Wet shot peening method

Abstract

To provide a wet shot peening method which can impart high compressive residual force even to workpiece having high hardness, while suppressing breakage of a projection material.SOLUTION: A wet shot peening method includes a shot peening step of jetting a slurry obtained by mixing liquid and a projection material together with compressed air, using a wet blast device having a nozzle, and making the projection material included in the slurry collide with workpiece, wherein yttria stabilization zirconia is used as the projection material.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a wet shot peening method.

Background Art

[0002] Peening treatment that can impart compressive residual stress to a workpiece is widely used as a treatment for strengthening workpieces such as machined parts that require strength and durability (which may also be referred to as workpieces). Examples of peening treatments include shot peening treatment (see Patent Document 1) in which a projection material is made to collide with the surface of the workpiece, laser peening treatment (see Patent Document 2) in which a pulsed laser is irradiated onto the surface of the workpiece, and water jet peening treatment (see Patent Document 3) in which a liquid is jet-injected at high pressure in a liquid and energy is applied to the surface of the workpiece using cavitation.

[0003] As shot peening treatment in which a projection material is made to collide with a workpiece, in addition to hard shot peening (which may also be referred to as dry shot peening) treatment in which only the projection material is made to collide with the workpiece by an air nozzle or an impeller (a turbine), shot peening treatment (see Patent Document 4) in which a slurry in which a liquid and a projection material are mixed is jetted together with compressed air and the projection material contained in the slurry is made to collide with the workpiece is known. Since shot peening treatment can impart compressive residual stress to a workpiece by a relatively simple method and can increase the strength, it is widely used in the field of peening treatment.

[0004] Recently, higher strength and durability have been demanded for machined parts. In response to this demand, the hardness of machined parts targeted for peening treatment has also become higher. By the way, in the shot peening treatment proposed so far, sufficient compressive residual stress has not been imparted to machined parts (workpieces) with high hardness, and there is still room for improvement. In addition, in order to impart sufficient compressive residual stress, it is necessary to increase the collision energy of the projectile during shot peening treatment, such as by increasing the injection pressure (projection pressure) of the projectile. However, as the collision energy increases, the projectile is more likely to be damaged. In particular, the higher the hardness of the workpiece targeted for shot peening treatment, the more likely the projectile is to be damaged during shot peening. Since the crushed projectile cannot be reused, the crushing of the projectile leads to high costs in shot peening treatment. In addition, the crushed projectile has sharp corners, and when the crushed projectile collides with the workpiece, the surface is damaged and the applied compressive residual stress is released. Not only that, when stress such as bending or tension acts on the workpiece, stress concentration occurs at the damaged part, significantly reducing the strength of the workpiece.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0006] The main object of the present invention is to provide a wet shot peening method capable of imparting high compressive residual stress while suppressing the crushing of the projectile even for a workpiece having high hardness.

Means for Solving the Problems

[0007] The wet shot peening method according to an embodiment of the present disclosure includes a wet shot peening step of using a wet blasting apparatus equipped with a nozzle, injecting a slurry in which a liquid and a projectile are mixed together with compressed air, and causing the projectile contained in the slurry to collide with a workpiece, wherein the projectile is yttria-stabilized zirconia. In the above method, the hardness of the workpiece may be 600 HV or more in terms of Vickers hardness and less than the Vickers hardness of the yttria-stabilized zirconia. In the above method, the nozzle may be a wide nozzle having a slit-shaped nozzle opening.

Advantages of the Invention

[0008] According to the wet shot peening method of the present invention, it is possible to impart high compressive residual stress while suppressing the crushing of the projectile even for a workpiece having high hardness.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0010] Hereinafter, preferred embodiments for carrying out the present invention will be described. Note that the following embodiments do not limit the invention according to each claim, and not all combinations of features described in the embodiments are essential for the solution means of the invention.

[0011] <<Wet Shot Peening Method>> An example of a wet shot peening method according to an embodiment of the present invention (hereinafter referred to as the wet shot peening method of one embodiment) will be described.

