May 24th – Hardening the Blade

May 24^th – Hardening the Blade

Well…the knife has sat overnight annealing in the bucket of Mica.  All of it’s molecules are nicely stacked up and we now have one really brittle knife.  I mean REALLY brittle…if I were to drop this thing on the concrete floor it would probably shatter.

What we want is a tough blade that can handle a lot of stress besides just cutting.  We want to be confident that I could use the blade as crowbar if I had too.  One of the reasons…hell…the only reason we through all that metal in a full tang blade is so the user will be reassured that if the blade is needed as a wedge tool it wont snap under pressure.

This is kinda important…if these issues were not particularly important to me…say I was making a smaller knife, or a decorative knife…I would be much more inclined to reduce the amount of metal over all for weight, and sink a small tang into a beautiful antler or flared piece of wood for a handle. 

That is not the purpose of this knife though…it is a full purpose tool…and thus needs a full tang.

Ok…so how do we harden it?

First…we need to heat the blade back up again until it looses its magnetism. This is the critical temperature that tells me it is ready for the next step.

Right out of the forge...still holding it's colored heat

that next step is to dump it into this big ol’ bucket of motor oil.  This quickly cools the metal down to below magnetism and freezes the surface molecules in their “starting to jumble” state.  It also produces fire and looks really friggen cool to anyone who happens to be passing by my shop!

AGHHH!!!  Fire!  I hope it doesn't catch on my grass skirt!

The metal is “flash cooled” with the oil and the blade quickly looses any residual heat.  During this time if a file is rasped against the blade it will leave a colored mark.  That color is the heat drifting away.  The color equates to the relative temperature.  With the blade still too hot to touch the color is a deep purple.  As the heat leaves the blade, subsequent passes with the file will leave “bluer” colors, till eventually we get a yellowish straw color.  When we see that straw color we are done…..and back into the oil quench it goes until the blade is at room temperature.

Once we pull it out of the oil the blade is caked and coated with gunk.  We need to clean it up, and we do that with a polisher disk.  We really don’t have to remove any more metal now….we only want to get the gunk off.

There it is!  The blade is basically done.  Next we have to shape out our side handles!