JPH02311268A - Shot peening method - Google Patents

Abstract

PURPOSE:To execute a shot peening process of constant arc height to a material to be processed by controlling projection time so as to make a product of an average grain size of a shot peening grain raised to the third power and projection time a constant value, according to the variation in the average grain size of the operating shot peening grain. CONSTITUTION:In a device in use, relation between a motor current I and an average grain size d of a shot peening grain is experimentally examined, so as to preliminarily determine constants (a), (b), whereby projection time t can be calculated based on an aimed arc height value h and on the motor current I by an operation means B without measuring average grading of a shot each time. By executing a shot peening process of the projection time t, irrespective of a variation in a shot peening grain size, process intensity of the aimed arc height value h can be obtained constantly.

Description

[Detailed description of the invention] [Purpose of the invention]

(Industrial Application Field) The present invention relates to a shot peening method used to project shot peening particles onto the surface of gears, shafts, springs, etc. to improve the fatigue strength of the parts.
More specifically, the present invention relates to a shot peening method that is used to reduce variations in fatigue strength of parts by controlling the degree of processing by shot peening particles to a constant level. (Prior art) Shot peening treatment involves projecting metal particles called shot peening particles onto the surface of a workpiece by rotating an impeller to work harden the surface and generate residual compressive stress in the surface layer. Therefore, it is widely applied to gears for transmissions, structural members for aircraft, etc. as a means to improve fatigue strength. In actual work of such shot peening treatment, production control is performed by measuring arc height as an index of treatment intensity. This arc height is as described in pages 4 to 6 of "Shot Peening Work Standards" published as the revised 3rd edition by the Japan Spring Manufacturers Association on September 1, 1980. The degree of peening treatment is evaluated as the amount of warpage of the test piece when shot peening is applied to a test plate of a certain size (Almen strip), and this arc height has a good correlation with the fatigue strength of the workpiece. (Problem to be solved by the invention) However, in this type of shot peening treatment, the arc height gradually decreases as the shot peening particles are worn out, damaged, etc. due to repeated treatments. Shot is replenished, but as a result of this difference in arc height before and after replenishment, there is a problem in that the fatigue strength of the product varies, and resolving this variation has been one of the challenges in shot peening processing. (Objective of the Invention) The present invention has been made to solve the conventional problems as described above, and it is possible to improve the shot peening particles by adjusting the projection time according to the change in the particle size of the shot peening particles. The arc height can be kept constant even if the grain size of The purpose of the present invention is to provide a shot peening method that can improve

