Tuesday, September 27, 2011

How to use Thermal Cycling in Heat Treating a Knife

Thermal Cycling Steps for a Hand Forged Knife

I have a knife that needs to be hardened and tempered. But I read your thermal cycling article and thought that this would be a time to test it. My question is, is when you thermal cycle the steel is that all that needs done or if the knife needs to be tempered after the thermal cycling process? Is all I need to do for the knife is the thermal cycling process that you describe on your website? could you give me instructions for heat treating a knife and axe using the thermal cycling process?

The thermal cycling process is part of the annealing process. So this means it is done before hardening and tempering.
The thermal cycling process changes depending for what type of steel that you used for the knife. Below are the steps for 5160.

The steps
  1. Bring to a bright orange, allow to air cool on an insulating surface such as fire brick or kaowool or vermiculite. This is important for knives so that they don't warp in the cooling process.
  2. Bring to medium orange, the cool as above.
  3. Bring to bright red and slow cool covered in vermiculate, kaowool etc. You may want to heat a bar and leave under it for added thermal mass. Allow to cool over night or until room temperature
  4. After cool it is annealled and ready for the hardening process. Now at this point I do my primary grinding. Shaping the knife but not putting an edge on. I do this after hardening and tempering.
  5. Hardening - depending on the steel and the quench media require air, oil, water etc.
    I heat the knife to an even medium red temperature and check to see if it is non magnetic. If not I will go a bit hotter. If it is non magnetic I will test until the magnet just pulls, then quench completely in the quench solution. At this point the whole blade will be quite hard and brittle. Don't drop it!
  6. Next is the first tempering. I place in a small oven at 425 degrees F and bake it for 1 hour. Best if you have a digital thermometer to check this temperature accurately.
  7. Last step of the tempering process. After the hour in the oven the knife will be evenly tempered to edge hardness all the way through the blade. The best blades have a softer back. So I now take a shallow pan of water (about 1/2 an inch deep) and place the edge down and heat the back of the blade with a torch and you should see the colors move to the edge but stop at the water level. Blue on the back and dark straw on the edge.
  8. Last step. Final grinding and polishing always keeping the blade cool so that you don't mess up the temper already acheived. I usually do one or two passes on the sander then dip in water.
I hope this helps.
David Robertson
Ontario Artist Blacksmith