The most common use of shot peening is to improve the fatigue characteristics of metal parts. Important parameters of shot peening must be controlled to obtain the most favorable, repeatable and reliable fatigue improvement. One of the most important parameters in the shot peening process is the shot itself. Without a projectile of the correct quality, all other shot peening parameters are irrelevant and the expected improvement in fatigue and consistency will not be achieved. Especially during processing, I was curious how much attention was paid to this most important parameter. The control of shot peening media includes the selection of the media and the control of the media in the process. This is the easiest way to provide total fatigue and consistency through shot peening.

Shot peening media has six important characteristics: size, shape, hardness, density, abrasion resistance, and material
size

The size of the medium is most commonly used to measure and evaluate the characteristics of the medium. A lot of information has been obtained in the past some narratives about the relationship between shot size and shot strength. When the velocity of the projectile remains the same or increases, the size of the projectile becomes larger, the particle mass will increase, and the strength of the shot peening will also increase.

The size of the projectile is usually evaluated using a screening test. In sieving tests, a class of 4 to 5 sieves is stacked according to their opening diameter (the largest is at the top and the smallest is at the bottom). Pour the pellets into the top sieve, and stack the sieve in a shaker for a while. After shaking, the weight of the media remaining on each sieve is measured, recorded, and compared to acceptable specifications. Table 1 shows the screening tests for two different media.

Table 1 Screening test according to MIL-S-13165C-

Screening tests are currently used to evaluate the size and shape of the media (3). In the screening test, computer surveillance uses a camera to project a layer of projectiles. The computer then measures the size and shape of each particle examined. A single inspection area contains many particles. It is expected that as many areas as possible can be inspected. Thousands of particles can be detected in a short period of time. Measurements of diameter (projectile size) and particle roundness (projectile shape) can be collected and entered into a computer software program. These software programs can produce and output data in various forms, such as statistical formats (mean, standard error, histogram, amplitude of change, maximum, minimum, etc.) or statistical process control formats (process control tables, distribution analysis, process Ability study, percentage above or below specified limit). The simple example in Table 2 is a format derived from the statistical output of a shape analysis.

Table 2Shape analysis-at least 1250

The sieving test can depict the general concept of shot peening media, but the shape analysis can provide more explanatory information, which can more clearly define the characteristics of shot peening media. The more descriptive data provided by the shape test, the better the user will be aware of the actual size and stability of the media being evaluated. For example, the analysis data in Table 1 will show that the sizes of the two media are very close. Since 18 shots per square inch cast steel shot 330 is 17.2% vs. 1.2% wire cut shot, indicating that the cast steel shot is slightly larger point. Both media are within the specified range. In Table 2, almost the same samples were used for the shape test, which clearly shows that the round cut of steel wire shots is larger and more consistent than the average cast steel shot size.

From Table 1 and Table 2, the shot peening strength comparison in Table 3 can be obtained. All shot peening parameters are the same, only the media are different. Saturation was performed by peening 25 Almen test pieces for each medium. Before shot peening, the flatness of each Almen test piece selected was ± 0.0005 inches. The statistics in Table 3 show that the medium CW35 is slightly larger and more consistent, so that a higher and more consistent shot peening strength value can be generated, which is consistent with the dimensional determination of the shape analysis in Table 2.

Table 3. Shot Peening Strength-A

Shot peening of gears and suspension springs of wantanabe and Chevrolet cars with round wire cut shots of similar size and hardness to cast steel shots can achieve more consistent fatigue life. There is no doubt that sharp-edged broken particles can adversely affect fatigue resistance.
shape

The second most commonly used measure of a medium's characteristics is its shape. The shape of the shot peened medium is round and free from potentially destructive sharp edges. Figure 1 clearly shows the surface damage (cut or tear) caused by particles with broken edges. Simpson points out that fatigue life is affected by particles with sharp edges that break. As the percentage of damaged particles increases, the fatigue life decreases accordingly. American Chevrolet Automobile and the available data (Table 4) show that the medium with less broken particles has higher consistency and higher average fatigue life than the medium with more broken particles. The shape analysis is more detailed and descriptive than the traditional visual evaluation methods. Using an automatic detection system again, the evaluation of the medium shape will be more accurate (3). For example, the use of such an automatic shape evaluation system in the shape analysis, such as the shape analysis, is very correct, and it can eliminate some common visual There are human errors and subjectivity in the detection. Using the current technology, it can be done when installing a shape analysis system on the shot blasting machine, and the size and shape of the medium can be measured while the machine is running.
hardness

Hardness is a very significant characteristic of the medium. The effect of medium hardness was discussed in the previous four international shot peening conferences. In order to obtain the best residual compressive stress (size and depth), the hardness of the recommended medium is not less than the hardness of the shot peened workpiece. When shot peening a workpiece with a hardness of HV450, this hardness is the middle value of the hardness range (HV390-510) of the selected cast steel shot. For workpieces with a hardness higher than HV450, a higher hardness medium is required.
But what about the Almen test? The hardness of the Almen test piece specified in the standard is HV430-510, and the average value is HV470. Arnold clearly showed that the arc height value and saturation point of the shot peened workpiece with the hardness of HV400 medium were lower than that of the shot peened with high hardness medium. (For example, 640HV is the standard hardness of cut pellets). The Aljo data indicate that the arc height of Almen test strips will increase with the increase of the hardness of the medium.
The author suggests that to achieve the best process control, the hardness of the medium must be as consistent as possible, and the minimum is HV510, which is the maximum hardness of Almen test strips.
density

