JTknives:
When you heat high carbon steel "white hot" and quench in water or oil, the result is an almost glass hard piece of metal which will snap in a heartbeat if any side force is applied.
We gunsmiths buy annealed (softened) spring and other types of hardenable steel stock to fabricate springs and parts. We bend/shape/machine the softened steel into the desired shape. Then it must be first hardened and then tempered, two separate steps. First I do like you described---heat the metal really hot and quench in water or oil. Now it's glass hard and must be tempered or "drawn" so it will function as a spring, firing pin, or whatever. Here's two quick methods to temper your hardened steel, in your case your knife blank.
1. Sand and oil. Put an inch of so of dry sand in a bread pan large enough for the knife blank. Saturate the sand with 30 weight motor oil and use your propane torch to set it on fire (it's hard to light). You could do this outside in your barbeque grill or anywhere the smoke won't bother someone (and there isn't a lot of smoke). When the oil soaked sand in the pan (poetic, huh?) is burning well, lay the knife blank on it, sift a bit more sand over the blank and perhaps add another small squirt of oil so it's all burning. Let this burn until it burns out then cover with a piece of wood or sheet metal and let it cool several hours. You now have your knife blank heat treated to a near perfect spring temper. It will take an edge and still be resilient. The 30 weight oil and sand mixture burns at between 800-900 F, hence the resulting spring temper.
2. Second method and ONLY to be done outside or in a VERY well ventilated area so you don't breathe fumes is to use molten lead which will also provide an excellent spring temper. Use a camp stove or grill to melt enough lead in an old frying or bread pan so you can submerge the knife blank completely it it. The steel blank will float on top of the molten lead like a cork on water so you have to use something metal to hold the blank down and keep it submerged. Keep the blank submerged in the molten lead until the lead stops sticking to it. I like to go at least five minutes. That's it, you now have a spring tempered blade. When you remove the blade from the lead lay it down on a piece of wood and let it air dry. I like to cover the item with an old dry shop towel when it's cooled enough so the wood is no longer smoking and the towel won't catch fire. Let it cool for several hours covered up.
WARNING! Make DOUBLE DARNED CERTAIN SURE WHATEVER YOU MELTED THE LEAD IN IS NEVER USED FOR ANY OTHER PURPOSE AND ESPECIALLY NOT FOR COOKING!!! Best thing is to leave the cooled, solidified lead in the container for future use or give pan and all to a buddy who molds bullets or fishing sinkers. Also make ABSOLUTELY sure no moisture of any kind gets into the molten lead! One drop of saliva, perspiration, or a single raindrop will cause the most violent explosion you've ever been close to! I kid you not! It's hard to believe how far ten or fifteen pounds of lead will spatter if one single drop of water gets into it. Be warned. The oil/sand is the safest method but the lead works great also provided you're careful and stay downwind from the fumes.
The methods I've described don't equal what's obtainable with a heat treat furnace used by an expert but they do work and work well. I'm a gunsmith with a lot of years in that trade. I have MANY old guns out there that have functioned for years with springs and other critical parts I've made using the above methods.
BTW, a shiny non stainless knife blank tempered with the burning oil method comes out gorgeously blued. This can be left or polished off.
I quench at 1550F 