The Gupta period Delhi Iron Pillar near Kutub Minar in New Delhi, India is testimony to the high level of skill achieved by the ancient Indian iron smiths in the extraction and processing of iron. This Pillar has attracted the attention of corrosion technologists because it has withstood corrosion for the last 1600 years.

The presence of relatively high phosphorus in the range of 0.25% in the forge welded Delhi Iron Pillar plays a major role in its excellent corrosion resistance. The presence of phosphorus leads to the formation of a protective passive film on the surface, which provides the Pillar its exceptional corrosion resistance properties. However, in modern steel making process, the phosphorus content is controlled to 0.05% because phosphorus segregation to grain boundaries reduces ductility of steel.

A detailed study was undertaken by a PhD student, Gadadhar Sahoo, under the guidance of Professor R Balasubramaniam of the Materials and Metallurgical Engineering Department of Institute of Technology of Kanpur India to understand possible industrial applications of phosphoric irons. The first aim was to render the phosphoric irons ductile and the second aim was to locate a modern application wherein the corrosion resistance of phosphoric irons could be put to good use.

Ductility is a very important property from the point of practical application of any engineering material. It is known that phosphorus segregation to the grain boundaries makes these locations weak and results in poor ductility.

Phosphorus will not be present in regions where carbon is located in the iron matrix because phosphorus is a substitutional solute element whereas carbon is an interstitial solute element, which is achieved by maintaining a small amount of carbon in phosphoric irons. Intelligent use of the phase transformations in the iron phosphorus system was used to locate these carbon atoms along the grain boundaries to keep the phosphorus atoms away from these locations.

This is well known in physical metallurgy. Austenite has a higher solubility for carbon than phosphorus and therefore all the carbon is pushed to the grain boundary region while the phosphorus is removed from the grain boundary region. After a suitable soaking time at high temperature, the phosphoric irons can be air cooled to room temperature. The beneficial aspect of this treatment is that phosphorus, which was removed from the grain boundary regions, does not go back to these regions during air cooling because phosphorus requires time to diffuse to the grain boundary regions. In this manner, a high temperature soaking in the two phase region and subsequent air cooling should result in good ductility for phosphoric irons.

Rebars used for reinforcement of concrete in structural applications need to possess the necessary strength, ductility and corrosion resistance and the prevalent technology uses thermo mechanical treatment to impart these properties.

Phosphoric irons can be processed by a similar method utilizing the existing arrangements, with the major difference being the ingot soaking, bar quenching and further cooling arrangements have to be fine tuned to produce phosphoric iron with a tough surface and a strong interior. This can be achieved by suitable design criteria.

The idea for developing phosphoric irons originated from the study of the Metallurgical Wonder of India – the Delhi Iron Pillar and now we believe in the old adage that “The best of the new is often the long forgotten past.”

The commercial trials for making phosphorus iron rebars are yet to take place but the patent for the idea and technology has been applied in most of the important countries.