ismoketoomuch wrote...
"...A genuine, classic katana... has a hard edge, which will chip, if carelessly used, in particular against hard things.... The tempering processes which made it possible to completely harden steel, throughout, and yet make it tough, springy and almost unbreakable, were discovered in Europe. The japanese never did."
That doesn't make sense to me. Tempering by definition is a re-heating process which reduces the hardness and brittleness of the work-piece. The purpose of tempering is to acheive the kind of balance between hardness flexibility and strength which you seem to be talking about. There is no logical reason that Europeans would be able to achieve that balance and the Japanese not.
Well, they didn't. Where to start, where to stop?...
“Tempering” is often used incorrectly, as by that guy in one of the essays Lotion Soronnar linked to (and it wasn't the only detail that was somewhat wrong, but the gist of that essay was still nice).
You, however, have the right idea, but no idea of how difficult or complex it is. Simple tempering, as in just destroying hardening or partially destroying hardening, is quite simple, provided you're a member of a secret society. But the Japanese didn't do that either. Tempering in order to create what I'd like to call 'modern hardened steel' is something entirely different. You then also need to be a magician and a member of an even more secret society. And that type of hardened steel was exclusive to Europe until the advent of industrialization.
I've done quite a lot of this kind of work. But I always used modern equipment. And I've always also used modern steel. Nothing essentially wrong with that, since rough correspondents can be had. But it's still a great deal of cheating, because I know exactly the composition and carbon content of the steel. And I have exact tables for heat treating, and ovens which keep exact temperatures. (The *real* Japanese sword smiths still use traditional kiln-made steel. Their awesome craft is totally 100% genuine, even today.
I do not know how they did it in Solingen back in 1400 and later. Many years ago I tried to find out, but as far as I came then, it simply didn't seem to be known. The exact methods forgotten. One problem is that much of this is researched and written by historians and people mainly qualified as blacksmiths, and they do not seem to have the knowledge to understand what is required. Maybe, if I searched for information today, I would find information that makes sense.
Anyway, I'll try to explain anyway, from my own knowledge and experience. If I were to try to make this kind of durable and tough hardened steel, with medieval equipment, my first try, experiment, to do this, would be as follows:
I'd have a heated container with water, and a heated container with oil. I'd bring the water to boiling, then bring it back to just under. I'd try to keep the oil at constant high temperature by judging the amount of smoke. The smoking oil would need to be kept distant from the charcoal forge, preferably outside, or it'll burst into flames. And I'd have tools to transfer the blade in a careful manner, avoiding any knocks or taps.
Then I'd heat the blade vigorously, all the way to bright red and slightly beyond, maybe a bit orange red. And then I'd keep it there, soak it, for like 20-25 minutes at that temperature (for a first attempt). Then I'd transfer it to the hot water and rapidly and completely quench it there in one go, all the way until it stops generating steam by itself. Then I'd lift it out, let it steam off, a moment, until it's dry, then immediately submerge it in the hot oil. I'd let it rest in the oil for about two hours, keeping the oil constantly hot. Then I'd take it out and let it cool slowly. Then I'd put it into the hot oil again for another two hours. Cool slowly again, and the blade is ready to evaluate. If the results are not good, I'd try tweaking the process.
And that's just a theory. It could work though. It depends on the steel. I repeat, I have no idea how they actually did it in medieval times. But what I just described should have most ancient smiths (and a few modern blacksmiths) screaming “madman” at me.
So what's behind this? Heat treating steel is the art of manipulating the crystal structures of steel. Not to make this too complicated, we'll stick to ferrite, pearlite, austenite, martensite, cementite, and bainite.
Ancient steel smiths all over the world made roughly the same discoveries and employed roughly the same methods to control the quality of their steel. They would, for instance, manage the carbon content of their block of steel (from the kiln) by repeatedly hammering and folding the steel. By this operation they 'stir' the steel into homogeneity, gets rid of impurities, and reduce the carbon content (because the carbon will burn off faster than the steel). I have no idea how they judged the carbon content, but they did. Even the early Viking smiths nailed it pretty good.
