Gary,
I'm not asking you to change your process only adjust it within the Frozen limits.
Typically PSI has a +/- 5 PSI, a similar tolerance would apply to shot flow in pounds per minute. You machines should have an interface that allows the operator to make these subtle changes. Just to be clear, what I'm saying is of your frozen process calls for 50 PSI +/- 5 you can set your machine to run at 50,51,52..... as long as during the process is never drops below 45 or goes over 55. You have to have this flexibility, your machine is physically changing all the time as all peening machines do. Example your blast hose and nozzles get larger with use. Obviously at some they would need to be replaced as part of your preventive maintenance plan. Also, temperature and barometric pressure can cause variation.
As for the AMS 2431/1 shot in your machine. Your right it does work harden with use that’s why we don't use very much of it at all. Your used shot could possibly now be AMS 2431/ 2 shot. Have you had the suspected shot in the machine hardness tested?
Why did you choose AMS 2431/1 shot? I'm guessing cost, as very few OEM's required regular hardness shot. If at all possible I would switch to AMS 2431/8 Conditioned Cut wire shot. Although it is more costly up front it lasts many times longer. Also, CCW never fractures it just gets smaller with use. It's hardness range by specification is 55-65 just like AMS 2431/2 however I have never seen it vary by more than 3 points. Meaning it's much easier to add to your machine since it hardness variation is considerably tighter than cast steel shot thereby making saturation curves easier to reproduce.
AMS 2430 is currently being revised. The intent is to change figure 1 and 2 to show only acceptable shapes anything else would be bad.
What type of shot classification system does your machine have? If you have just a size classifier your may have problems removing some non-round particles. You need to run the shot through a spiral device to remove these.
Feel free to call me to discuss if you need to. I sent a reply to you via this website with my contact info.
-Walter
