how much will a clay hardened blade bend up

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unky_gumbi

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Aug 28, 2009
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I have been going over my blade drawings to decide which ones to scan into the computer, and amongst them I have a more traditional Tanto. I have it drawn with a curve, but I know that when these things get Htd with a clayed spine they tend to bend upwards. Is there any way to estimate how much a tip will move up during quenching over a given length, or is this pretty much hit and miss. I am not sure if I will ever make this blade, but if I ever do it will most likely be done in W2
 
It's been my experience that clayed blades, when properly austenized, will rise up when quenched in water, and bend down when quenched in fast oil.
 
That is the way it usually works.

The amount of sori is hard to guess, so most folks leave a little meat to grind away if needed and adjust the curve.
 
So its more of a guessing game, that you learn with experience then:(

I do find it odd that water tends to bend them up and oil bends them down, is there a reason for this? The only think I can think of is that the water is faster than a fast oil, but the idea of it reacting differently confuses me.
 
In water it first bends down then back up, at least in Wally Hayes Katana DVD it showed that. He did an interrupted quench in warmed water. Maybe someone could explain why that happened. It did in the DVD but I'm at work and can't check it until later.
 
This may be overly simple, but could it be since hot metal expands and before the quench the metal is in an expanded state, then as the blade edge cools first it contracts causing a bend toward the edge then as the spine cools (slower than the edge allowing the carbon to move out of the iron crystal centers) it contracts even more causing the bend to move back toward the spine.
 
A VERY simplified explanation is that the different structures have different volume ( density).
As the edge cools down past the pearlite nose, it contracts, bending the blade down. The soft austenite spine under the clay yields easily to this stress.
Next the super-cooled austenite edge converts to martensite, which takes up more space than the austenite did, so the edge lengthens...and the blade curves upward dramatically and suddenly.
Then the spine becomes pearlite, and it cools, contracting. This final stress can literally tear the blade apart. The edge is now fully converted brittle martensite, and the spine is tough pearlite...in the tug of war, the edge looses easily.
The steel needs to be the shallowest hardening alloy possible. As little Mn as possible, and none of the normal other toughening elements, like Cr....and no carbide formers but the iron.
 
Thanks for that explanation Stacy! I've heard it explained various times and never as easy to understand as that.
 
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