Tempering is usual the act of making steel softer.
To get it harder you would have to heat it above it's critical temperature. Depending on the steel that is 1700-1850 F and then quench it (Depending on the steel the quenching media could be water,oil or air) I am guessing oil or water. At that point they would be very brittle, and of course then you would temper it back to a hardness of less than what it is but greater than it was.. Same old story too hard they are brittle and a bitch to get sharp, too soft they sharpen easy but bend readily