JP2002205270A - Spring shot peening method and spring - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a shot peening process capable of most effectively increasing the durability of a spring in consideration of operating conditions actually performed. SOLUTION: Ceramic beads are projected on a spring at a speed of 50 m / s or less. Or, while projecting ceramic beads, compressive residual stress σ on the surface after shot peening
Relative hardness H _S of the _R spring may be a method of adjusting the shot peening condition to satisfy the formula _{[σ R ≧ 2.675 × H S} -955]. In these cases, it is desirable to use ceramic beads having an average particle size of 0.3 mm or less. In addition,
When shot peening is performed a plurality of times, any one of the above methods is performed in the last shot peening. In this case, it is desirable that the shot peening other than the last round be performed by a conventional method (a steel shot grain having a diameter of about 0.1 to 1.2 mm is projected at a speed of 60 to 90 m / s).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a shot peening method for manufacturing a spring having excellent durability (fatigue resistance).

[0002]

2. Description of the Related Art As a method of dramatically improving the durability of a spring, shot peening is an indispensable treatment especially for high-strength springs such as suspension springs for automobiles and valve springs for engines.

[0003] Shot peening is a process in which small particles are projected onto the surface of an object to be processed. However, while performing the same process, burrs generated during cutting and molding processes and heat treatment are not included. It differs greatly from the shot blasting performed for the purpose of cleaning the surface by removing the scale (hard oxide film) formed on the surface in terms of conditions such as strength. That is, the shot peening process is performed under the condition that the outermost surface is plastically deformed for the purpose of generating compressive residual stress on the surface of the spring. Conventionally, as small particles suitable for such conditions, steel wires such as cut wires, cast steel balls, and white pig iron balls, which are obtained by cutting a steel wire into a short length, are often used.

[0004]

SUMMARY OF THE INVENTION Regarding the spring itself,
Improvement by changing the composition of the material steel, improvement in the forming process, improvement in heat treatment, surface treatment (for example, nitriding)
Various improvements such as the improvement of the fatigue life have been made, and the fatigue life has been improved. In order to cope with such improvement of the material itself, that is, high strength, also in the shot peening process, stress peening in which shot peening is performed while applying stress to the processing target spring, or a state where the temperature of the processing target spring is increased (100 to 100). A technique such as warm shot peening for performing shot peening at about 300 ° C.) has been developed and has been effective in improving the fatigue life of a spring.

As for shot grains used in the shot peening process, use of ceramic grains such as glass, alumina and zirconia, and small grains using metals other than iron in addition to steel grains have been proposed. For example, Japanese Patent Application Laid-Open No. 6-158158 discloses a method of subjecting a coil spring to nitriding treatment and then projecting ceramic shot grains having a hardness higher than the surface hardness. Further, Japanese Patent Application Laid-Open No. 9-57629 discloses a method of using a spherical body having an average diameter of 1 mm or less made of an inorganic substance having a specific gravity of 16 to 20. As an example of such an inorganic substance, W
(Tungsten) and / or W ₂ C.

[0006] However, although ceramic grains can be generally used for shot blasting such as descaling, they are considered unsuitable for shot peening, which involves projecting the steel surface to the extent of plastically deforming the steel surface. In other words, ceramic grains are more fragile than steel grains, so the sharp corners of shot grains that break during repeated use give flaws to the surface of the object to be treated, and this becomes the starting point of fatigue failure, which in turn lowers fatigue strength. there's a possibility that. Therefore, when selecting shot grains, it is necessary to consider not only the effect of the laboratory but also the actual operation after a long period of operation.

[0007] The present invention provides a shot peening process that can most effectively increase the durability of a spring in consideration of the actual operating conditions.

[0008]

That is, a method of shot peening a spring according to the present invention comprises the steps of:
It is characterized by projecting onto a spring at a speed of 50 m / s or less.

In addition, the method of projecting ceramic beads and adjusting the shot peening conditions so that the compressive residual stress σ _{R on} the surface after shot peening satisfies the following equation with respect to the hardness H _{S of the} spring: Good. σ _R [MPa] ≧ 2.675 × H _S [Hv] −955

[0010] These methods may, of course, be performed in combination.

In these cases, the object of the present invention can be better achieved by setting the average particle size of the ceramic beads used to 0.3 mm or less.

When the shot peening is performed a plurality of times, any one of the above-described methods is performed in the last shot peening. In this case, shot peening other than the last round is performed by the conventional method (diameter 0.1 to
It is desirable to project steel shot grains of about 1.2 mm at a speed of 60 to 90 m / s).