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above object, the inventors of the present invention have conducted extensive studies on shot peening treatment intensity, that is, various factors that affect arc bite, and conducted many experiments on the interrelationships among them. As a result of investigating and analyzing the data, it was found that the arc height h can generally be expressed by the following formula. h=c 1 og (dja v2 * t) where,
C is a constant determined by the shot peening device used, ■ is a constant determined by the impeller rotation speed of the shot peening device, d is the average particle diameter of the shot, and t is the projection time. Further, the present inventor focused on the relationship between the average particle diameter of shot peening particles and the motor current of the impeller motor, and found that there is a linear relationship between the average particle diameter of shot peening particles and the motor current of the impeller. is recognized, and from the above-mentioned relational expression between the arc height and the average particle diameter of shot peening particles, the arc height can be expressed as a function of the motor current of the impeller motor and the projection time, and the motor current of the impeller motor is detected and the above By controlling the projection time so that the function remains constant, we have obtained a completely new finding that the arc height can always be controlled to be constant even if the particle size of the shot peening particles changes. The shot peening method according to the present invention is based on the above-mentioned knowledge, and is based on the relationship between the cube of the average particle size of the shot peening particles and the projection time, depending on the change in the average particle size of the shot peening particles during operation. In one embodiment, the shot peening particles have a linear correlation between the average particle diameter of the shot peening particles and the average particle diameter. An embodiment of the shot peening apparatus for carrying out the shot peening method according to the present invention is characterized by having a configuration in which the projection time is controlled as a function of the motor current of an impeller motor having a shot peening method according to the present invention. calculation means for calculating and outputting a projection time from the arc height value and the motor current value of the impeller motor detected by the motor current detection means; The shot peening particles used in the present invention are not only at room temperature, but can also be cooled or heated. The present invention is characterized in that the above-described configuration is used as a means for solving the conventional problems mentioned above. (Operation of the Invention) Hereinafter, embodiments of the present invention will be described in detail along with the operation using an example in which an impeller is used as a means for spraying shot peening particles. In the above relational expression % formula % (1), since C and V are constants determined by the shot peening device and the impeller rotation speed, d'
It is clear that if t, that is, the product of the average particle diameter of the shot and the projection time, is made constant, the arc height h will also be constant. Therefore, C and V are determined by conducting an experiment in advance, and projection is performed so that d' t becomes a constant value determined by the constants c and v and the target arc height value according to the change in the grain size of the shot. By controlling the time t, the shot peening process can always be performed at the target arc height (ilIh). Also, the above relational expression (1) is as follows: t=10°/d' v2 -...(1)'
The average particle diameter d of the shot peening particles has a correlation with the motor current of the impeller motor, and is generally expressed by the linear equation of the motor current I, % (2) (a, b is a constant determined by the shot peening equipment], and therefore the projection time t is written as: (3). By experimentally investigating the relationship with the average particle diameter d and determining the constants a and b in advance, the projection time t can be adjusted to the target arc height value without having to measure the average particle diameter of each shot each time. By performing the shot peening treatment for this projection time t, it is possible to always obtain a treatment intensity equal to the target arc height value regardless of fluctuations in the shot peening particle diameter. In one embodiment of the shot peening apparatus used to carry out the shot peening method according to the above, the projection time is calculated based on the manually input target arc height value and the output of the motor current detector of the impeller motor. and a motor drive means for cutting off power supply to the impeller motor when the projection time has elapsed after the start of rotation of the impeller motor, and the calculation means calculates the manually input target arc height value. The projection time t is automatically calculated from the motor current of the impeller motor detected by the motor current detector according to the relational expression (3), and the motor driving means responds to the output of the projection time t from the calculation means. Since the impeller motor is rotated only for the time t, the shot peening process is always performed fully automatically at the target arc height value even if the particle diameter of the shot varies. (Example) First The figure shows a shot peening machine with an impeller rotation speed of 200 Or p m and a projection time of 240
The relationship between the arc height (h) and the average particle diameter d of shot peening particles was investigated when shot peening treatment was performed under the conditions of sec. In addition, this arc height h is Almen strip A gauge (plate thickness 1.29
5±0.025 mm), and the average particle diameter d was measured by sampling shot peening particles of about 1 kg, with an opening of 7001 Lm and 500 ILm. The shot peening particles were determined by the following formula from the weight of the shot peening particles remaining on each sieve when they were sieved through four stages of 300 JLm and 100 gm. In addition, Wr is the total weight of the sample, W7oO9W5°. , W3゜. , Wl. . Each has an opening of 700 lumens,
500 lm, 300 islands m. The weight of shot peening particles remaining on a sieve of 100 gm, WtooP, is the weight of shot peening particles that have passed through a sieve with an opening of 100 lm. The result shown in Figure 1 can be approximated as h=0.311ogd3+0.6...-(4) since the impeller rotation speed is constant at 200 rpm and the projection time is constant at 240 seconds, and the above general formula By comparing with (1), the constants C and v2 are 0.31 and 0.31, respectively.
