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Response To:
Microstructure ()

David Weaver
...when I mention microstructure, I'm borrowing some from Larrin's talk and knife people talk, but just looking at visible micrographs of grain doesn't tell the toughness story. For someone uneducated like me (one materials science class, and half of it was plastics), I'm really looking to link a tangible process to results, and they don't need to be intangible results - I'm just looking for the tangible ones.

Heating a japanese chisel through malleting with zero corner loss, zero edge loss.

There are definitely things that go on below the visible spectrum as I can't even tell when you can see bainite vs. martensite, which is which sometimes. And even Larrin said something about having to go further to tell the difference. The crystalline structure is similar.

Further, you can end up with things like fine grain, but retained austenite, which will either need to be converted to martensite to be tempered (by cooling well below room temperature - like liquid nitro), or it'll just be there and a tool will be somewhat soft and lose the goodness of most of it being tempered martensite.

For the guy on the ground like me, it's a *whole lot* easier just to make some chisels and then see first how hard they come out of the quench, and second how much I have to temper them before they stop chipping. If they're still boggling hard and should be taken back further, that generally means I have a lot of room to work with. Good wood files are like that. 1095 isn't - 1095 gets really hard, but then doesn't exhibit good chipping behavior until it's borderline too soft for my tastes. On a knife, the tiny chipping isn't a problem (one is mostly slicing). On a plane iron, it makes 1095 last about as long as 1084, but 1084 doesn't chip, so what's the point of using 1095 if it lasts the same but part of the time it doesn't leave as nice of a surface.

The thermal cycles and refinement that the higher end guys do with 52100 make the difference for someone who will test those things to the limit. I don't think we need that much, but we do need to start with a steel that isn't almost impossible to get a good result with (e.g., spicy white is supposedly easy to work with as it doesn't have many high heat carbides in it - it's an ideal candidate. To the extent that toward the tang, it's not tough enough, then I can intentionally partially harden it there. If the bevel comes off on a chisel in chunks, then we'll have a real problem to deal with. Time will tell).

Separately, with all of this - I was fascinated with the whole idea of cutting nails with a knife, so I made a file knife quickly and hardened and tempered it and then left a gradual bevel on it. Not surprisingly, it chipped. The damage went further than the chip about half of the chip's depth and I got all of that out and steepened the bevel a little bit to about 20 degrees per side. My cheesball lightly forged file knife clipped through nails no problem after that....they're a lot softer than we think! I'm convinced that a lot of the impressive torture demos are much about edge geometry (nobody is cutting through nails 10dps without anything at the bevel).

The O1 lack of toughness with such a fine structure is interesting only because it's easier to work with than 52100 in terms of thermal needs (it's close to really good just with a heat and quench, and it responds to forge thermal cycling really well) - they look a lot alike on a micrograph, but something is different below the visible level.

The alloying in O1 allows me to get very high hardness with simple soy oil, but to push hardness on 52100, I need parks 50. Parks is stinky and I can never hide the smell from the mrs. I liked 52100 in plane irons even though I didn't "optimize the microstructure". Hopefully, the chisel experience is similar and not more like 1095 as I'm not ever going to send things out for commercial heat treat and understand that if you want some of the long optimization schedules, price goes up quickly.

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