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The ceramic beads used in the method according to the present invention are ZrO ₂ (zirconia) and SiO ₂
(Silica) as the main component, and additional MgO and C if necessary
aO-, a ceramic containing _{HfO 2, Y 2 O 3,} CeO 2 or the like, those produced by a spray or the like of blown or melt such as molten After air-steam (e.g., U.S. Patent No. 4,10
No. 6,947, Japanese Patent No. 2594023). As described in these, generally, ZrO ₂ / SiO ₂ = 2: 1
Although the mixing ratio is of the order of magnitude, the ceramic beads that can be used in the present invention are not strictly limited thereto, and it is sufficient to use those commercially available as “ceramic beads”. As an example, the product name ZIRSHO
Ceramic beads sold as T (Nippon Saint-Gobain Co., Ltd.) have the following composition and properties.

Chemical composition: ZrO ₂ 67% SiO ₂ 30% Other 3% Crystalline analysis value: Zirconia 67% Vitreous 33% Material properties: True specific gravity 3.85 Bead specific gravity 3.76 Bulk specific gravity 2.3kg / L Micro hardness (Vickers) 7-9GPa (7000-9000N / mm2) 500g load Rockwell hardness 50-65RC

In the spring shot peening method according to the present invention, the reason why the projection speed of the ceramic beads is set to 50 m / s or less is that, when the projection is performed at a higher speed and collides with the surface of the object to be treated, This is because there is a high probability that the ceramic beads are broken by energy, and the fragments are highly likely to cause deep flaws on the surface of the processing object.

The following relational expression σ _R [MPa] ≧ 2.675 × H _S [Hv] −955 between the compressive residual stress σ _R of the spring surface after shot peening and the hardness H _S of the spring is given by the present inventors. Have performed experiments on a large number of springs, and have determined that a spring subjected to shot peening treatment to satisfy this relational expression has high fatigue strength. The details will be described later.

It is desirable to use ceramic beads having an average particle diameter of 0.3 mm or less,
Larger pieces are more likely to break when colliding with the surface of the spring to be treated, and when used repeatedly, the sharp corners of the fragments are more likely to cause sharp flaws on the surface of the spring to be treated. . Further, by reducing the surface roughness (maximum roughness, average roughness) after the shot peening treatment as much as possible, an effect of improving durability can be obtained.

The shot peening method according to the present invention comprises:
It may be performed by itself, or may be performed in combination with the conventional shot peening method. In this case, the method according to the present invention is always performed in the last round. This is for the following reasons. When shot peening is performed by the method according to the present invention, the depth from the surface where the compressive residual stress is maximized is slightly smaller than that obtained by the conventional method, as described later. Therefore, after the residual stress is generated at a relatively deep position by performing the shot peening by the conventional method, the shot peening is further performed by the method according to the present invention, thereby increasing the residual stress at a position closer to the surface. A compressive residual stress can be applied. Thereby, the effect of improving the durability can be greatly increased.

[0019]

EXAMPLES In order to clarify the effects of the shot peening method according to the present invention, an actual spring was manufactured and subjected to a fatigue test, and its surface hardness distribution and surface compression residual stress distribution were measured.

The springs used in the experiments are valve springs (A, B, C) and suspension springs (D) for automobile engines. The materials and specifications of the springs are shown in FIG. 1, and the manufacturing method is shown in FIG. The test springs A and B have been subjected to a nitriding treatment (S3).
The test springs C and D were not subjected to nitriding treatment.

Each of two or three test springs was subjected to shot peening under the conditions shown in FIG. FIG. 4 is a graph showing the relationship between the surface hardness H _S of each test spring and the surface compressive residual stress value σ _R after the shot peening treatment. Shot particles using ceramic beads (A1, B1, C1, D1, D2; these are hereinafter referred to as developed products) and shot particles using conventional steel shot particles (A2, B2, C2, D3). Hereinafter, these are referred to as comparative products.) Means that they are clearly separated from each other at the line of [σ _R = 2.675 × H _S −955] and the hardness of the test spring is the same.
The developed product has a higher surface compressive residual stress value.