[0012] The wet shot peening method referred to in this specification means a shot peening process using wet blasting. Wet blasting means a processing (treatment) method in which a slurry obtained by mixing a liquid and a projectile is sprayed onto a workpiece together with compressed air, and the projectile contained in the slurry is made to collide with the workpiece. The slurry used in wet blasting has a liquid as the main component. The main component referred to here means that the total volume of the liquid with respect to the total volume of the slurry is greater than 50% by volume. This also applies to the slurry used in the wet shot peening method of one embodiment.

[0013] The wet shot peening method of one embodiment includes a wet shot peening step of using wet blasting to make the projectile contained in the slurry collide with the workpiece. The wet shot peening method of one embodiment includes a wet shot peening step of using a wet blasting device equipped with a nozzle to spray a slurry obtained by mixing a liquid and a projectile together with compressed air, and making the projectile contained in the slurry collide with the workpiece. According to the wet shot peening method of one embodiment, compressive residual stress can be imparted to the workpiece, and the surface of the workpiece can be modified. Examples of the surface modification of the workpiece include surface modification that plastically deforms the surface layer of the workpiece and surface modification that changes the crystal structure of the surface layer of the workpiece. By performing surface modification of the workpiece, the fatigue strength and hardness of the workpiece can be increased.

[0014] Further, according to the wet shot peening method of one embodiment, the surface of the workpiece can be uniformly modified even for a workpiece having unevenness on the surface or a workpiece having a curved surface. Also, due to the presence of the liquid contained in the slurry, scattering of the projectile when the projectile collides with the workpiece can be suppressed.

[0015] By the way, recently, there has been an increasing demand to impart a higher compressive residual stress to a workpiece having a high hardness (a workpiece with a high hardness). In order to impart a high compressive residual stress to the workpiece, it is necessary to increase the collision energy when the projectile contained in the slurry collides with the workpiece and to impart energy that causes sufficient plastic deformation and compositional change to the surface layer of the workpiece. However, as the hardness of the workpiece increases, energy loss is likely to occur in the collision energy when the projectile contained in the slurry collides with the workpiece, and there may arise a problem that a sufficient compressive residual stress cannot be imparted to the workpiece due to this energy loss. In other words, there may arise a problem that the collision energy does not act on the above-mentioned plastic deformation and compositional change and is consumed for deformation and destruction of the projectile, and a sufficient compressive residual stress cannot be imparted to the workpiece. Further, when the projection pressure (injection pressure) of the slurry is increased to increase the speed at which the projectile collides in order to increase the collision energy, there may arise a problem that the projectile is likely to be crushed.

[0016] Therefore, in the wet shot peening method of one embodiment, yttria-stabilized zirconia is used as the projectile. In other words, in the wet shot peening method of one embodiment, a slurry in which a liquid and yttria-stabilized zirconia are mixed is used. Yttria-stabilized zirconia as used in the present specification means zirconia whose crystal structure is stabilized by yttria.

[0017] Yttria-stabilized zirconia has properties such as high hardness and specific gravity compared to conventional projection materials. By spraying a slurry onto a workpiece and causing yttria-stabilized zirconia having such properties to collide with the workpiece, high collision energy can be imparted to the workpiece. Also, due to its high hardness, yttria-stabilized zirconia can suppress the loss of collision energy and efficiently impart collision energy to the workpiece. Further, since yttria-stabilized zirconia has a small elastic deformation when colliding with the workpiece, the loss of collision energy due to elastic deformation can be suppressed. According to a wet shot peening method of one embodiment in which the use of yttria-stabilized zirconia enables suppression of the loss of collision energy, high compressive residual stress can be imparted even to a workpiece with high hardness.

