Shot peening method

Abstract

To provide a shot-peening method capable of making an object made of a metal material longer in a lifetime.SOLUTION: A shot-peening method includes a process S2 of performing first shot peening to impart residual stress to a surface 2 of an object 1 made of a metal material to a first depth d1; and a process S4 of performing second shot peening to impart residual stress to the surface 2 of the object 1 to a second depth d2 deeper than the first depth d1 after the first shot peening. The first shot peening is carried out before the object 1 is used, and the second shot peening is carried out after the surface 2 of the object 1 cracks.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a shot peening method. [Background technology]

[0002] Shot peening, in which high-hardness projectiles (shot) are projected onto the surface of an object made of a metal material, is known (see, for example, Patent Document 1). Shot peening is effective in extending the life of products made of metal materials. For example, damage to die-casting dies is often caused by heat checking (heat cracking) that occurs due to repeated thermal expansion and contraction. Shot peening can impart residual stress (compressive residual stress) to the surface of the die, thereby suppressing heat checking. If shot peening can extend the life of the die, it can reduce the die costs included in the manufacturing costs of the product. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-235318 Summary of the Invention [Problem to be solved by the invention]

[0004] The present disclosure provides a shot peening method that can further extend the life of an object made of a metallic material. [Means for solving the problem]

[0005] A shot peening method according to one aspect of the present disclosure includes a step of performing a first shot peening to impart residual stress to a surface of an object made of a metallic material at a first depth, and a step of performing a second shot peening after the first shot peening to impart residual stress to a second depth deeper than the first depth in the surface of the object.

[0006] In this shot peening method, the first shot peening imparts residual stress to the surface of the object, thereby achieving the effect of suppressing heat checking. Even if the effect of the first shot peening decreases during use of the object, the second shot peening imparts residual stress to the surface of the object, thereby achieving the effect of suppressing heat checking again. In particular, the first shot peening imparts residual stress at a first depth, whereas the second shot peening imparts residual stress at a second depth that is deeper than the first depth. Therefore, it is possible to suppress further propagation of cracks that have progressed deeper than the first depth. As a result, the lifespan of objects made of metallic materials can be further extended.

[0007] In a shot peening method according to one embodiment, the particle size of the shot material used in the second shot peening may be larger than the particle size of the shot material used in the first shot peening. The velocity of the shot material used in the second shot peening may be larger than the velocity of the shot material used in the first shot peening. The hardness of the shot material used in the second shot peening may be greater than the hardness of the shot material used in the first shot peening. In these cases, the second depth can be easily made deeper than the first depth.

[0008] In one embodiment of the shot peening method, the object may be made of an iron-based alloy or a titanium-based alloy. Since iron-based alloys and titanium alloys are widely used, there is a great need for a shot peening method that can further extend the life of the alloy.

[0009] In one embodiment of the shot peening method, the target object may be a die-casting die or a transmission gear. Because die-casting dies are used at high temperatures, the effect of suppressing heat checks is likely to decrease. Transmission gears are components that are subjected to heavy loads. Furthermore, particularly when used in construction machinery, replacement is difficult due to the high operating rate and large size of the machinery. Therefore, there is a strong need for a shot peening method that can further extend the life of die-casting dies and transmission gears. [Effects of the Invention]

[0010] According to the shot peening method according to the present disclosure, the life of an object made of a metallic material can be further extended. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a flowchart illustrating a shot peening method according to an embodiment. [Figure 2] 10A and 10B are schematic cross-sectional views for explaining an affected layer and a heat check in an object. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. In the description, the same elements or elements having the same functions are designated by the same reference numerals, and redundant description will be omitted.

[0013] The shot peening method according to the embodiment is a method for performing shot peening on an object made of a metal material. The object is made of, for example, an iron-based alloy or a titanium-based alloy. The iron-based alloy is, for example, a steel material. Specific examples of the steel material include a medium-carbon quenched material having a carbon content of 0.5% to 0.6%, and a high-carbon carburized material having a carbon content of 0.8% to 1.1%. The medium-carbon quenched material is used, for example, as a spring material or a mold material for aluminum die casting. The high-carbon carburized material is used, for example, as a gear material. All of these steel materials are martensitic steels having a martensitic structure.

