Home Forums General Discussion Media, Shot, Beads, etc. optimal operating mix sieve testing results

Hello,

First, I would avoid using the term “operating mix” when discussing shot peening media. That term is generally associated with abrasive blast-cleaning operations, where a broad mixture of new, worn, and broken particles may be intentionally maintained to support cleaning efficiency.

Shot peening is a controlled special process. The media-size distribution must be tightly controlled in accordance with AMS2430 and the applicable AMS2431 media specification. A general rule such as “a minimum of 80% above a 0.8 mm sieve” may be appropriate for a cleaning operation, but it does not adequately control shot peening media. Both oversized and undersized particles must be controlled because changes in the size distribution can affect intensity, coverage, surface condition, and the resulting residual-stress profile.

I would also consider using only an air-wash separator to be a poor choice for controlling media in a shot peening application. An air-wash system can remove dust and some lighter particles, but it does not provide precise control of particle size and cannot reliably remove all broken, elongated, or otherwise non-round particles.

You stated that the process operates at a high intensity. High-intensity peening generally causes more rapid media breakdown, increasing the rate at which undersized and non-round particles enter the media supply. Without adequate screening and shape separation, these degraded particles can accumulate even though an Almen verification reading remains within the required range.

At a minimum, I recommend adding a mechanical sieve-screening system to the machine to continuously or routinely remove undersized and oversized particles. I would also consider adding a spiral separator, either integrated into the machine’s media-recovery system or used offline at established intervals, to remove non-round and damaged particles. The inspection frequency should be based on the demonstrated media-breakdown rate and the volume of media circulated.

Before concluding that the established sieve requirement is too restrictive or that the Almen strip is “too saturated,” I would need clarification on the following:

1. After performing the sieve test, are the unacceptable particles determined from the original representative sample or from a particular fraction separated during sieving? Sieve analysis determines particle-size distribution by weight, while particle-shape inspection is a separate evaluation performed on the appropriate sample or sieve fraction.
2. What original intensity value or values were established from the saturation curve?
3. What intensity range is specified on the engineering drawing?
4. When performing the production intensity verification, is the resulting arc height both:
* Within the engineering drawing’s specified intensity range; and
* Within ±0.0015 inch of the original intensity established from the saturation curve?
5. How long has it been since a complete saturation curve was performed using the current equipment configuration, process parameters, and media-size distribution?

A conforming verification arc height does not establish that a nonconforming media-size distribution is acceptable. Sieve analysis, particle-shape inspection, and Almen intensity verification control different characteristics of the process. An acceptable Almen reading confirms the energy delivered at the strip location under that particular test condition, but it does not demonstrate that the media distribution, coverage rate, surface effects, or residual-stress results remain equivalent to those obtained during the original process qualification.

An Almen strip also should not normally be described as “too saturated.” Saturation refers to the behavior of the complete saturation curve and is used to determine process intensity and saturation time. A single verification strip cannot establish whether the saturation curve or the process response has changed.

If the historical media-size limits cannot be maintained, the first step should be to investigate the media-reclamation system, separator performance, media-addition practices, and inspection frequency. If a different size distribution is proposed, it should be validated by performing a new saturation curve, confirming coverage, and demonstrating continued compliance with AMS2430, the applicable media specification, and the engineering drawing before revising the established limits.