[0018] As described above, although yttria-stabilized zirconia can impart high collision energy to a workpiece, it is damaged by the accumulation of heat generated when colliding with the workpiece, and there is a tendency for crushing to progress easily. In particular, as the collision energy increases, the heat accumulated in the yttria-stabilized zirconia increases. Therefore, it can be said that it is important to suppress the accumulation of this heat in order to sufficiently suppress the crushing of yttria-stabilized zirconia. In a wet shot peening method of one embodiment, the action of the liquid contained in the slurry can suppress the accumulation of heat in the yttria-stabilized zirconia, and as a result, the crushing of the yttria-stabilized zirconia can be effectively suppressed. Specifically, a wet shot peening method of one embodiment in which a slurry obtained by mixing a liquid and yttria-stabilized zirconia is used and this slurry is projected onto a workpiece can achieve a higher effect of suppressing the crushing of yttria-stabilized zirconia than a shot peening process (hereinafter referred to as a hard shot peening process) in which yttria-stabilized zirconia is projected onto a workpiece using compressed air or an impeller without using a slurry mainly composed of a liquid. According to a wet shot peening method of one embodiment, both imparting high collision energy to a workpiece in a trade-off relationship and suppressing the crushing of the projection material can be realized.

[0019] In one embodiment, the mechanism by which the wet shot peening method can enhance the effect of suppressing the crushing of yttria-stabilized zirconia compared to the hard shot peening process is not necessarily clear at present. However, it is presumed that the cooling effect of the liquid contained in the slurry can enhance the effect of suppressing the crushing of yttria-stabilized zirconia. Specifically, the liquid contained in the slurry is cooled by the decrease in air temperature due to the expansion (volume increase) of the compressed air during projection. Since the slurry in which this cooled liquid is mixed with yttria-stabilized zirconia is projected onto the workpiece, the cooling effect of the liquid contained in the slurry can suppress the thermal damage that yttria-stabilized zirconia receives during projection (collision). It is presumed that the suppression of this thermal damage acts to suppress the crushing of yttria-stabilized zirconia. More specifically, it is presumed that the liquid contained in the slurry serves to cool the frictional heat generated between yttria-stabilized zirconia and the workpiece surface that may occur during projection and the heat generated by the plastic deformation of the workpiece surface, thereby suppressing the thermal damage received by yttria-stabilized zirconia. According to the wet shot peening method of one embodiment, the lifespan of yttria-stabilized zirconia can be extended, and yttria-stabilized zirconia can be reused repeatedly. The wet shot peening method of one embodiment can be said to be a shot peening method that can suppress the crushing of yttria-stabilized zirconia and enable peening treatment at low cost.

[0020] There is no limitation on the particle size of yttria-stabilized zirconia, but it is preferably 30 μm or more and 300 μm or less. The particle size of yttria-stabilized zirconia as referred to in the present specification means the particle size (mode diameter: the diameter with the highest frequency) at the peak portion in the particle size distribution obtained by measurement with a particle size distribution measuring device. Note that the peak position of the compressive residual stress in the depth direction of the workpiece changes depending on the particle size of yttria-stabilized zirconia. Specifically, the smaller the particle size of yttria-stabilized zirconia, the more the peak of the compressive residual stress moves to the surface of the workpiece. On the other hand, the larger the particle size, the more the peak of the compressive residual stress tends to move to the inside (depth direction) of the workpiece. Also, wet shot peening can achieve a higher compressive residual stress in the surface layer of the workpiece than hard shot peening (dry shot peening). This can be presumably because wet shot peening uses a slurry mainly composed of a liquid, and the presence of this liquid can reduce stress relaxation due to heat.

[0021] There is no limitation on the shape of yttria-stabilized zirconia, and examples thereof include spherical, irregular, and polygonal shapes. Among them, spherical yttria-stabilized zirconia can efficiently impart collision energy to the workpiece and can more effectively suppress the crushing of yttria-stabilized zirconia, which is preferable.

[0022] The slurry may contain other components together with the liquid and yttria-stabilized zirconia. Examples of other components include rust preventives and dispersants. The slurry containing a rust preventive can effectively suppress the corrosion of the workpiece. An alkali-based rust preventive is suitable as the rust preventive. The slurry may contain other projection materials other than yttria-stabilized zirconia.

