JPH10128665A - Manufacture of metal part having high fatigue strength - Google Patents

Abstract

PROBLEM TO BE SOLVED: To introduce high compressive residual stress with small surface roughness by previously applying lubricating oil to the surface of a metal part with a film thickness which is a specified multiple of a diameter of a shot ball or less, and then shot-peening by use of the shot ball with the above diameter. SOLUTION: An deterioration of surface roughness of a material to be processed is caused by metallic adhesion between the material to be processed and the shot ball, it is necessary to previously apply lubricating oil to the surface of the material to be processed for prevention of adhesion. The film thickness lower limit value of lubricating oil is determined from the viewpoint of balancing between the roughness of the surface of the material to be processed and hardening. The film thickness upper limit is 1.6 times as larger as the diameter of the shot ball. Even if the film thickness of lubricating oil exceeds 1.6 times as large as the diameter of the shot ball, hardening is saturated, but the time required by processing is prolonged. Further it becomes difficult to apply lubricating oil to the surface of the metallic part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing metal parts having high fatigue strength, such as gears for construction machines, automobiles and industrial machines, various shafts, and joints of welded structures.

[0002]

2. Description of the Related Art In general, high fatigue strength is applied to metal parts for machine structures, such as gears, various shafts, and welded joints of construction machines, automobiles or industrial machines, and to which bending and shear stresses are repeatedly applied. Required.

In order to improve the fatigue strength, a metal component is usually subjected to a surface treatment for hardening its surface or imparting compressive residual stress to the surface. An industrial method for introducing compressive residual stress to a surface is a shot peening treatment method.

[0004] The "shot peening method" is a method in which a shot ball is driven by compressed air or the like and projected on a metal part to apply a compressive residual stress to a corresponding portion by local plastic deformation of the surface and restraint from the surroundings. This is a processing method for increasing the fatigue strength of metal parts. Note that the residual stress is not necessarily introduced into the compression region by shot peening. For example, when a residual tensile stress exists in advance such as in a welded portion, the value of the residual tensile stress is reduced by the shot peening process, but whether or not the residual tensile stress reaches the compression range depends on the welding conditions and the shot conditions. Regardless of the presence or absence of the tensile residual stress, the residual stress of the material to be processed is shifted to the compression side by the shot peening treatment as compared to before the treatment, and the fatigue strength is improved.

[0005] In recent years, in connection with the weight reduction of machines and structures,
There is a strong demand for improved fatigue strength of metal parts, especially steel parts. As one of the countermeasures, it is desired to introduce a large compressive residual stress higher than the conventional level into steel parts. In order to increase the compressive residual stress in the shot peening method, it is necessary to increase the projection energy density.

Incidentally, in the shot peening treatment method, since a compressive residual stress is generated by local plastic deformation, an increase in the surface roughness of a material to be treated as a metal component, that is, deterioration of the surface roughness is inevitable. It is known that the deterioration of the surface roughness causes concentration of stress or strain, and is a great barrier for improving fatigue strength in order to facilitate generation of fatigue cracks.

From the viewpoint of introducing a high compressive residual stress, the effect is higher as the projection energy density in the shot peening treatment is higher. However, fatigue cracks are liable to occur due to deterioration of the surface roughness, and the fatigue strength is lower than the compressive residual stress value. Does not improve as expected.

In order to solve such a problem, there has been proposed a shot peening treatment method which introduces a high compressive residual stress while preventing the deterioration of the surface roughness as described below.

[0009] One of them is subjected to a first-stage shot peening treatment using a shot ball having a large diameter of 1.0 mm or more, and thereafter, a particle size of 0.1 mm is used in order to level the surface irregularities generated in the first stage. There is a method of performing a second shot peening process using a shot ball of 3 mm or less (Japanese Patent Application Laid-Open No.
184627).

[0010] Similarly, the shaving step at the root of the gear that has been subjected to shaving is eliminated by the first-stage shot peening, the surface roughness is improved by the second-stage shot peening, and the compressive residual stress of the steel part is reduced. (Japanese Unexamined Patent Publication No. Sho 61-170511)
No.).

