Going back to my apprenticeship days IIRC heating and slow cooling tempered steel. Heating and fast cooling (plunged in to water) hardened steel. This still depended on the carbon content. You could heat mild steel with a very low carbon content and then put in in to a carbon dust bath, the surface absorbed carbon and this case hardened it. I think you would have to heat it and fast cool it after the carbon bath.
As what Al203 said, I'm probably being over cautious on the hardening of steel bit I guess. I've heard of needing to cool it as part of the process but don't know as much Al203. Either way, good luck. Every time I resort to drilling it seems to go wrong for me!