[0023] There is no limitation on the concentration of yttria-stabilized zirconia in the slurry, and it may be determined in consideration of the projection pressure of the slurry containing yttria-stabilized zirconia, the processing time required for wet shot peening, production efficiency, etc. For example, when the concentration of the projection material in the slurry is lowered, the apparent density of the slurry can be reduced. Assuming that the projection pressure of the slurry is the same, the projection speed of the projection material can be increased by lowering the concentration of the projection material in the slurry. That is, under the same projection pressure conditions, the lower the concentration of the projection material in the slurry, the higher the peening effect can be obtained. In addition to this, by lowering the concentration of the projection material in the slurry, the amount of compressed air required to obtain an equivalent peening effect can be reduced. On the other hand, when the concentration of the projection material in the slurry is lowered, the amount of particles of the projection material projected per unit time decreases, and the processing time required to obtain an arbitrary peening effect increases. Considering this point, the mass of yttria-stabilized zirconia with respect to the total mass of the slurry is preferably 10% by mass or more and 70% by mass or less. Also, the volume of yttria-stabilized zirconia with respect to the total volume of the slurry is preferably 3% by volume or more and less than 50% by volume. By using a slurry having such a concentration, the slurry can be sufficiently accelerated by compressed air, and the peening effect can be made better. Also, the processing time required for wet shot peening can be shortened, and the productivity can be made good. In the present specification, when it is said that the peening effect is good, it means that a high compressive residual stress can be imparted to the workpiece. By imparting a high compressive residual stress to the workpiece, the fatigue strength of the workpiece can be increased, and a high peening effect can be obtained.

[0024] The specific gravity of yttria-stabilized zirconia as an example is 5.8 or more, preferably 6 or more, in terms of true specific gravity.

[0025] The hardness of yttria-stabilized zirconia as an example is 900 HV or more, preferably 1000 HV or more, more preferably 1200 HV or more, in terms of Vickers hardness.

[0026] (Wet blasting device) The wet blasting device used in the wet shot peening method of one embodiment is not limited as long as it is equipped with a nozzle (which may also be referred to as a gun) and can inject a slurry in which a liquid and a projectile are mixed together with compressed air from this nozzle.

[0027] As an example, the nozzle of the wet blasting device includes a slurry inlet for introducing the slurry, a compressed air inlet for introducing compressed air, and an opening (which may also be referred to as an injection port) for injecting the slurry onto the workpiece. As an example, the wet blasting device includes, in addition to the nozzle, a holding mechanism for holding the workpiece, a nozzle rotation mechanism, a nozzle movement mechanism, and the like.

[0028] The wet blasting device in a preferred form includes a wide nozzle (which may also be referred to as a wide gun) having a slit-shaped nozzle opening. By using the wet blasting device of this form, it becomes possible to uniformly process a wide range in one wet shot peening process. Furthermore, according to the wet shot peening method of one embodiment using yttria-stabilized zirconia as the projectile, it becomes possible to more uniformly process a wide range in one wet shot peening process due to the synergistic effect of the wide nozzle and yttria-stabilized zirconia.

[0029] (Projection conditions) There is no limitation on the projection conditions of the slurry, and the projection pressure of the slurry, the projection distance from the nozzle to the workpiece, the area coverage, the projection angle of the slurry, etc. can be determined as appropriate. As an example, the projection pressure of the slurry is 0.2 MPa or more and 0.5 MPa or less. As an example, the projection distance is 50 mm or more and 400 mm or less. When using general steel members such as S45C and SS400 as the workpiece, the area coverage is preferably 100% or more and 600% or less. When using a workpiece that has been subjected to either carburizing treatment or nitriding treatment, or both, as the workpiece, the area coverage is preferably 100% or more and 2000% or less. The area coverage referred to in the specification of the present application means a value obtained by expressing the total area of the impact marks of the projection material as a ratio to the area of the entire processed surface of the workpiece. The projection angle as an example is 30 degrees or more and 90 degrees or less, preferably 60 degrees or more and 90 degrees or less, and more preferably 80 degrees or more and 90 degrees or less.