If we take a piece of this steel with a carbon content of about 0,7-0,9 %, it will mostly consist of ferrite and pearlite. Ferrite cannot dissolve more than a very minute quantity of carbon, so most of the carbon is outside the crystals. If we heat it though, the steel will gradually change to austenite. Austenite is not a stable crystal form at lower temperatures. It will however dissolve a higher amount of carbon. Suppose we now rapidly cool the steel again? So fast that the carbon does not escape and ferrite cannot be formed? And austenite is not stable? Martensite is formed instead, that is what happens. This also results in a very jumbled structure with the crystals sort of interlocking. So the steel is no longer malleable. It has become rigid, hard.
This is what all cultures discovered. Then they also discovered all the problems. This steel is ridiculously brittle. If the steel is heated for long, and then rapidly cooled, it will become so brittle it can literally fall apart into pieces just as you're looking at it. All cultures handled this problem exactly as a modern blacksmith does. They would hold back the hardening process. By not heating for so long, by not quenching so so completely and hard. This reduces the amount of martensite being formed. They would make partially hardened steel. They also did something else. They made steel with lower carbon content. This would also form less martensite, and would still do so subjected to the exact same hardening as the higher content steel.
From this they progressed to make blades of composite steel. You combine low-carbon and high-carbon steel in a way that doesn't completely mix the steels. The Japanese did it the most difficult way, folding the combined steel a certain number of times (probably 12, but I don't know) to create a laminate of thousands of flat sheets. This demands extreme skill from the smith and his helpers. Each fold must be made cleanly and very rapidly, or the laminates will not adhere strongly enough.
Others, the Vikings and Europeans, and India and Arabs, braided and twisted their steels. This is more foolproof to do with good results, but does not result in quite the same directional and edge qualities as the Japanese method. Probably for that reason, the Europeans at some time started to forge separate harder edges and weld together with the blade. At some time the Japanese started to do the same, resulting in an even better cutting blade. The Japanese would also harden their edge harder. They accomplished this by holding back the quenching, for the rest of the blade, with an insulating layer of clay. This clay would be thinner for the edge, resulting in more rapid cooling. They would also make it thin in small portions, sort of forming small teeth of harder steel imbedded in somewhat milder steel. This made the edge somewhat more durable.
These composite blades did a lot to achieve a compromise of stiffness, flexibility and resistance to breaking. But it's ultimately a combination of not so good or strong steels. The hard steel is hard enough, but not strong, and only being intertwined with the mild steel makes it resistant to breaking. And steel hard enough for edges and points is still brittle, prone to breaking and chipping.
This is because the sufficiently hard steel has excess martensite, making it brittle. And it may also have retained austenite. Retained austenite makes the steel weaker. Makes it less strong, as in bending, wearing and breaking more easily.
So what was discovered 1400 in a neighbouring town to Solingen (don't remember the name, Solingen is what became famous), and possibly co-discovered somewhere in Italy around the same time, is that storing the hardened steel at moderately high temperature for a good while, made it much better. So much better that you could go completely crazy with the hardening, and still end up with a very durable steel. In fact you must go crazy, all in, with the hardening, or the steel won't be as strong. These temperatures and times are very sensitive for pure carbon steels, making the actual process quite difficult. Modern steels for hardening are alloyed to make them much more forgiving and having greater tolerances in the heat treating.
But they did it. So what happens? The slow tempering at moderate temperature reduces the excess martensite, forming cementite, and reduces the retained austenite, forming pearlite. The resulting steel is extremely strong, and both hard and tough at the same time.
The toughest steels are made by also forming bainite, but this they couldn't do in medieval times so we'll leave that.
Modifié par bEVEsthda, 21 octobre 2013 - 09:24 .