Ann: Quarterly Report and Appendix 5B , page-27

"Furthermore, Stainless Steel is highly susceptible to fatigue failure if there is any cyclic loading (continual movement)"
Yes in Young's modus of elasticity charts , continual flexing of stainless results in cracks while mild steel is able to take the shock and return to its previous strength. The agitators in the dairy industry are all stainless and seem to perform fine, you can incorporate "soft drives" to reduce shock loadings something as simple as a flex plate/clutch between the shaft and the coupling and drive shafts as seen on 120Kw PTO shafts of tractors running brush slashers.
From memory the ball mill motor was American made.