Specific example: my bicycle takes an allen wrench (hex key) that has a centre hole in it (for security purposes, a pin in the socket prevents a normal key from being used. So I made a spare by drilling a hole into a normal hex bit.
The steel was too hard to drill, so I treated it by heating it to dull red and then quenching in water. After that, it drilled very easily, and everything worked out fine and now it won't be a disaster if I lose the original.
Except, I imagine that the treatment has left the bit in a weakened state.
The question is, what should I do to undo the treatment?