||[Nov. 25th, 2011|03:49 pm]
I decided today was the day to try forging some of that wootz I made up. And, at the least, I can confirm that this is, by all descriptions, a very wootz-like steel.|
More behind the cut.
Two cakes, both spark tested out as high carbon, but not so much that it was cast iron. One has a bit of vanadium in it, the other was made in the same crucible, with with no vanadium in the melt itself. Otherwise, the ingredients were just iron and carbon, as close to what people used as steel for millennia. Ok, so many thought that phosphorus or sulfur wold do the same thing as carbon, but we'll ignore that for now.
The first task was to split each in two -- I wanted to save half of each for future projects. That their subtly different compositions should provide some contrast against each other didn't hurt, either. Since the chop saw and bandsaw were proving slow and tedious, I resorted to hot-cutting. This was when the first bit of wootz-like character came to light.
Wootz, when you heat it above it's forging temperature, greatly weakens in structure. The crystalline structure of wootz is of iron carbide crystals forming in a matrix of iron so saturated in carbon that graphite and even buckytubes will precipitate out. This higher-carbon matrix has a lower melting point than the carbide crystals. This is because it is hyper-eutectic mix of iron and carbon. The crystals are variable in size depending on initial cooling rate, alloying agents, and so on. It's traditional to show an iron/carbon phase diagram at this point, so here you go:
Thank you University of Vermont.
Now, on this chart you can see how as the carbon content increases, until about 4.3%, the melting temperatures decreases. Since there is more carbon by far than the iron can form carbides with, you end up with the aforementioned carbide crystals in a cast iron matrix. Heat it until the cast iron melts, but the carbides are still solid, and hit it with a hammer and the whole thing falls apart into white-hot sand-like fragments in a shower of sparks, useless. Heat it to below this temperature, though, and you can work it, if you're gentle and lucky.
Fortunately, I was just wanting to make smaller, flatter pieces for the katana project. I deliberately overheated just a bit, and used a hot cut to groove both sides, then got the cake to crack almost exactly in two along that groove. Mostly. Close enough for this project, at least.
Flattening out the chunks proved difficult but not impossible. It has to be between a dull red (i.e. cherry) and orange-red to work properly, and I can tell I'm going to have to work out a bunch of carbon (no worries, with at least 9 folds to go, I'm probably going to lose a great deal of it). When it works right, it has that squishiness when forged that I'm coming to recognize. A few pieces just did the fall-apart-into-sand thing, before I finally got the concept of the proper working temperature straight in my head. Several pieces just cracked into smaller pieces -- I'll keep these, but cook them in a reducing atmosphere for a while before I do anything with them, drive off some carbon -- while others flattened out ok. I'm guessing the variation is more due to my lack of experience than any variation in the ingot.
After flattening out the big pieces to about 1/2" from the original 1.5", I decided my back wasn't happy, so quit for now. Nice to make progress, though. Since the tendency to fall apart spectacularly, yet still have a narrow forging window (I'd guess maybe 100-200C, somewhere in the 900C/1650F range) is classic wootz behavior, causing no end of grief to the Crusaders who picked up and tried to forge the stuff, it's yet another sign that I may actually have made it right.
In other project news, the replacement blade for my brother's b-day present (UPS lost the original, as I found out a few weeks ago) is done, just need to remove the oxide and anodize it.
Edit: almost forgot to include a pic of the wootz: