How to Differentially Harden Steel

The process of differentially hardening steel principally applies to the production of blades. The method increases the hardness of the cutting edge without making the whole blade brittle. By differentiating the area that will be treated, the metallurgist aims to create a balance of toughness in the spine, to absorb the impact of the blade, and hardness of the edge, to cut through objects. To accomplish this, the blade, already formed, is put through a heat treatment with a clay buffer on the spine and then cooled, or quenched, using water or oil. On a molecular level heating and quickly cooling optimizes the crystalline structure of the steel, harnessing its unyielding characteristics to strengthen the blade edge.

Things You'll Need

  • Torch (or another source capable of producing a high temperature)
  • Water (or oil)
  • Clay (cement)
  • Blacksmith's tongs
  • Putty knife (various sizes)
  • Leather gloves
  • Protective clothing
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Instructions

    • 1

      Mix the clay putty in a bucket so that it is the consistency of icing and free of lumps. Forging experts like Bob Engnath have utilized a high alumina refractory cement, which provides a high resistance to heat, but a general clay cement will provide adequate resistance.

    • 2

      Apply the mixture to the blade using the putty knife, covering the the spine and any parts of the heel that you do not want to subject to heat tempering.

    • 3

      Allow the putty to dry before subjecting the sword to heat so that none of the application is lost in the process.

    • 4

      Heat the steel blade to a critical temperature of around 1500 F or until the metal turns yellow in color, with the torch. Handle the hot steel with tongs, and be sure to wear protective eye gear, a welders mask and thick leather gloves.

    • 5

      Quench the treated blade quickly by submerging it in a bucket of water or oil for 10 to 20 seconds. According to Engnath, if you hear a ping noise, the condition of the quenched metal has exceeded the differentiated treatment, compromising the blade.

    • 6

      Chisel off the applied putty and polish the tempered blade to your liking.