GILLESPIE introduces the density and wear resistance-service life and crush resistance in turn. High-density media (wire cut pellets) also have the best service life and crush resistance. The service life and crushing resistance of cast steel shots, ceramic shots, and glass shots are the same (the ceramic shots are followed by ceramic shots, followed by glass shots) in the same range. In the next part of the abrasion resistance, it is pointed out that the density of stainless steel wire cut shots is slightly larger than that of wire cut shots, and the wear resistance is also measured to be slightly larger than that of wire cut shots.
Media with the highest and most consistent density will produce the highest and most consistent shot strength and the lowest possible surface breakage.
Abrasion resistance

One of the most important and complex characteristics of abrasion-resistant shot peening media. It can be defined as the ability of shot peening media to maintain shape and size during use. The size of the media gradually decreases. The life of the shot peening medium is the time and the number of revolutions that the medium can withstand before its size becomes unavailable. When the medium becomes damaged particles with sharp edges, the shape of the medium is also damaged. Breaking force refers to the time that the medium can resist breaking.
The wear resistance of the medium will affect the fatigue resistance of the shot peened workpiece, environmental concerns (dust generation and discharge), equipment maintenance, and the cost of the medium. The potential hazards of damaged, sharp-edged particles are described in the section above. The importance of dimensional consistency for fatigue body resistance has been demonstrated in the dimensional section.
Table 5 shows the measurement results of dust produced by three media—stainless steel wire shot rounds of similar size and hardness, steel wire round shots, and cast steel shots. Table 5 also shows the service life of these three media. It is not surprising that the medium with the longest service life (stainless steel wire cut rounds) produces the least amount of dust. The medium with the lowest service life produces the most dust. It is clear that the higher the abrasion resistance of the shot peening medium, the lower the amount of dust generated by the media damage.

Table 5. Dust generation rate-steel shot media

Obviously, the pellets broken into sharp edges are more resistant to rupture than the media that do not form broken particles will cause more wear on the shot blasting machine and accessories. Gillespie compares the life of shot blasting media with their resistance to crushing. In any case, the higher the service life of the medium, the stronger the crush resistance it has.
In order to accurately evaluate the cost of the consumed medium in the shot peening process, one point that must be considered is the wear resistance of the medium and its initial cost. Table 6 shows the comparison of the three media costs based on the initial cost and wear resistance. The lower the breaking rate of the medium, the better the abrasion resistance (lifetime).
In Table 6, the initial cost of the medium with the lowest cost per round (stainless steel wire cut rounds) is the highest. This is due to its higher wear resistance (low crushing rate)

Table 6- Cost Comparison-Steel Shot

Texture
The last feature discussed is material. There are many materials for making shot peened shots. Glass beads, ceramic beads, cast steel shots, steel wire shot rounds and stainless steel wire cut rounds are some of the common media used for shot peening to improve fatigue resistance. Glass beads, ceramic beads, and stainless steel shots are commonly used to treat iron-free metal workpieces to avoid iron contamination. Premier Projectiles conducted an iron residue test using three steel shots.
The sample panel is made of aluminum alloy plate 7075 (commonly used in aircraft manufacturing). This plate is shot peened with a new medium of the same size and hardness (0.35mm, HV600-700). X-ray photoelectron spectroscopy was used to analyze the surface of the plate after shot peening in three media (cast steel shot, steel wire shot rounding, and stainless steel wire shot rounding). The X-ray photoelectron spectroscopy analysis was also used to compare the flat plate operation without shot peening.
In the process of X-ray photoelectron spectroscopy, the surface of the material is irradiated with X-rays, and the X-rays only interact with the atoms on the surface. Emitting various electronic energy. The level of energy determines which energy is released from that atom. A computer model can be used to calculate the atomic concentration of the elements present.
Figure 3 summarizes the measurement of iron residues under four conditions. Unblasted samples present a small amount of iron (0.55%), which is usually present in 7075 alloys. The stainless steel shot has a residual iron content of 2.06%. The stainless steel shot, like the typical 302 alloy, is composed of nickel and chromium, and it does not suffer from iron oxidation deformation (rust). The residual iron content of the steel cut pellets is 3.34%, which is combined with oxygen to form iron oxides. The cast iron shot iron has the largest residual amount of 30.91%, and it also combines with oxygen to generate oxides.
To avoid the possibility of pitting or corrosion problems, many shot peening specifications require that aluminum or non-ferrous alloys be decontaminated after wire shot peening. Of course, according to the residual amount of the surface of the workpiece, necessary purification.
Abrasion resistance and initial cost in the material portion of the shot peening medium are other major factors. These important factors were discussed in the previous section.
to sum up

The use of shot peening in order to achieve the best increase in fatigue resistance, shot peening media can be formulated as follows.

1. Size-The size of the media used must be the same.

2. Shape-The selected medium can resist the breakage and deformation of sharp edges.

3. Hardness-The hardness of the medium used for strengthening cannot be less than the hardness of the workpiece and Almen test piece.

4.Density-Density media is more resistant to wear and tear.

5. Abrasion resistance-the selected medium has the longest service life to resist crushing force

6. Material-The selected medium has strong abrasion resistance and minimizes surface residue.