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.

How to harden a wood carving knife blade

As for how to harden a wood carving knife blade, you need to heat it to a red hot temperature and then quickly cool the blade. The exact temperature is called the critical temperature and it varies depending on the exact steel alloy. It is the temperature that the austenitic phase transformation occurs.

On simple carbon steels you can roughly 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 it is typically just under the critical temperature and ready to be quenched. At this temperature the steel will glow a dull red if viewed under low light conditions while at higher temperature you can end up burning off the carbon in the steel making it unable to reach its full hardness and potentially melt the edges of your blade.

More exotic alloys may have critical temperatures that vary from the point where the steel is no longer attracted to a magnet and more complicated heat treatment requirements. For this article we will assume you are working with a simple high carbon steel. If you are dealing with fancier steel alloys you should look up the exact temperature to use and consider a temperature controlled forge to get the full benefit of the more advanced alloy.

For small blades an oxy-acetylene torch can be used to harden the blade. For larger ones like adze, machetes or swords you will need a proper forge to heat the entire blade to an even red heat, or 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 and enough tanks to run an oxy-acetylene heating rosebud is prohibitively expensive and large. I have also seen air-propane torches used but I am not sure if it is economical without a forge to help insulate it.

When using an oxy-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 and iron from being burned off on the surface of the steel that would result in an oxidizing atmosphere. 

Steps to harden a wood carving knife

1. Heat the wood carving knife 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 its just below the critical temperature. Get the blade slightly hotter then this.
   
   
2. When the blade reaches the desired temperature along the entire blade, dunk the blade quickly in a large metal container of liquid 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. Also make sure that the cutting edge of the blade does not cool before you quench it. You must be very quick to quench once removed from the heat source.
   
   
3. Temper the wood carving knife blade by 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) at 350 to 450f for 1/2 hour to 2 hours. 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.

Knife Hardening Tips and additional details:

As for what liquids to use to quench with, 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. Other better hardening oils exist, but Canola oil can be easily sourced from your local grocery store, has a very high smoke point and is very cheap. Water can be used and will cool much faster then oils but may produce more distortion and even crack some steels.

Different steels have different recommended quench liquids and if you don't know exactly what allow you are using, oils are more likely to produce the result you want then water.

If you are hardening a very large blade in oil you may still get some smoke especially 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.

As for tempering, 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.

Normalizing your knife blade

Normalizing your wood carving knife blade is another important step. Before you harden, but after grinding and forging you should consider normalization. Normalizing is the act of heating steel above the critical temp and allowing it to slowly cool from that temperature. It helps relieve stress in the blade imparted by forging and grinding. It is very important for straight knives to keep them from warping during heat treatment to normalize them first. It can also improve the grain structure in the final hardened work to normalize them before hardening, producing slightly more durable blades after hardening.

To normalize your wood carving knife blade, heat it above the critical temperature and then allow it to cool in air back to room temperature. Its generally recommended to repeat several times as each time will cause more stresses to be relieved. 3 times is generally considered good and ideally each time should be done to a lower peak temperature then the last although this may be hard to achieve without a temperature controlled forge.

The difference between normalization and annealing your wood carving knife is normalization cools at a faster rate and won't produce the dead soft steel that annealing does. Some steels can even significantly harden when just air cooled, but generally you want harder for knives.