JP2001079766A - Projection material for shot peening - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the compressive residual stress of a machined layer to the inside while keeping the surface roughness of a machined face satisfactory by forming a projection material for shot peening into a spherical shape with the sphericity of a specific value or below. SOLUTION: When the sphericity of the projection material for shot peening exceeds 50 μm, the plastic deformation on the face of a work becomes nonuniform, and the distribution of compressive residual stress is reduced. The sphericity is preferably set to 20-30 μm to further improve a peening effect, to reduce the cost, and to improve productivity. The Vickers hardness is preferably set to 700-1,000 HV. When the hardness of the projection material for shot peening is increased, the absorbed energy by this projection material when shot is decreased, and large projection energy can be applied to the surface of the work, thereby larger compressive residual stress can be applied not only to the surface portion but also to the inside of a machined layer in cooperation with the good sphericity of the projection material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shot peening process for improving the strength of a part and the like, and more particularly to a shot material used in the process.

[0002]

2. Description of the Related Art Structural members subjected to repeated stress include:
Fatigue strength is required in addition to tensile strength. For example, an airframe, a wing, a turbine wing, a suspension spring, a valve spring, a connecting rod, and a gear of an automobile. In particular, mechanical parts and the like that are required to have high performance and high durability with small size and light weight require further improvement in fatigue resistance.

[0003] Fatigue fracture of a metal material often occurs after a repetitive stress exceeding a fatigue limit is applied for a certain period of time and then a crack is propagated from the starting point of the material surface. Therefore, reinforcement of the material surface is effective for improving fatigue resistance, and various surface modifications (treatments) have been conventionally performed. Shot peening is one of them. Shot peening is effective for stress corrosion cracking resistance and wear resistance, but is often used for the purpose of enhancing fatigue strength. Shot peening is often used in addition to carburizing and quenching and nitriding, because it is easily applied to products and has a large effect on fatigue resistance.

[0004] In shot peening, in general, a shot material (shot) having a particle size of about several tens of μm to several mm is projected on a work material by an air-type or centrifugal peening machine, and the surface of the work material is cooled. (Warm) processing. As a result, the surface layer of the workpiece is plastically worked into a satin finish, the hardness near the surface layer increases due to work hardening, and a compressive residual stress appears near the surface due to the extension of the surface layer. Due to the work hardening and the increase in compressive residual stress in the surface layer, the fatigue strength of the member is greatly improved. In particular, the influence of the latter is large. As described above, since shot peening is very effective in improving the fatigue resistance of a member, various proposals regarding shot peening have been made conventionally.

[0005] In particular, there are the following with respect to the projection material. Japanese Patent No. 2619339 discloses a high-hardness cut wire shot obtained by using a pearlitized wire drawing material. According to this, a shot (projection material) having a hardness equal to or higher than that of a high-strength member subjected to carburizing and quenching or the like can be obtained, so that a compressive residual stress can be effectively applied to such a member. . Japanese Patent Application Laid-Open No. 9-57629 discloses that the specific gravity is 16-2.
0, a projection material having a particle size of 0.1 mm or less, a hardness of 1200 HV or more, and containing 80% by weight of W and / or W ₂ C is disclosed.

Japanese Unexamined Patent Application Publication No. 10-100609 proposes a shot material having a smaller particle size (30 to 250 μm). By increasing the specific gravity of the shot material, these two increase the projection energy per piece and make the shot material a small particle size and high hardness.
This is to improve the compressive residual stress while improving the surface roughness of the processed surface.

[0007]

As described above, the compressive residual stress and the surface roughness applied to the workpiece are important in performing shot peening. However, Japanese Patent Application Laid-Open No. 9-5762
No. 9 and Japanese Patent Application Laid-Open No. 10-100609 disclose that the projection material which is a consumable uses a large amount of material such as tungsten, which is not realistic in terms of cost.

Further, the shot material disclosed in Japanese Patent Application Laid-Open No. H10-100609 needs to be used for shot peening in the second and subsequent stages because of its small particle size. Therefore, from the viewpoint of productivity, it cannot always be said that it is a preferable shot material. Moreover, the above prior art is based only on the following general properties of shot peening.

[0009] The compressive residual stress increases as the projection energy increases and as the shot material becomes harder. However, in this case, the distribution of the compressive residual stress proceeds to the inside, and the compressive residual stress in the surface layer decreases on the contrary. In particular, the larger the particle size of the blast material, the greater the effect. When the hardness of the shot material is equal to or greater than the hardness of the workpiece, the hardness of the shot material has little effect on the compressive residual stress.

The surface roughness increases as the projection energy increases and as the particle size increases. Further, as the hardness of the blast material increases, the surface roughness decreases. However,
When the shot material becomes harder than the work material, the hardness of the shot material has a small effect on the surface roughness.

According to investigations by the present inventors, only proposals relating to the specific gravity, grain size, hardness, and material of a shot material for shot peening could be found. In particular, the shape of the blast material is merely a substantially spherical shape, and no specific proposal has been made. The present invention has been made in view of such circumstances, and an object of the present invention is to provide a new blasting material capable of further improving the fatigue resistance of a workpiece and performing efficient shot peening.

[0012]

Therefore, the present inventors have taken a completely different viewpoint from the prior art and focused on the shape of the projection material.
Diligent research was conducted. As a result of repeated systematic experiments, they found that the difference in the shape of the shot material had a great effect on the peening effect, and came to develop the shot material of the present invention.

That is, the shot peening shot material of the present invention is characterized in that it has a spherical shape with a sphericity of 50 μm or less. By setting the sphericity of the shot peening projection material to 50 μm or less,
The compression residual stress could be made larger than before.

Although the details of this mechanism are not clear, it is considered as follows. First, consider a case where a conventional shot material such as a cut wire shot or a cast steel shot is used. Such a shot material has various shapes, one by one. In particular, the cut wire shot has a shape like a rugby ball or a cocoon, although the corner is rounded. When the blast material having such a shape collides with the surface of the workpiece, a large force is likely to act on only a part thereof.
In other words, focusing on one shot mark, it is considered that only a very small portion of the processed surface undergoes large plastic deformation, and the plastic deformation becomes extremely non-uniform. For this reason, it is considered that the distribution of the compressive residual stress becomes non-uniform, the portion having a low compressive residual stress increases, and the overall compressive residual stress decreases.

On the other hand, when the projectile of the present invention is used,
Due to the good sphericity, the force acting on the surface of the workpiece when the projection material collides is uniform regardless of the direction of each projection material. Therefore, the plastic deformation occurring on the processed surface of the workpiece becomes substantially uniform, and the distribution of the compressive residual stress becomes uniform.
Therefore, it is considered that as a result, the overall compressive residual stress also increased. This is a result of focusing on the shape of the projection material, and is not due to an increase in the projection energy. For this reason, a large distribution of compressive residual stress was obtained even near the machined surface.

Further, since the sphericity of each shot material was good, the surface roughness of the processed surface after shot peening was also good. Since the surface roughness has a great influence on the fatigue resistance, the use of the shot material of the present invention can improve the fatigue resistance from this viewpoint. Moreover, even if the shot material is made small in particle size and two-stage shot peening is not performed, the surface roughness is good and the compressive residual stress near the surface of the workpiece is large, so that the contribution to the improvement in productivity is also large. As described above, when the shot material of the present invention is used, both the compressive residual stress and the surface roughness after shot peening can be improved, and the peening effect can be improved more than before.

Here, the sphericity refers to a value expressed by (maximum diameter-minimum diameter) / 2 for one shot material. If the sphericity exceeds 50 μm, the plastic deformation of the surface of the workpiece becomes non-uniform, and the distribution of compressive residual stress decreases. In order to further improve the peening effect, reduce costs and improve productivity, the sphericity should be 20 to
More preferably, it is 30 μm. Further, the shot peening shot material of the present invention has a Biccus hardness of 700 to 10.
00HV is more preferable. By increasing the hardness of the shot peening projection material in this way, the absorbed energy of the main projection material at the time of a shot is reduced, and a large projection energy can be given to the surface of the workpiece. For this reason, combined with the good sphericity of the shot material, a larger compressive residual stress can be given not only to the surface portion but also to the inside of the processed layer, which is more effective.

