Great answers (all over the place!)
The "which steel" question is probably most appropriate.
I have a choice between drill rod and a something out of a random pile of unknown round stock (chair legs and other odd and ends that came from the guy I got my lathe from).
I also have a heat treating oven, quenching oil, torches etc. So both hardening and tempering would not be difficult.
I imagine (possibly incorrectly) that as long as the bar stock is substantial enough twisting won't be an issue.
I've also got a mill so the flats / square end will be easy.
I'm guessing that the "nuts/screws" in the chuck are hardened - hence the key would get mauled up before damaging them. So in a perfect world, the key end would be hard enough but not harder than the chuck and the shaft would be stiff enough to not twist but not brittle (probably not going to be able to snap it with hand force anyway)
I'm doing this project to try out some process steps before doing trickier stuff. (and I also need a key for this chuck).
