Hi,
Damascene swords are made by hammering a blade to twice its length, covering it with carbon dust, and folding it back onto itself, and hammering it again to weld the two sheets together. This means the soft (0.5% carbon) iron cores are interleaved with hard (5.0% carbon) case-hardened layers.
The process is repeated, so 2 folds give 4 layers, 6 folds give 64 layers etc, so a Samurai sword with 10 folds gave 1024 layers, the equivalent of a 2048-layer sandwich of hard and flexible steels !
The carbon is present in iron carbide forms, Austenite, Martensite, Perlite etc, whose proportions and molecular sizes depend on the temperature attained and quenching.
Cast iron is totally fragile, so all iron used has been wrought, whether through the ancient 'direct' process' or post 15th C
'indirect process'.
Sorry, but the above is not accurate. Firstly, the metallurgy of iron and its carbon alloys. Carbon is virtually insoluble (about 0.02%) in Ferrite, the form in which iron exists at room temperature. Any carbon above that 0.02% will form Cementite (Iron Carbide) which will form in layers alternating with Ferrite, this is known as Pearlite. At higher temperatures, the crystal structure of iron changes to the form known as Austenite and carbon can dissolve in it more readily; the temperature at which this happens starts at 911 degrees C for pure iron, down to 723 degrees C for 0.8% carbon and above. The quenching process locks the iron into its high-temperature form, and in this state it is known as Martensite - extremely hard but also extremely brittle. By reheating in the tempering process, enough of the Martensite is allowed to change back to the Ferrite crystal structure to make it more resilient but retain enough hardness to be useful.
The range of carbon content that will give you a usable steel for cutting edges is about 0.4-0.5% up to 1-1.2%; below that and it's too soft for a good edge, above that and it's too brittle (I'm not counting modern alloy steels here where other metals have been introduced into the mix, some of those can have higher carbon contents). Wrought iron for general smithing work usually contains less that 0.1% carbon
Beyond about 2% carbon, you're into the realms of cast iron - at a little over 4%, you get the lowest possible melting point for iron (~1150 degrees C) hence the preferred composition for casting as you don't need to put as much heat in, but the resulting iron is brittle. From the 17th century onwards, wrought iron was increasingly made by the puddling process, which involves blowing air through molten cast iron while moving it around with iron rods; the carbon gets burnt out and the largely carbon-free iron accumulates on the rod until there's enough to remove and forge out into bar form. There will be residual strings of slag left in the finished iron, hence the corrosion effects I've referred to earlier in this thread.
Turning to the process of making a laminated steel, commonly but incorrectly called Damascus, this historically tended to be done with lower carbon content irons, the good stuff above 0.5% carbon being put aside for cutting edges. How this higher grade steel was made varies from place to place and through time; some might be created naturally during the bloomery smelting process (as is the case in traditional Japanese smelting) or manufactured by heating bars of wrought iron surrounded by powdered charcoal in sealed containers for several hours. You can create a very thin layer of steel on hot iron by sprinkling powdered carbon on it (case hardening) but not one that will survive much grinding and sharpening.
The lamination is usually done in one of two ways. The Japanese technique involves making a piled block of low and medium-carbon steel, forge-welding it together, then almost cutting it in half, folding it over and forge-welding it back to itself, and repeating several times. Only when the swordsmith feels there are enough layers does he forge the block out to sword length, then weld the cutting edge of good steel on. The technique that evolved in Northern Europe, known as pattern-welding, involved making up rods of between 3 and 9 layers of irons of differing composition, forge-welding the together, then twisting them. The twisted rods would be forged back to square and welded together to form the sword core, then the cutting edge(s) of good steel welded on.
True Damascus steel as used in the near and middle east for swordmaking, as I’ve mentioned earlier is a different beast. It’s a high-carbon (1 to 1.2%) steel called Wootz that was made in parts of India by remelting bloomery iron in sealed crucibles with charcoal, but crucially there were minute traces of some other metals like vanadium in the original iron that caused some interesting crystallisation effects. The swordsmiths of Damascus and other blademaking centres in the Muslim world discovered that this steel, when carefully forged, could produce subtle “watered silk” patterns when polished and etched, as well as taking a seriously sharp edge! There is also some evidence to suggest that there was a European technique of producing high-quality steel by oxidising molten cast iron, but it seems to have been a closely guarded secret that died out in the middle ages, and cast steel did not reappear until Huntsman perfected the crucible steel process in the late 18th century.
All of this is going way off-topic, however….
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