It is found that it is 3587. That is, in the case of this shot peening apparatus, the relationship between the arc height h, the average particle diameter d of shot peening particles, and the projection time t is h=0.311og (0,3587d'-t)...
(5) The above equation (4) can be written as follows. The right side of the above equation (4)7 becomes a constant value by determining the target arc height value, and depending on the consumption of shot peening particles, the product of the cube of the average particle diameter of shot peening particles and the projection time t By controlling the projection time t so that djt becomes the constant value, it becomes possible to perform shot peening processing in which the arc bite is always at the target value. Table 1 shows that the target arc height value is 0.45 mm.
That is, it shows the actual measured value of the arc height when the shot peening process is performed by controlling the projection time t so that a3t becomes 78.9 from the formula (5). The average particle diameter of the shot peening particles was measured by the sieving method described above before each shot peening. l As can be seen from Table 1, as the average particle size of shot peening particles decreases, by increasing the projection time t so that the value of d3t becomes 78.9, the average particle size can be reduced to 0.
It was confirmed that the arc height could be kept within the range of 0.45±0.01 mm even though the arc height varied from 0.45±0.01 mm. FIG. 2 shows an investigation of the relationship between the average particle diameter d of shot peening particles and the motor current I of the impeller motor using the shot peening apparatus used in the above example. ' From this figure, the relationship between the average particle diameter d and the motor current I is approximated as d=0.147I-0,56...(6),
The above formula (5)7 and (6) can be obtained. Table 2 shows the arc when the target arc height value is also 0.45 mm, the motor current bias of the operating impeller motor is measured, the projection time t is determined according to equation (7), and shot peening is performed. This shows the actual measured height value. The motor electrician calculated the projection time for the next process using the current value immediately before the end of the process. As can be seen from Table 2, by determining the projection time t in the shot peening process based on the motor current bias of the impeller motor, the arc height can be adjusted to 0.45±0.
．． It was confirmed that the treatment strength could be kept within a range of 0.01 mm, and that a constant treatment strength could always be obtained. In this example,
Processing time can be calculated from the motor current of the impeller motor, and there is no need to measure the average particle size of shot peening particles each time, so calculation of processing time is quick and extremely practical. FIG. 3 shows a control circuit in an embodiment of a shot peening apparatus used to carry out the shot peening method according to the present invention. , an impeller motor 3 that rotates the impeller 2, an ammeter 4 that detects the motor current of the impeller motor 3, and a first
motor current detection means A consisting of a trigger 6 that captures the current value of the ammeter 4 in response to a signal from the digital timer 5; and an A/D converter 7 that converts the current value captured by the trigger 6 into a digital current value I; further comprising an arithmetic circuit 8 which is an arithmetic means B for calculating the projection time t according to the function (3) from the current bias from the motor current detection means and the manually input target arc height value; Turn on the impeller motor 3.
A second switch for instructing ON/OFF of the switch 9 based on the switch 9 to be turned OFF and the output from the arithmetic circuit 8.
A motor drive means C consisting of a digital timer 10 is provided. When the shot peening apparatus 1 with the above configuration is started,
First, a starting program is activated to rotate the impeller 2 and take in the motor current imbalance of the impeller motor 3 by the trigger 6, and automatically input it to the arithmetic circuit 8. Next, when the target arc height value is manually input into the arithmetic circuit 8 using a keyboard or numeric keypad, the projection time t is automatically calculated according to the function (3). The calculated projection time t is input to the second digital timer 10 as the shot peening processing time, and the switch 9 is turned off when seconds elapse after the start of the shot peening processing.
The impeller motor 3 is activated and the impeller motor 3 is automatically stopped. Also, at this time, t-x is input to the first digital timer 5, and a trigger is applied to the projection end X extraction section.
The motor current value I' at that time is taken in and input to the arithmetic circuit 8 as data for the next shot peening process. At this time, if a new target arc height value is not input, the projection time t' in the next shot peening process is automatically calculated in the same manner as above, assuming that the target arc height remains unchanged, and the second digital timer 10 The impeller motor 3 is configured to automatically stop t' seconds after the start of the next shot peening process. In this way, the shot peening process is repeated. Although X is set in advance, it is desirable to set it to a small value in order to obtain a value as close as possible to the motor current value during the next shot peening process. Such an automatic control device is incorporated into the shot peening device used in the above example, and the target arc bite value is set to the same <0.45 mm, and each constant of the device is set to c=0.
31. v2=0.3587. a=0.147. b=-0
, 56 was subjected to shot peening treatment, and it was confirmed that the arc height values were all within the range of 0.45±0.01 mm. Note that the average particle diameter of the shot peening particles at this time was initially 0.74 mm, but it had been refined to 0.62 mm by the time all the treatments were completed. As described above, in the shot peening apparatus used to carry out the shot peening method according to the present invention, a decrease in the particle size of shot peening particles is detected by the motor current, and the projection time of shot peening particles is automatically extended accordingly. Therefore, shot peening treatment can always be performed at a constant intensity. As described above, in the above-mentioned embodiment of the present invention, shot peening treatment is always performed at a constant intensity by lengthening the shot projection time in accordance with shot damage and consumption due to wear. This was designed to reduce the variation in strength and raise the lower limit of the design strength, but damage and consumption of shot peening particles prolongs the processing time, and in order to increase the frequency of replenishing new shot peening particles, Needless to say, this is not desirable in terms of cost. As a result of various studies on preventing damage to shot peening particles from the above-mentioned viewpoint, the present inventor found that if the effect of heat on the processed product is not so great, shot peening particles should be heated above the ductile-brittle transition temperature of the material. It has been found that the rate of shot wastage can be significantly reduced by heating the shot to a temperature preferably in the range of 80°C to 200°C. FIG. 4 shows the procedure for carrying out such hot shot peening. In the figure, a workpiece 11 is installed on a rotary table 12 and rotates together with the rotary table 12. The shot peening particles 13 are heated by a shot peening particle heater 14 and supplied from a shot peening particle output pipe 15 onto an impeller 16 rotating at high speed.