As a further improvement method of these methods, the first stage treatment is performed with shot balls having a hardness of HRC 50 or more and a particle size of 0.2 to 1.0 mm, and then the shot particles having a particle size smaller than the first stage shot balls are obtained. It has been proposed to perform a second-stage process so that the ball has an optimum arc height value (Japanese Patent Laid-Open No. 7 (1999) -76).
-178668). Note that the arc height value is an index serving as a measure of the effect of shot peening when shot peening is performed on a metal plate, and is the warpage height of the metal plate generated by shot peening, that is, camber.

[0012] Although each of these methods achieves a certain effect and achieves the object, all of them are two-stage processing methods, so that the processing time is prolonged, the productivity is impaired, and the compressive residual stress is significantly increased. It is difficult to do.
Unexamined Japanese Patent Publication No. Hei 7-17 which can introduce a relatively large compressive residual stress
The method disclosed in JP-A-8668 does not change to the basic configuration of two-stage processing. For this reason, when there is only one processing device, a step of replacing shot balls occurs, and it takes a lot of processing time,
If two processing apparatuses are arranged in series to shorten the processing time, a large capital investment will be required.

[0013]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a metal part having high fatigue strength by an inexpensive and simple shot peening method capable of introducing a high compressive residual stress with a small surface roughness. To provide.

[0014]

A feature of the present invention is to utilize a technical idea that has not existed before, that is, a method of applying a lubricating oil to a surface of a metal component to be processed and then performing shot peening. It is in.

The present inventor investigated various shot peening conditions and searched for a method of introducing a high compressive residual stress without deteriorating the surface roughness, and as a result, the following items could be confirmed. .

(A) One of the causes of the deterioration of the surface roughness due to the shot peening treatment is metal adhesion between the shot ball and the material to be processed. That is, when the shot ball collides with the material to be processed, metal adhesion occurs at the contact surface between the two materials, and when the shot ball rebounds and separates, adhesion marks are formed on the surface of the material to be processed. Is one of the major causes of deterioration.

(B) By applying a lubricating oil to the surface of the material to be treated in advance, the surface of the material to be treated can be hardened while preventing metal adhesion for the reasons described below.

(C) Regarding the upper limit of the film thickness of the lubricating oil,
There is an appropriate range in terms of oil slick efficiency and workability.

The present invention is a combination of the above items, and has been completed after an experiment on an enormous number of shot peening conditions. The gist of the present invention is a method of manufacturing a metal part having high fatigue strength as described below.

That is, the lubricating oil has a high fatigue strength in which a lubricating oil having a thickness of 1.6 times or less of the diameter of the shot ball is previously applied to the surface of the metal part, and then the shot ball having the diameter is shot peened. Manufacturing method of metal parts ".

In the above description, "metal" corresponds to steel, copper, aluminum, etc., but steel is a main object of application, especially since steel is frequently used. The “film thickness” is calculated based on the used oil amount and the surface area of the target metal component. in this case,
The thickness of the lower part of the metal part tends to be thicker,
The thickness of the lubricating oil may not be the same in all portions of the surface of the metal component, and may be 1.6 times or less the diameter of the shot ball at the portion where the thickness is maximum.

The term "lubricating oil" refers to all lubricating oils and the like, for example, JIS VG46 grade.

Next, the role of the lubricating oil in the present invention will be described in detail.

Each shot ball has the kinetic energy shown in equation (a).

SE = (1/2) · m · v ² = (1/12) · π · D ³ · ρ · v ² (a) (where SE: shot Kinetic energy per ball, m:
Shot ball mass, v: shot ball speed, π: pi,
(D: shot ball diameter, ρ: density) When the lubricating oil is present on the surface of the material to be processed, the shot ball flies in the lubricating oil while receiving a drag force D represented by the equation (b). D = (1/2) · ρ · v ² · C _D · F · · · (b) (where F is the projected area of the shot sphere on a plane perpendicular to the flow, (C _D : resistance coefficient, ρ: density, v: shot ball speed) When the shot ball passes through the lubricating oil and actually reaches the surface of the workpiece, the kinetic energy is reduced from the initial value by the work due to the drag. That is, the lubricating oil has a buffering effect on the collision of the shot ball.

