The most dramatic moment in making a Japanese sword is the quenching. In part, that’s because if you have failures in your welds, the blade may suddenly delaminate, turn into a pretzel, or crack.
Japanese swords get their characteristic temper-line (Hamon) due to differential hardening of the steel – the leading edge of the blade is very hard because it is cooled quickly in water and it forms large martensite crystals; the back and sides are less hard because they are shielded with a layer of fire-clay that slows down the cooling.
Harder steel takes a sharper edge, and holds it longer, but it’s more brittle. The whole process is a way of tuning the blade’s internal crystallography and the stresses in the metal – quenching changes the curvature of the blade as you watch it, which is a bit nerve-wracking and very cool. Michael Bell keeps a blade in the studio that delaminated during quenching – it’s a pretty ugly thing – none of us want to see that happen to something we just spent days working on.
This is a bit of video I finally go around to editing, of Michael Bell quenching my wakizashi blade. A few notes:
- Up until about 0:26, he is moving the blade in and out of the forge to “soak” the heat through the steel as evenly as possible. If the steel is not evenly heated, there’s a much higher chance it’ll deform dramatically.
- At 0:29 when he heads over to the quenching trough, you can see he appears to be touching the yellow-hot metal with his left hand. He’s holding a small magnet, and is testing along the length of the blade to make sure the steel is demagnetized (“normalized”) – it goes into a form of Austenite, around 1500F, or 750C. The magnet is a good way of making sure it’s evenly heated, though Michael was mostly doing that to show us – at this point, he can tell steel temperature by looking at it.
- The trough of water is not cold; it’s about 130 degrees or so. To bring the trough up to temperature, Michael put a big steel block into the forge and got it red hot, then popped it into the trough to warm the water. If you quench a blade in cold water, you may over-stress it.
- At 0:48 you can clearly see the blade has taken on a different curvature.
The shock of going into the water blows most of the cement off the edge, but the cement on the sides and back stays long enough to slow the cooling.