Damascus steel
2008/9 Schools Wikipedia Selection. Related subjects: Engineering
Damascus steel is a hot- forged steel used in Middle Eastern swordmaking from about 1100 to 1700 AD. Damascus swords were of legendary sharpness and strength, and were apocryphally claimed to be able to cut through lesser quality European swords and even rock. The technique used to create original Damascus steel is now a matter of historical conjecture. Many raw materials, and the metalsmiths' recipes, are no longer available. The foundation for Damascus Steel is Wootz Steel, which originated in India and later spread to Persia. From the 3rd century to 17th century, India was shipping steel ingots to the Middle East for use in Damascus Steel.
The general term "Damascus" refers to metal with a visible grain pattern, sometimes with a texture. Modern damascus is a lamination of folded steels selected with cosmetic qualities, with grinding and polishing specifically to expose the layers. True Damascus patterns are formed when carbon trace elements form visible swirls in the steel mix, that change properties when work hardened (forged).
Before the advent of modern metal alloys cast and hot rolled to construction beam sizes, swordmakers of antiquity produced steel by the handful. Melting and casting a good alloy the size of a sword was difficult. Hollywood has described a fictional event where a crusader throws down his cast sword that shattered, for a damascene sword, taking home the folded hard and soft steels, changing European swordmaking forever. In actuality, folding/forging was well known. But this discovery of better metallurgy happened at the beginning of the age of alchemy, and so the legend of Damascus Steel was born.
Several other steelmaking techniques, such as wootz steel also result in patterned surfaces and have often been sold as Damascus steel Damascened steel and sometimes watered steel(Japan). The most common technique today for producing these materials is pattern welding, which is widely used for custom knife making. Modern damascus is usually two tool steels, one stainless and one carbon. One with higher nickel, the other appearing more grey so that alternating steels produce light-dark stripes. Treating or pickeling the steel after polishing enhances the pattern with a controlled tarnish that does not affect one of the steels. Folding and twisting while hammer forging controls the striped pattern and is often trademarked. Skilled swordsmiths can manipulate the layered patterns to mimic the complex designs found in the surface of the original, medieval Damascus steel. Some knife artists begin with stacking steel wires and through folding can produce repeating images along a blade, such as a crossed U.S. flags. Stacking wires is a specialization of the Cable Damascus technique, a new age development. The advent of steel wire rope (1830's) provided mid-west blacksmiths a way to make corn harvester's machetes (cable knives).
One explanation of the legendary properties of Damascus steel is that the pattern consists of alternating bands of very hard but brittle iron carbide or cementite and softer more flexible iron. Another possibility is that the steel contains a small amount of vanadium, which would theoretically strengthen the blade. The legendary steel may have been a happy accident by way of the limited production methods. Original Damascus steel billet was formed from a small disk that was hammer folded/forged into its final shape. Unlike northern European methods, the ferro-smelting technique in Persia during the Middle Ages involved small bowl-type crucibles with lids, baked in a mound-type oven often used for bread. Controlling the air contact to the melt, as well as trace elements found locally, all combined to produce a steel blade noticeably better than its contemporaries.
In late 2006, a group of scientists headed by Peter Paufler found direct evidence of nanotubes and nanowires in a sample of a 17th century sword forged from Damascus steel. The complex process of forging and annealing is thought to have accounted for the nano-scale structures.
The origins of the name Damascus remains somewhat controversial. Damascus steel was originally made using ore with a certain chemical composition from a mine that is now exhausted, so attempts at reproduction are difficult at best.
It would seem obvious that the name Damascus refers to swords forged in Damascus, but there are several other possible sources of the name. One is the name of the swordsmith himself: the author al-Beruni refers to swords made by a man he names Damashqi. Another author, al-Kindi, refers to swords made in Damascus as Damascene. This word has often been employed as an epithet in various Eastern European legends (Sabya Damaskinya or Sablja Dimiskija meaning "Damascene sword"), of which perhaps the best known are the Serbian legends of Prince Marko, a historical figure of the late 14th century in what is now the Republic of Macedonia.
Manufacture
The original Damascus steel swords may have been made in the vicinity of Damascus, Syria, in the period from 900AD to as late as 1750AD. Damascus steel is a type of steel alloy that is both hard and flexible, a combination that made it ideal for the building of swords. It is said that when Damascus-made swords were first encountered by Europeans during the Crusades, it garnered an almost mythical reputation—a Damascus steel blade was said to be able to cut a piece of silk in half as it fell to the ground, as well as being able to chop through normal blades, or even rock, without losing its sharp edge. Recent metallurgical experiments, based on microscopic studies of preserved Damascus-steel blades, have claimed to reproduce a very similar steel via possible reconstructions of the historical process.
When forming a batch of steel, impurities are added to control the properties of the resulting alloy. In general, notably during the era of Damascus steel, one could produce an alloy that was hard and brittle at one extreme by adding up to 2% carbon, or soft and malleable at the other, with about 0.5% carbon. The problem for a swordsmith is that the best steel should be both hard and malleable — hard, so as to hold an edge once sharpened, but malleable so it would not break when hitting other metal in combat. This was not possible with normal processes.
Metalsmiths in India and Sri Lanka perhaps as early as 300BC developed a new technique known as wootz steel that produced a high-carbon steel of unusually high purity. Glass was added to a mixture of iron and charcoal and then heated. The glass would act as a flux and bind to other impurities in the mixture, allowing them to rise to the surface and leave a more pure steel when the mixture cooled. Thousands of steel making sites were found in Samanalawewa area in Sri Lanka that made high carbon steel as early as 300BC. (Juleff, 1996). These steel making furnaces were built facing western monsoon winds and wind turbulence and suction was used to create heat in the furnace. Steel making sites in Sri Lanka have been dated to 300BC using carbon dating technology. The technique propagated very slowly through the world, reaching modern-day Turkmenistan and Uzbekistan around 900AD, and then the Middle East circa 1000AD.