[0014] Titanium alloys include, for example, α-titanium alloys (e.g., Ti-5Al-1Mo-1V), α-β-titanium alloys (e.g., Ti-6Al-4V), and β-titanium alloys (e.g., Ti-14-3-3-3). Among titanium alloys, the α-β titanium alloy Ti-6Al-4V, commonly known as 64 titanium, is the most widely used. Applications include implants such as artificial joints, and aircraft frame components.

[0015] The target object is, for example, a die-casting die or a transmission gear. A typical die-casting die is made of SKD61 (JIS standard). JIS SKD61 corresponds to X40CrMoV5-1 in ISO standard (ISO4957:1999). Nitriding is typically applied to die-casting dies after quenching and tempering. Nitriding methods include salt bath nitrocarburizing, gas nitriding, and plasma nitriding. Salt bath nitrocarburizing is the most commonly used method due to its low cost and ability to be applied to large parts. Salt bath nitrocarburizing is a process in which a compound layer is formed on the steel surface and then nitrogen diffuses into the surface, resulting in the presence of a compound layer. This compound layer acts to suppress adhesion between iron and aluminum. There is no clear boundary between the parent steel and the compound layer; the composition (chemical composition) changes continuously from the parent phase to form the compound layer. While the compound layer is hard, it is also brittle and prone to cracking, which is said to cause cracks at the grain boundaries of the parent steel.

[0016] Let's continue the explanation using aluminum alloy die-casting dies as an example. Approximately 70% of aluminum alloy die-casting die failures are caused by cracks known as heat checks. The thermal cycle of the die from the high-temperature molten aluminum and the low-temperature mold release agent causes the crystal grains to expand and contract. As a result, heat checks occur at the grain boundaries. Several studies have suggested that applying residual stress through shot peening is effective in preventing heat checks. X-rays are used to evaluate the residual stress. Heat checks are cracks that originate at grain boundaries. X-rays, on the other hand, evaluate the residual stress within the crystal grains. It is unclear how residual stress within the crystal grains contributes to heat check prevention. However, in hindsight, it is believed that shot peening can actually prevent heat checks.

[0017] Die-casting dies are constantly at a temperature of around 550°C due to the heat they receive from the molten aluminum. This temperature is roughly the same as the recrystallization temperature of steel. Even if residual stress is imparted by shot peening, the stress will be released in this temperature range. Therefore, it is a bit of a stretch to think that the reason for suppressing heat checking is solely due to residual stress caused by shot peening.

[0018] Shot peening is a process that causes plastic deformation. Shot peening results in the refinement of crystal grains. Heat checks occur at grain boundaries. Therefore, the refinement of crystal grains and the complexity of their paths are considered to be effective in terms of reducing the opportunity for heat checks to occur. Crystal grain refinement does not appear in the numerical results of residual stress measurement using X-rays. The half-width, a numerical value obtained from X-ray stress measurement, is a parameter that indicates crystallinity. The larger the half-width value, the finer and more complex the crystal grains.

[0019] As mentioned above, die-casting dies are constantly exposed to the recrystallization temperature of steel. Even if shot peening results in grain refinement, the recrystallization of the grains progresses during use of the die-casting die, reducing the effectiveness of the shot peening. To extend the life of a die-casting die, shot peening at an early stage is of course effective. However, the effectiveness of shot peening will eventually decrease. Therefore, if shot peening is performed again during use to refine the grains, the life of the die-casting die can be expected to be further extended. Here, this type of shot peening is called re-shot peening.

