JP2646728B2 - Manufacturing method of high strength bolt - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a high-strength bolt by subjecting a serrated portion of the bolt to shot peening.

(Prior Art) FIG. 1 shows an example of a serration bolt. In the figure, 1 is a head, 2 is a lower part of a neck, 3 is a serration, and 4 is a screw.

The serration portion 3 is formed with several spiral grooves.

The serration bolt is used by inserting a serration portion into a bolt hole of a conrot body of the engine by press fitting.

Conventional serration bolts are manufactured in the head, lower neck,
The shaft portion (other than the serrated portion and the threaded portion) was processed by cold forging, then the serrated portion and the threaded portion were cold-rolled, and then quenched and tempered to obtain a finished product.

(Problems to be Solved by the Invention) Recent engines tend to have a high output and a high rotation like a turbo engine or a supercharger engine, for example.

Along with this, the load stress applied to the engine connecting rod bolt (serration bolt) has also become severe.

Therefore, in the conventional serration bolt, the bending fatigue strength of the bolt tends to be insufficient.

As countermeasures, high-quality serration bolts are used, and the shape of the serration groove is examined from a technical point of view. However, it is still a decisive means in terms of stability and reliability. The fact is that it is hard to say.

It is also conceivable to use a connecting rod one rank higher, but this is not a countermeasure because the weight is increased and fuel consumption is reduced.

As described above, at present, the development of serration bolts that can cope with high-output and high-speed engines is urgently required.

The present invention has been made in view of the above circumstances, and provides a method of manufacturing a serration bolt capable of coping with a high-output, high-speed engine without changing the material and size of the conventional serration bolt and the shape of the groove of the serration. It is something you want to do.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a serration portion of a serration bolt which has been subjected to quenching and tempering, wherein the shot grain hardness Hv is 700 to 900 and the shot grain size d is serration. The value of d / R is 0.18 for the minimum fillet radius R of the part
The shot peening is performed by projecting shot grains having a size of ~ 3.3 for a predetermined time.

(Action) By performing shot peening on the serrated portion of the serrated bolt that has been quenched and tempered as described above, a compressive residual stress is generated.

And the diameter d of the shot grain is the minimum radius R of the fillet.
The value of d / R is 0.18 to 3.3 and its hardness Hv is 700 to 900
The use of a shot grain that results in a compressive residual stress guaranteed over the entire serration.

(Example) Hereinafter, an example of the present invention will be described in detail. The inventor of the present invention has found that since conventional serration bolts are manufactured by quenching and tempering after machining by cold forging and cold rolling, tensile residual stress is generated on the surface of the completed serration bolt. As expected, the low bending fatigue strength
It was determined that this was due to the tensile residual stress.

Then, the serration bolt after quenching and tempering was subjected to shot peening to generate a compressive residual stress on the surface of the serration bolt, and an experiment was conducted on the bending fatigue strength.

 The specifications of the experiment are as follows.

In Fig. 1, the upper limit tolerance is 8 mm for the outer diameter of the serration.
0.010mm lower limit tolerance 0.015mm, trough diameter 7.2mm, groove pitch 1.5m
m, the width of the peak was 0.8 mm, the radius from the peak to the valley was 0.5 mm, the twist angle of the groove (ridge) was 60 ° C., the number of threads was 8, and the total length 1 was a serration bolt having a length of 49 mm.

As shown in FIGS. 2 and 3, the serration bolt was divided on the center line, and the R of the fillet portion was measured. As a result, R = 0.17 mm, 0.07 mm, 0.18 mm, and 0.27 mm were obtained.

The bolt is processed in JIS SCM440H (Cr-
After cold forging and cold rolling, quenching and tempering were performed so that the internal hardness became about Hv350.

The bending fatigue test was performed by simulating a failure mode on an actual engine and using a hydraulic vibrator with a connecting rod bolt incorporated therein. The repetition rate at this time is 20
00 cpm and the number of repetitions were 10 ⁷ to 10 ¹⁰ .

The specifications of shot peening use a direct pressure type air blast machine, the nozzle diameter is 5 mm, the injection pressure is 5 kg / c
m ² , the injection distance is about 15 cm, and the injection time is about 60 seconds.

In the experiment shown in FIG. 4, the shot grain hardness was Hv800, and how the shot grain size was determined in relation to the bending load kg (change in bending stress) applied to the serration bolt at that time and the repetitive load. It is an effect or effect.

From the results of this test, it was found that it was more advantageous to perform shot peening with a shot particle having a small size.

The experiment in FIG. 5 shows how the grain size and hardness of shot particles affect the bending fatigue limit of serration bolts.

According to this experiment, the shot grain size was small and hardness
Higher Hv proved to be advantageous.

FIG. 6 shows the relationship between the residual stress value of the serration bolt and the shot projection time, which was tested using shot grains having a hardness of Hv800.

From this experiment, it was found that the residual stress value did not increase any more in about 90 seconds and the residual stress was saturated irrespective of the particle size of the shot grains.

In this experiment, the residual stress value was measured at the groove of the serration because it was impossible to measure the fillet R.

When the experiment in FIG. 6 is compared with the experiments in FIGS. 4 and 5 described above, the smaller the particle size in the previous experiment, the better, whereas the experiment in FIG. The larger the grain size, the larger the residual stress value. Contrary to the previous experiment, the larger the grain size, the more advantageous it is.