[0030] In a wet shot peening method according to an embodiment, the wet shot peening process may be performed multiple times. The wet shot peening process as an example includes a first wet shot peening process and a second wet shot peening process that is performed after the first wet shot peening process.

[0031] The particle sizes of the yttria-stabilized zirconia used in the first wet shot peening process and the second wet shot peening process may be the same or different. In a preferred form of the wet shot peening process, the particle size of the yttria-stabilized zirconia used in the second wet shot peening is made smaller than the particle size of the yttria-stabilized zirconia used in the first wet shot peening process. According to the preferred form of the wet shot peening process, while imparting sufficient compressive residual stress to the workpiece, the surface roughness of the workpiece can be reduced. In other words, the smoothness of the workpiece surface can be increased. Further, the compressive residual stress applied to the workpiece in the second wet shot peening process can be accumulated on the compressive residual stress applied to the workpiece in the first wet shot peening process, and higher compressive residual stress can be applied to the workpiece. The particle size of the yttria-stabilized zirconia used in the first wet shot peening process may be read as the projection pressure of the slurry in the first wet shot peening process, and the particle size of the yttria-stabilized zirconia used in the second wet shot peening process may be read as the projection pressure of the slurry in the second wet shot peening process. Another wet shot peening process may be performed between the first wet shot peening process and the second wet peening process.

[0032] In the wet shot peening method of one embodiment, the wet shot peening process may be performed multiple times. The wet shot peening method of one embodiment performs the wet shot peening process multiple times (1, 2, 3 ··· n - 1, n times), and the particle size of the yttria-stabilized zirconia used in the wet shot peening process performed last among the multiple times (the nth wet shot peening process) is made smaller than the particle size of the yttria-stabilized zirconia used in the wet shot peening process performed immediately before this (the (n - 1)th wet shot peening process). The particle size of the yttria-stabilized zirconia may be read as the projection pressure of the slurry.

[0033] (Workpiece) Although there is no limitation on the workpiece, the wet shot peening method according to one embodiment is suitable for workpieces having high hardness or those requiring high strength. In particular, it is suitable for workpieces that have been subjected to either or both of carburizing treatment and nitriding treatment. Examples of carburizing treatment include gas carburizing treatment, vacuum carburizing treatment, plasma carburizing treatment, etc. The carburizing treatment may be read as carburizing quenching and tempering.

[0034] The workpiece subjected to carburizing treatment is imparted with compressive residual stress by changing the crystal structure. The change in the crystal structure is a processing-induced transformation (martensite transformation) from austenite to martensite. The martensite transformation is reversible, and when the crystal structure returns to its original state (when the reversible reaction progresses), the applied compressive residual stress is released (the compressive residual stress decreases). Therefore, it can be said that it is important to suppress the crystal structure from returning to its original state in order to impart sufficient compressive residual stress to the workpiece subjected to carburizing treatment. A factor for the crystal structure to return to its original state is the temperature rise on the surface of the workpiece. By the way, the workpiece subjected to carburizing treatment has high hardness, and in order to provide a sufficient peening effect, it is necessary to project a large number of projectiles onto the surface of the workpiece. In such a peening process, the surface of the workpiece is heated by the repeated collisions of the projectiles during projection, and the temperature of the surface of the workpiece tends to rise. On the other hand, in the wet shot peening method according to one embodiment, the cooling effect of the liquid contained in the slurry can suppress the temperature rise of the workpiece. Therefore, in the wet shot peening method according to one embodiment, sufficient compressive residual stress can be imparted even to the workpiece subjected to carburizing treatment.

