One of the misconceptions about hardening and tempering is the time factor. Generally, unless you're working with some alloy that demands being held at certain temperatures for certain times, it goes like this:
When you get a piece to hardening temperature, that's it. It doesn't have to be held there for a particular length of time, the critical factor is simply that the piece, or the sections of it needed, have reached the required temperature. Take it out of the fire and quench it in the appropriate medium, water, oil or air.
With the carbon steels I mostly use, I generally quench twice. Meaning bring it up to heat, quench. After it cools enough take it out of the oil, clean it off & let it cool to ambient temp. Repeat. For whatever reason, it often does seem to make stronger blade to harden it twice. And as I mentioned once before, some demand it: 52100 seems to get the best result by three quenches, spaced 24 hours apart. Why? I have no idea, but it does seem to work better.
Tempering is the next step. If you have the aforementioned heat-treating furnace you can use it. You can use the forge or a burner and move the piece back & forth over the heat and watch the color change, pulling the piece out and quenching it at the temp needed. What some smiths do is take a piece of heavy steel or iron pipe cut lengthwise, a foot long at most, and set it over the fire. Let it get good & hot, and while it's heating clean the oil off the blade and shine it up so you can see the color change. Then, for a single-edge blade, start working the spine back & forth along the pipe. The blade will soak up heat, and by moving it around you can keep it even; as before, quench when the color is right. Or, you can use a commonly available household device to do it.
Stick it in the oven.
Yes, that wonder of the modern age, the oven! Capable of turning out a loaf of bread with a crisp crust, turning turkey into dinner, and yes, even tempering knives! However, there are some warnings here, especially if you have a spouse or live-in significant other. First, CLEAN THE DAMN THING OFF! Quenching oil does NOT smell like food baking when it gets hot. In fact, it stinks. So if you leave it on the blade, you will a: piss off the housemate, b: stink up the place, and c: get orders to clean out the oven and/or "You expect me to cook DINNER in that? WIth that stink in it?!?" Possibly including threats to your health. So, after it cools down completely after the last quench, take it to the sink(watch the sensitivities of the spousal unit) and wash it with either a degreaser or dishwashing soap( I personally use Dawn). Get it good & clean, and then hit it lightly on at least one side to shine it up a bit. Then into the oven.
If you're not sure about what temp to use, the oven is very nice in that you can start low, say 375F, and go up from there. Once you know what temp will give the steel in question the hardness you want you can just set it there. Following a procedure I first read I can't remember where, I do it this way:
Set the heat at your desired temp, and set the knife in. I try to set it up so the blade is either edge up or down, not laying on the side. If the oven is already hot, start your timing here, if not give it time to get up to temperature and start then. Again, the critical factor is simply that the entire piece has to get up to the temperature. A thick piece will take longer than a thin one. You don't really gain anything by leaving a piece at temp for hours, though it won't hurt it either. For knives, I leave it in for 45 minutes, maybe an hour for a thick blade(just to be on the safe side). At that time I take it out and set it on a cooling rack(the one you use for cake will work fine as long as the primary cook doesn't get pissed). Leave it until it cools to room temp, then back into the oven for 45 minutes more. I temper three times, and unless the oven is needed after the last one I just turn off the heat and leave it there until cool.
That's it. Nothing magical, although the result can seem that way.
More to follow when time allows
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