FIGS. 5, 8, 11 and 14 are graphs plotting the fatigue test results of each test spring, ie, the test stress amplitude and the number of repetitions up to breakage. Each graph shows the results obtained by statistically processing the test results of each test spring.
A 0% break probability line (the number of repetitions at which 10% of the total number of samples breaks) was entered. FIG. 17 shows the fatigue strength (statistical fatigue strength) of each test spring obtained from the 10% break probability line. As is clear from these results, all of the test springs (developed products) subjected to the shot peening treatment by the method according to the present invention show higher fatigue strength than the comparative products.

The distribution of hardness measured from the surface of each test spring in the depth direction is shown in FIGS. 6, 9, 12 and 15, and the distribution of compressive residual stress measured from the surface in the depth direction. Are shown in FIGS. 7, 10, 13, and 16. In the value of the compressive residual stress on the outermost surface, in any type,
The value of the developed product exceeds the value of the comparison product. 13 and 16, the depth at which the compressive residual stress is maximized is smaller (shallower) in the developed product than in the comparative product. Since the maximum (shear) stress is generated at the outermost surface when the coil spring is compressed, it is needless to say that the maximum residual stress is desirably present closer to the outermost surface. Excellent durability of the developed product (Fig. 17)
Is thought to be explained by this phenomenon.

[Brief description of the drawings]

FIG. 1 is a table showing the materials and specifications of test springs.

FIG. 2 is a flowchart showing a manufacturing process of a test spring.

FIG. 3 is a table showing a shot peening method for each test spring.

FIG. 4 is a graph showing the relationship between the surface hardness of a test spring and the value of surface compressive residual stress after shot peening.

FIG. 5 is a graph of a fatigue test result of test springs A1 and A2.

FIG. 6 is a graph of hardness distribution in the depth direction from the surfaces of test springs A1 and A2.

FIG. 7 is a graph of compressive residual stress distribution in the depth direction from the surfaces of test springs A1 and A2.

FIG. 8 is a graph showing the results of a fatigue test of test springs B1 and B2.

FIG. 9 is a graph of hardness distribution in the depth direction from the surfaces of test springs B1 and B2.

FIG. 10 is a graph showing the distribution of compressive residual stress in the depth direction from the surfaces of test springs B1 and B2.

FIG. 11 is a graph of fatigue test results of test springs C1 and C2.

FIG. 12 is a graph of hardness distribution in the depth direction from the surfaces of test springs C1 and C2.

FIG. 13 is a graph of a compressive residual stress distribution in a depth direction from the surfaces of test springs C1 and C2.

FIG. 14 is a graph showing fatigue test results of test springs D1, D2, and D3.

FIG. 15 is a graph of hardness distribution in the depth direction from the surfaces of test springs D1, D2, and D3.

FIG. 16 is a graph showing the distribution of compressive residual stress in the depth direction from the surfaces of test springs D1, D2, and D3.

FIG. 17 is a table of statistical fatigue strength obtained from a 10% break probability line of each test spring.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tomohiro Nakano 68, Kamioshita, Narumi-cho, Midori-ku, Nagoya-shi Chuo-Hatsujo Co., Ltd. (72) Inventor Masami Wakita 68, Kamioshida, Narumi-cho, Midori-ku, Nagoya

Claims (9)

[Claims] 1. A shot peening method for a spring, which comprises projecting ceramic beads at a speed of 50 m / s or less. 2. A method of projecting ceramic beads,
A shot peening method for a spring, characterized in that shot peening conditions are adjusted such that the compressive residual stress σ _R of the surface after shot peening satisfies the following expression with respect to the hardness H _S of the spring. σ _R [MPa] ≧ 2.675 × H _S [Hv] −955 3. The spring shot peening method according to claim 1, wherein the ceramic beads have an average particle diameter of 0.3 mm or less. 4. A shot peening method for a spring, wherein one or more shot peenings are performed, and the last shot peening is performed by the method according to any one of claims 1 to 3. 5. A spring which has been subjected to shot peening by the method according to claim 1. 6. A spring characterized by being made of a hard drawn wire such as a piano wire, a hard steel wire, a high silicon piano wire, etc., and subjected to shot peening by the method according to claim 1. 7. The method according to claim 1, wherein the material is an oil-tempered wire.
A spring characterized by being subjected to shot peening by any one of the methods (1) to (4). 8. A spring obtained by subjecting an oil-tempered wire to a material, performing a nitriding treatment, and then performing shot peening by a method according to any one of claims 1 to 4. 9. A spring characterized by being subjected to hot forming, quenching and tempering, and then shot peening according to any one of claims 1 to 4.