question about tempering

From a heat treatment point of view, quenching isn't necessary, but to prevent you overheating the piece, and allowing the heat to travel further "up" the tool than desired, its a good idea.
If you use a furnace, and soak the piece at the temper temperature, then there's no need to quench.
If you use a blowtorch, and judge the temp by the colour, then a quench is a *real* good idea.
There is no difference to the result of the temper. (Unless you lose your temper, of course.)
HTH Jeff

Jeff said it all; I'll say it a different way.
What you want to do in tempering is to bring the work up to your temper temperature (400-600F depending on what you are making) but not exceed this temperature. If there is any thickness to the item, it should soak for awhile at that temp.
If you're using an oven (with a thermometer that reads correctly!), like Jeff says, just put it in and soak. If you use a torch and are judging temper colors, you need to heat slowly. This allows the heat to penetrate and gives the piece some soak time. It also helps keep from overshooting. Practice a few times if you are using a torch.
Steve Smith
Damned if i know wrote:

Your final condition of the steel is determined by what the part will do. I temper 0-1 down to almost soft to full hard depending on what ballance of toughness and brittleness needed for the job. If it's a die with some meat on it, it stays full hard. If it's a small punch or machine part that will see some side force, it goes straw to blue. What will your part do?

You never quench steel just to cool it off. You quench to drop the temperature very fast when hardening.

Well, yes and no. Someone here mentioned that differential tempering with a torch, or in a forge, requires a quick quench to prevent the temperature from rising too high in parts you want to leave hard.
But that's a tradeoff. If you look into the charts and graphs on tempering in a metallurgical handbook or in a practical heat-treating handbook like _Tool Steel Simplified_, you'll see that you sacrifice a lot of toughness by quick-quenching to temper the steel. O-1 should be held at its tempering temperature for several hours regardless of thickness (and much longer if the steel is very thick, because the time is measured from the point at which the whole piece is up to tempering temperature) to achieve full toughness at a given hardness level. A reasonable compromise between performance and energy cost is a couple of hours per inch of thickness.
Ed Huntress

As I remember Ed Huntress advised a while back: A general purpose procedure for O-1 drill rod or hardenable flat stock is: Heat the stock to a temperature where it loses its magnetic attraction (cherry red in most cases), quench, and place in a pre-heated 350 degree oven for at least 1 hour. This has worked very well for me in making small cutting parts such as taps and dies. Supposedly, larger pieces would require longer soak times in the tempering oven, but 1 hour is a good time for ordinary "shop size" tools.
This procedure is for "carbon" steel only and is not recommended for HSS. I recently got an older copy of "Tool Steel Simplified" in which a 350 degree, 1 hour "soak" was recommended for tempering a common carbon steel to Rockwell hardness 63/64.
Ed, you may have comments.
Bob Swinney

Nothing much to add, except that longer is better. But the advantages of longer soaking times to temper fall off very sharply after a couple of hours. The difference between one hour and two hours isn't very much, for that matter.
Ed Huntress