Sunday, August 25, 2013

BC1 - HARDENING

HARDENING THE KNIFE

Hardening high-carbon knife steel is essentially heating the blade to red hot and quickly cooling it (known as quenching) in water or oil. In some cases salt or air can be the cooling medium. For the particular steel I am using, a 30 minute soak at about 1000°C (yellow hot) then an oil quench is recommended by the manufacturer.

I started by preparing the area; getting the forge and quench tank setup, grabbing my welding gloves, vice-grips, safety glasses and so on. The oil can be vegetable oil, motor oil, olive oil, etc. I happened to have some motor oil kicking around so that is what I used. The smoke from an edible oil will be less offensive than motor oil, so after my first time, I will recommend some el-cheapo cooking oil.

 Inside the forge, I needed to move the blade around and get as much as I could a nice even yellow colour. The magnet is on top of the forge (see picture above) so I can check a carbon steel's magnetic properties before quenching.

When the steel is a pale yellow colour, maintain this colour as best you can for 30 minutes. If this were a carbon steel blade, we'd get it red hot and apply it to the magnet. If it is non-magnetic, give it one more shot of heat, say 10 seconds and then dive it into the quench tank.


When the hot steel is plunged into the oil, it takes a few seconds for the vibrations in the steel to stop. I give it a gentle stir or sway back and forth for about 15 seconds.

Once quenched, the blade will look burnt. That's normal. If you want to repeat the process again you can. I repeated this heating and quenching process three times, each time checking with the magnet before quenching.

The steel is now very hard now, but also very brittle. 


When the blade is cool to touch, we need to clean it up for tempering. Tempering is a softening process that reduces the brittleness and allows for sharpening the cutting edge.

This is the oil that was baked on to the blade during quenches.
 Clean up the faces with 220 grit sand paper.







Clean up of the tang edges with 220 grit.

















Once cleaned up, we need to temper in an oven at 230 to 250°C (450 to 475°F). The heat cycles are 230°C for two hours, then cool to room temperature, then a second soak at 230°C for two hours. The steel should come to a yellow straw colour.

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