How to Heat-Treat Knife Blades

Knife strength can be improved with heat treatment.

Heat treatment is a metal fortification technique that gives high carbon steel increased strength and durability. When subjected to extremely high temperatures followed by rapid cooling, a chemical change takes place within the steel that rearranges its atomic structure. Carbon atoms push iron atoms within a steel blade further apart, resulting in a drastic hardening effect. The end product is a cutting tool with strong resistance to wear and tear, which will remain useful for an extended period of time.

Things You'll Need

• Steel knife blade
• Heat-resistant gloves
• Tongs
• Furnace or barbecue pit
• Charcoal
• 2 quarts cooking oil
• Baking pan
• Cloth
• File

Instructions

1. 

   Use a simple, outdoor furnace or barbecue pit for heat treatment.

   Heat charcoals inside of a large furnace or barbecue pit. Disperse a large amount of charcoal so that the knife blade heats uniformly.

2. 

   Heating coals evenly will ensure proper heating of a knife blade.

   Heat the knife blade until it has reached a minimum temperature of 1,475 degrees Fahrenheit, or glows a bright cherry red.

3. Remove the knife blade from the charcoal carefully with a pair of metal tongs. Submerge the blade into a pan of cooking oil, blade edge first. Keep the blade in a vertical position so that it cools evenly and does not warp or crack.

4. Remove the knife blade from the cooking oil and lay it on a flat surface to air dry. Allow the blade adequate drying time and thoroughly remove any excess oil with a clean, dry cloth.

5. 

   Temper a knife blade in a conventional kitchen oven.

   Temper the knife blade by setting an oven to 400 degrees Fahrenheit and allowing the blade to heat for 20 to 30 minutes.

6. 

   Scratching your knife blade with a file is one method for measuring results.

   Remove the knife blade from the oven to cool. To check the accuracy of your results, scratch the knife blade with a small file. If you see any marks, repeat the heat-treatment process until desired results are achieved.

Tips & Warnings

• Completely shape your blade to your liking before attempting this process, since it should not be reshaped after it has undergone heat tempering.

• For thicker knife blades, such as double-edged blades, leave in the oven for one to two hours.

• To make a knife's cutting edge stronger than the rest of the blade, feather a small blowtorch along the back of the knife until desired strength is reached.

• The lighter the blade becomes in appearance during heating, the harder it will become. Straw yellow is the highest heat level a blade can reach before it turns brittle.

• Do not attempt to handle hot materials without wearing proper hand and face protection.

• Keep a fire extinguisher nearby in case of accident or emergency.

• Steel knife blades should not be submerged in water, since doing so will increase the likelihood that they will warp or crack.

References

• Charles W. Davidson College of Engineering: MIL-HDBK-5H (PDF)
• Macabee Knives: Heat Treating Knives
• Primitive Ways: How to Make Your Own Steel Knives

Resources

• British Artist Blacksmiths Association
• John C. Campbell Folk School