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 (shots) are projected onto the surface of an object made of a metallic material, is known (for example, Patent Document 1). Shot peening is effective in extending the life of products made of metallic materials. For example, damage to die-casting dies is often caused by heat checking (heat cracks) 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] JP 2011-235318 A 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 first depth on a surface of an object made of a metallic material, and a step of performing, after the first shot peening, a second shot peening to impart residual stress to a second depth deeper than the first depth on 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 obtaining the effect of suppressing heat checking. Even if the effect of the first shot peening decreases while the object is in use, the second shot peening imparts residual stress to the surface of the object, thereby obtaining 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. From the above, it is possible to further extend the life of objects made of metallic materials.

[0007] In the shot peening method according to one embodiment, the particle size of the shot material for the second shot peening may be larger than the particle size of the shot material for the first shot peening. Also, the projection velocity of the shot material for the second shot peening may be larger than the projection velocity of the shot material for the first shot peening. Also, the hardness of the shot material for the second shot peening may be larger than the hardness of the shot material for 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 target 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 high demand for a shot peening method that can further extend the life of the alloy.

[0009] In the shot peening method according to one embodiment, the target object may be a die-casting die or a transmission gear. Since a die-casting die is used at high temperatures, the effect of suppressing heat checks is likely to decrease. A transmission gear is a part that is subjected to a large load. In particular, when used in a construction machine, replacement is difficult due to the high operating rate and the large size of the machine. Therefore, there is a high demand for a shot peening method that can further extend the life of the die-casting die and the transmission gear. Effect 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 description of the drawings]

[0011] [Figure 1] 2 is a flowchart illustrating a shot peening method according to an embodiment. [Diagram 2] 1 is a schematic cross-sectional view for explaining an affected layer and a heat check in an object. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 duplicated 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 and 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, so-called 64 titanium, is the most widely used. Applications include implants such as artificial joints, and aircraft framework members.

[0015] The object is, for example, a die-casting die or a gear for a transmission. A typical die-casting die is made of SKD61 (JIS standard). SKD61 of the JIS standard corresponds to X40CrMoV5-1 of the ISO standard (ISO4957:1999). Nitriding is generally applied to die-casting dies after quenching and tempering. Types of nitriding include salt bath soft nitriding, gas nitriding, and plasma nitriding. Among them, salt bath soft nitriding is widely used because it is low cost and can be applied to large objects. Salt bath soft nitriding is a process in which a compound layer is generated on the steel surface and then nitrogen diffuses, so a compound layer is always present on the surface. This compound layer has the function of suppressing adhesion between iron and aluminum. There is no clear boundary between the parent phase steel and the compound layer, and the composition (chemical composition) changes continuously from the parent phase to form the compound layer. Although the compound layer is hard, it is also brittle and easily cracked, and it is said to cause cracks in the grain boundaries of the parent phase steel.

[0016] The explanation will be further continued using the case of an aluminum alloy die-casting die as an example. Approximately 70% of the causes of damage to aluminum alloy die-casting dies are cracks called heat checks. The thermal cycle that the die is subjected to from the high-temperature aluminum molten metal and the low-temperature mold release agent causes the crystal grains to expand and contract. As a result, heat checks occur from the grain boundaries. There are some documents that say that applying residual stress by shot peening is effective in suppressing heat checks. In this case, a method using X-rays is used to evaluate the residual stress. A heat check is a crack that starts from the grain boundaries. In contrast, X-rays evaluate the residual stress within the crystal grains. It is unclear how the residual stress within the crystal grains contributes to suppressing heat checks. However, in hindsight, it is considered to be a fact that heat checks can be suppressed by shot peening.

[0017] Die casting dies are always at a temperature of around 550 degrees due to the heat they receive from the molten aluminum. This temperature is almost 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 only reason for suppressing heat checking is the residual stress caused by shot peening.

[0018] Shot peening is a process that causes plastic deformation. As a result of shot peening, grain refinement occurs. Heat checks occur from grain boundaries. Therefore, the refinement of grains and the complexity of paths are considered to be effective in terms of reducing the opportunity for heat checks to occur. Grain refinement does not appear in the numerical values ​​obtained as the result 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 numerical value, the finer and more complex the grains are.

[0019] As mentioned above, die casting dies are constantly exposed to the recrystallization temperature of steel. Even if the crystal grains are refined by shot peening, the recrystallization of the crystal grains progresses during use of the die casting die, and the effect of the shot peening decreases. To extend the life of a die casting die, it is of course effective to perform shot peening at an early stage. However, the effect of shot peening decreases. Therefore, if shot peening is performed again during use to refine the crystal 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 a step S1 of setting first shot peening conditions for the first shot peening, a step S2 of performing the first shot peening, a step S3 of setting second shot peening conditions for the second shot peening, and a 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 the 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. The maximum residual stress is the maximum value of compressive residual stress that can be imparted to an object. The maximum residual stress differs depending on the object. The first shot peening conditions set in step S1 include, for example, the hardness of the projection material, the particle size of the projection material, and the projection speed of the projection material.

[0022] Heat checking is caused by stress due to thermal expansion and contraction of crystals, so only the surface where the stress is maximum is the starting point of destruction. Originally, shot peening is 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 of the object, etc.

