Can you break a black iron fitting by overtightening?
Yes, you can break cast iron fittings from over-tightening but you rarely do. That said, if you use teflon tape on cast iron fittings, they tend to break more often. Years ago, I learned that the use of teflon tape for the threads of an oil burner pump will easily break the pump's casting. (found that out on a very expensive pump). My only question is; are you sure that you have cast iron fittings and not black iron fittings (malleable iron) which are more common?