How to Harden Stainless Steel Throwing Knives

Quick, cheap manufacturing processes can account for big differences in the quality of stainless steel throwing knives. While higher-quality knives are made individually from forged steel, many commercially-bought knives are stamped from sheets of steel and then sharpened before being shipped. Such knives can bend when thrown at a target board with great force because the steel has not undergone a sufficient hardening process, by which the molecular structure of steel is made less malleable by partial crystallization. This requires the steel be brought from a high temperature to a low temperature very quickly, and produces knives that can withstand greater force than unhardened steel. The downside to this process, though, is that hardened steel will shatter if enough force is exerted.

Things You'll Need

• Quenching tank Mineral oil Water Coal forge Coal Lighter Newspaper Tinder Kindling Throwing knives Tongs Diamond file Steel rasp Anvil

Instructions

1. Fill your quenching tank with a combination of water and mineral oil, using a 1 to 8 ratio. The oil is necessary to prevent the steel from developing micro fractures as it is rapidly cooled.

2. Prep the forge by filling the pan at the center with coal. Make a small hole in the middle. Place in several wads of newspaper and light them. Once the flame has caught, feed tinder such as grass and small twigs until the fire begins to grow. Once this is done, feed it kindling wood or larger dried branches until the fire can sustain itself. Pile the coal closely around the fire until the coal begins to cook. This process may take up to half an hour. What you are doing is converting coal into coke. While coal produces thick black smoke and heats unevenly, once it has been roasted it becomes coke, which releases no smoke and creates a lot of heat. Once the coke is cooked you will want to open the flue or turn on the air circulator of the forge to get the coke red-hot.

3. Use your tongs to thrust the steel knives completely into the center of the mound of coals. You don't want them to get hot enough to actually bend out of shape, but hot enough to begin to discolor to a dark dirty brown. This will take about 5 minutes, or less depending on the size of your forge. A larger forge can build heat quicker.

4. Remove the blades from the coals with the tongs and lay them out on the anvil. Before you can cool them, you must knock off the carbon crust imparted by the coke in order to maintain shiny looking knives. Do this with a light scraping from a steel rasp.

5. Plunge the cleaned knives in the quenching tank, and stand back a safe distance because the steam can scald you. The water should hiss and bubble for a few minutes as the knives are rapidly cooled and hardened. While the steel is still warm and more malleable, you can sharpen the edges with a diamond file. With this done, simply turn off the air circulator to bank the forge and let it go out.