|success at last
||[Aug. 15th, 2009|11:55 pm]
Today I did something I wasn't sure I could pull off: I forge welded a piece of 1-1/4" steel cable into a solid billet, then upset that billet end-on into a disc.|
long rambling blacksmithing geekery behind the cut
I could claim I was successful on my first attempt, but that would belittle the many stages of getting here. Learning how to work a new material is always a challenge, and forge welding cable damascus was no exception.
I've had this cable, special improved plough steel, aka 1095, for some time now, but hadn't done much with it until a couple of weeks ago when I started working with it again. Since my first few test pieces over a year ago had proven to be quite unsatisfactory, with more of the strands of cable than not having failed to weld cleanly, I had gone back to basics. Hand hammering, coal forge, patience, and doing it by the book.
The decision to go with coal is because it can generate heat well in excess of what a gas forge can -- a coal forge is really only limited in energy output by the quality of your coal and the amount of oxygen you can shove in there. Coal ranges from almost 90% carbon in some Anthracites to half or less in some industrial coals, the lignites and brown coals. Aromatic hydrocarbons comprise most of the rest, and they do not burn as cleanly or as hot as the carbon, so you'd think that you'd want to use the highest carbon, anthracite, for forge work.
The thing is, though, if you have a bituminous coal, low sulfur, with about 15-20% volatiles, something interesting happens as you burn off the volatiles, or coke it. As they boil out, the coal expands into a foamy structure, and sticks to itself in large clumps, or if you make your fire correctly, sheets. As the last of the aromatics burn off, you're left with a lightweight foam of nearly pure carbon, with a few percent of various stuff that becomes the ash and clinker.
If you can get your fire built right, using the sheets of coke on top with plenty of coke beneath, you get a hollow fire, basically an oven made of white-hot glowing carbon. Turn the air up high, and you can get to steel-burning temperatures trivially. Keep it a bit lower, low enough to maintain a neutral to reducing atmosphere, and you have a perfect welding environment: hot, a reducing atmosphere, and with heat coming in from all sides.
That said, my first attempt was not the most successful attempt in the world. It's standard practice to tighten up the twists in a wire rope before welding it -- it both improves the pattern and helps everything to hold together while you're welding it. You have to be careful, though.
To prepare, some people like to take the rope apart and clean out all the grease; some even line each sub-cable with nickle foil to enhance the pattern. Most, however, go the route I chose: just weld as is. Grease is, after all, really nothing but oil thickened with soap, mixed with a bit of dirt and iron oxide. If you know your welding theory, that's an almost ideal welding flux as is. I use borax on the outside to protect and flux that surface, but on the interior the grease first drives out the oxygen as it vaporizes off, then the nonvolatiles melt, and the iron oxide reduces to iron and liquefies the silicates. This also helps keep things lubricated while twisting.
To twist, you first forge weld up each end of the cable, then bring the whole piece to a good almost-welding yellow-orange heat. To make the twisting easier, while welding the ends I forged them into squares, so the twisting wrench and vice have something to hold onto. As quickly as you can, before it loses heat, you move it to a vice, clamp in one end, and twist the other end until it's as tight as you like.
It's almost absurd: I have a hobby in which shearing 1-1/4" steel cable using little more than finger strength is a possible hazard. One, I must admit, I managed to succumb to. While twisting up the first billet, I managed to over-twist it just a bit. I could feel the resistance growing, the smooth steady resistance of metal stretching smoothly changing to the creaky jerky feedback of metal strained to its breaking point. I thought I had stopped in time.
To actually weld, you get one end of the billet to heat, fluxing generously, and start tapping with a big hammer. Work the weld up the billet until you're past halfway, swap ends, and keep the welding going until you reach the other end.
A bit past the halfway point, as I started to weld the metal felt wrong. The metal moved under the hammer in a way that didn't feel right. Instead of a solid thump and a bit of a ring from the anvil, it squished and made a squeaky noise. A quick check showed the metal was sheared through, hanging on only by a few wires.
I tossed it in the annealing tank to cool down, and next time started over with a new piece of cable. This time I was pretty sure I got the twisting and welding right, but I wanted to make sure before moving on to the shaping phase.
I dragged out two bits of the failed piece, and decided that the damage was limited to within an inch of the break on either side. Selecting the longer of the two pieces, I cut the bad end off with the chopsaw and polished the end to make sure it was welded solid. Sure enough there were indications of cold shuts in the very core. I got it to heat and went over the piece again, reducing the rope down to about 3/4", or almost half of its original diameter. To make sure that I had accomplushed my goal, however, required a test.
I turned a piece of that welded up wire rope about 4" long by 3/4" wide into a flat disk, currently 2.5" across, by upsetting it. By hand, using hammer and tongs, though I did switch to a gas forge for the upsetting process.
Upsetting something that long and thin into a disk is actually a pretty standard journeyman test, and one I believe I passed today -- even more so by starting with a bundle of wires. If there had been cold shuts or other flaws it would have split apart, or started to show conspicuous gaps. At one point, the upsetting started to go off-center turning the column into a leaning tower of pizza cutter, but I was able to recover. It took several heats, and while I wanted to get it to the full size, I stopped at about 2.5" wide by ~5/16" thick.
(Virginia in August heat + humidity + 6 hours blacksmithing = exhaustion)
Tomorrow, I'm going to punch a hole in the middle, forge it out a bit more, and hopefully forge in the bevels. Since this has to have a lot of decarburization, I'm going to need to leave it thick and grind off several hundredths off the front and back, and probably at least a sixteenth off the edge, probably more to get to clean metal. But that's a post for another day.