India has been reputed for its iron and steel since ancient times. Literary accounts indicate that steel from southern India was rated as some of the finest in the world and was traded over ancient Europe, China, the Arab world and the Middle East. Studies on wootz indicate that it was an ultra-high carbon steel with 1-2% carbon and was believed to have been used to fashion the Damascus blades with a watered steel pattern. Wootz steel also spurred developments in modern metallographic studies and also qualifies as an advanced material in modern terminology since such steels are shown to exhibit super-plastic properties. This paper reviews some of these developments.
There are numerous early literary references to steel from India from Mediterranean sources including one from the time of Alexander (3rd c. BC) who was said to have been presented with 100 talents of Indian steel. The archaeological evidence from the region of Tamil Nadu suggests that the Indian crucible steel process is likely to have started before the Christian era from that region. The process involved preparation of porous iron, hammering it while hot to release slag, breaking it up and sealing it with wood chips in a clay container, and heating it until the pieces of iron absorbed carbon from the wood and melted. The steel thus produced had a uniform composition of 1–1.6% carbon and could be heated and forged into bars for later use in fashioning articles, such as the famous medieval Damascus swords.
Superplasticity is a phenomenon whereby an elongation of several hundred percent can be observed in certain alloys in tension, with neck free elongations and without fracture. By contrast most crystalline materials can be stretched to no more than 50-100 per cent. Superplasticity occurs at high temperatures and superplastic materials can be formed into complex shapes.
Super plastic material essentially comprises of a two-phase material of spherical grains of extremely fine grain size of not more than 5 microns at the working temperature. Such ultrafine grained materials exhibit grain boundary sliding yielding superplastic properties. The explanation of the superplasticity of the steel is that the typical microstructure of ultra-high carbon steel with the coarse network of proeutectoid cementite forming along the grain boundaries of prior, can lead to a fine uniform distribution of spheroidised cementite particles (0.1 μ m diam.) in a fine grained ferrite matrix. Such steels are also found to have strength, hardness and wear resistance. Recent studies have indicated that ultra-high carbon steels exhibit superplastic properties. As pointed out by Wadsworth and Sherby, by 1975 Stanford University had found that steels with 1-2.1% C i.e. ultrahigh carbon steels could be both superplastic at warm temperatures and strong and ductile at room temperatures. These steels were in fact similar in carbon content to the Damascus steels.
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