|Some blacksmithing has to be in the dark . . .
||[Aug. 19th, 2009|09:43 pm]
Sometimes, to truly see, you have to turn off the lights. |
rambing swordmaking stuff again.
I started the heat treating process tonight on a sword.
The steel is 5160, please forgive me if I've written about it too much before. It is my favorite steel for tricky stuff like this sword. It is cheap enough that I don't mind the cost of the occasional inevitable failure, and often use it to just play around with when I'm trying out ideas. It is much more forgiving in forging and heat treatment than the 10xx steels, at least in my opinion, and tough as it gets as a steel. They make truck springs out of the stuff, it's canonical for it. It's specifications are simple, roughly C .60 Mn .85 Cr .80, plus minimums for contaminants. When I want a nice hamon, I'll go 10xx or W1, for pretty patterns I'll use O1 and L6, but for an experimental sword design, it's the right material.
I started this a while back, but haven't touched it in a long while. I had pretty much gotten it all forged out and straightened, but it still had a bit of spine bend and a wavy edge, as well as a little bit of a twist, so I spent the better part of two hours getting everything lined up and as smooth as possible. It's still not perfect, so I'm going to try The Secret of Three Sticks in a Vice to get the last bits of bend out before grinding. If that doesn't work, I still have a few tricks up my sleeve.
A proper sword blade tapers both in thickness and width as you go from pommel to tip. This one starts out at 5/16" and tapers down to about 1/16th over some 32". It doesn't have to be an even taper, or even consistent, but overall, that should be the flow. This one starts out at 5/16" and tapers down to about 1/16th over some 32". This poses a problem when heat treating, as the thicker the metal is, the longer it'll take to get to temperature. Ideally, you'd want to heat the back and edge to the same temperature by immersing it in a conductive medium, such as a molten salt, held at the correct temperature in a pot. These systems cost thousands of dollars, though, and yellow-hot molten salt containment isn't something I'm sure I want to jury rig right now.
So, I substitued observation and attention to detail for technology. After preheating the 3-burner gas forge and turning it down to its lowest level, a point at which it pops and growls as the flame fronts travel up and down the burner tubes, I turned off the lights. With the doors closed, it was too dark, so I opened up both doors until the light was dim enough to see the slightest glow in the steel, while being bright enough that I wouldn't trip over stuff. Like a good chef making a sauce, I made sure my tools were right at hand, I had a clear path from forge to tank, and everything was set.
First, to relieve the stresses of forging, I normalized 3 times. This consists of heating to the critical point and then letting the piece cool down in air, which relieves stresses in the steel. To find the critical point, which can change by several degrees from batch to batch, you can use several different methods, but my favorite is to watch the steel for a "shadow" effect as the phase transition move down the blade, verified with a magnet. I normalized 3 times, then annealed by heating to the same temperature and burying it in vermiculite.
As the steel heated, the glow, which no camera can really capture, changes from moment to moment. You keep the steel moving, as no forge heats evenly, and heat the handle end first. It's thick enough that by the time it comes to temperature all the way through, it's got enough heat to carry for long enough to to the rest of the blade. Then you start putting in more blade, and so on, moving the glow and its shadow down the blade, trying to keep everything at that just past the shadow glow, and once the whole blade, butt to tip is at the right temperature, over to the vice or tank to cool. It will take several hours for the piece to finish cooling down, it's not quire a true spheroid anneal, but close enough for my purposes.
Most beginners heat steel too hot when heat treating -- you can't see the phase changes in even weak light, and somehow they think the cherries in the standard "heat it cherry red" aren't even the primary brightness of a maraschino or even the bright yellow of a rainier cherry. Once you can see what's going on, though, you understand it's the dark, almost-black red that's the temperature you're looking for.
Turning off the lights and working by the glow of steel and forge is often a moving experience; it requires surety of motion, vigilance, preparation and understanding of what you're trying to accomplish. The intensity of focus comes naturally -- it's the source of light, your eyes, once attuned, focuses in on the moving shadow. You see a hint of unevenness in the light as sections, particularly the thin edge, cool and overheat, and so you move it faster or in a different directions, flip, pull it out and confirm you've got an even glow, then it's done.