AMS-S-13165 was cancelled years ago and is superseded by AMS2430U, which does not have this reference data. This reference data is also going to be removed in the next revision of AMS2432. This information has been moved to ARP7488. The data there is a recommendation
not a requirement. I don't know where this data originally came from, but I find the information suspect.
I brought this very issue up at our last SAE specification meeting earlier this month. I am seeing more and more drawings calling out a shot size that is not correct for the desired intensity. In my opinion at somepoint the velocity of the peening media becomes so great that peening is no longer being performed and abrasive blasting is. There is a reason there a multiple sizes of peening media.
Shot size is very critical. Every shot peening machine propels a constant number of pounds of shot per hour, which is dependent upon the horsepower of the centrifugal wheel or the volume of air used per hour in an air blast machine.
The number of particles in a pound is therefore a function of the particle size.
A pound of S-660 shot contains 14,000 particles. A pound of S-330 (half the diameter of S-660) contains 110,000 particles, 8 times as many as S-660. Therefore a wheel that throws one pound, would throw 14,000 impacts of S-660 or 110,000 impacts of S-330. Conversely 1 particle of S-660 would have 8 times the impact energy of 1 particle of S-330, due to the E = 1/2MV2 kinetic energy formula.
However, a small shot particle may not possess sufficient energy to peen effectively and produce the compressive stresses to the depth required.
It is important to select the smallest size of shot that will produce the proper Almen intensity since small shot will saturate the surface more rapidly. The purpose of this size discussion is to point out the tremendous increases in speeds possible by using the smallest size of abrasive that produces adequate cleaning or shot peening. Processing time is money and fast coverage will reduce cost."