How to harden and temper knife blades
This post will answer the question of how to harden knife blades as well as temper them. We will also answer why to harden and anneal a knife blade.
Let's start with the why you harden steel. when you harden steel, you trap the carbon and iron in stronger but more brittle structures. When you temper it you allow carbon to dissolve into the iron and form more durable structures at the cost of some hardness.
As for how to harden a knife blade, you need to heat it to a red hot temperature. You can check if the temperature is high enough by using a magnet. when it reaches a high enough temperature the magnet will no longer stick to the steel. at this temperature the steel will glow a dull red if viewed under low light conditions.
For small blades an ox-acetylene torch will work to harden the blade. For larger ones like machetes or swords you will need a proper forge to heat the entire blade to an even red heat, or at least an oxy-propane rosebud to be able to produce enough heat as acetylene has a very low limit on how fast you can draw gas from it.
When using an ox-acetylene torch I recommend using a slightly fuel rich flame, such that you can see a slight orange feather in the flame. this will help cover the blade in carbon and prevent carbon from being burned off on the surface of the steel.
Heat the blade to a dull red heat. Start at the tang of the blade and work the heat up towards the tip. It will be no longer attracted to magnets when it reaches sufficient heat.
When the blade reaches the desired temperature along the entire blade, dunk the blade quickly in a large metal container of canola oil to quench it to harden it. Make sure the red hot state extends somewhat into the tang so the tang has proper strength and won't bend on you. Canola oil is better then motor oil because of much lower amount of smoke produced and likely less dangerous smoke compared to the additives in motor oil burning off.
If you are hardening a very large blade you may still get some smoke or if you are using motor oil. I recommend using your torch to ignite the smoke rising off the oil as burning it off will reduce the amount of smoke and smell.
Next is how to temper a knife. Its simple really, one simply heats the knife in a temperature controlled environment like a home oven (another good reason to use a food safe oil like canola instead of motor oil). I recommend you at least wipe the blades clean of oil before this step, and use a detergent/de-greaser like simple green if you used a motor oil to prevent contaminating your oven.
I personally temper my wood carving knives at 400f (204c) for 30 minutes, but you may find higher temps up to 450f (232c) desirable for more durability on some blades or down to 350f (176c) for more hardness/edge retaining ability depending on the knife.