If you are going to rework an old arbor and the rework area is hardened, take the arbor to a heat treater and have them induction anneal just the area you need to machine, that should preserve the remaining hardness on the taper.
does that really work? you can induction harden, but anneal? usually you hold for an hour or more and even if not you cool slowly. by that time the whore part is annealed. or is there a trick to it?