The fact that the particle size is larger is more advantageous for a portion where the surface on which shot particles collide is flat, and that the particle size is larger for a sharp portion such as the fillet R portion. The smaller is the more advantageous.

Therefore, when the bending fatigue strength of the entire serration bolt is considered, it can be said that there is a predetermined relationship between the particle size of the shot grain and the fillet R of the serration.

In this experiment, the minimum value of the fillet R was 0.07 mm, which is the most difficult part for shot peening.
The value of d / R is 0.7 to 2.9 in relation to.

And the fillet R at the end of the serration is
Since it is 0.06, d / R is 0.83-3.3.

From Fig. 5, the ratio when the bending fatigue strength of the serration bolt without shot is 1 is 1.
Since only two or more are required, those having a particle diameter of 200 μ also satisfy this condition. From this, it is effective in the range of the particle size of 50 μm to 200 μm.

Accordingly, the range from the minimum value to the maximum value of d / R in the range of 0.06 mm to 0.27 mm of the fillet R is 0.18 to 3.3.

Next, when looking at the hardness of the shot grains, as shown in FIG.
00 satisfies the ratio of 1.2 even when the particle size is 200 μm. For Hv700, the particle size is 150 for 60 seconds shot time.
μ is the limit, but if the shot time is extended, the particle size becomes 200
μ also satisfies the ratio 1.2.

Next, with regard to the shot time, the experimental results are shown for Hv800 in FIG.

In this experiment, when the shot time is 30 seconds, the residual stress value is in the process of increasing again, and becomes saturated in 90 seconds.

From the above experimental results, the following conclusions can be made.

First, with regard to the hardness of shot grains, Hv 700 or less is of course effective in improving bending fatigue strength, but in order to obtain a predetermined effect, the shot projection time must be lengthened. However, if productivity is considered in view of the balance with other processing steps such as cold forging and cold rolling, it is not a measure to increase the time. Therefore, Hv700 or more is appropriate in practice.

In addition, a sufficiently good effect can be obtained with hardness Hv900 and Hv9
If it is more than 00, the shot grain life is remarkably reduced, so that Hv900 or less is appropriate.

Therefore, the shot grain hardness is most preferably Hv700 or more and Hv900 or less.

Next, as for the particle size of the shot particles, those having a size of 50 μm to 200 μm can be used, and the relationship with the radius of the fillet R portion affects.

As a result of the experiment, when the particle diameter of the shot grains is d and the radius of the fret is R, a value of d / R of 0.18 to 3.3 is appropriate.

Next, regarding the shot projection time, from the experimental results for hardness Hv800, compressive residual stress useful for improving bending fatigue strength is still insufficient for 30 seconds or less, and compressive residual stress is saturated for 90 seconds or more .

In addition, if the shot projection time is long, the serration portion is worn, so that the processing tolerance for press-fitting the serration bolt into the connecting rod body hole cannot be maintained, which is a problem.

Also, as can be seen from FIG. 5 (shot projection time 60 seconds), the shot projection time can be determined by a selective combination of the shot grain size and hardness.

For example, under the conditions of hardness Hv700 and particle size 100μ, a shot projection time of 60 seconds is sufficient.

The shot projection time is 3 by the experiment of average hardness Hv800
It is appropriate to set the time between 0 and 90 seconds.

(Effects of the Invention) As described in detail above, according to the present invention, the hardness of shot grains is Hv 700 to 900, and the grain size d
The shot peening process is performed by projecting a shot of 0.18 to 3.3 to the fillet R for a predetermined time, so that a predetermined compressive residual stress can be generated in the entire serration portion.

As a result, without changing the material of the serration bolt, changing the shape of the serration bolt, and changing the dimensions of the serration bolt, the ratio when the bending fatigue limit of the serration bolt without shot is set to 1 can be increased to 1.2 or more. Serration bolts that can handle high-power, high-speed engines were obtained.

Also, as long as the above shot peening conditions are satisfied,
Since a predetermined bending fatigue strength is obtained, there is an excellent effect that its reliability is high.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an example of a serration bolt, and FIG.
The figure is a longitudinal sectional view of the serration bolt used in the experiment divided along the center line, FIG. 3 is a partially enlarged view of the serration part of FIG. 2, and FIG. 4 is the shot grain hardness Hv800, shot shot time 60. Fig. 5 is a graph showing the results of an experiment on the effect of the shot grain size on the bending load applied to the bolt under the condition of seconds, and Fig. 5 shows the shot grain size and hardness under the condition of a shot projection time of 60 seconds. Graph showing the results of an experiment on the effect of blasting on bending fatigue strength. Fig. 6 shows the effect of shot blasting time and shot particle size on the residual stress value at the serration groove surface using shot grain hardness Hv800. It is a diagram showing the result of having experimented on the influence. 1 ... head, 2 ... lower neck, 3 ... serration part, 4
... Screw part.

Claims (1)

(57) [Claims] 1. The serration bolt has a quenched and tempered serration bolt having a shot grain hardness of Hv700 to 900 and a shot grain size d of a minimum fillet radius R of the serration part.
A shot grain having a d / R value of 0.18 to 3.3 is shot for a predetermined time to perform shot peening.