To harden the spring requires two steps. First bring to red hot and then
plunge in water. This should make the spring very hard and brittle.

I would recommend using oil instead of water.

I have tried quenching steel in water. I had serious problems with
surface stress cracks. My experiement consisted of heating 1 inch
diameter discs, 1/4 inch thick (cut from drill rod) to 1500 degrees
in an oven and quenching in water.

I have heated smaller drill rod (3/8 inch) with a propane torch and
quenching in water without too much trouble, but I don't think I got
anywhere near the correct soak temp with the propane torch.

chuck