Crucible steel

What is steel?

Key to the formation of any steel is the conversion of carbon dioxide, leaving elementary iron with a high concentration of carbon.

One common source of carbon is wrought iron.

South Asian steel

The first form of crucible steel was wootz, developed in India some time around 300 AD. In its production the iron was mixed with glass and then slowly heated and then cooled. As the mixture cooled the glass would bond to impurities in the steel and then float to the surface, leaving the steel considerably more pure. Carbon could enter the iron by diffusing in through the porous walls of the crucibles. Carbon dioxide would not react with the iron, but the small amounts of carbon monoxide could, adding carbon to the mix with some level of control. Wootz was widely exported throughout the Middle East, where it was combined with a local production technique around 1000 AD to produce Damascus steel, famed throughout the world.

As early as the 17th century, Europeans knew of India's ability to make crucible steel from reports brought back by travelers who had observed the process at several places in southern India. Several attempts were made to import the process, but failed because the exact technique remained a mystery. Studies of wootz were made in an attempt to understand its secrets, including a major effort by the famous scientist, silica from the glass that gave wootz its unique properties.

Blister steel

Main article: cementation process

Nevertheless it was possible to produce quality steel in Europe, by importing the highly valued Swedish iron. Although it was not understood at the time, the Swedish ore contained very low levels of common impurities, leading to higher quality irons and steels from otherwise identical techniques applied to other ores. Swedish cementation process.

English crucible steel

A new technique was developed in England by Benjamin Huntsman, a clockmaker in search of a better steel for clock springs. It was only in 1740 after he moved to Handsworth near Sheffield, and after years of experimenting in secret he perfected his process. Huntsman's system used a blister steel broken into lumps of about ½ kg, and a flux to help remove impurities. The pots are removed after about 3 hours in the furnace, impurities skimmed off, and the molten steel poured into ingots. Sheffield's Abbeydale Industrial Hamlet has preserved a water-wheel powered, scythe-making works dating from Huntsman's times, which is still operated for the public, several times per year using crucible steel made on the Abbeydale site.

Before the introduction of Huntsman's technique, Sheffield produced about 200 tonnes of steel per year based on Swedish ore. The introduction of Huntsman's technique changed this radically; one hundred years later the amount had risen to over 80,000 tonnes per year - almost half of Europe's total production. This discovery enabled Sheffield to develop from a small township into one of Europe's leading industrial cities.

Crucible steel elsewhere

Another form of crucible steel was developed in 1837 by the Russian engineer, Pavel Anosov. His technique relied less on the heating and cooling, and more on the quenching process of rapidly cooling the molten steel when the right crystal structure had formed within. He called his steel bulat; its secret died with him. In the United States crucible steel was pioneered by William Metcalf.

Conclusion

Crucible steels remained the world's best, although very expensive, for some time. The introduction of the Bessemer process replaced it in large scale production however, able to produce steel of similar (or better) quality for a fraction of the time and cost. The Bessemer process and more modern methods instead remove carbon from the pig iron, stopping before all the carbon is removed, something that was not managed at earlier periods.

Crucible Steel didn't disappear when the Bessemer process was introduced. It is still the preferred method for specialty steel production. Especially tool steel. While more expensive them Bessemer and other later methods of steel production it is still among the most precise. So rather then disappearing it became a niche market.

See also

• Delhi Iron Pillar