How to heat-treat knife blades

Written by jessica conner
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How to heat-treat knife blades
Knife strength can be improved with heat treatment. (knife blades image by Horticulture from Fotolia.com)

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.

Skill level:
Easy

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Things you need

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

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Instructions

  1. 1

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

    How to heat-treat knife blades
    Use a simple, outdoor furnace or barbecue pit for heat treatment. (olden steel tradicional furnace image by Olga Khoroshunova from Fotolia.com)
  2. 2

    Heat the knife blade until it has reached a minimum temperature of 802 degrees Celsius, or glows a bright cherry red.

    How to heat-treat knife blades
    Heating coals evenly will ensure proper heating of a knife blade. (coals image by Oleg Guryanov from Fotolia.com)
  3. 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. 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. 5

    Temper the knife blade by setting an oven to 204 degrees Celsius and allowing the blade to heat for 20 to 30 minutes.

    How to heat-treat knife blades
    Temper a knife blade in a conventional kitchen oven. (oven temperature control in red image by laurent dambies from Fotolia.com)
  6. 6

    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.

    How to heat-treat knife blades
    Scratching your knife blade with a file is one method for measuring results. (rasp (engineers file) image by AGphotographer from Fotolia.com)

Tips and 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.

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