A member requiring higher strength has a higher hardness. The shot peening shot material of the present invention is particularly effective for such a member having a high hardness. Here, the reason why the Viccus hardness is set to 700 to 1000 HV is that if the Biccus hardness is less than 700, the amount of projection energy absorbed by the projection material increases, the compressive residual stress applied to the workpiece decreases, and the fatigue resistance increases. This is not preferable in increasing the value. On the other hand, when the Vickus hardness exceeds 1000 HV, there is no problem in improving the fatigue resistance of the workpiece, but the life of the shot material is reduced. In order to further improve the fatigue resistance of the workpiece, the Biccus hardness of the projection material is set to 800 to 90.
0 HV is more preferable, and it is particularly preferable that the hardness is equal to or higher than the hardness of the workpiece.

It is more preferable that the shot peening shot material of the present invention has a particle size of 0.3 to 1.5 mm. By setting the particle size of the shot peening shot material in this way, the fatigue resistance of the work material can be further improved.
Here, the particle size refers to an average value of diameters when one shot material is measured from at least two orthogonal directions. If the particle size is less than 0.3 mm, appropriate projection energy cannot be obtained, the compressive residual stress is reduced, and it is not suitable for improving the fatigue resistance of the workpiece. Also, the processing efficiency is not good.

On the other hand, when the particle size exceeds 1.5 mm, shot marks and projection energy formed on the processed surface increase, and the surface roughness deteriorates. In order to improve the fatigue resistance of the workpiece by giving appropriate projection energy and good surface roughness, the particle diameter of the projection material is set to 0.5 to 0.5 mm.
9 mm is more preferable.

Further, the shot peening blasting material of the present invention is prepared by adding 0.95 to 1.10% of carbon, 0.15 to 0.35% of silicon, 0.50% or less of manganese, More preferably, it contains 0.025% or less, sulfur 0.025% or less, and chromium in an amount of 1.30 to 1.60%, with the balance being iron and unavoidable impurities.

This composition is the same as that used as a high carbon chromium bearing steel. By using a material of such a component as the shot peening shot material of the present invention, a shot material excellent in hardenability, hardness, wear resistance, life, and the like was obtained. In addition, by using a commercially available steel ball material for bearings, it is convenient to reduce the manufacturing cost.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Shot peening is often used in the fields of aviation, military, space, automobile, chemistry, etc. because it can improve fatigue resistance, stress corrosion cracking resistance and the like. For example, aircraft fuselage, wings, turbine wings, automotive suspension springs, valve springs,
Connecting rods, gears and the like. In particular, it is preferable to perform shot peening on the small and light mechanical parts and the like that require high performance and high durability using the shot peening shot material of the present invention. Thereby, the fatigue resistance of the mechanical parts and the like is further improved.

Here, as an example, shot peening using the shot material of the present invention for a valve spring and a carburized gear of an engine will be described. Valve springs are a typical example of shot peening. The valve spring is a compression coil spring that biases the valve so that the valve closes the supply / exhaust port of the cylinder head. Usually, an automobile engine rotates several thousand revolutions per minute, and a motorcycle engine rotates 10,000 to 20,000 revolutions per minute. In addition, both the shear stress caused by the load itself and the shear stress caused by the torsional moment act inside the coil, and the repetitive stress is large. Therefore, a large fatigue strength is required.

By subjecting such parts to shot peening using the shot material of the present invention, it is possible to reliably apply a compressive residual stress to the surface and the inside of the wire without impairing the surface roughness of the valve spring. As a result, the fatigue strength is significantly improved. In particular, it is suitable for use in valve springs used in harsh environments such as racing. Further, in order to further improve the fatigue strength, multiple stages of shot peening may be performed.

[0026] Carburized gears are one in which shot peening is frequently used. This is because not only the effect of shot peening itself but also martensite of retained austenite increases hardness and compressive residual stress and further improves fatigue strength. Carburization is generally performed in a CO gas atmosphere. In this case, near the surface (10 mm from the outermost surface)
(0-200 μm), a surface abnormal layer containing a large amount of retained austenite is formed. And, this surface abnormal layer causes a reduction in the strength of the carburized gear.