An even more important function of the lubricating oil is to absorb the heat energy generated at the portion where the shot ball comes into contact with the surface of the metal component by a reaction such as evaporation of the metal component, thereby suppressing the occurrence of metal aggregation. At this time, it is considered that the gas in which the lubricating oil is decomposed and evaporated also contributes to preventing agglomeration by intervening in a portion where the shot ball and the metal component surface are in contact.
These effects are effective even when the film thickness of the lubricating oil is as thin as a fraction of the shot ball diameter.

At a very early stage after the shot peening begins, most of the lubricating oil is scattered off the surface of the metal part by a shot ball, and thereafter, a high-energy shot is applied to the surface of the metal part covered with a thin oil film. The collision of the ball begins. At this point, the metal surface is not so hardened due to the buffering action of the lubricating oil.

When the thickness of the lubricating oil is reduced and the buffering action is reduced, a high-energy shot ball plastically deforms the metal surface to cause hardening and heat energy generation. Even if heat is generated at this stage, it is taken away as heat of vaporization of the lubricating oil and does not directly lead to a large temperature rise. Therefore, while the lubricating oil is present, the metal surface is hardened and a residual stress is applied to the compression side without causing metal aggregation.

Thereafter, when the shot ball collides with the absence of lubricating oil, the collision is closer to an elastic collision than to the first unhardened surface. That is, the shot ball tends to bounce off as it is without dissipating energy as heat energy there. As a result, the rate at which kinetic energy is converted to heat energy or the like on the surface is reduced, and metal adhesion is less likely to occur. As a result, a high compressive residual stress is applied while maintaining good surface roughness, so that fatigue strength is improved.

According to the above-described two-stage processing method, it is necessary to change the shot conditions at the first stage and the second stage. In particular, changing the diameter of the shot ball requires a large number of steps.
In contrast, the lubricating oil of the present invention can achieve the same effect without such a change.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the reasons for limiting the present invention will be described.

1. Lubricating oil The type of lubricating oil is not particularly limited. For example, commercially available mineral oils, rust-preventing oils having a viscosity at 40 ° C. of about 10 to 20 cSt, and the like are applicable.

The deterioration of the surface roughness of the material to be treated is caused by metal adhesion between the material to be treated and the shot sphere. Must be kept. The lower limit of the film thickness of the lubricating oil is determined from the viewpoint of balancing the roughness and the hardening of the surface of the material to be treated. When the time change of the curing phenomenon was measured, it was confirmed that work hardening was progressing at an extremely early stage, and that the surface roughness was good as long as the lubricating oil was applied, no matter how thin the film thickness was. Therefore, in the present invention, only the application is required, and the lower limit is not set.
However, in order to more reliably enjoy the effects of the present invention, it is desirable that the film thickness of the lubricating oil be 0.1 mm or more.

The upper limit of the film thickness is set to 1.6 times the shot ball diameter as described above. Even if the film thickness of the lubricating oil exceeds 1.6 times the shot ball diameter, the above effect is saturated, and the time required for the treatment is simply prolonged. Further, it becomes difficult to apply the lubricant to the surface of the metal part, and the harmful effect of polluting the working environment due to the dripping flow from the surface cannot be ignored.

2. Coating method There is no limitation on the method of applying the lubricating oil. Brushing and spraying may be used. Further, a simple shape such as a flat plate may be applied by a roll around which a felt ground containing lubricating oil is wound.

3. Shot Peening Treatment The conditions of the shot peening treatment may be an ordinary method for imparting compressive residual stress to the surface of the metal material, or a stronger condition as described later. The conditions of shot peening include the projected ball speed, projected weight per unit time,
It is determined by the projected weight per unit area and the projected density (coverage). As usual conditions, for example, the projection speed = 80 m / sec, the projection weight per unit time = 150 kg / min, the projection weight per unit area =
For example, 500 kg / m ² , coverage = 200%, and the like.

