Tempering and Hardening

This is an excerpt from the section on Finishes in The DIY Blacksmithing Guide.  

Hardening and Tempering

Hardening and tempering could have their own large section, but I’ll hit the basics here. It’s used for heat-treating metal to be used as tools. First you harden. Then you temper. The most common application is with blades.

Hardening in 5 Steps

1. Fill a large metal tub or pot with either used motor oil (this flames up pretty good) or linseed oil and turpentine. It should be deep enough to submerge your piece and wide enough to move it in circles and figure-eights.
2. Take your fully-forged piece and set it in (gas) or on (coal) your pre-heated forge. 
3. Have a long cow magnet handy to test magnetism periodically.
4. When it’s glowing red-orange (about 1400-1500 degrees fahrenheit, 760-815 centigrade), touch the long magnet to it. If it’s no longer magnetic (there’s no attraction between the metal and the magnet), quench it in the metal tub with the oil in it.
5. I recommend swirling it in figure-eights rather than back-to-front. It ensures that it’s evenly quenched. 

Tempering in 3 Steps

1. Tempering is a gradual, low heat softening of the metal. This allows for hardness and the ability to sharpen the blade. (If you try to sharpen a blade that has only been hardened, you’ll wear your arms out before you make much progress. The file will slide off without removing any metal.)
2. Paying close attention and with the sharp side of the blade pointed away from the heat source, slowly heat the knife to blue (600 degrees fahrenheit, 315 centigrade) at the spine and straw-colored at the blade edge (approximately 450 degrees fahrenheit, 260 centigrade). This softens the spine more than the blade edge which you want to stay hard. 
3. Quench it with the same figure-eight motion as you used to harden it. This locks in the temper. The cutting edge can now be sharpened and still retain its hardness and durability.

*I’ve included a DIY Blacksmithing Tempering Color Chart as a Bonus with your purchase. Post it in your shop or forging area for reference.

<End Excerpt>

A portion of that color chart is seen here:

The chart is in full color, showing the temperature ranges on actual pieces of metal. You won't be guessing on colors from a colored pencil drawing or something computer-generated. 

Typos aside, if you're interested in seeing more of The DIY Blacksmithing Guide just click the cover below:

Thanks! Next time I'll be talking about Two Methods of Knifemaking.