain and the second shot grain are made ≤HRC65 in Rockwell hardness. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

    The present invention relates to a shot peening treatment method that can be carried out after carburizing the surface of a steel member such as a gear to significantly improve the fatigue characteristics of the steel member.

  Conventionally, steel members such as gears have been required to have high fatigue strength, and in particular, due to the recent demand for higher output and smaller engines, the load on steel members has been increasing, so that fatigue strength is further increased. Improvement is demanded. As a technique for improving such fatigue strength, a technique is known in which a steel member subjected to carburizing treatment or a steel member reheated after carburizing treatment is subjected to shot peening treatment to harden the surface and generate high compressive residual stress. It has been.

In this shot peening treatment method, the peak position of the compressive residual stress is set near the surface where fatigue starts, by performing shot peening using fine iron-based shot particles on a steel member that has been subjected to vacuum carburization. A technique for improving the strength is known (for example, see Patent Document 1).
In addition, after performing the first shot peening with the first shot particles, the second shot peening is performed using the small second shot particles having a predetermined particle size ratio with respect to the first shot particles, thereby reducing the compressive residual stress. A technique for improving the distribution in the depth direction and improving the fatigue strength is known (for example, see Patent Document 2).
Furthermore, after the first shot peening is performed with the first shot particles having a predetermined particle size, the second shot peening is performed using the second shot particles that are very small compared to the first shot particles, thereby achieving high compression residual. A technique for improving fatigue strength by introducing stress to the surface is also known (see, for example, Patent Document 3).
Japanese Examined Patent Publication No. 6-72254 Japanese Patent No. 2723150 JP 2002-30344 A

However, in shot peening using the fine iron-based shot particles among the above-described shot peening treatments, the hardness of shot particles used is as low as HRC50 or more and less than 58 in terms of Rockwell hardness. The value itself was low and the fatigue strength was not sufficient.
Also, in the two-stage shot peening using a small second shot particle corresponding to a predetermined particle size ratio of the first shot particle, the distribution of compressive residual stress is improved, but the first shot particle and the second shot particle Since the hardness is as low as HRC50 or more and less than 58 in terms of Rockwell hardness, there is still a problem that the value of the compressive residual stress itself is low and sufficient fatigue strength cannot be obtained.
Furthermore, in the two-stage shot peening using the second shot particle which is very small compared to the first shot particle, the hardness of the first shot particle and the second shot particle is as high as HRC60, but the particle size of the first shot particle Is 400 to 800 μm, and the peak position of compressive residual stress shifts to the inside. Therefore, when fine particles with a diameter of 60 μm or less that can minimize the influence of the surface notch are used, discontinuity occurs in the compressive residual stress distribution, The required residual stress at a depth of 20 to 30 μm is lowered, and there is a problem that sufficient fatigue strength cannot be obtained.

The problem to be solved by the present invention is as described above. Next, means for solving the problem will be described.
That is, in Claim 1, in the shot peening treatment method performed after carburizing the surface of the steel member, the first shot particles having a hardness of Rockwell hardness of HRC 60 or more and a particle size of 200 μm or more and 400 μm or less. The first shot peening is performed by the second shot particles having a Rockwell hardness of HRC 60 or more and a particle size ratio of 0.1 to 0.3 with the first shot particles. Two shot peening is applied.
According to a second aspect of the present invention, the hardness of the first shot particles and the second shot particles are both Rockwell hardnesses of HRC 65 or less.

Since this invention was comprised as mentioned above, there exists an effect shown below.
That is, in Claim 1, in the shot peening treatment method performed after carburizing the surface of the steel member, the first shot particles having a hardness of Rockwell hardness of HRC 60 or more and a particle size of 200 μm or more and 400 μm or less. The first shot peening is performed by the second shot particles having a Rockwell hardness of HRC 60 or more and a particle size ratio of 0.1 to 0.3 with the first shot particles. Since two shot peening is performed, the value of the compressive residual stress produced | generated on the surface of a steel member and its vicinity can be increased significantly, and sufficient fatigue strength can be provided to a steel member.
Further, in claim 2, since the hardness of each of the first shot particles and the second shot particles is HRC65 or less in Rockwell hardness, the hardness of the shot particles is suppressed as low as possible. In addition to preventing high costs due to the use of appropriate shot particles, surface roughness can be minimized by minimizing the formation of indentations on the steel member surface due to collision of shot particles, thereby improving the surface quality.