[0020] Fig. 1 is a flowchart showing a shot peening method according to an embodiment. As shown in Fig. 1, the shot peening method according to the embodiment includes step S1 of setting first shot peening conditions for the first shot peening, step S2 of performing the first shot peening, step S3 of setting second shot peening conditions for the second shot peening, and step S4 of performing the second shot peening. The second shot peening corresponds to re-shot peening. Steps S1 and S2 are performed, for example, during the manufacture of an object. Steps S3 and S4 are performed, for example, during maintenance of the object. Each step will be described below.

[0021] In step S1, conditions for imparting maximum residual stress to an object are set as first shot peening conditions. Maximum residual stress is the maximum value of compressive residual stress that can be imparted to an object. The maximum residual stress varies depending on the object. The first shot peening conditions set in step S1 include, for example, the hardness of the shot material, the particle size of the shot material, and the shot velocity of the shot material.

[0022] Since heat checking is caused by stress due to the thermal expansion and contraction of the crystal, the surface where the stress is greatest is the only starting point for destruction. Shot peening is originally a process that applies stress to the surface. Therefore, in the first shot peening performed on a newly manufactured object, appropriate first shot peening conditions are set according to the hardness, etc. of the object.

[0023] In shot peening, the residual stress imparted to the target can be increased by increasing the hardness of the shot material. However, if the hardness of the target does not match the hardness of the shot material, increasing the hardness of the shot material may actually reduce the residual stress imparted to the target. In other words, to impart the maximum residual stress to the target, it is necessary to optimize the balance between the hardness of the target and the hardness of the shot material.

[0024] To impart maximum residual stress to the object by the first shot peening, for example, the hardness of the shot material is set higher than the hardness of the object within a range of 50 HV (Vickers hardness) to 250 HV. By setting the hardness at 50 HV or higher, residual stress can be imparted to the surface of the object. If the hardness is set higher than 250 HV, the energy of the shot is used to erode the surface of the object, making it impossible to effectively and stably impart residual stress to the surface of the object. As the amount of erode increases, the amount of change in the dimensions of the object also increases. By setting the amount of erode of the object to 5 μm or less, residual stress can be effectively and stably imparted to the surface of the object. Furthermore, change in the dimensions of the object can be suppressed. Furthermore, deterioration of the surface roughness of the object can be suppressed.

[0025] However, if the hardness of the object is lower than 750 HV, it may not be possible to impart sufficient residual stress to the surface of the object. The hardness of the object means, for example, the hardness of the surface portion up to a depth of 0.050 mm from the surface of the object.

[0026] As with hardness, increasing the particle size of the shot material can increase the residual stress imparted to the target. As the particle size of the shot material increases, the peening-affected layer is formed deeper. However, if the particle size of the shot material is too large, the residual stress imparted to the target may actually decrease. Damage to the target may increase, causing embrittlement of the material itself, deterioration of the surface roughness of the target, or dimensional changes. For example, surface roughness depends on the particle size of the shot material. Furthermore, if the particle size of the shot material is too large, there is a risk that the processing will not be possible due to the capacity of the blasting equipment. Furthermore, the number of shots per unit flow rate decreases, resulting in a longer processing time.

[0027] The particle size of the shot material is, for example, the diameter of the shot material if the shot material is spherical. To impart maximum residual stress to the object by the first shot peening, the particle size of the shot material is, for example, 50 μm or more and 1000 μm or less. By setting the particle size of the shot material to 50 μm or more, it is possible to modify the area just below the surface of the object. Although it depends on the hardness and application of the object, by setting the particle size of the shot material to 1000 μm or less (1 mm or less), it is possible to suppress deterioration of surface roughness. The depth (thickness) of the peening-affected layer obtained when the particle size of the shot material is 1000 μm is about 500 μm. It is difficult to make the peening-affected layer any thicker because it would worsen the surface roughness.

[0028] As with hardness, increasing the projection velocity of the shot material can increase the residual stress imparted to the object. However, if the projection velocity of the shot material is too high, the residual stress imparted to the object may actually decrease. This may result in embrittlement of the material itself, deterioration of the surface roughness of the object, or dimensional changes. To impart maximum residual stress to the object by the first shot peening, the projection velocity of the shot material is, for example, 50 m / s or more and 150 m / s or less. When using an air peening device with a nozzle, the above projection velocity can be achieved by setting the projection pressure (air pressure) to, for example, 0.05 MPa or more and 0.5 MPa or less.

