PART FIVE: THERMAL PROCESSING - YAKI IRE
 

This is the critical step in developing the beauty and functionality of the blade. Most will refer to this process as hardening and it started already when I thermal cycled the blade in what has become known as normalizing. At this point I will clay-coat the blade in preparation for quenching and hardening the blade. That is called differential hardening (hard edge - soft back). And to finish the process the blade will be tempered or drawn back a little to reduce the brittleness of the edge.

In order to gain as much knowledge as possible from this steel I am planning on using three different clays applied to the blades in different sections and with different thicknesses. I will also vary the soak time at austenizing temperature in the heat treating forge to see how that affects the steel.

I will start by mixing my own clay formula: 50% fireclay + 50% charcoal.

I have developed a set of simple tools for laying down the clay: a brush and a spatula with some shaping of the tips to form the clay. A little plastic box to mix the clay to the consistence needed. A hard plastic board to serve as an easel.

Here I am mixing my clay to a thick paste consistency first.

The second formula that I will be using is well-known by everyone: satanite.

And the third formula is based on a recommendation from Don Fogg: an anti-scale compound that is quite plastic and easy to work with.

Then Don's anti-scale compound. Thick coat and thin coat again

After letting the clay dry over night I am ready to quench on the next evening. There will be no pictures of the actual quench. Sorry about that but I was by myself and this step requires all my concentration. But here is a picture of the setup. I have a long heat treating oven made out of a water tank, a pyrometer to guide my temperature. The final decision as to when the blade is ready to be quenched for me is a visual one based on the colors of the blade and not the reading on the pyrometer. And I have a fish tank for tanto-size blades. The fish tank contains water saturated with salt and some soap at about 110 degree F. One blade was heated to 1450 degrees F and the temperature was held there for 5 minutes then quenched. The other blade was quickly brought up to 1480 degrees and then quenched. I had thought about trying to form nie crystals. The Japanese create those by allowing the martensite crystals to grow. My understanding is that they do so by using very high temperature for the quench. Higher temperatures that I feel comfortable with. I guess you could just hold the blade at austenizing temperature for a very long time to allow the crystals to grow and form nie without overheating the blade and risking a crack. But after all the work I put into these blades I was not quite ready to try that yet.

Another useful tool is a blade holder. A piece of rebar to which I have welded two attachments on the ends to hold the blade. One small sized for tantos and the other for larger blades.

And at this point yaki ire (quenching) has been completed and the blades are in the oven cooking at 300 degrees F for one hour and a half to temper the edge and reduce the brittleness of the martensitic edge. Prior to putting them in the oven I have checked the edge with a file to see that they hardened properly.

Next morning this is what they look like.

After a quick clean-up I noticed that the satanite and the anti-scale compound did not leave any residue but my clay mix interacted with the steel a little deeper and left some residue (carburization?).

I now have two twin blades ready for polish.

TO PART SIX