Join Date: Jul 2020
Research On A Mid 1st millennium BC High Carbon Sword, South India
An iron sword from an Iron Age megalithic burial at Thelunganur in Tamil Nadu, India, was examined using metallographic techniques. The sword was made of ultra‐high‐carbon steel with a fairly uniform microstructure consisting primarily of fine cementite particles in a ferrite background free of notable non‐metallic inclusions. The morphological control, however, was not perfect and frequently allowed cementite to precipitate in the form of a network along austenite grain boundaries. It was also observed that carbide particles of varying size and shape often caused microscopic layers to develop, forming a visible pattern to the naked eye on the polished and etched surface of the iron sword. This pattern likely inspired the later development of various surface markings such as the damask. This paper presents a detailed account of the analytical data to show that the iron sword under consideration was an early example of high‐carbon steel employed in the manufacture of a functional object where the divorced eutectoid transformation technique, rediscovered recently, was used for the control of cementite morphology. It is also proposed that technologies for making and handling high‐carbon steel were in existence at a much earlier date than previously supposed.
Carbon concentration and its distribution pattern constitute two key factors that determine the functional properties of an iron sword, which can be optimized by taking the balance between strength and ductility. The selection of raw materials therefore plays an important role in the establishment of a particular sword‐making technology. If high‐carbon steel were used to take advantage of its high strength, it would be necessary to improve the low‐impact resistance arising from its high carbon concentration.
India was famed for the early production of a special high‐carbon material termed crucible steel. In his survey research on traditional Indian crucible steel, wootz, Bronson (1986) concluded that it was produced by a wide range of processes, not only just as super steel but also as a material for common use. Despite its fame gained as the raw material for Damascus blades, Bronson noted that crucible steel was not a material of choice among warriors, primarily due to its brittleness. This brittleness arises from its high carbon concentration, which is generally > 1%. (Chemical compositions in this paper are based on weight fraction.) Such high‐carbon steel contains a substantial amount of iron carbide (Fe3C), termed cementite, which is brittle even at temperatures up to its melting point. This brittleness poses serious difficulties in fabrication or service and cannot be overcome without a high level of technological sophistication.
Raw materials similar in carbon levels to Indian crucible steel were produced elsewhere in the world using a variety of techniques, most of which involved the use of cast iron in one way or another. Such high‐carbon materials were then employed as intermediaries to be made into finished items with processes that caused a substantial reduction of their carbon level (Park 2004, 2005, 2008). Evidence was found, however, that the technology for processing high‐carbon steel in India was rather unique and focused on controlling the shape, size and distribution of the cementite phase (Park and Shinde 2013b).
In a certain sense, therefore, the discovery of proper techniques for handling such high‐carbon materials signifies the beginning of crucible or other high‐carbon technologies, and Indian ironworkers should thus also be recognized for their in vention of such processing methods.
Bronson (1986) traced the beginning of Indian crucible technology to the second century ce , at the earliest, by refuting the prevailing hypotheses of earlier dates based on certain classical sources of the 5th‐century bce onwards. He also rejected Hadfield's interpretation that some of the iron objects excavated from the Sirkap site at Taxila, Pakistan, were made of crucible steel (Marshall 1945). In a subsequent review article drawing on an extended body of literary and archaeological evidence, Craddock (1998) reinforced most of Bronson's conclusions. Craddock, however, regarded the Taxila iron objects examined by Hadfield as an example of crucible steel, and suggested that crucible processes were practised by the first centuries of the first millennium ce . On the other hand, recent studies (Srinivasan 2007; Srinivasan et al . 2009), based on the investigation of crucible fragments and metal particles recovered from ancient sites in Tamil Nadu (Rajan 1997), proposed that the crucible process was likely practised as early as the mid‐first millennium bce .
Despite substantial debate over the origin and production of Indian crucible steel, little is known of thermomechanical treatments developed for the fabrication of such high‐carbon materials into finished products. Much work (Verhoeven and Peterson 1992; Verhoeven et al . 1996) has been done on Damascus swords to understand the mechanisms responsible for the emergence of their unique surface patterns.
However, the information from such research cannot be used to infer technologies practised in Indian antiquity. In this respect, the recent work of Park and Shinde (2013b) is significant as it reported on a small piece of unprocessed high‐carbon steel and small finished items evidently made from such a high‐carbon material. In addition, they produced radiocarbon data placing the date of manufacture to the last few centuries of the first millennium bce . The objects these authors examined, however, were ordinary items weighing approximately 10 g, or less. Their data, therefore, cannot be extended to address the technological and chronological aspects associated with the use of high‐carbon steel in the manufacture of important functional items, such as swords.
Similarly, the analysis of a Sasanian sword dating to the sixth to seventh centuries ce (Lang et al . 1998), one of the earliest examples of such products thus far examined, is also far from sufficient.
A unique opportunity to resolve this lack of information is provided by an iron sword (Fig. 1) recently recovered from an Iron Age megalithic burial site at Thelunganur in the Salem district of Tamil Nadu, India (Fig. 2). The site can be dated to the sixth century bce or earlier based on typological grounds (Rajan 2004; Rajan and Yatheeskumar 2012), indicating that the sword could be an early archaeological example of a high‐carbon material successfully processed into a finished product. The results presented below are expected to shed a new light upon the discussion of Indian high‐carbon steel in terms of its technology and chronology.
SUMMARY AND CONCLUSIONS
The metallographic examination of a double‐edged iron sword recovered from a ritual context of an Iron Age megalithic site at Thelunganur in Tamil Nadu, India, showed that it was forged out of high‐carbon steel with a carbon content of 0.9–1.3%. The specimens examined were clean and free of any notable non‐metallic inclusions. Their microstructures were fairly uniform and consisted primarily of fine spherical cementite particles in the ferrite background. The high carbon content, absence of slag inclusions and uniform microstructure—rarely obtained in bloomery smelting—are features characteristic of crucible steel produced from a fully molten state. In addition, the formation of ferrite bands in one specimen given the proper thermal treatment confirms that the material examined was the product of a solidification reaction. It may therefore be suggested that the blade under investigation could possibly be one of the earliest examples of a crucible steel product.
More importantly, the carbide phase precipitated in the form of fine spherical particles provides clear evidence of special thermomechanical treatments applied in the processing of such a brittle material for fabrication. The treatments likely involved the divorced eutectoid transformation technique, which was rediscovered in the 20th century ce for the effective exploitation of high‐carbon steel.
The shape and size of cementite were not perfectly uniform, which in some cases caused a laminated structure to develop in the microstructure, creating colorful patterns visible on the polished and etched surface. This pattern, once noticed, may have inspired the later development of varying patterns leading to the famous damask.
The above results show that technologies needed to facilitate both the production and the processing of crucible or other high‐carbon steel were largely established in the Thelunganur region of Tamil Nadu by the time the sword was made, approximately around the mid‐first millennium bce . The application of such technologies, however, was apparently not always perfect and might often end up with a tragic and premature failure of crucible steel products.
Only put the intro and conclusion of the link here. But its very well researched article.