From these conditions, a projection energy density described later can be obtained as (projection energy density) = (kinetic energy per shot ball) × (number of shot balls colliding with a unit area during the processing period). it can. As a standard of the above-mentioned "condition stronger than normal conditions", about twice the projection energy under normal conditions can be exemplified.

[0038]

EXAMPLES Next, the effects of the present invention will be described with reference to examples.

As test materials, thick steel plates having a plate thickness of 20 mm having three strength levels were used. It was named TS40, TS60 and TS80 according to each tensile strength level.

Next, among the conditions of the shot peening treatment, the particle diameter of the shot ball is 0.5 mm centering on 0.8 mm.
m, 1.0 mm steel balls were used. The hardness of all steel balls was equal and the HRC value was 62. Usually, as mentioned above,
The projection condition of the shot ball is often controlled by a projection speed, a projection weight per unit time, a projection weight per unit area, and a coverage. Here, since these factors are changed at many levels, the projection energy density of a shot ball colliding per unit area of the material to be processed was used as an index for organizing the entire data.

The surface residual stress after the shot peening treatment was measured by the X-ray method.

Table 1 shows the conditions for measuring the residual stress.

[0043]

[Table 1]

The direction of the axial force in the fatigue test was adjusted to the direction of the surface residual stress measurement shown in Table 1.

FIG. 1 is a drawing showing the shape of an axial fatigue test piece conforming to JIS used in the fatigue test.

The surface roughness of the material to be treated after the shot peening treatment was evaluated by a stylus type surface roughness measuring device. Ra (μm) was adopted as an evaluation index.

After evaluating the residual stress and the surface roughness, an axial fatigue test was performed to evaluate the fatigue strength of the material to be treated. An electrohydraulic universal fatigue tester was used as the fatigue tester, and the evaluation was performed in the air at room temperature. The test was performed by the load control method, and the load ratio (minimum load / maximum load) was a sinusoidal waveform of 0.1, and the repetition rate was 5 Hz. A relationship between the nominal stress and the number of fracture cycles, that is, an SN curve was created for each treatment material, and a stress corresponding to 2 × 10 ⁶ fracture cycles was defined as fatigue strength.

Tables 2 to 6 summarize the evaluation results for each test material and each shot particle size.

[0049]

[Table 2]

[0050]

[Table 3]

[0051]

[Table 4]

[0052]

[Table 5]

[0053]

[Table 6]

From these tables, it can be seen that Test Nos.
4, 14-17, 27-30, 40-43, 53-56
In any case, it can be seen that when the projection energy density exceeds the reference projection energy density or 1.2 times the reference projection energy density, the fatigue strength is significantly reduced. On the other hand, in the example of the present invention in which the film thickness of the lubricating oil is 1.6 times or less of the shot ball diameter, even when the projection energy density is 1.2 times or 1.5 times the reference projection energy density, the fatigue is reduced. It can be seen that the strength is on a rising trend and that the absolute value of the fatigue strength is considerably larger than that of the comparative example. That is, by setting the film thickness of the lubricating oil on the surface of the metal component within the range of the present invention, a large compressive residual stress is applied to the material to be processed under a high projection energy density condition while suppressing the deterioration of the surface roughness. Shot peening can be performed.

[0055]

According to the present invention, even if the shot ball projection energy density on the material to be processed is increased by a simple and inexpensive method of applying lubricating oil, deterioration of surface roughness can be suppressed, fatigue strength can be improved, and cost can be reduced. Since weight reduction and fuel economy of automobile parts and steel structures are realized, it has a very useful effect not only on related industries but also on the general public.

[Brief description of the drawings]

FIG. 1 shows the shape of an axial force fatigue test piece.

Claims (1)

[Claims] 1. A shot sphere having a diameter of 1.6 on a surface of a metal part.
A method for producing a metal part having high fatigue strength, wherein a lubricating oil having a film thickness of twice or less is applied in advance, and then shot peening is performed using a shot ball having the diameter.