Therefore, when shot peening is performed on the carburized gear, the retained austenite is transformed into martensite by plastic strain. Due to this martensitic transformation, the volume of the surface layer expands, the compressive residual stress increases, and the hardness also increases. When shot peening is performed on the carburized gear by using the shot material of the present invention, since the shot material has good sphericity, plastic deformation is almost uniformly applied to the processed layer of the carburized gear. As a result, the martensitic transformation of retained austenite is uniformly promoted,
The compressive residual stress and hardness are further increased near the surface layer of the carburized gear. Moreover, at this time, since the shot material of the present invention has good sphericity, the surface roughness does not deteriorate. As described above, when the shot peening is performed on the carburized gear using the shot material of the present invention, the fatigue resistance is particularly improved, which is preferable.

[0028]

The present invention will be described in detail with reference to examples of the present invention and comparative examples. (Example) Manufacture and sorting of blasting material JIS G4805SUJ Bearing steel is drawn and φ1.0
mm, and the drawn wire has the same length (1.0 mm) as its diameter.
mm) and then projected onto a rigid wall to round the edges.

Then, this was ground (polished) to obtain a predetermined sphericity. This grinding includes a fixed upper plate, a lower plate having a grindstone surface on its upper surface and rotating, a drive device for rotating the lower plate, a projecting material inlet and an outlet for taking out the ground material after grinding. This was performed using a grinding machine. The shot material whose edge has been rounded is put into the inlet and guided to the gap between the upper panel and the lower panel. Since the lower plate is rotated with respect to the upper plate by the driving device, the blast material guided between them is ground while rolling on the grindstone surface of the lower plate while being in contact with the upper plate. Then, when a predetermined sphericity was achieved, it was taken out from the outlet. Thereafter, quenching and tempering were performed to obtain a predetermined Viccus hardness. The quenching was performed by heating in a vacuum quenching furnace at 850 ° C. for 120 minutes and then rapidly cooling in quenching oil. Tempering is carried out by an electric furnace.
After performing at 0 ° C. × 180 minutes, it was air-cooled.

Next, in order to control the particle size of the shot material thus obtained, roller sorting was performed using a roller sorting machine as shown in FIG. This roller sorting means that two rollers 1 in which the projection material is rotatably arranged at a predetermined interval.
By rolling on 0 and 20, the blast material is sorted by its diameter. The two rollers 10 and 20 have the same inclination θ with respect to the horizontal plane, and the interval A at the upper portion thereof is slightly larger than the interval B at the lower portion. That is, A> B. (A <B may be satisfied, and the roller may be slightly tapered so that the roller shaft itself is arranged in parallel.) In this embodiment, the basic diameter (target value) of the blast material to be selected is determined. φ
0.915 mm, the gap difference A-
B = 2 to 10 μm.

Therefore, when the projectile is rolled from above,
The projection material having a diameter smaller than the predetermined diameter is sieved at the upper portion, so that only the projection material having a desired diameter or more is obtained.
In this method, only the lower limit of the diameter is managed. However, since the diameter does not exceed the diameter of the drawn wire before cutting, the management of the upper limit is unnecessary.

(Comparative Example) As a shot material of the comparative example, a cut wire shot generally used was used. This is done by drawing a hard steel wire of JIS-G3506-SWRH82A, drawing a wire of φ1.0 mm, cutting the drawn wire to the same diameter (1.0 mm), and then projecting the wire onto a rigid wall. By performing this, the edges are rounded.

(Measurement) (1) Sphericity The sphericity was measured as follows. One of a large number of shot materials was randomly extracted from the V-groove measurement block.
Place it in the groove, securely fix the dial gauge to the arm tip of the magnet base, contact the measuring part (tip part) of the dial gauge from above the blast material, read the needle runout of the dial gauge after reset It is. By rotating the projectile so that the entire projectile can be measured on average,
Similar measurements were made at 0 points. Among these measured values, the minimum value and the maximum value were selected, and ２ of the difference was defined as sphericity, which was 4.5 μm. Table 1 shows this sphericity.
Shown in

The sphericity was also measured for other production lots for confirmation.
Since it was 0 μm, 3.2 μm, and 4.0 ± 0.8 μm, it was confirmed that the sphericity was very good regardless of the lot. In addition, since the sphericity of the comparative example has a large variation and is meaningless to measure, the approximate values are shown in Table 1.

