sword heat treating

Kevin McGovern

Kevin McGovern

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Jul 31, 2015
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Let's say, hypothetically, that a guy had a great idea to have a get together where he and his buddies heat treat a katana. This hypothetical (but fetchingly handsome) guy would use a forge to cycle and aus heat the blade. Yet (as handsome and charming as he is) this gent is stumped on how he would temper his sword (on the off chance it didn't break in the quench). Thoughts?
 
How precise do you want the temper to be? If you want a long, near perfect soak, I would say build an oven. If a little higher temperature variance is fine, you could use some large metal (copper would be best but aluminum should work) plates as a heatsink. Coupled with an IR thermometer and an easy to control heat source (heat gun may work) that would probably do it. I might heat the plates over temp in your forge and as they cool through your desired range, place the blade between. Then use a torch or two ro some heat guns to maintain the heat as it naturally cools.

Other than that, I think building an oven is probably your best option. Use some non papered, hard rockwool or similar insulation, some sort of open air heating element and a $15 PID (with included TC). I would probably try for a long, ceramic shielded element, if I was going to build this thing permanently, If I was going for a one and done setup, I would buy a nichrome wire and wind a quick element. I think some of the cheapy PIDs have a 15A SSR built in, which is plenty to get the internal temp of a 1 cuft oven to 500 F.
 
I temper katana in my kitchen oven.
Set the oven to 400F and let it heat for 30 minutes. Stick the sword in diagonally with the nakago sticking out the top corner. If you really want to, you can put some kaowool along the small crack at the door top ( I don't bother.) Let soak for an hour and flip the sword around the other way ( nakago in first). Heat for another hour and remove to cool. Repeat for the second temper. If the nakago is all that sticks out, you don't even have to flip the sword. Just do two 1 hour tempers. Then torch temper the nakago. Traditionally, the nakago is very soft.

The other way is to torch temper the whole sword. While the description sounds complex, it is pretty fast and easy. It won't equal an oven tempering of a couple hours, but it will take the brittleness out of the sword. It is equivalent to the traditional tempering done before modern equipment.
Clean the blade up with a 120 grit belt/paper/stone to show bright steel. Take a regular plumbers propane/butane torch and work it along the mune watching the temper colors go down. work about 4" to 6" at a time. Have a very damp folded towel on the table/bench top and as the gold/straw color reaches the edge in the area you are working, lower the edge into the wet towel. This will stop the temper from going too far. Go down the whole length and try for an even temper. If you are going to cut with the sword, grind a quick edge and test it for chippiness. If too chippy, temper again, letting the gold/yellow stay a tad longer before cooling that area.
 
This is one of the reasons I built my Fogg drum forge. When I asked this question last year, it was because I knew I was going to be on FiF and had no way to heat treat a sword properly. In the drum forge with a 1" burner I can hold 1500F on something 36" long, and with a 1/2" burner I can hold 400F. I put some fire brick in where the burner enters to act as a baffle and keep any flame from directly contacting a sword.

Unfortunately I didn't get the chance to use it as intended but will soon on the odachi I'm filing away at. Though I have been using it regularly for heat treating and occasionally for tempering, just nothing a full 36" long.
 
It's on the lower end for an odachi. 38" I believe. I may have to stretch my drum if it does not fit the diagonal.
 
That should finish out at around 45-48" when the koshirae is fitted. I'd go with a 15" tsuka.
 
O-dachi - large/great sword.

Some folks like to call them "horse killing swords". They had blades starting at 3 skaku (36"). In general any large sword is an O-dachi.

O-dachi were mainly used in open field battles where their extra weight and length was an advantage. No-dachi is the term for a field sword used by infantry.
Other O-dachu were ceremonial and were of ridiculous size ... some as long aas 10 feet.
 
The late John White had a mini-Fogg forge that he used to HT his clayed up bowie blades of W2. He could hold the temp to within 2-3 degrees all day long. That let him go fairly low in the aus temp and get a LOT of hamon activity.
 
Sam Salvati used a steel tube full of oil and heated that for tempering. That was in a class here, no idea how he does it at home. A friend took the class, I bought a new shop instead. I really wanted to go to that one.

-Clint
 
People regularly set their shops on fire doing that. It is a bad idea to put that type of advice out on a public forum. Hot oil tempering is a professional and industrial method and a lot more involved than setting a cylinder of oil on a hot plate.

Look up the recent threads on the issue or watch FIF. On guy set his whole shop ablaze.
 
Yeah, low temp salt seems much safer than oil and no harder to achieve. That being said, you can build a tempering oven (tempering only mind you) for very little compared to the cost of a full HT oven. I would think a low temp salt pot might be constructed easily in a similar way
 
Too close to the flash point ! In an industrial setting you would have many controls. But in a small shop ? too dangerous !
 
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