This process was further refined in the Middle East using locally produced steels. The exact process remains unknown, but allowed carbides to precipitate out as micro particles arranged in sheets or bands within the body of a blade. The carbides are far harder than the surrounding low carbon steel, allowing the swordsmith to make an edge which would cut hard materials with the precipitated carbides, while the bands of softer steel allowed the sword as a whole to remain tough and flexible.
The banded carbide precipitates appear in the blade as a swirling pattern. By manipulating the ingot of steel in a certain way during forging, various intentional patterns could be induced in the steel. The most common of these was a pattern of lateral bands, often called 'Muhammad's Ladder', most likely formed by cutting or forging notches into the surface of the ingot, then forging it into the blade shape (this is the method Pendray (below) used to reproduce the pattern).
A team of researchers based at the Technical University of Dresden that uses x-rays and electron microscopy to examine Damascus steel discovered the presence of cementite nanowires and carbon nanotubes. Peter Paufler, a member of the Dresden team, says that these nanostructures give Damascus steel its distinctive properties and are a result of the forging process.
Loss of the technique
For reasons that are not entirely clear, but possibly because sources of ores containing trace amounts of tungsten and/or vanadium needed for its production were depleted, the process was lost to the middle-eastern metalsmiths circa 1750AD. It has been eagerly sought by many since that time.
It has long been argued that the raw material for Damascus steel swords was imported from India, because India was the only known centre of crucible-fired steels like wootz. However this conclusion became suspect when the furnaces in Turkmenistan were discovered, demonstrating at least that the technique was moving out from India. The wootz may have been manufactured locally in the Damascus area, but so far no remains of the distinctive wootz furnaces have appeared. The work of Verhoeven et al. supports the hypothesis that the wootz used was from India, as several key impurities that appear to give Damascus steel its properties point to particular ores available only in India.
The Russian bulat steel has many similar properties, at least in nature if not in process. Recently various groups have claimed to have recreated steel with properties consistent with true Damascus blades, through experimental archaeology, though even they admit they cannot be certain how it was originally created. Verhoeven et al. (1998) argued that the keys are ores with certain trace elements, controlled thermal cycling after the initial forging, and a grinding process to reveal the final damask pattern. A somewhat different technique was proposed by Wadsworth and Sherby (1980; also 2001).
The recent discovery of carbon nanotubes in the steel's composition has also brought to light a new hypothesis which might explain the loss of the technique. Carbon nanotubes (perhaps the strongest and stiffest material known), while occurring randomly in nature (simple campfires produce some nanotubes), require fairly high-tech, high-energy production methods to be made useful as structural materials. Therefore, ancient smiths, with the level of technology at their disposal, could hardly control the formation of these nanometer-scale carbon structures. Some element of random chance (forging, alloy composition, heat treatment, smelting process, environmental particularities, etc.) might have been responsible for the formation of these structures, which could not only explain some of their "legendary" qualities, but also the reason why, to this day, these properties have never been successfully emulated.
Attempts at reproduction
From the very start, the superior capabilities of Damascus swords attracted significant attention, and many attempts were made to reproduce either the performance or the appearance of the Damascus blades. Since pattern welding was a widespread technique, and produced surface patterns similar to those found on Damascus blades, many people believed that Damascus blades were made using a pattern welding technique. This belief was challenged in the 1990s when J. D. Verhoeven and A. H. Pendray published an article on their experiments on reproducing the elemental, structural, and visual characteristics of Damascus steel.
Verhoeven and Pendray started with a cake of steel that matched the properties of the original wootz steel from India, which also matched a number of original Damascus swords they had access to. The wootz was in a soft, annealed state, with a large grain structure, and many beads of pure iron carbide which were the result of the hypereutectoid state of the wootz. They had already determined that the grains on the surface of the steel were grains of iron carbide, so their question was how to reproduce the fine iron carbide patterns they saw in the Damascus blades from the large grains in the wootz.
Although such material could be worked at low temperatures to produce the striated Damascene pattern of intermixed ferrite and cementite bands (in a manner identical to pattern-welded Damascus steel), any heat treatment sufficient to dissolve the carbides would destroy the pattern permanently. However, Verhoeven and Pendray discovered that in samples of true Damascus steel the Damascene pattern could be recovered by aging at a moderate temperature. Their investigations found that certain carbide forming elements (chief of which was Vanadium), which in the wootz were concentrated in the carbide regions and were formed into a striated pattern during forging just as the iron carbide itself, did not disperse until higher temperatures than those needed to dissolve the carbides. Therefore, though a high heat treatment could remove the visible evidence of patterning associated with carbides it did not remove the underlying patterning of the carbide forming elements, and a subsequent lower temperature heat treatment (at a temperature at which the carbides were again stable) could recover the identical structure by the binding of carbon by those elements.
Pattern welded "Damascened" steel
It used to be believed that Damascus steel was made using pattern welding because the layering revealed by etching a pattern-welded blade in acid is similar to that of Damascus steel.
Pattern welded steel is commonly sold today as "Damascus steel", though it appears that the original Damascus steel was not created with that technique. Pattern welded Damascus is made out of several types of steel and iron slices, which are then welded together to form a billet. The patterns vary depending on what the smith does to the billet. The billet is drawn out and folded until the desired number of layers are formed. The end result, if done well, bears a strong resemblance to the surface appearance of a true Damascus blade, though the internal structure is completely dissimilar.