Changing blade curvature in the quench

Storm Crow

Storm Crow

Joined
Apr 12, 2006
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I'm working on a chunky quasi-wakizashi for the USN Gathering coming up with alarming rapidity, and thought I would document something interesting that happens when I do a quasi-Japanese blade. My apologies for the quality of the cell phone pictures.

One of the factors to take into consideration when making a Japanese-style blade is the sori, or amount of curvature that is formed during heat treatment. In a traditional blade using tamahagane steel in a water quench, the blade is straight when it come out of the fire and curves first down toward the edge then up toward the spine. It can be very stressful and there's a high rate of traditional blades tearing themselves apart in the quench. With an oil quench, a blade tends to get negative sori, curving toward the edge.

So, with this 5160 wakizashi, I gave it more curvature than I wanted and let the negative sori straighten it. The soapstone mark on the anvil shows where the spine was prior to the quench. I've never had a blade crack from sori in a canola quench.



While at the base of the blade near the blade/tang transition there has been almost no change, about 2/3 of the way down it has moved about a 1/4". It seems like this happens by far the most with the quasi-Japanese blades I do. It's like they know... ;) I've gotten pretty good at gauging how much extra curvature to have to get the blade to straighten up to where I want it, but I don't do too many in this style.

 
Very interesting, James.
Someone is going to be very happy with this short sword.
rolf
 
You are not the first one ! The amount of curvature is very complex but those trying the same with oil get the same surprise as you did !! NO I would not even think of trying to predict the curve no matter what
as the process is far too complex !!
 
Mete - Not a surprise to me; I'd read about it before experiencing it. I like my Japanese-style blades to have just a slight curve, so I give it a little extra when shaping and let it pull itself straighter in the quench. Works pretty well. :)
 
Good stuff, Storm Crow. Have you heard of the heat-benders who work in shipyards using huge torches and water to curve hull plates? There are established torching patterns to curve the giant plates predictably. Within the welding/fabrication world, it's considered a high art form!
 
Yes Mecha but they don't try to do it with hardened tool steel ! I've seen that and the results were funny !
 
Lycosa - I can't recall having a blade ever crack in a canola quench. I had a handful of bush swords where the steel cracked as I started to roll the integral socket handles, but after making sure not to work the steel too cold on my power hammer and to give the handle pre-form a normalizing cycle after thinning and spreading it out, I haven't had any issues. It really makes me wonder what the issue was on Forged in Fire after hearing (no access to broadcast or cable channels, so I haven't seen it) that more than one blade cracked in the heat treatment.

Mecha - That would be something to see! Definitely a high art. I've played around just a bit with thin sheet metal and a cutting torch watching it warp back and forth, but doing it in a controlled fashion on a large scale is something else.
 
James- That's a good question. I'd like to why some of those blade cracked.
I remember when Gary Bradburn made me a forged knife some years ago and he told me that if my blade survived the quench, he'd finish my knife. I never thought to ask him what would cause the blade to crack or what quench he was using.
Hi Sam.
rolf
 
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