No, it just broke at the weakest point. Below the studs were firmly gripped by the head. Avove it was reinforced by the nut. So the weakest point was between, and it broke through the core of the thread. Unlikely the orginal studs had any special heat treatment, or enough carbon or alloying elements to effect any. So heating them up and cooling them probably had little effect on strength.
I also noted quite a lot of white powder packed in between the studs and head on my Brum, though no clear evidence of leaking. Some of those were gripped fast.
So you had access to a welder but did not try resistance heating the stud to break the grip (heating the top/nut is not enough), pity. Brutal as it sound I think you would have found it simpler and safer than hacksaw blades and drilling out. Perhaps next time you go to remove the cylinder head! 😀