[0029] In step S2, the object is subjected to first shot peening under the first shot peening conditions set in step S1. As a result, as shown in FIG. 2(a), a peening-affected layer 3 is formed on the surface 2 of the object 1. Strain is introduced into the peening-affected layer 3 by the first shot peening, and residual stress is imparted to the peening-affected layer 3. In other words, the first shot peening is a process of introducing strain into the surface 2 of the object 1 at a first depth d1, which is equal to the thickness of the peening-affected layer 3, and imparting residual stress. In the peening-affected layer 3, the crystal grains are refined and the crystal grain boundaries are made complex. The peening-affected layer 3 is both a strain-introduced layer into which strain has been introduced, and a residual stress-applying layer into which residual stress has been imparted.

[0030] After step S2, when the object 1 is used for a certain period of time, heat checks 4 appear on the surface 2 of the object 1, as shown in Figure 2(b), and progress toward the interior of the object 1. As mentioned above, the operating environment of the die-casting mold is in a temperature range where recrystallization occurs, so the crystal grains recover from the effects of the first shot peening. In other words, the effects of grain refinement and strain introduction in the object 1 decrease. In the process, heat checks 4 also progress. As shown in Figure 2(c), heat checks 4 eventually progress beyond the peening-affected layer 3.

[0031] In step S3, second shot peening conditions for the second shot peening are set. The second shot peening conditions include, for example, the hardness of the shot material, the particle size of the shot material, and the projection speed of the shot material. The second shot peening is performed on the object 1 in a state in which the heat check 4 has progressed. In order to further extend the life of the object 1, it is necessary to form the peening-affected layer 3 to a position deeper than the heat check 4. In other words, the second shot peening conditions are set so that the peening-affected layer 3 becomes deeper than the first shot peening conditions.

[0032] For example, the particle size of the shot material for the second shot peening is set to be larger than the particle size of the shot material for the first shot peening conditions. The velocity of the shot material for the second shot peening is set to be larger than the velocity of the shot material for the first shot peening. The hardness of the shot material for the second shot peening is set to be harder than the hardness of the shot material for the first shot peening. At least one of these settings may be performed in step S3. For example, if the particle size of the shot material for the second shot peening is set to be larger than the particle size of the shot material for the first shot peening conditions, the velocity and hardness of the shot material for the second shot peening may be the same as the velocity and hardness of the shot material for the first shot peening.

[0033] In step S4, second shot peening is performed on the object 1 after step S2 under the second shot peening conditions set in step S3. As a result, as shown in FIG. 2(d), residual stress is imparted to the surface 2 of the object 1 at a second depth d2 deeper than the first depth d1, forming a peening-affected layer 3 with a thickness equal to the second depth d2. The second shot peening causes plastic flow due to the impact of the shot material, which acts to close any heat checks 4 that have already occurred. Because this effect is weak inside the object 1, the heat checks 4 may not be completely eliminated, and some may remain. However, because the heat checks 4 occur from the outermost surface of the object 1, the effect on the outermost surface is important.

[0034] As described above, the shot peening method according to the embodiment includes a step S2 of performing a first shot peening to impart residual stress to a first depth d1 on the surface 2 of the object 1 made of a metallic material, and a step S4 of performing a second shot peening after the first shot peening to impart residual stress to a second depth d2 deeper than the first depth d1 on the surface 2 of the object 1.

[0035] In this shot peening method, the first shot peening imparts residual stress to the surface 2 of the object 1, thereby suppressing heat checking 4. Even if the effect of the first shot peening decreases during use of the object 1, the second shot peening imparts residual stress to the surface 2 of the object 1, thereby again suppressing heat checking 4. In particular, the first shot peening imparts residual stress at a first depth d1, whereas the second shot peening imparts residual stress at a second depth d2 that is deeper than the first depth d1. Therefore, it is possible to suppress further propagation of cracks that have progressed deeper than the first depth d1. As a result, the life of the object 1 made of a metallic material can be further extended.

