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Quality Standards
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HARDNESS
Media hardness is a well-known characteristic. In all four previous International Conferences on Shot Peening, the influence of media hardness was discussed. It is always recommended that, for compressive residual stress (magnitude & depth), media should be at least as hard as the part being peened. This is particularly true when peening parts above hardness HV 450, which is the center of the normal hardness range (HV 390 – 510) of cast steel shot. For parts above HV 450 in hardness, a harder media should be used.
But what about the Almen Strip? The standard specified hardness of Almen strips is HV 430 – 510 with an average of HV 470. Arnaud (7) clearly showed that peening with media at hardness HV 440 gave much lower are heights & saturation points than higher hardness media (e.g., 640 HV that is a standard hardness for cut wire shot). In fact, Arnaud’s data indicates that Almen strip arc heights will increase with increasing media hardness. |
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DENSITY
Density is a characteristic of peening shot that is included in most media specifications, but is rarely measured by the user. In fact many users do not know how to measure density of their media. A density measurement can be a quick and valuable indication of the presence of large amounts of voids, porosity or other internal defects in shot particles. Particles with significant internal defects will have low density and tend to shatter and breakdown much faster than solid sound particles thus reducing average particle size and increasing the amount of unacceptable sharp edge broken pieces.
Another importance of density has to do with potential Kinetic Energy of a particle of shot. Kinetic Energy (KE) of a moving object is a product the mass of the object and its velocity. Mass of a particle is the product of its density and volume (8). As particle density increases, particle mass also increases.
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DURABILITY
Durability of shot peening media is one of its most important and complex characteristics. It can be defined as the ability of shot peening media to maintain its size and shapes during use. Media size breaks down by becoming smaller. Useful Life (UL) of peening media is how long, or how many cycles, media can endure before becoming unacceptable in size. Media shape deteriorates by forming sharp edge broken pieces. Resistance to Fracture (RTF) is how long media can resist fracturing or breaking.
Media durability influences Fatigue Resistance of parts peened, environmental concerns (dust generation and disposal), equipment maintenance and media costs.
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MATERIAL
The final characteristic that should be discussed is the material. There are a number of different materials from which to make shot peening shot. Glass beads, Ceramic beads, Cast Carbon Steel Shot, Carbon Steel Conditioned Cut Wire shot and Stainless Steel Conditioned Cut Wire Shot are the most commonly used media when peening for improved Fatigue Resistance. Glass beads, Ceramic beads and Stainless Steel shot are normally used to avoid Iron contamination of non- ferrous metal parts. Bakul Shot Company performed a test program to measure differences in Iron residue left by the three steel media.
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 | Weight Limit for Cut Wire Shot | |
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| Shot (inch) | Size (mm) | Weight of 50 Random pieces | | CW 62 | 1.6 | 1.090 - 0.330 | | CW 54 | 1.4 | 0.720 - 0.880 | | CW 47 | 1.2 | 0.480 - 0.580 | | CW 41 | 1.0 | 0.310 - 0.390 | | CW 35 | 0.9 | 0.200 - 0.240 | | CW 32 | 0.8 | 0.140 - 0.180 | | CW 28 | 0.7 | 0.100 - 0.120 | | CW 23 | 0.6 | 0.050 - 0.070 | | CW 20 | 0.5 | 0.040 - 0.050 | | CW 17 | 0.45 | 0.040 - 0.060 | | CW 14 | 0.35 | 0.020 - 0.040 | | CW 12 | 0.30 | 0.010 - 0.025 |
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| Wire Diameter Used For Cut Wire Shot | |
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| Shot Size | Mean Wire (Inch) | Diameter (mm) | | CW 62 | 0.062 | 1.6 | | CW 54 | 0.054 | 1.4 | | CW 47 | 0.047 | 1.2 | | CW 41 | 0.041 | 1.0 | | CW 35 | 0.035 | 0.9 | | CW 32 | 0.032 | 0.8 | | CW 28 | 0.028 | 0.7 | | CW 23 | 0.023 | 0.6 | | CW 20 | 0.020 | 0.5 | | CW 17 | 0.017 | 0.45 | | CW 14 | 0.014 | 0.35 | | CW 12 | 0.012 | 0.30 |
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