Or use a grade 8 or grade 10 bolt, like GG said.
Bolts are made to bolt, not to be machined easily. Moreover, there's no requirement on the bolt manufacturer to use any one material or another -- just to get an end result.
So getting a bolt and machining it will be luck of the draw -- ordering steel of your desired alloy at your desired temper will be much more controlled.
Steve is used to using nice rigid machines, so he fails to mention that not only do you need carbide steel cutters to machine the stuff he's talking about, you also need good equipment. If you've got a cheap Chinese lathe, machine flexibility may prove to be an extra challenge.
I suggest that if this is your first crank, than you just make the thing out of plain leaded steel. It'll be ridiculously easy to machine, and the only issue with it not being tempered is that it may wear faster. Then, when you know how to machine cranks, start using more challenging material.
(Oh -- and consider getting a tool post grinder. Yes, you have to be careful of grit in your ways, but you'll have a way of grinding shafts.)