(2) Biccus Hardness The Biccus hardness is a Vickers hardness specified in JISZ2244.
The hardness was measured according to the hardness test method. That is,
Using a Viccus hardness tester, measure
Test using a diamond-shaped pyramid indenter to prevent damage
The load makes a depression in the mirror-polished plane of the sample. Soshi
From the diagonal length d of this dent and the test load P,
Therefore, the calculated value was defined as Biccus hardness. Hv = 1.8544 P / d^Two  The test load was 500 g and the load time was 5 seconds.
Based on this, it was measured for one shot material of the example.
Table 2 shows the Viccus hardness. From Table 2 it can be seen that
It can be seen that the shot material has a substantially constant hardness up to the inside.

The "Viccus hardness" in the present invention is represented by the Viccus hardness near the surface of the blast material.
Further, since the material and the shape of the projection material are uniform, it can be considered that the other projection materials have the same hardness. Table 1 shows this Viccus hardness measured for the examples and comparative examples.

(3) Particle size Please explain the following. Using a stereoscopic (optical) microscope, the particle diameter of the projection material was set to 2
The diameter in the direction was measured, and the average of both was defined as the diameter in that direction. This was repeated 10 times while changing the measurement direction of the blast material. When the average of these 10 data was taken as the particle size, it was φ0.915 mm. Table 1 shows the particle size.
Shown in

For confirmation, the other lots were also measured to find that φ0.9149 mm, φ0.9151 mm,
Since the diameter was 0.9495 mm and all were 0.91 or more, it was confirmed that sorting was performed well and that particles having a uniform particle size were obtained. In addition, since the particle diameter of the comparative example has a large dispersion | variation and there is no meaning of a measurement, the approximate value was shown in Table 1.

(4) Weight The weight was determined by arbitrarily extracting 20 shot materials and using an electronic balance for Examples and Comparative Examples. This was repeated twice each. Table 3 shows the results and the average value. From Table 3, it can be seen that the variation in weight of the examples is very small. This is considered to be because the sphericity of the examples was good and the particle diameter was uniform. Therefore, when the shot material of the embodiment is used, uniform shot peening can be performed on the surface of the work material. The weights of the examples and comparative examples in Table 1 show the average weight per shot material.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

[Table 3]

(Evaluation) (1) Apparatus Shot peening was performed using a pneumatic shot peening apparatus equipped with an injection nozzle. The table on which the test piece was placed was rotatable with respect to the injection nozzle.

(2) Conditions The shot peening projection test was performed on the shot materials of the example and the comparative example under the same conditions using the above-described apparatus.
Specifically, the projection pressure was 4.0 kgf / cm ^2, and the distance from the nozzle of the apparatus to each test piece (workpiece) was about 12 kgf / cm ^2.
0 mm. The projection angle was perpendicular to the processing surface of the test piece. The table on which the test pieces were placed was rotated at 12 revolutions per minute so that shot peening was performed uniformly on each test piece. The projection time was 60 seconds.

Under these conditions, the peening strength was measured. In the case of the embodiment, the arc height value is 0.788 mmA
And 0.835 mmH. In the case of the comparative example, the arc height values were 0.800 mmA and 0.715 mmH. The specifications of the almen strip and the measurement method using an almen gauge were in accordance with JSMA (standard). The coverage at this time was 300% in both the example and the comparative example. The measurement was performed by comparison with a sample using a magnifying glass of 10 magnification.