Next, embodiments of the invention will be described.
FIG. 1 is a flowchart showing a manufacturing process of a steel member subjected to shot peening according to the present invention, FIG. 2 is an explanatory diagram showing the effect of the number of repetitions on the residual stress distribution, and FIG. 3 is a residual stress at the time of only one stage of shot peening. FIG. 4 is an explanatory diagram showing the effect of shot particle hardness and particle size on the distribution, FIG. 4 is an explanatory diagram showing the effect of shot particle size on the residual stress distribution, and the residual stress distribution after two-stage shot peening, FIG. 5 is an explanatory diagram showing the effect of shot particle hardness on the maximum compressive residual stress, and FIG. 6 is an explanatory diagram showing the effect of the particle size of the first shot particle on the depth from the surface of the high compressive residual stress region. FIG. 7 is an explanatory diagram showing the effect of the particle size of the first shot particles on the surface roughness, and FIG. 8 is an explanatory diagram showing the effect of the particle size ratio between the first shot particles and the second shot particles on the fatigue life. , FIG. FIG. 9A is an explanatory diagram showing a heat cycle in vacuum carburizing treatment, FIG. 9A is an explanatory diagram of a heat cycle of heat treatment 1, FIG. 9B is an explanatory diagram of a heat cycle of heat treatment 2, and FIG. 10 is a fatigue test. It is a block diagram which shows the structure of an apparatus. Here, the high compressive residual stress region indicates a layer having a compressive residual stress higher than −1400 MPa.

First, the manufacturing process of the steel member which performs shot peening according to the present invention will be described with reference to FIG.
As the material, alloy steel such as SCM420H suitable for carburizing treatment is prepared, cut into a predetermined size (step S1), and then processed into a rough product by cutting or hot forging (step 2). At this time, if necessary, a normalizing process is performed to return the crystal structure coarsened by heat to the standard structure (step S3). Next, a steel member is obtained by processing the rough shape product into a product shape (step S4) by performing predetermined processing such as turning, gear cutting, etc. for a gear, and subsequently, the steel member A vacuum carburizing and quenching process (step S5) and a tempering process (step S6) for removing brittleness and distortion caused by the quenching are performed, and then a two-stage shot peening process according to the present invention is performed.

  The two-stage shot peening treatment is performed by first shot peening with first shot particles having a Rockwell hardness of HRC 60 or more and a particle size of 200 μm or more and 400 μm or less, and then the hardness is Rockwell hardness. The second shot peening is performed with small second shot particles having an HRC of 60 or more and a particle size ratio of 0.1 to 0.3 with respect to the first shot particles.

The reason why the hardness and particle size of each shot particle are limited in this way is based on the following results which the present inventors have earnestly studied.
When a stress is repeatedly applied to the steel member, microcracks are generated, and the cracks extend starting from the microcracks, leading to fatigue failure. Therefore, in order to further improve the fatigue strength, the microcrack generation limit stress, which is the starting point of fatigue, is increased to suppress the microcrack generation itself, and the microcrack is prevented from extending and growing into a fatigue crack. It is important to increase the compressive residual stress at the starting point of microcracks and to suppress the propagation of cracks while minimizing the decrease in compressive residual stress associated with microcracks. I can say that.

  Here, in this embodiment, since the steel member is subjected to vacuum carburization, there is no surface abnormal layer due to grain boundary oxidation, so the microcracks are generated from the surface. For this reason, it is important to simultaneously increase the compressive residual stress on the surface and the compressive residual stress at the tip of the compressive residual stress reduction region near the surface.

  From this point of view, various methods for obtaining a higher fatigue strength have been studied, and it has been found that it is extremely effective to increase the hardness of the shot particles and simultaneously reduce the particle size. In other words, by making the shot particles hard, the value of the generated compressive residual stress itself can be increased, and with regard to the particle size, if the particle size is large, the compressive residual stress is generated with a wide distribution width, Since the peak position moves closer to the inside of the steel member, the particle size is conversely reduced, and the peak position is set at the front end of the compressive residual stress lowering region closer to the steel member surface.