The light is projected toward the object to be processed 11 and collides with the surface of the object to be processed 11 at high speed. All of the above shot peening processing is performed within the shot peening chamber 17 so that the shot peening particles 13 are not scattered outside the shot peening chamber 17. The shot peening particles after shot peening are transported to a particle sorter 19 through a recovered shot peening particle transport pipe 18 . The particle sorter 19 removes particles smaller than a predetermined particle size and sends the remaining shot peening particles to the shot peening particle heater 14 . The shot peening particle heater 14 heats the shot peening particles sent out from the particle sorter 19 and the shot peening particles replenished from the new shot peening particle replenishing machine 20 to a predetermined temperature to subject a predetermined amount to shot peening. The particles are sent to the particle output tube 15. In this example, shot peening particles with an average particle diameter of 0.6 mm and a hardness of HRC58 are heated to 100±5°C, and shot peening particles with a surface hardness of HRC61 are heated at a projection speed of 70 mm/see.
A shaft made of 0M420 carburized, quenched and tempered steel was subjected to shot peening treatment. FIG. 5 shows the amount of shot peening particles consumed when the above-mentioned shot peening is performed for one hour in comparison with when the shot peening particles are not heated. By using such a hot shot peening method, the amount of shot peening particles consumed could be reduced to 1/2 or less compared to the conventional shot peening method. In this case, the heating temperature of the shot peening particles may be higher than the ductile-brittle transition temperature of the shot peening particle material;
°C or higher, and the upper limit of the heating temperature is 200 °C or lower in consideration of tempering softening of shot peening particles.Even in shot peening methods using such high-temperature shot peening particles, By controlling the projection time in accordance with the change in the shot peening particles in a relationship established between the average particle diameter of the shot peening particles and the projection time, variations in the arc height of the workpiece are reduced. It has been found that the variation in fatigue strength of the processed object can be further reduced. In addition, as a result of further detailed study of this type of shot peening method, the present inventor found that in conventional shot peening methods that do not control the temperature of the workpiece, steel balls, etc. are projected onto the workpiece at a higher speed. When heat treatment is performed, the heat generated by plastic deformation on the surface of the workpiece becomes too large, and this heat may cause the work-hardened surface of the workpiece to recover at the same time. That's not to say that it won't go up. Therefore, in the case of such a workpiece, it may be possible to mix steel balls with temperature-controlled water that is jetted at high speed, and to make the steel balls collide with the surface of the workpiece at high speed. FIG. 6 is a diagram showing an example of a case where such a measure is taken. In the figure, shot peening particles 21 collide with a workpiece 24 placed on a rotary table 23 while being mixed with jet water 22 . After the collision, the shot peening particles 21 and water are collected in the recovery chamber 25, and the shot peening particles 21 are sedimented and transported to the shot peening particle sorting and supplying device 27 through the transmission pipe 26. The recovered water is controlled to a predetermined temperature by a temperature control device 28, and is sent to a water injection device 29, where it is given a predetermined flow rate and is injected. The shot peening particles sorted by the shot peening particle sorting and supplying device 27 are replenished with shot peening particles by a new shot peening particle supplying device 30, and mixed in a predetermined amount with the running water injected from the water injection device 29. . In this way, the flowing water and shot peening particles are mixed in the water jet pipe 31 and sprayed onto the object 24 to be treated. In the tree control example, shot peening particles with an average particle diameter of 0.5 mm were sprayed at a speed of 100 at a temperature of 5 ± 1°C.
The mixture was mixed in flowing water at a rate of m/sec, and shot peening was applied to a rotary bending test piece with a notch made of 5Cr420 carburized, quenched and tempered steel. Figure 7 shows the results of a fatigue test using the rotary bending test piece with the notch that was subjected to shot peening as a wood countermeasure. Also shown are the fatigue test results when shot peening was performed on the notched bending test piece at room temperature (20°C) under the same conditions. As is clear from FIG. 7, the fatigue strength was improved by about 10% by using the shot peening method for wood treatment. This example of countermeasure does not specifically limit the cooling medium (which is also a transportation medium for shot peening particles) to water, nor does it specifically limit the temperature of this medium. For example, the temperature may be controlled at -20'O with salt water. At least the temperature of this medium should be at most 20°C, and even in the shot peening method performed by controlling the temperature of the object to be treated, the shot peening particles are adjusted according to changes in the shot peening particles during operation. By controlling the projection time based on the relationship between the average particle size and the projection time, it is possible to reduce variations in the arc height of the workpiece, and to reduce variations in the fatigue strength of the workpiece. It was recognized that it could be made even smaller.