[0035] Moreover, according to the wet shot peening method of one embodiment, the crystal structure of the workpiece can be sufficiently deformed (sufficiently plastically deformed) by the yttria-stabilized zirconia contained in the slurry. According to the wet shot peening method of one embodiment, due to the synergistic effect between the above-described effects based on the properties of yttria-stabilized zirconia and the above-described effects based on the cooling effect of the liquid contained in the slurry, it is possible to impart extremely high compressive residual stress even to a workpiece with high hardness, for example, a workpiece that has been carburized.

[0036] The wet shot peening method of one embodiment is suitable for workpieces with a Vickers hardness of 600 HV or more. Although there is no upper limit, it is preferably performed on workpieces with a Vickers hardness less than that of yttria-stabilized zirconia.

[0037] Hereinafter, as a workpiece, a gear tooth of a gear that can be suitably used in the wet shot peening method of one embodiment will be specifically described as an example.

[0038] For the tooth surface, in the wet shot peening process, a first wet shot peening process and a second wet shot peening process performed after the first wet shot peening process are carried out. It is preferable that the average particle diameter of yttria-stabilized zirconia contained in the slurry used in the first wet shot peening process is larger than the average particle diameter of yttria-stabilized zirconia contained in the slurry used in the second wet shot peening process. In particular, the average particle diameter of yttria-stabilized zirconia contained in the slurry used in the first wet shot peening process is 100 μm or more and 500 μm or less, and the average particle diameter of yttria-stabilized zirconia contained in the slurry used in the second wet shot peening process is preferably smaller than the average particle diameter of yttria-stabilized zirconia contained in the slurry used in the first wet shot peening process. In particular, it is more preferable that the average particle diameter of yttria-stabilized zirconia contained in the slurry used in the second wet shot peening process is 30 μm or more and less than 200 μm. By performing such a wet shot peening process on the tooth surface, the fatigue strength and hardness near the tooth surface can be improved. In addition, the surface roughness of the tooth surface can be sufficiently reduced, and as a result, the wear resistance can be improved.

[0039] For the tooth root, in the wet shot peening process, it is preferable to use a slurry in which yttria-stabilized zirconia having an average particle diameter of 100 μm or more and 500 μm or less is mixed with a liquid. By performing the wet shot peening process using such a slurry, sufficient compressive residual stress can be applied not only to the surface of the tooth root but also deep from the surface of the tooth root.

[0040] Also, with respect to the tooth margin, similar to the tooth surface, the wet shot peening process may be performed multiple times. The wet shot peening process for the tooth margin includes a first wet shot peening process and a second wet shot peening process performed after the first wet shot peening process. The average particle size of yttria-stabilized zirconia contained in the slurry used in the first wet shot peening process is 100 μm or more and 500 μm or less, and the average particle size of yttria-stabilized zirconia contained in the slurry used in the second wet shot peening process is preferably smaller than the average particle size of yttria-stabilized zirconia contained in the slurry used in the first wet shot peening process. In particular, it is more preferable that the average particle size of yttria-stabilized zirconia contained in the slurry used in the second wet shot peening process is 30 μm or more and 200 μm or less. In this form, the compressive residual stress on the tooth margin surface layer can be further increased, and the tooth margin strength can be further improved.

[0041] In the wet shot peening process for the tooth surface and the tooth margin, the multiple times (1, 2, 3 ··· n - 1, n times) can be read as such, the first wet shot peening process can be read as the (n - 1)-th wet shot peening process, and the second wet shot peening process can be read as the n-th wet shot peening process.

[0042] <Recovery process> The wet shot peening method of one embodiment may include a recovery process of recovering the projection material after the wet shot peening process. The projection material referred to here includes yttria-stabilized zirconia and any other projection material used optionally. In the recovery process, removal of the crushed projection material may be performed. The crushed projection material can be separated from the uncrushed projection material by centrifugation using a centrifuge or the like.

Examples

[0043] Hereinafter, the present invention will be described more specifically based on examples, but the present invention is not limited to these examples at all.

