I second what Mark E. said, but depending on what type of tool steel you have, un-tempering by torch may not be the way to go.
While this would be fine for a lot of steels, some steels air harden and could get so brittle that it cracks or shatters.
If possible, find out what kind of tool steel you have. If there's no sure way of knowing, you could always cut off a small piece to test.