[0023] In shot peening, the residual stress imparted to the object can be increased by increasing the hardness of the shot material. However, if the hardness of the object 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 object. In other words, in order to impart the maximum residual stress to the object, it is necessary to optimize the balance between the hardness of the object and the hardness of the shot material.

[0024] In order to impart the maximum residual stress to the object by the first shot peening, for example, the hardness of the blast material is set to be higher than the hardness of the object within a range of 50 HV (Vickers hardness) to 250 HV. By setting the hardness to 50 HV or more, residual stress can be imparted to the surface portion of the object. If the hardness is set to be higher than 250 HV, the energy of the blast is used to erode the surface of the object, so that residual stress cannot be effectively and stably imparted to the surface portion of the object. If 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 portion of the object. In addition, the change in the dimensions of the object can be suppressed. Furthermore, the 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 from the surface of the object to a depth of 0.050 mm.

[0026] As in the case of hardness, the residual stress imparted to the target can be increased by increasing the particle size of the shot material. 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 changes in dimensions. For example, the surface roughness depends on the particle size of the shot material. Also, if the particle size of the shot material is too large, it may not be possible to process the target due to the capacity of the projection equipment. Furthermore, the number of shots per unit flow rate decreases, so the processing time becomes longer.

[0027] The particle size of the shot material is, for example, the diameter of the shot material when the shot material is spherical. To impart the 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 making the particle size of the shot material 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 making the particle size of the shot material 1000 μm or less (1 mm or less), it is possible to suppress the deterioration of the 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 thicker than this because the surface roughness will deteriorate.

[0028] As in the case of hardness, the residual stress imparted to the object can be increased by increasing the projection velocity of the shot material. However, if the projection velocity of the shot material is too high, the residual stress imparted to the object may be decreased. There is also a risk of embrittlement of the material itself, deterioration of the surface roughness of the object, or changes in dimensions. 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 using 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 a 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. In the peening-affected layer 3, strain is introduced by the first shot peening, and residual stress is imparted. That is, the first shot peening is a process of introducing strain to the surface 2 of the object 1 at a first depth d1 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 complicated. The peening-affected layer 3 is a strain-introduced layer into which strain is introduced, and also a residual stress-imparting layer into which residual stress is imparted.

[0030] When the object 1 after step S2 is used for a certain period of time, as shown in FIG. 2(b), a heat check 4 appears on the surface 2 of the object 1 and progresses toward the inside of the object 1. As described above, the environment in which the die casting mold is used is a temperature range in which recrystallization occurs, so the crystal grains recover from the effects of the first shot peening. In other words, the effects of refining the crystal grains and introducing strain in the object 1 decrease. In the process, the heat check 4 also progresses. The heat check 4 eventually progresses beyond the peening-affected layer 3, as shown in FIG. 2(c).

[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 shot 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 such 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, the 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, and a peening-affected layer 3 having a thickness equal to the second depth d2 is formed. The second shot peening causes plastic flow due to the impact of the shot material, and acts to close the heat check 4 that has already occurred. Since this effect is weak inside the object 1, the heat check 4 is not alleviated, and some may remain. However, since the heat check 4 occurs from the outermost surface of the object 1, the effect on the outermost surface is important.

[0034] As described above, the shot peening method of the embodiment includes a step S2 of performing a first shot peening to impart residual stress to a first depth d1 on surface 2 of an 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 surface 2 of the object 1 at a second depth d2 deeper than the first depth d1.

[0035] In this shot peening method, the first shot peening imparts residual stress to the surface 2 of the object 1, thereby obtaining the effect of 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 obtaining the effect of suppressing heat checking 4 again. In particular, the first shot peening imparts residual stress at the first depth d1, whereas the second shot peening imparts residual stress at a second depth d2 deeper than the first depth d1. Therefore, it is possible to suppress further propagation of a crack that has progressed deeper than the first depth d1. From the above, it is possible to further extend the life of the object 1 made of a metallic material.

[0036] The particle size of the shot material of the second shot peening may be larger than the particle size of the shot material of the first shot peening. Moreover, the projection velocity of the shot material of the second shot peening may be larger than the projection velocity of the shot material of the first shot peening. Moreover, the hardness of the shot material of the second shot peening may be harder than the hardness of the shot material of the first shot peening. This makes it possible to easily 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 demand for a shot peening method that can further extend the life of the alloy.

[0038] The object 1 is a die-casting mold. Since die-casting molds are used at high temperatures, the effect of suppressing heat checks is easily reduced. 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 parts that are subjected to heavy 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 gist 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 after step S4 is used, a new heat check 4 is generated again from the surface 2 and progresses toward the inside of the object 1. The new heat check 4 may combine with the heat check 4 remaining inside and progress. Therefore, when re-shot peening is performed after step S4, it is necessary to impart residual stress at a depth deeper than the depth at which residual stress was imparted by the shot peening performed immediately before. Therefore, when re-shot peening is performed successively, for example, the particle size of the projection material is 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 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 a crack is generated on the surface of the object. Shot peening method.

2. 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 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 a crack has occurred in the surface of the object, and serves to close the crack. Shot peening method.

3. 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 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 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 a crack is generated on the surface of the object. 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.

Images