|back in the shop
||[Jul. 11th, 2011|01:07 am]
Got some shop time in today, got through a necessary but not terribly exciting stage in making an absurdly accurate historical example of a blade: sorting through the blister steel. Each piece is heated to forging temperature and forged a bit to verify workability. Too much carbon and/or phosphorus can turn steel into something that works like cottage cheese -- which is to say, not at all. Still hot, the piece of steel is then quenched and attempted to be broken. This helps determine carbon content, as well as depth of carburization. As expected, some pieces showed less than full penetration, but every piece except for one stray piece that missed a trip in the carburizing retort showed at least a surface penetration of 1/16", the the thickest pieces being 1/4" thick. This is plenty of carbon for this purpose, and combined with the last batch gives me enough blister steel to make a couple of swords.|
My experience with this kind of steel is that it has good potential for contrast - between the different sources for the wrought iron and the different levels of carburization, there's enough difference to make the contrast fairly dramatic.
I tried to video this session, using the flip camera and a tripod, but somehow managed to position it so it caught little but the back of my head. It was pretty boring, so I don't think anyone missed much. I did take some stills, and some closeups using a USB microscope.
Some pictures behind the cut.
This is the steel pre-testing. The shorter, thicker stuff was sold to me as having come out of a colonial-era bank vault, making it over 200 years old. The thinner stuff was hammered out of 3/4" wrought iron bar from a bridge, date uncertain. I'm not sure why the vault iron came out of the retort so dark, the bridge material came out shiny, with all the surface rust reduced back to iron.
After testing, I sorted the steel pieces according to how deep the carbon had appeared to penetrate.
In practice, this worked out to sorting by thickness -- even though I had run the thicker stuff much longer in a retort, it still had far more undercarburized iron in the center.
Even the thinner stuff has a core of low carbon. As it all gets welded together and folded, the carbon should migrate, though there'll still be a visible difference, and it may well cause a lot of "activity" on the hamon if I go Japanese. If I go European, it'll have a nice contrast for any pattern welding.
This is the thinner blister steel, polished to .5 micron and etched in ferric chloride. Note the darker etched high-carbon steel outer areas, with the brighter low-carbon inside. The lines in the steel are from it's origins as wrought iron, those are trace inclusions of ferrosillicate slag, carbon, and so on. It's pretty dirty stuff, and I will be folding it a few times to work out any inclusions (I hope).
This is the thicker blister steel, polished to .5 micron and etched in ferric chloride. Again, note the darker high-carbon outer layer, the lines from previous forgings during the making of the wrought iron, and the many inclusions and flaws. I'll probably fold this a few times, then use it for the back or core -- it won't have enough carbon to be good edge steel, but it will have nice contrast, and after a few folds the crap should mostly be worked out.
A couple quick additions: 150x micrographs, unetched, of the blister steels. Note all the inclusions -- and, yes, those hideous scratches are actually .5 micron or so, essentially invisible and putting the lie to the whole "mirror polish" concept.
This is the thick piece, showing the strands and sheets of slag, mostly ferrosillicon with added iron oxide dissolved in. The relative large size of the included slag combined with the laminate structure show that this was pretty crappy iron, only barely worked before being rolled out into sheet.
This is from the thinner piece, showing comparatively finer strings of slag, more evenly distributed. Zooming in shows that there's an almost fractal distribution of slag, indicating this bar, used as a supporting member on a bridge somewhere, had been worked for the equivalent of several more heats compared to the thicker bar.