  However, the single stage shot peening with such high hardness small diameter shot particles still leaves the surface compressive residual stress low. Then, when the improvement method of the surface compressive residual stress was examined, it turned out that it can improve by performing two-stage shot peening using the 1st shot particle and the 2nd shot particle which has a particle size ratio of a specific range. That is, the particle size of the second shot particle is made smaller than that of the first shot particle in order to increase the compressive residual stress on the surface, and in this case, in order to optimize the distribution of the compressive residual stress, the appropriate range of the particle size ratio is set. Is prescribed.

  Here, FIG. 2 shows a change in the depth direction distribution of residual stress when a roller pitching test is performed on a steel member subjected to one-step shot peening with high hardness small diameter shot particles having a particle size of 55 μm by HRC63. .

According to this, microcracks occur early in the test (number of repetitions: 1 × 10 ⁴ ), and the compressive residual stress near the surface is released, and the compressive residual stress gradually decreases as the number of repetitions increases. Then, it can be seen that the depth from the surface at which this decrease is not recognized, that is, the region where the compressive residual stress is reduced due to the occurrence of microcracks exists from the surface to about 15 μm.

  Further, the depth of the residual stress of a steel member subjected to only one stage of shot peening with three types of shot particles having a hardness HRC53 of 600 μm, a hardness of HRC63 of 600 μm, and a hardness of HRC63 of 250 μm The change in the direction distribution is shown in FIG.

  When the hardness is increased from HRC53 to HRC63 with shot particles having the same particle size of 600 μm, the compressive residual stress increases with a wide distribution width, but the increase in the vicinity of the surface is not sufficient, and the peak The position also moves from the surface to the inside (see curves 1 and 2). Therefore, when the particle size is reduced from 600 μm to 250 μm with shot particles having the same hardness HRC63, the peak position of the compressive residual stress moves greatly from the inside toward the surface to a depth of about 15 μm (see curves 2 and 3). ). That is, it can be seen that the compression residual stress in the compression residual stress reduction region can be increased by increasing the hardness and miniaturization of the shot particles.

  Further, the second shot peening is performed with the second shot peening with the hardness HRC63 and the second shot particles with the particle size of 55 μm, following the first shot peening with the high hardness small diameter first shot particles with the particle size of 250 μm. FIG. 4 shows a change in the distribution of residual stress in the depth direction of the steel member subjected to.

  The compressive residual stress distribution in the case of two-stage shot peening is a shape that combines the compressive residual stress distribution in the case where only one stage of shot peening is performed with each stage of shot particles. It can be seen that the compressive residual stress of the surface layer is significantly higher than the compressive residual stress distribution of the first shot peening due to (see curves 3, 4, and 5).

  Based on the above findings, the effects of the hardness and size of the shot particles on the fatigue strength were investigated, and it was found that the hardness of the first shot particles and the second shot particles is preferably HRC 60 or higher. If it is less than HRC60, the loss of collision energy increases due to deformation of shot particles at the time of collision, sufficient compressive residual stress cannot be obtained, and a significant improvement in fatigue strength cannot be expected.

  In addition, the hardness of the first shot particles and the second shot particles may be limited to a range of HRC 60 or more and 65 or less. If HRC65 is exceeded, special shot particles such as high-speed steel, amorphous steel, and cemented carbide will be required, resulting in high cost. Furthermore, the formation of indentations on the steel member surface due to impact becomes significant, resulting in surface roughness. This is because it becomes larger.

  The particle size of the first shot particles is preferably 200 μm or more and 400 μm or less. If it is less than 200 μm, a high peak value of compressive residual stress is obtained near the surface, but the distribution width is narrow. This is because high fatigue strength cannot be obtained. On the other hand, if it exceeds 400 μm, the distribution width of the generated compressive residual stress becomes wide, but the surface roughness becomes large.

  Furthermore, when using high-hardness shot particles of HRC 60 or higher as in the present invention, the particle size of the second shot particles is 0.1 to 0.3 in terms of the particle size ratio with the first shot particles. Is preferred. If the particle size ratio is less than 0.1, the difference between the compressive residual stress generated by the first shot particles and the compressive residual stress generated by the second shot particles is too large, and the compressive residual stress after two-stage shot peening This is because a discontinuity occurs in the distribution of, and the increase in the compressive residual stress value at the required 20-30 μm depth is small, and high fatigue strength cannot be obtained. On the other hand, when the particle size ratio exceeds 0.3, the difference between the compressive residual stress generated by the first shot particles and the compressive residual stress generated by the second shot particles is small, and the surface layer formed by the first shot particles This is because the compressive residual stress cannot be backed up by the second shot particles.

  The appropriate particle size ratio in the present invention is 0.1 or more and 0.3 or less as compared with the appropriate particle size ratio in the case of using conventional low hardness shot particles, for example, the appropriate particle size ratio in Patent Document 2 above. However, this is because the overall value of compressive residual stress is greatly increased by the first shot peening with the high-hardness small-diameter first shot particles. The compressive residual stress also needs to be increased to the same extent, but for that purpose, extremely high compressive residual stress is introduced on the surface by using smaller second shot particles than in the case of low hardness shot particles. I guess it must be done.

Examples of the present invention will be described below.
First, three types of alloy steels having the compositions shown in Table 1 were processed according to the manufacturing process shown in FIG. 1 to produce gears (module: 2.25, number of teeth: 13, tooth width: 28 mm), Test gears A to D were used. Note that the test gears A and B are alloy steels having the same composition with different heat treatment conditions.

  As shown in FIG. 9, in heat treatment 1 of Table 1, after vacuum carburizing at 980 ° C. for 2 hours, oil quenching was performed at 850 ° C. (step S5), followed by air cooling at 160 ° C. for 2 hours. Then, tempering is performed (step S6). In heat treatment 2, the amount and thickness of carburization are increased by extending the time of vacuum carburization in heat treatment 1 from 2 hours to 3.5 hours.

  In this embodiment, such a vacuum carburizing process is applied, but a vacuum ion carburizing process or the like may be applied in addition to the vacuum carburizing process, and when the collision energy of shot particles acts on the starting point of microcracks. If the carburizing treatment can make the surface abnormal layer serving as a barrier of the thickness as thin as possible, the effect of the present invention can be effectively exhibited.

  Next, the test gears A to D are subjected to shot peening treatment with various shot conditions changed, and the surface roughness Rmax (maximum roughness: μm) is measured for the test gear after the treatment. A fatigue test was conducted.

  As shown in FIG. 10, in the fatigue test apparatus 6, the test gear 8 is meshed with the counterpart gear 9 (module: 2.25, number of teeth: 62, tooth width: 28 mm) in the gear box 10, The test gear 8 is linked to the input shaft 12 from the motor 7, and the output shaft 13 of the counterpart gear 9 is linked to the power absorption type load device 11. The test gear 8 can be given a constant load of 84 MPa with a rolling contact pressure Ke.

  In this constant load state, the test gear 8 is rotated at a constant speed of 180 rpm by the motor 7, and the mating gear 9 to be engaged is rotationally driven. Then, the motor 7 is driven until the teeth of the test gear 8 are damaged by progressive pitting, the number of meshing repetitions until the defects are generated is determined as the fatigue life, and each of the effects on the fatigue life is determined. The effects of shot peening conditions were compared. In addition, all the defect | deletion forms in this test were due to the tooth surface fatigue.

  The results of investigating the influence of the hardness of shot particles on the maximum compressive residual stress and the influence of the particle size of the first shot particles on the depth from the surface of the high compressive residual stress region are shown in FIGS. .

  In FIG. 5, the change in the maximum compressive residual stress obtained when the test gear: A, the particle size of the first shot particle (hereinafter referred to as “D1”): 300 μm is changed is shown as a curve 14. Show.

  According to the curve 14, the maximum compressive residual stress increases rapidly when the hardness of the shot particles becomes HRC60 or higher, but if the shot particle hardness is less than HRC60, the maximum compressive residual stress is as low as 1000 MPa, and a great improvement in fatigue life cannot be expected. . On the other hand, if it exceeds HRC65, the surface roughness becomes large and there is a concern about the deterioration of the surface quality. Therefore, the shot hardness is desirably HRC65 or less.

  FIG. 6 shows that the test gears: A, D1 and the particle size ratio of the second shot particles (hereinafter referred to as “D2”) D2 / D1: substantially constant at 0.20 to 0.25, Hardness of one shot particle and second shot particle: The change in depth from the surface of the high compressive residual stress region when D1 is changed under the basic condition of HRC63 is shown as a curve 15, FIG. The change in surface roughness when D1 is changed under the same condition is shown as a curve 16.

