How to Harden a Knife Blade

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Knife blades can become dull over time as they are used regularly. Whether it is a steel blade being used in the kitchen or one being used to cut and chop material outdoors, blades need to be hardened to keep them strong. Hardening steel blades is essential and it can be done by heating them. Heating blades causes the molecules to realign themselves into a pattern that is stronger and allows the blade to become harder. This is done by heating the blade once, then re-heating to relieve stress.

Things You'll Need

  • High-grade steel blade
  • Motor oil or vegetable oil
  • Protective gloves
  • Eye protection
  • Steel wool
  • Gas torch
  • Kitchen oven
  • Lighter
  • Calipers
  • Magnet
  • Light your gas torch until it is burning clean and effectively. Use your calipers or pliers to clamp the steel blade of choice into a strong hold. Holding the calipers, clamp the blade into the center of the torch's flame so it receives the full heat of the flame. You will want to allow the blade to heat until it reaches an orange-to-red coloration. You will not want the blade to reach a bright red color. Once the blade has reached a point where you think it is ready, you can test it with a magnet just to be sure. When a blade is ready to be hardened, it will become non-magnetic to show that the molecules have been altered.

  • Poor motor oil or vegetable oil into a shallow pan to accommodate your blade. After your blade is heated to the point it loses its magnetism, "quench" the blade into the oil to allow it to cool. Be sure to dip the blade vertically so that both sides cool at the same pace. This will prevent the blade from warping. Hold the entire blade in the oil for about five seconds to fully quench. Remove the blade from the oil in order to allow it to fully cool back to the environment's temperature.

  • Clean off all of the remaining oil and corrosion that occurred during the heating process. You can do this by using your steel wool and rubbing it off, once the metal blade is cool to the touch. After doing this step, the blade should have a brighter appeal to it and look much better.

  • Preheat your oven to about 425-475 degrees F in order to temper the blade. Once the oven is preheated, place the blade into the oven and leave it for about 1 hour to ensure it gets fully tempered. If you do not want to use an oven, you can re-apply the torch or lighter heat source to the edge of the blade. If you take this step instead, allow for the edge to appear shiny and get a slight amount of yellowish color. Once this has been accomplished, allow the blade to cool at room temperature. If the blade appears warped, you can pound it out and repeat the process if needed.

Tips & Warnings

  • Water can be used to quench high quality hard steel blades, but can cause metal to cool too quickly and make it crack. Be careful when quenching and use oil to quench it slower.
  • Motor oil does the job best, but vegetable oil can be used as a substitute if motor oil is not available.
  • Whenever working with high-heat sources such as torches, be very careful and use protective eye and hand gear.
  • Take extra steps towards making sure you are ready if something goes wrong. Have a fire extinguisher ready and work away from objects that could catch fire.

References

  • Photo Credit BananaStock/BananaStock/Getty Images Hemera Technologies/Photos.com/Getty Images Zedcor Wholly Owned/PhotoObjects.net/Getty Images Jupiterimages/liquidlibrary/Getty Images Jupiterimages/Comstock/Getty Images
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