[0044] Slurries (1) to (4) and (A) to (C) each containing water and the following projection materials were prepared (the symbols in parentheses correspond to the symbols in parentheses of the following projection materials). The concentration of the projection materials was 5% by volume in all cases. The specific gravity of projection materials (1) to (4) is 6.05, and the Vickers hardness is 1250 HV. The specific gravity of projection material (A) is about 3.9, and the Vickers hardness is about 1800 HV. The specific gravity of projection materials (B) and (C) is 3.85, and the Vickers hardness is 700 HV. Projection material (1) ··· Yttria-stabilized zirconia (particle size: 30 μm) Projection material (2) ··· Yttria-stabilized zirconia (particle size: 50 μm) Projection material (3) ··· Yttria-stabilized zirconia (particle size: 180 μm) Projection material (4) ··· Yttria-stabilized zirconia (particle size: 250 μm) Projection material (A) ··· Alumina (particle size: 30 μm) Projection material (B) ··· Zirconia (particle size: 50 μm) Projection material (C) ··· Zirconia (particle size: 180 μm)

[0045] As the workpiece, SCM415 (Vickers hardness of about 750 HV) subjected to carburizing and quenching treatment was used, and wet shot peening using each of the slurries prepared above was performed on this workpiece. For the wet shot peening, a wet blasting apparatus having a wide nozzle manufactured by Marco Co., Ltd. was used. The conditions for the wet shot peening were a slurry projection pressure of 0.4 MPa and an area coverage of 600%. After the wet shot peening, the measurement of the compressive residual stress imparted to the workpiece was performed. For the measurement of the compressive residual stress, an X-ray residual stress measuring device (μ-X360n) manufactured by Pulstec Industrial Co., Ltd. was used.

[0046] Figure 1 is a graph showing the relationship between the depth from the surface of the workpiece and the compressive residual stress when wet shot peening is performed using slurry (1) and slurry (A) containing a projectile with a particle size of 30 μm. Figure 2 is a graph showing the relationship between the depth from the surface of the workpiece and the compressive residual stress when wet shot peening is performed using slurry (2) and slurry (B) containing a projectile with a particle size of 50 μm. Figure 3 is a graph showing the relationship between the depth from the surface of the workpiece and the compressive residual stress when wet shot peening is performed using slurry (3) and slurry (C) containing a projectile with a particle size of 180 μm. As shown in Figures 1 to 3, in this evaluation, the compressive residual stress is compared for each particle size. This is because, as shown in Figure 4, the depth at which the compressive residual stress reaches its maximum varies for each particle size of the projectile. Therefore, by comparing under the condition of the same particle size (and the same volume% (number of projectiles)), the superiority of the compressive residual stress by the projectile can be evaluated. Figure 4 is a graph showing the relationship between the depth from the surface of the workpiece and the compressive residual stress when wet shot peening is performed using slurries (1) to (4) in which the projectile is yttria-stabilized zirconia.

[0047] As is clear from the results of Figures 1 to 3, it was confirmed that when wet shot peening is performed using yttria-stabilized zirconia, a higher compressive residual stress can be imparted to the workpiece compared to the case of using other projectiles. Also, in the evaluation of Figure 2, after wet shot peening, the degree of crushing of the projectile was visually confirmed. It was confirmed that yttria-stabilized zirconia (50 μm) has less damage to the projectile compared to zirconia (50 μm).

Claims

1. A wet shot peening method, comprising: using a wet blasting apparatus equipped with a nozzle to inject a slurry in which a liquid and a projectile are mixed together with compressed air, and causing the projectile contained in the slurry to collide with a workpiece, the method including a wet shot peening step; using yttria-stabilized zirconia as the projectile; a wet shot peening method.

2. The hardness of the workpiece is 600 HV or more in terms of Vickers hardness and less than the Vickers hardness of the yttria-stabilized zirconia; the wet shot peening method according to Claim 1.

3. The nozzle is a wide nozzle having a slit-shaped nozzle opening; the wet shot peening method according to Claim 1 or 2.

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