if it's an application that has no load bearing or other safety implications (i.e. not part of a car that is used to drive, stop, corner, or hold any critical bits on), go your hardest (no pun intended).
It can be fun to harden stuff, put it in the vice and check what you've achieved in harder, but less ductile.
A little viking quenching medium might be interesting too.
Frontier blacksmiths and gunsmiths could harden iron'steel and make files. Many hardened screws and bolts - that nice case hardened blue.
Also , somehow they knew how to make spring steel out of iron/steel for gun lockwork.
I'd look into areas of old timey guy's techniques.
The art of hardening has moved to science. With controlled temperatures, neutral atmospheres, and proper quenching and tempering cycles you will get the optimal properties. But this is not for the home heat-treater. Just heating something red-hot and quenching it will give a brittle bolt. There's not even a guarantee that it will be hard enough.
The only time I've done this is when I wanted to make my sandblast nozzles more abrasion-resistant. Toughness was not an issue.
Leave the heat treating to the experts and buy a Grade 8 (or Grade 5) bolt.
thanks for all the input, I just needed a bolt for a clutch puller I couldn't find local, made one on lathe heated with torch and quinched,it worked but I wont try it again with same bolt cuz I don't trust it but in a jam I figured what the heck and gave it a shot
for futre reference:
Did you start with mild steel, i.e. 1020? If it was and you quenched it in water then you gained nothing. There is not enough carbon in the mix to gain any hardness. Quenching in oil can gain some carbon.
Case hardening is a surface treatment only. You are driving carbon into the surface to a depth of 10's to 100's of microns. Case hardening does not affect the bulk hardness of the work piece. Basically, case hardening was effective when you needed a hard wearing surface but you wanted the bulk of the work piece to retain it's flexibility. In the case of something like a bolt, case hardening would be of very little usefulness.
The advice others have given is sound, go buy some grade 8 fasteners. Metal hardening has because an exact science, the chances you will achieve similar results on your own are very remote.