  According to the curve 15, in order to prevent a crack from propagating by causing a high compressive residual stress to exist at a position deeper than the compressive residual stress reduction region shown in FIG. 2 that occurs during operation, D1 of φ200 μm or more is required. I know that there is. On the other hand, from the curve 16, when D1 exceeds φ400 μm, D2 becomes φ100 μm or more under these conditions, the surface roughness increases and the surface quality deteriorates.

  Further, FIG. 8 shows fatigue life when the particle size ratio D2 / D1 is changed under the basic conditions of test gear: A, D2: 55 μm, hardness of the first shot particle and second shot particle: HRC63. Is shown as a curve 17, and plots 18, 19, and 20 are also written as comparison data with the curve 17.

  According to the curve 17, there is an appropriate range for the particle size ratio between the first shot particles and the second shot particles. In the high hardness conditions according to the present invention, for example, HRC63, this appropriate range is 0.1 or more and 0.3. It can be seen that the fatigue life is greatly reduced regardless of whether it is smaller or larger than the appropriate range.

  Further, when the curve 17 and the plots 18, 19, and 20 are compared, even if the test gear has various alloy steel compositions and heat treatment conditions, sufficient fatigue can be obtained by performing the shot peening treatment according to the present invention. It can be seen that the lifetime is secured.

  That is, in the shot peening treatment method performed after carburizing the surface of the steel member, the first shot peening is performed with the first shot particles having a hardness of Rockwell hardness of HRC 60 or more and a particle size of 200 μm or more and 400 μm or less. Next, the second shot peening is performed with small second shot particles having a Rockwell hardness of HRC 60 or more and a particle size ratio of 0.1 to 0.3 with respect to the first shot particles. The value of the compressive residual stress produced | generated on the surface of a steel member and its vicinity can be increased significantly, and sufficient fatigue strength can be provided to a steel member.

  Further, since the hardness of the first shot particle and the second shot particle are both HRC65 or less in Rockwell hardness, the hardness of the shot particle is kept as low as possible, thereby using the special shot particle. In addition to preventing an increase in cost, surface roughness can be minimized by minimizing the formation of indentations on the steel member surface due to shot particle collisions, thereby improving the surface quality.

  The present invention is a shot peening treatment method capable of greatly improving the fatigue strength of a steel member, and is not limited to a steel member subjected to carburizing treatment, but a surface treatment capable of imparting compressive residual stress in the vicinity of the surface by shot particle collision energy. For example, in addition to carburizing treatment, the present invention is applied to various steel members subjected to carbonitriding treatment, plating treatment of a carbon-enriched layer on the steel member surface, and the like. It is possible.

It is a flowchart which shows the manufacturing process of the steel member which performs the shot peening process concerning this invention. It is explanatory drawing which shows the influence of the repetition number which acts on residual-stress distribution. It is explanatory drawing which shows the influence of the hardness of a shot particle and a particle size on the residual stress distribution at the time of shot peening of only one step. It is explanatory drawing which shows the influence of the particle size of the shot particle which acts on a residual stress distribution, and the residual stress distribution situation after two-step shot peening. It is explanatory drawing which shows the influence of the hardness of the shot particle which acts on the maximum compressive residual stress. It is explanatory drawing which shows the influence of the particle size of the 1st shot particle | grains which acts on the depth from the surface of a high compressive residual stress area | region. It is explanatory drawing which shows the influence of the particle size of the 1st shot particle | grains which acts on surface roughness. It is explanatory drawing which shows the influence of the particle size ratio of the 1st shot particle and the 2nd shot particle which has on a fatigue life. FIG. 9A is an explanatory diagram showing a heat cycle in vacuum carburizing treatment, FIG. 9A is an explanatory diagram of a heat cycle of heat treatment 1, and FIG. 9B is an explanatory diagram of a heat cycle of heat treatment 2. FIG. It is a block diagram which shows the structure of a fatigue test apparatus.

Claims (2)

  In the shot peening treatment method performed after carburizing the surface of the steel member, the first shot peening is performed with the first shot particles having a hardness of Rockwell hardness of HRC60 or more and a particle size of 200 μm or more and 400 μm or less, Next, the second shot peening is performed with small second shot particles having a Rockwell hardness of HRC 60 or more and a particle size ratio of 0.1 to 0.3 with respect to the first shot particles. The shot peening processing method. 2. The shot peening processing method according to claim 1, wherein the hardness of each of the first shot particles and the second shot particles is Rockwell hardness of HRC 65 or less.