How to harden & temper mild steel

Written by greg johnson
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How to harden & temper mild steel
Harden steel to make knives like these. (steel kitchen knives image by Warren Millar from Fotolia.com)

Hardening steel is an essential part of any blade-making process. When exposed to heat, carbon molecules in steel realign themselves in a harder, stronger pattern, allowing a blade to hold an edge. Two processes are necessary to harden steel: the initial hardening, or heat treating, and tempering.

Skill level:
Moderate

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Things you need

  • 1095-grade steel, blade thickness
  • Charcoal grill or gas torch
  • Lighter
  • Motor oil
  • Pliers or calipers
  • Steel wool
  • Leather gloves
  • Eye protection
  • Kitchen oven

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Instructions

  1. 1

    Light charcoal or a torch. Use pliers to insert your steel blank into the centre of the coals or torch flame. Allow the steel to become a deep cherry red and remove it from the heat. Do not let the steel become bright red or orange.

  2. 2

    Poor motor oil into a shallow pan. Quench the steel in the oil. Dip the blank vertically, since rapid cooling on one side can cause the steel to warp. Slide the steel into the oil until the blank is covered, and hold for five seconds. Remove the steel and set aside to cool completely. The steel is now heat-treated.

  3. 3

    Clean off the oil when the steel is cool to the touch, and use steel wool to remove corrosion on the surface. The steel should be bright and reflective.

  4. 4

    Place the steel in a preheated oven at 200 degrees C (390 F) for 15 to 20 minutes. Remove and allow to cool. This will temper the steel. To temper steel without an oven, place the cleaned blank back in the heat source you used for Step 1. Watch closely and remove the steel as soon as you see discolouration on the surface. Normally, the steel shows yellow striations. Allow the steel to cool at room temperature.

Tips and warnings

  • Water should not be used to quench hot steel, since the water will cool the metal too quickly and may cause it to crack.
  • Vegetable oil can be used instead of motor oil.
  • Always wear eye and hand protection during the heat-treating process. Keep a fire extinguisher close in case the oil ignites.
  • Work outdoors and away from structures to reduce fire risk.

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