[0036] The particle size of the shot material used in the second shot peening may be larger than the particle size of the shot material used in the first shot peening. Furthermore, the projection velocity of the shot material used in the second shot peening may be larger than the projection velocity of the shot material used in the first shot peening. Furthermore, the hardness of the shot material used in the second shot peening may be larger than the hardness of the shot material used in the first shot peening. This makes it easy to make the second depth d2 deeper than the first depth d1. In other words, the peening-affected layer 3 formed by the second shot peening can be easily made thicker than the peening-affected layer 3 formed by the first shot peening.

[0037] The object 1 is an iron-based alloy or a titanium-based alloy. Since iron-based alloys and titanium alloys are widely used, there is a high need for a shot peening method that can further extend the life of the alloys.

[0038] The object 1 is a die-casting mold. Because die-casting molds are used at high temperatures, the effect of suppressing heat checks tends to decrease. Therefore, there is a high demand for a shot peening method that can further extend the life of die-casting molds. The object 1 is also a transmission gear. Transmission gears are components that are subjected to extremely high loads. In particular, when used in construction machinery, replacement is difficult due to the high operating rate and large size of the machinery. Therefore, there is a high demand for a shot peening method that can further extend the life of transmission gears.

[0039] The present invention is not necessarily limited to the above-described embodiment, and various modifications are possible without departing from the spirit and scope of the present invention.

[0040] The shot peening method may further include a step of performing re-shot peening after step S4. When the object 1 is used after step S4, new heat checks 4 are generated again from the surface 2 and progress toward the inside of the object 1. The new heat checks 4 may combine with the heat checks 4 remaining inside and progress. Therefore, when re-shot peening is performed after step S4, it is necessary to impart residual stress to a depth deeper than the depth at which residual stress was imparted by the immediately preceding shot peening. Therefore, when re-shot peening is performed successively, for example, the particle size of the shot material will be successively increased. [Explanation of symbols]

[0041] 1...object, 2...surface, 3...peening affected layer, 4...heat check, d1...first depth, d2...second depth.

Claims

1. performing a first shot peening process to impart residual stress to a surface of an object made of a metallic material at a first depth; and performing, after the first shot peening, a second shot peening to impart residual stress to the surface of the object at a second depth that is deeper than the first depth, The first shot peening is performed before the object is used, and the second shot peening is performed after cracks occur on the surface of the object. Shot peening method.

2. performing a first shot peening process to impart residual stress to a surface of an object made of a metallic material at a first depth; and performing, after the first shot peening, a second shot peening to impart residual stress to the surface of the object at a second depth that is deeper than the first depth, The first shot peening is performed before the object is used, and the second shot peening is performed after cracks occur on the surface of the object, and has the effect of closing the cracks. Shot peening method.

3. performing a first shot peening process to impart residual stress to a surface of an object made of a metallic material at a first depth; and performing, after the first shot peening, a second shot peening to impart residual stress to the surface of the object at a second depth that is deeper than the first depth, The first shot peening is performed before the object is used at a recrystallization temperature of the metallic material, and the second shot peening is performed after the object is used at the recrystallization temperature of the metallic material. Shot peening method.

4. performing a first shot peening process to impart residual stress to a surface of an object made of a metallic material at a first depth; and performing, after the first shot peening, a second shot peening to impart residual stress to the surface of the object at a second depth that is deeper than the first depth, The second shot peening is performed after the object has been used. Shot peening method.

5. The second shot peening is performed after cracks are generated on the surface of the object.

5. The shot peening method according to claim 3 or 4.

6. The crack is a heat check.

6. The shot peening method according to claim 1, 2 or 5.

7. the crack is deeper than the first depth and shallower than the second depth; 7. The shot peening method according to claim 1, 2, 5 or 6.

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