Hi Manolo,
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Originally Posted by Manolo
..... Would such blades exhibit a high austenite content?
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Thank you for your appreciative words. I think that both Ann and Greg,as well as the others deserve a special thanks for their extermely valuable contribution. I certainly learned a lot about Wootz from this thread.
Now for your question: It is hard to explain all the ins and outs of the heat treatment of steel within the constrains of a thread like this, but I'll try, albeit at the risk of oversimplification.
Carbon steel at room temperature consists of a mixture of near pure fairly malleable iron, known as Ferrite, and very hard iron carbides, known as Cementite. In this state, steel is moderately soft and can be bent and worked fairly easily.
Hardening by Quenching and tempering:
Steel is heated to a temperature at which its crystal structure changes and becomes as soft and malleable as lead. This structure is called Austenite and it can dissolve all of the carbides that we mentioned above, forming a solid solution of carbon in iron. Here I should mention that solids can dissolve in other solids, not just liquids - Hard to believe, but the process is complex and you'll have to take my word for it.
When heated Austenitic steel is rapidly cooled, as when quenched in water, the carbon cannot come out of solid solution, as it would under slower cooling, and the Austenitic crystal structure changes to one that is very resistant to deformation, on account of the carbon atoms trapped in it.
This new crystal structure is called Martensite and it is very hard and brittle.To render it usable, it is usually tempered by reheating, so as to allow some of the carbon atoms to come out of solid solution and thus reduce both its extreme hardness and brittleness. The carbon that is thus removed from the Martensite forms tiny spheroids of Cementite and results in a structure sometimes known as Sorbite, but more commonly called tempered Martensite. If Martensitic steel is tempered at high temperatures for a very long time it reverts to its original unhardened structure of Ferrite plus Cementite.
OK - Those are the raw basics. Now for the problems.
Plain carbon steel (without additional alloying elements) with less than about 0.4%C cannot be cooled fast enough to transform the Austenite into Martensite, but between 0.4%C and 0.8%C there are no great problems.
However, once the carbon content exceeds 0.8%C, known as the eutectoid composition, then upon quenching the tendency of the Austenite is to stay as it is and not transform into Martensite - Contrary to its usual high temperature `habitat', this Austenite remains as such at room temperature and is known by metallurgists as `retained Austenite', that is retained after the quench.
If we quench hypereutectoid steel (>0.8%C) not all of the steel remains as Austenite. In practice, depending on by how much the 0.8%C is exceeded, we tend to get a mixture of Martensite (hard and brittle) together with retained Austenite. Now remember that as I said at the start, Austenite is very soft and malleable. At the risk of gross oversimplification, now you should think of the retained Austenite as if it wasn't there, because it is so weak. The net result is a Martensitic sword blade with what amounts to all intents and purposes as `strength gaps' all over and inside it. Really disastrous for strength. Of course, the real picture is more complex, but at this level we need not overly concern ourselves with metallurgical minutiae.
In the heat treatment of modern high carbon steels there are strategies to minimize the problem posed by retained Austenite; For example with cutlery high carbon stainless steels such as 440C, any retained Austenite is converted into Martensite by cooling to very low temperatures.
However, the ancients, not understanding what went on inside the steel, would have had only two options (that I can think of):
a) Stick with hypoeutectoid steels (0.4%C-0.8%C), not easy to do as they could not analyze for carbon, nor knew about its critical role; And
b) if having to heat treat higher carbon content steels, they would have had to be extremely careful to quench from the lowest possible temperature at which Austenite forms. This so as to minimize the dissolution of the segregated carbides back into the Austenite and thus raising its carbon content, which would lead to retained Austenite. I imagine that by trial and error this temperature could be judged by the colour of the hot steel, but my guess is that they would have turned out a lot of bad blades.
I hope that this helps. If you would like to obtain more information see the entries in Wikipedia, as it gives a fairly good account.
I also hope also that you can see why the manufacture of a well hardened sword was more often than not a stroke of luck and why such swords had such exalted and legendary status.
Cheers
Chris