Actually, that depends on the kind of loading (i.e. static or impact).
It is true that to solve for stress, one only needs to divide F/A.
However, in an impact situation, you force is actually mass times acceleration. Therefore,
The wording of the question sort of led me to think that infraray was considering the use of an arbour press or such-like, so I assumed the impact speed to be negligible. It would be a little more complicated if he were using a gun of some kind, because there'd probably be deformation of the tool head and more heat production. That's a little out of my league.