(3) Results Carburized material (JI 70 mm, H 19 mm, thickness 10 mm)
(S-SCM420), using a test piece having a surface hardness of 730 HV, the evaluation was performed under the above-described apparatus and conditions. Compressive Residual Stress Compressive residual stress of each test piece after shot peening was measured for Examples and Comparative Examples. As a measuring method, a general X-ray residual stress measuring method using X-ray diffraction was used as a non-destructive method. FIG. 1 shows a graph of the measurement result. From this, it can be seen that, when the shot peening was performed using the shot material of the present invention, a large compressive residual stress was uniformly generated not only on the processed surface portion of the test piece (workpiece) but also on the inside.

Surface Roughness The surface roughness of each test piece after shot peening was measured for the examples and comparative examples. Table 1 shows the results. Here, Ra is the center line average roughness, Rmax is the maximum height, Rz
Represents a ten-point average roughness, and the units are all μm.

[0048]

[Table 4]

FIG. 2 shows the state of the machined surface after shot peening in Examples and Comparative Examples. FIG. 2 (a)
Is the surface of the test piece before shot peening, (b) is the surface of the test piece of the example after shot peening, and (c) is the surface of the test piece of the comparative example after shot peening. . From these, it can be seen that when shot peening is performed using the shot material of the present invention, the surface roughness is significantly improved. Moreover, when shot peening was performed using the shot material of the present invention, the surface roughness was better than the surface of the test piece before shot peening. FIG. 3 shows the entire shot peening shot material of the present invention.

(Others) A roller sorter as shown in FIG. 4 was used to control the particle diameter of the shot material of the present invention. Such a strict control can be performed, and the significance is that the shot material is significant. Is greatly affected by sphericity. Further, the roller sorter is not limited to use only during the production of a shot material, but is also very effective for removing worn, deformed and damaged shot material during shot peening. That is, since the sphericity of the shot material can be maintained within a certain range even during shot peening, a desired compressive residual stress can be applied without roughening the surface roughness of the shot-peened mechanical component.

As described above, when shot peening is performed on a mechanical part using the shot material of the present invention, its mechanical properties can be strictly controlled by the synergistic effect of sphericity and particle diameter. In particular, the shot material of the present invention is suitable for shot peening of mechanical parts that require strict control of strength, fatigue resistance, surface roughness, and the like.

[0052]

When shot peening is performed using the shot material of the present invention, the compressive residual stress of the processed layer can be increased to the inside while the surface roughness of the processed surface is kept good. And
The fatigue resistance of the workpiece after the shot peening can be significantly improved.

[Brief description of the drawings]

FIG. 1 is a graph showing the distribution of compressive residual stress between an example of the present invention and a comparative example.

FIG. 2 is a diagram showing the surface roughness after performing shot peening using an example of the present invention and a comparative example.

FIG. 3 is a diagram illustrating a shot material according to an embodiment of the present invention.

FIG. 4 is a conceptual diagram illustrating a state of selecting a shot material.

[Explanation of symbols]

 10, 20 rollers θ Angle between roller and horizontal plane A Upper distance between rollers B Lower distance between rollers

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[Procedure amendment]

[Submission date] May 19, 2000 (2005.1.
9)

[Procedure amendment 1]

[Document name to be amended] Drawing

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[Correction method] Change

[Correction contents]

FIG. 2

[Procedure amendment 2]

[Document name to be amended] Drawing

[Correction target item name] Figure 3

[Correction method] Change

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FIG. 3

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kanehisa Hattori 481, Tori, Omagari, Nitta-ji, Oji-machi, Kaifu-gun, Aichi Pref.

Claims (4)

[Claims] 1. A shot peening blast material having a sphericity of 50 μm or less. 2. The shot peening blasting material according to claim 1, wherein the shot peening material has a Viccus hardness of 700 to 1000 HV. 3. The method according to claim 1, wherein the particle size is 0.3 to 1.5 mm.
Or the shot peening projection material according to 2. 4. The composition of claim 1 wherein the carbon content is 0.95 to 1.10%, silicon is 0.15 to 0.35%, manganese is 0.50% or less, phosphorus is 0.025% or less, and sulfur is 0.025%. %Less than,
The shot peening shot material according to any one of claims 1 to 3, wherein the shot peening material contains 1.30 to 1.60% of chromium, with the balance being iron and unavoidable impurities.