Instructables

Step 9: Hardening the blade

If you are going slowly or want to use a file instead of a grinder, let the knife cool slowly and do your shaping now.

Me, I'm trying for the quick way : ) I'll get all the forge time done first, and use the grinder instead of a file to shape the knife.

To harden the knife, you have to get the WHOLE blade ORANGE. Not red, orange. To do things right, you should test the orange heat with a magnet because when the metal is ready to be hardened (it has to do with which crystaline shape the iron is in and how the carbon is held in that latice) it becomes non-magnetic. If the magnet doesn't stick, you are ready to quench.

Again, no helper with the camera, so no pics of the actual quench : P

After the quench, test the edge of the blade by trying to file it. If the file works, the metal wasn't hot enough, try again (I had to). If the file skates across without biting into the metal, you did it!

A word about quenching. In this non-critical piece, I'm using water for a fast quench. Some steels need a slower quench like oil or they get TOO hard and can break VERY easily, as in the stress of fast cooling will shatter them in the water or if you drop them or even twist them to much.

You should, after hardening, temper the steel. This take a little of the hardness out but puts some flexibility back in. Two hours in the oven at 400-500 degrees F. should do it. Otherwise you could try to flame temper it-make the edge shiny and heat it from the back just enough so that you get a little bit of color (yellow, purple is probably to far). This knife didn't harden enough for me to worry about it (the file bit a little after hardening, but not much).

Looking at the blade, it's a little warped. I could reheat it, pound it flat and try another hardening but it's getting late, I'm hungry and the warp isn't to bad so I'll leave it.
 
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spenny19724 years ago
to have more control over the tempering process, heat a 1/4" thick rounded steel plate up to 430- 530 degrees farenheit and rub the body of the blade around until the polished edge of the blade begins to change colour.rubbung both sides of the piece around. dark "straw" yellow for knives and light purple for larger cutting implements like swords. when desired colour is reached quench along edge of blade(for single edge knives)and straight down for double edged pieces.(to prevent warping).
hanzokendo5 years ago
Drawing the back side of the blade along a pipe will weep away the heat causing less trauma to the crystalline structures during the quench, giving the knife a softer spine.
jtobako (author)  hanzokendo5 years ago
"Trauma" to the crystalline structure would be work hardening. If I wanted to do a one step quench with a softer spine (differential hardening) I would only quench the blade portion, leaving the spine out of the quench to cool more slowly. I suspect (from personal experience) that the time taken to try to cool the spine would cool the blade to much for proper hardening due to the difference in cross section (the thin blade cools faster than the spine, so it has to get into the quench FAST).