hi, nitraded hardening is a surface chemical process, its usually about .0003 thick, once you polish the crank its gone. the crank doesn't get soft by turning it!! if that was the case, couldn't turn any crank !!!
Most nitriding actually gets you a hardened surface about .010" deep.
Most production cranks have induction hardened bearing surfaces, which can be turned without issue. But not forever, they aren't hardened all the way through. If you're cutting a big amount, say .030 or larger, you may run into areas that aren't as hard as the original surface was.