I recently did some plumbing work in my house which involved using threaded (NPT) black iron fittings in the 1" and 2" sizes.
At the outset I was concerned that I could overtighten them and crack the female side. But in a few cases I had to really torque down to get the right orientation and had no problems. To get an appropriate number of threads turned in (based on what I've read) they really required a lot of force! In the 2" size it was extremely tough using 18" wrenches to do the last 1/4 turn or so.
In contrast a few copper NPT/sweat adapters definitely seemed malleable and I held off more on those.
In all cases I checked threads, wire brushed, etc. so the fittings should have gone together without impediment.
Is breaking a black iron fitting actually possible / likely? Assume you are using an appropriate sized wrench and don't have superhuman strength.
Note - in my specific situation I used teflon tape coated with Rectorseal for these joints. (Using both was based on advice from lengthy internet "research". This might have been "belt and suspenders" but has worked perfectly.)
But this question is meant to be more broad then just that specific case, so feel free to answer considering different joint preparations if that is appropriate! Thanks