THE DELHI IRON PILLAR- THE CROWN JEWEL OF ANCIENT INDIAN METALLURGY.

Every new visitor to New Delhi makes a bee line to the famous Qutub Minar, the red and buff sand stone structure built in the 12th Century by the Delhi king Qutbud-Din Aiback and his successors. Adjacent to the Minar is the Quwwat-Ul-Islam mosque built by him in 1198 AD using carved columns and architectural members of 27 Hindu and Jain temples he plundered and destroyed. On the court-yard of this mosque stands the Delhi Iron pillar, a massive iron column erected there in 1233 AD. In spite of standing there for the past 800 years or more, the iron pillar shows no sign of rusting. It is 7.3 meters tall with about one meter below the ground. Its diameter at the bottom is 48 centimetres and 29 centimetres at the top. There is a decorative bell at the top. It weighs around 6.5 tons. This rust less wonder is a marvellous example of the metallurgical knowledge and workmanship of ancient India. History The Delhi pillar bears a Sanskrit-Brahmi inscription from which it was learnt that the pillar was originally erected as a flagstaff for the Hindu God Vishnu by King Chandragupta II (375-413 AD) at Vishnupadagiri, which is the present Udayagiri, a place 50 km east of Bhopal. Iltutmish, the Delhi Sultan invaded the Malva region in 1233 AD, and shifted the pillar to its present location. It is thus seen that this iron pillar is about 1600 years old. Metallurgical Investigations Because of its rust less nature, many scientists studied this pillar. The first metallurgical study was done by the British metallurgist Hadfield in 1912. National Metallurgical Laboratory, Jamshedpur evaluated it in 1961. Prof. T. R. Anantharaman published a study in 1996. The latest and most exhaustive study was published by Prof. R. Balasubramaniam of Kanpur IIT in 2002. Chemical composition In ancient India Iron was produced in puddle furnaces where high quality iron ore reacts with charcoal and the iron lumps formed is taken out and hammered to remove the slag particles. In making the huge iron pillar many such lumps were hot –hammered. Invariably, the chemical composition will be non-uniform which has been revealed by the several chemical analyses carried out on the Delhi iron pillar. The average chemical composition is carbon-0.15 wt.%, phosphorus- 0.25 wt.%, silicon-0.05 wt.%,copper & nickel- 0.03 wt.%, sulphur, nitrogen and manganese- traces. The important point to note is the high phosphorus content which must have come from the method of iron making. Microstructural studies The microstructure of the Delhi iron pillar showed non-uniform distribution of slag inclusions, varying grain sizes, extensive deformation markings in the (ferrite) grains, and an irregular distribution of pearlite (a constituent containing carbon & iron). The slag particles were of microscopic size and contained essentially fayalite (Fe2SiO4 ), iron oxide particles and some carbon particles. Manufacturing technology The individual iron lumps were forge-welded to produce the huge body of the pillar. The diameter of the pillar was increased by side-wise hammering of lumps on the pillar which was kept in a horizontal position and rotated using clamps. Forge-welding technology using hand-held hammers was perfected to yield the massive iron pillar with very good mechanical properties. A massive decorative bell capital which consisted of seven distinct parts was then joined to the main body of the pillar using central inserts. At the top of the bell capital a hollow slot is seen in which a wheel may have been present originally. Corrosion resistance--To continue