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I've been fascinated by pattern-welded (or Damascus) steel for a long time now. Modern pattern-welded steel refers to a technique where two dissimilar metals are forged together and then etched to bring out the difference between the steels. You can see my modest attempts at this technique on my "metalworking" page. Professional examples can be found here, here, here, here and here. Damascus steel swords can be seen here, here, here or here. A similar, but more challenging technique forge welds precious metals together: Mokume. Artefacts made from Damascus steel date back to before 500 AD. There is a second type of Damascus steel, so-called Wootz. It is not made by forge-welding, but instead the two dissimilar metals are liquified and then solidified in a closed crucible. The exact technique probably originated in India got lost some time in the 18th or 19th century. To this day, nobody can reproduce this exact type of steel.
The more interesting is an article in the journal Nature on an historic blade from the 17th century. This article details how the authors used transmission electron microscopy to detect carbon nanotubes and cementite nanowires in the ancient steel. They attribute some of the superior properties of the steel to these nano-structures. I have appended the full text of the article in the extended news post, as you need a subscription for it. However, neither the references nor the figures are reproduced. The figures can be found (along with an excellent article in German) in a related SPIEGEL article. NewScientist and the Telegraph both have articles on these findings.
Full article at Nature (requires subscription).
It is believed that Damascus blades were forged directly from small cakes of steel (named 'wootz') produced in ancient India. A sophisticated thermomechanical treatment of forging and annealing was applied to these cakes to refine the steel to its exceptional quality. However, European bladesmiths were unable to replicate the process, and its secret was lost at about the end of the eighteenth century. It was unclear how medieval blacksmiths would have overcome the inherent brittleness of the plates of cementite (Fe3C, a mineral known as cohenite) that form in steel with a carbon content of 1–2 wt%, as well as how the steel's characteristic banding could have arisen from these plates.
Mechanical processing at the appropriate temperature can cause the steel's microstructure to become fine-grained, and superplastic behaviour is induced at higher temperatures1. Small additions of the elements vanadium, chromium, manganese, cobalt, nickel and others are known to facilitate the formation of cementite bands during thermal cycling at temperatures below the cementite formation temperature2 (about 800 °C). Moreover, the Damascene steel contains rare-earth elements and shows evidence of cementite nanowires in its microstructure3, 4, 5.
Using high-resolution transmission electron microscopy, we have now also detected carbon nanotubes in a specimen taken from a genuine Damascus sabre (sabre no.10 (ref. 6); sample kindly provided by E. J. Kläy of Berne Historical Museum, Switzerland) produced by the famous blacksmith Assad Ullah in the seventeenth century. Its microstructure has been investigated previously4, but the nanotubes only become apparent (Fig. 1) after dissolution of the sample in hydrochloric acid (for methods, see supplementary information). Some remnants (Fig. 1c) show evidence of incompletely dissolved cementite nanowires3, indicating that these wires could have been encapsulated and protected by the carbon nanotubes7.
Nanotubes can be formed catalytically8, as well as from hydrocarbons inside micropores at reduced temperatures9. We suggest therefore that our finding could link the distinctive banding seen in Damascus blades with 'impurities' contained in the steel2, 4. By empirically optimizing their blade-treatment procedure, craftsmen ended up making nanotubes more than 400 years ago.
According to an early report on Indian wootz production10, particular ingredients were mandatory — such as wood from Cassia auriculata and leaves of Calotropis gigantea, and ores taken from particular mines in India. The diminishing supply of some of these ores during the eighteenth century may have prevented bladesmiths, who would not have been aware of the importance of these alloying elements, from practising their ancient recipes.
Thermal cycling and cyclic forging cause catalytic elements to segregate gradually into planar arrays parallel to the forging plane11. These elements may give rise to the growth of carbon nanotubes, which in turn initiate formation of cementite nanowires and coarse cementite particles. As the nanoscale structure of Damascus steel emerges, a refined interpretation of its remarkable mechanical properties should become possible.
It is believed that Damascus blades were forged directly from small cakes of steel (named 'wootz') produced in ancient India. A sophisticated thermomechanical treatment of forging and annealing was applied to these cakes to refine the steel to its exceptional quality. However, European bladesmiths were unable to replicate the process, and its secret was lost at about the end of the eighteenth century. It was unclear how medieval blacksmiths would have overcome the inherent brittleness of the plates of cementite (Fe3C, a mineral known as cohenite) that form in steel with a carbon content of 1–2 wt%, as well as how the steel's characteristic banding could have arisen from these plates.
Mechanical processing at the appropriate temperature can cause the steel's microstructure to become fine-grained, and superplastic behaviour is induced at higher temperatures1. Small additions of the elements vanadium, chromium, manganese, cobalt, nickel and others are known to facilitate the formation of cementite bands during thermal cycling at temperatures below the cementite formation temperature2 (about 800 °C). Moreover, the Damascene steel contains rare-earth elements and shows evidence of cementite nanowires in its microstructure3, 4, 5.
Using high-resolution transmission electron microscopy, we have now also detected carbon nanotubes in a specimen taken from a genuine Damascus sabre (sabre no.10 (ref. 6); sample kindly provided by E. J. Kläy of Berne Historical Museum, Switzerland) produced by the famous blacksmith Assad Ullah in the seventeenth century. Its microstructure has been investigated previously4, but the nanotubes only become apparent (Fig. 1) after dissolution of the sample in hydrochloric acid (for methods, see supplementary information). Some remnants (Fig. 1c) show evidence of incompletely dissolved cementite nanowires3, indicating that these wires could have been encapsulated and protected by the carbon nanotubes7.
Nanotubes can be formed catalytically8, as well as from hydrocarbons inside micropores at reduced temperatures9. We suggest therefore that our finding could link the distinctive banding seen in Damascus blades with 'impurities' contained in the steel2, 4. By empirically optimizing their blade-treatment procedure, craftsmen ended up making nanotubes more than 400 years ago.
According to an early report on Indian wootz production10, particular ingredients were mandatory — such as wood from Cassia auriculata and leaves of Calotropis gigantea, and ores taken from particular mines in India. The diminishing supply of some of these ores during the eighteenth century may have prevented bladesmiths, who would not have been aware of the importance of these alloying elements, from practising their ancient recipes.
Thermal cycling and cyclic forging cause catalytic elements to segregate gradually into planar arrays parallel to the forging plane11. These elements may give rise to the growth of carbon nanotubes, which in turn initiate formation of cementite nanowires and coarse cementite particles. As the nanoscale structure of Damascus steel emerges, a refined interpretation of its remarkable mechanical properties should become possible.
Posted on Thursday 16 November 2006 - 08:48:48 comment: 0
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