【Effect of the invention】

As explained above, in the shot peening method according to the present invention, the product of the cube of the average particle size of the shot peening particles and the projection time is adjusted according to the change in the average particle size of the shot peening particles during operation. Since the structure is such that the projection time is controlled so that the This makes it possible to perform shot peening treatment with a constant arc height at all times, greatly contributing to eliminating variations in fatigue strength, etc. of the parts to be treated, and improving the lower limit of the design strength of mechanical parts. A significantly better effect is produced.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the average particle diameter of shot peening particles and arc height in an example of the shot peening method according to the present invention, and FIG. 2 is a graph showing the relationship between the average particle diameter of shot peening particles and the motor current of the impeller motor. 3 is a block diagram illustrating an example of a control circuit of a shot peening apparatus used to implement the shot peening method according to the present invention, and FIG. 4 is a shot peening method using heated shot peening particles. FIG. 5 is a graph showing the effect of heating shot peening particles on the consumption rate of shot peening particles in the shot peening method shown in FIG. An explanatory diagram showing the method, FIG. 7 is a graph showing the relationship between the repeated stress and the number of rupture cycles in the shot peening method shown in FIG. 6. Patent Applicant Nissan Motor Co., Ltd. Representative Patent Attorney Yutaka Oshio N (LLJ) LI AkXtl, -L(V) I
'1'=Special b-1

Claims (1)

[Claims] (1) Controlling the projection time so that the product of the cube of the average particle diameter of the shot peening particles and the projection time becomes a constant value in accordance with changes in the average particle diameter of the shot peening particles during operation. A shot peening method characterized by: