||[Dec. 27th, 2003|05:02 pm]
So, I'm currently stinking of charcoal fumes and heated oil, and holding in my hand a beautifully curved, hardened and (lightly) tempered katana blade. I'd been working on and off for the last month on the initial grinding, shaping, smoothing, and claying of the blade, it's good to see a critical and dangerous milestone pass.
To get to this point, I first took a piece of 1075, a high-carbon steel, straight and 1/4" thick by 1" wide x 36" long. This was a bit long for a katana, so I shortened it to 32". Normally at this point I would have forged it, but I had decided to make a fairly narrow blade, about 1" wide, and I decided to skip this step. Instead, I chucked up a 36 grit belt on the 4hp grinder and started in. In profile, I allowed a good 8" of tang, the rest blade, with a 1/16" step at the tang/blade junction. I then narrowed the width of the bade so that it was about 7/8" wide at the tip, a full 1" at the junction. In thickness, I took it down 1/8" towards the tip, spreading the taper over the last 2/3 of the blade. This gives a decent weight distribution, while leaving the sword sturdy enough for cutting practice if so desired.
The bevel starts 1/4" from the back of the blade, with 1.5" for the tip. Before heat treatment, I left a few hundreths of an inch of edge thickness left so as to minimize warping, provide some protection against decarburization, and to avoid the dreaded "ping" of a broken blade during the quench. I clayed using a medium bodied furnance cement, using a crenelated patern. Following the recommendation of Bob Engnath, I wrapped the clayed sword with wire, in this case some steel MIG wire I had lying around. Bad choice, I think, as the wire is tough and springy, next time I'm going to try something softer. The wire helped keep the clay sticking to the sword as it gyrates during heat treatment. It works!
I let the clay air-dry for a couple of days, then stuck it on top of a heater to finish drying. At this point, I was ready to heat treat. A friend asked if he could come over to work on some of his knives, so I asked if he wanted to help with the heat treatment -- he said sure. I always like to have company when I do this sort of thing, as a check against stupidity.
I dug a shallow trench, and laid a 2" x 4' pipe in it that I had drilled a bunch of holes in, then tamped the dirt back around it to hold it in place, leaving the holes exposed. Next, using some spare bricks, we created a long forge for the heat treatment. I hooked up a blower, lit the charcoal, and we were off.
After getting the charcoal going well, I laid the sword in the fire, the back side down. Slowly, over about an hour, I raised the temperature of the sword, moving it as little as possible, and making sure there was a good bed of lit charcoal under it at all times to minimize any oxidation and decarburization. After it was just barely glowing (I had erected a plywood screen so we were in shadow) I checked to make sure the whole sword was at temperature, flipping it over to ensure the edge was at critical, then in one quick, smooth, practiced motion moved it from fire to quench.
For a quench, I was using just plain water. The sword, straight as an arrow at this point, hit the water, then the acrobatics started. First, the blade started curving, but in the opposite direction from what you'd expect. Then, the hissing got louder, and it straightened out. Then the hissing got a lot louder of a sudden, and the blade curved, the clay coating cracking, but staying in place. Finally, the hissing dimished and the sword was quenched. I pulled it out and knocked off the clay -- it was perfectly curved, no cracks, no warping, no oddly bent sections, a nice smooth curvature to it.
After a brief excursion for lunch, I went ahead and set up a 2-burner camp stove and an iron box filled with vegtable oil for the tempering. Given the fragility of the hamon (temper line), it is a bad idea to temper too much, so I only tempered it to 360f. The stove took some coaxing (and disassembly, cleaning, and reassembly of the burner elements) to get going, but it finally did the job. To keep the sword away from any hot spots, I put some scrap metal in the bottom to keep the sword up. After about a half hour, the probe thermometer indicated 350, and I pulled out the hardened, tempered katana.
All in all, not a bad days work.