How to Harden & Temper Mild Steel
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.
Things You'll 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
Instructions
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1
Light charcoal or a torch. Use pliers to insert your steel blank into the center 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.
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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.
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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.
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4
Place the steel in a pre-heated oven at 400 degrees 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 discoloration on the surface. Normally, the steel shows yellow striations. Allow the steel to cool at room temperature.
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Tips & 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.
References
- Photo Credit steel kitchen knives image by Warren Millar from Fotolia.com
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Comments
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jthurst
Sep 15, 2010
Does this harden just the surface of the steel or does it harden it equally all the way through? -
forkknifespoon
Dec 22, 2009
ScubaSteve, Do you have a reference for how many BTU can be absorbed by water and oil? I'd like to refer to this info in a paper and would like a credible source for my teachers. Thanks.
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scubasteve4sq
Nov 07, 2009
also even if the boiling points were the same 1 gallon of water can absorb 7728 BTU's where 1 gallon of oil can only absorb 992 BTU's assuming 8 pounds per gallon. which is generous on the oil side and constant for water. thats about 1/8 the efficiency of heat transfer of water, and remember its actually much much worse than that since oil's latent heat of vaporization point is so much higher. water is a VASTLY SUPERIOR heat transfer liquid.
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scubasteve4sq
Nov 07, 2009
actually joe, you need to look up the definition of "latent heat" more BTU's are transfered during a state change than any other method, so water by shanging state to steam wouyld remove a much more massive ammout of btu's than oil would.
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joe1979Ca
Dec 17, 2008
WHAT? Water should never be used to cool steel because it would cool it to fast and crack? You are mistaken my friend. Water will not quench steel fast enough to harden the steel. The reason for this is that the boilling point of water is 212 degrees F at sea level. Boiling point is less at higher altitudes since atmospheric pressure is not compressing the molecules as much, oh never mind that part. Back to the subject, when the steel is above 212 F and comes into contact with water, the water instantly flashes to steam creating a vapor barrier between and steel and water and decreases heat transfer and increases cooling time enough to interrupt the hardening process and the steel will remain close to the original unhardened state, or will unharden previously treated steel. Oil on the other hand has a much higher boiling point, also changes with alttitude, and will pull much more heat fr
