At very high temperatures, iron begins to absorb carbon, which lowers the melting point of the metal, resulting in cast iron (2.5 to 4.5% carbon). The development of blast furnaces, first used by the Chinese in the 6th century BC but more widely used in Europe during the Middle Ages, increased the production of cast iron.
Molten iron run out of the blast furnaces and cooled in a main channel and adjoining moulds became referred to as pig iron because the large, central and adjoining smaller ingots resembled a sow and suckling piglets.
Cast iron is strong, but suffers from brittleness due to its carbon content, making it less than ideal for working and shaping. As metallurgists became aware that the high carbon content in iron was central to the problem of brittleness, they experimented with new methods for reducing the carbon content in order to make iron more workable.
By the late 18th century, ironmakers learned how to transform cast pig iron into a low-carbon content wrought iron using puddling furnaces (developed by Henry Cort in 1784). The furnaces heated molten iron, which had to be stirred by puddlers using a long oar-shaped tools, allowing oxygen to combine with and slowly remove carbon.
As the carbon content decreases, iron's melting point increases, so masses of iron would aggolerate in the furnace. These masses would be removed and worked with a forge hammer by the puddler before being rolled into sheets or rails. By 1860, there were over 3000 puddling furnaces in Britain, but the process remained hindered by its labor and fuel intensiveness.
One of the earliest forms of steel, blister steel, began production in Germany and England in the 17th century, and was produced by increasing the carbon content in molten pig iron using a process known as cementation. In this process, bars of wrought iron were layered with powdered charcoal in stone boxes and heated.
After about a week, the iron would absorb the carbon in the charcoal. Repeated heating would distribute carbon more evenly and the result, after cooling, was blister steel. The higher carbon content made blister steel much more workable than pig iron, allowing it to be pressed or rolled.
Blister steel production advanced in the 1740s when English clockmaker Benjamin Huntsman, while trying to develop a high quality steel for his clock springs, found that the metal could be melted in clay crucibles and refined with a special flux to remove slag that the cementation process left behind. The result was crucible - or cast - steel. But due to the cost of production, both blister and cast steel was only ever used in specialty applications.
As a result, cast iron made in puddling furnaces remained the primary structural metal in industrializing Britain during most of the 19th century.
The Bessamer Process and Modern Steelmaking
The growth of railroads during the 19th century in both Europe and America put great pressure on the iron industry, which still struggled with inefficient production processes. Yet steel was still unproven as a structural metal and production was slow and costly. That was until 1856, when Henry Bessemer came up with a more effective way to introduce oxygen into molten iron in order to reduce the carbon content. More...
