The typical trim in small floating ball valves is Austenitic Stainless Steel (304 or 316); this is what is stocked and readily available.I did some digging and found the ISO interpretations and the specific question of solid 316 SS balls was raised in MP INQUIRY #2005-03, where the issue of the low stress was also mentioned.
The Maintenance Panel responded that parts loaded solely in compression were a permitted exclusion to the standard since tensile stress was required for SCC. However, they also indicated that there was no threshold specified for tensile stresses.
So, if a ball valve "ball" saw only compression, it would be exempt from ISO but if it sees any tensile stress at all, it must comply with the environmental restrictions. I don't believe there is any significant tensile stress present but I'm sure that there is a tensile stress somewhere during normal operation (certainly in a floater anyway).

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