I know that this thread has been up here for a while, but I have to respond to this one. I am a retired tool and diemaker and have heat treated a variety of steels for over 30 years.
What most people refer to as tempering really isn't. There is a temperature range of generally about 75-100 degrees ie 1800-1875 for a lot of steels where the steel has to be heated to in order to harden fully. Once it is quenched (air, oil or water) and the temperature drops quickly enough it will be at its maximum hardness. This maximum hardness material, however, is very brittle due to the stresses caused by the quenching and once it has reached room temperature must be reheated and "soaked" at a temperature which is held for a number of hours depending on how hard you want the tool to be for final use. Generally any tool steel that reaches a quench hardness of 60 Rockwell on the "C" scale will be "Tempered" to a few points below this hardness. The tempering or normalizing relieves the shock caused by the quenching and makes the crystaline structure of the steel consistent and reduces the tendency to fracture during use. Most air hardening tool steels like A-2 need to be held at 300-400 degrees for two hours and then be air cooled and have the process repeated again to adequately stabilize the material. If the material is over an inch thick you have to add another hour for every additional inch thickness. Crucible Steel Company has an excellent guide for the heat treatment of steels.
I would suggest unless you want to spend the money to acquire a digital controlled oven and do your own heat treating, that any critical part you want to harden send it to a commercial heat treating company. They generally will heat treat based on a minimum fee for a certain number of pounds of materiel sent in for the same specification of hardness and tempering. They will have the capability of vacuum heat treating and doing an inert gas quench on any items you do not want scale on.