Cast steel shot will leave a ferrous residue on the surface that will eventually rust. I assume these parts are for the oil drilling industry, there very well may be an issue with magnetization if cast steel shot is used. Sensors used to guide the drill head can be negatively effected. In my experience peening API threads was done with SS Conditioned cut wire shot then followed up by peening with glass bead at a significantly lower intensity. This would clean the part and improve the surface finish. This additional step also could lead to increased fatigue life.
