Heat Treating

In my opinion, the single most important aspect of knifemaking is heat treating.

There are many methods used in the heat-treating process. I will touch on the ones that were commonly used in my shop. Heat treating consists of hardening and tempering, in the simplest of terms. There are those using molten salt pots, cryogenic tanks and various eclectic methods - I used what worked consistently for me over the years.

Normalizing and annealing

After forging a carbon steel blade to shape, the blade is brought to critical (non-magnetic) in the forge and allowed to air cool. I do this process twice - this is normalizing, and its purpose is to remove some of the stresses imparted to the steel during forging. The blade is then brought up to critical again and allowed to cool slowly in vermiculite. Once the blade is fully annealed (softened), it can be ground, filed and drilled.

Hardening

After annealing, the blade is ground from 50 grit to 220 grit. I usually leave the cutting edge a bit heavy so that I can remove any scale formed during heat treating. I put a rosebud tip on my oxyacetylene torch and heat just the cutting edge of the blade to critical, then quench the whole blade in oil. I usually repeat this process two or three times depending on the steel used.

Tempering

Hardened steel is brittle, so the blade must be tempered - this takes the brittleness out while imparting toughness. My tempering methods are quite simple: I clean up the blade with 220 belts, then place the hardened knife (right after hardening) in a toaster oven until the blade is a straw or light gold color - typically at 350–375°F for one to two hours. This process is repeated three times, with air cooling between cycles.

I then put a small tip on the torch and draw-soften the knife's tang by bluing. The tang is brought to an even deeper blue to make it dead soft. Using the torch this way allows a controlled differential temper: hard edge, soft spine, softer tang.

I have clay-tempered before but usually do not use that method. There are times I will heat the whole blade in the forge and only quench the cutting edge in oil - I use this on blades too large to heat effectively with a torch. These blades are tempered the same way.

Damascus blades

For pattern-welded Damascus blades I use a different heat treat. Most of my Damascus was forged from high- and low-carbon steels. These blades are fully heated and the whole blade quenched in oil. After clean-up, the blade is tempered in the oven; on a bowie I would often draw the spine to blue.

Both carbon and Damascus blades are cleaned up after tempering, ground, sharpened, tested, and then hand-sanded and finished.