How to Spring Treat Knife Blades

Heat treating knife blades is often a mystery to the beginner. Metallurgy is not magic, however. A good spring temper is the result of knowledge and practice. Different types of steel have different heat-treat procedures. Once a blade is raised to a given temperature, it is often necessary to stop the reaction between the metal, the air and any alloys present to maintain the desired effect. This is known as quenching. Medium- to high-carbon steels are often quenched in oil.

Difficulty: Challenging

Instructions

Things You'll Need:

• Vegetable oil
• Metal coffee can or bucket taller than knife blade length
• Bar of 1018 mild steel
• Steel color chart
• Dish soap
• 80-grit abrasive wheel or belt
• Right-angle grinder or belt sander
• Oven

1. Preheat vegetable oil to between 200 and 350 degrees Fahrenheit by heating a bar of 1018 mild steel to dull red. Plunge the bar into the oil to quench it.

2. Heat your knife blade to eutectic, which is bright orange in dim light. Eutectic is the point where the metal becomes non-magnetic. Quench knife vertically in the oil without striking the blade tip on the bottom.

3. Wash oil off blade with dish soap. Scuff entire blade with an 80-grit abrasive on a right-angle grinder or belt sander.

4. Preheat your oven to 450 degrees Fahrenheit. Place blade on rack in oven for one hour. The blade should reach a royal blue color. Allow blade to air cool for a second hour.

5. Repeat Step 4 three times. This is known as triple-drawing the temper. Tempering reduces brittleness, making the blade less likely to crack, shatter or snap during use.

Tips & Warnings

• According to MaterialsEngineer.com, heat treatment is "a combination of heating and cooling operations applied to a metal or alloy in the solid state to obtain desired conditions or properties."
• Technology instructor V. Ryan notes, "If steel is heated until it glows red and is quenched...it becomes very hard but also brittle. This means it is likely to break or snap if put under great pressure. On the other hand, if the red hot steel is allowed to cool slowly, the resulting steel will be easier to cut, shape and file as it will be relatively soft"

References

• Steel Heat Treatment Terminology
• V. Ryan: Hardening and Tempering

Resources

• Jock Dempsey: Steel Temper Colors
• Glossary of Blacksmithing and Metalworking Terms

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