Creation and Uses of Steel
Steel has a carbon content ranging from .2 to 1.5 percent, enough carbon to make it harder than wrought iron, but not so much as to make it as brittle as cast iron. Its hardness combined with its flexibility and tensile strength make steel far more useful than either type of iron: it is more durable and holds a sharp edge better than the softer wrought iron, but it resists shock and tension better than the more brittle cast iron. However, until the mid 1800s, steel was difficult to manufacture and expensive. Prior to the invention of the Bessemer converter (described below), steel was made mainly by the so-called cementation process. Bars of wrought iron would be packed in powdered charcoal, layer upon layer, in tightly covered stone boxes and heated. After several days of heating, the wrought iron bars would absorb carbon; to distribute the carbon more evenly, the metal would be broken up, rebundled with charcoal powder, and reheated. The resulting blister steel would then be heated again and brought under a forge hammer to give it a more consistent texture. In the 1740s, the English clockmaker Benjamin Huntsman, searching for a higher-quality steel for making clock springs, discovered that blister steel could be melted in clay crucibles and further refined by the addition of a special flux that removed fine particles of slag that the cementation process could not remove. This was called crucible steel; it was of a high quality, but expensive.
To sum up so far: wrought iron has a little carbon (.02 to .08 percent), just enough to make it hard without losing its malleability. Cast iron, in contrast, has a lot of carbon (3 to 4.5 percent), which makes it hard but brittle and nonmalleable. In between these is steel, with .2 to 1.5 percent carbon, making it harder than wrought iron, yet malleable and flexible, unlike cast iron. These properties make steel more useful than either wrought or cast iron, yet prior to 1856, there was no easy way to control the carbon level in iron so as to manufacture steel cheaply and efficiently. Yet the growth of railroads in the 1800s created a huge market for steel. The first railroads ran on wrought iron rails which were too soft to be durable. On some busy stretches, and on the outer edges of curves, the wrought iron rails had to be replaced every six to eight weeks. Steel rails would be far more durable, yet the labor- and energy-intensive process of cementation made steel prohibitively expensive for such large-scale uses.
The mass-production of cheap steel only became possible after the introduction of the Bessemer process, named after its brilliant inventor, the British metallurgist Sir Henry Bessemer (1813-1898). Bessemer reasoned that carbon in molten pig iron unites readily with oxygen, so a strong blast of air through molten pig iron should convert the pig iron into steel by reducing its carbon content. In 1856 Bessemer designed what he called a converter, a large, pear-shaped receptacle with holes at the bottom to allow the injection of compressed air. Bessemer filled it with molten pig iron, blew compressed air through the molten metal, and found that the pig iron was indeed emptied of carbon and silicon in just a few minutes; moreover, instead of freezing up from the blast of cold air, the metal became even hotter and so remained molten. Subsequent experimentation by another British inventor, Robert Mushet, showed that the air blast actually removed too much carbon and left too much oxygen behind in the molten metal. This made necessary the addition of a compound of iron, carbon, and manganese called spiegeleisen (or spiegel for short): the manganese removes the oxygen in the form of manganese oxide, which passes into the slag, and the carbon remains behind, converting the molten iron into steel. (Ferromanganese serves a similar purpose.) The blast of air through the molten pig iron, followed by the addition of a small quantity of molten spiegel, thus converts the whole large mass of molten pig iron into steel in just minutes, without the need for any additional fuel (as contrasted with the days, and tons of extra fuel and labor, required for puddling and cementation).
To sum up so far: wrought iron has a little carbon (.02 to .08 percent), just enough to make it hard without losing its malleability. Cast iron, in contrast, has a lot of carbon (3 to 4.5 percent), which makes it hard but brittle and nonmalleable. In between these is steel, with .2 to 1.5 percent carbon, making it harder than wrought iron, yet malleable and flexible, unlike cast iron. These properties make steel more useful than either wrought or cast iron, yet prior to 1856, there was no easy way to control the carbon level in iron so as to manufacture steel cheaply and efficiently. Yet the growth of railroads in the 1800s created a huge market for steel. The first railroads ran on wrought iron rails which were too soft to be durable. On some busy stretches, and on the outer edges of curves, the wrought iron rails had to be replaced every six to eight weeks. Steel rails would be far more durable, yet the labor- and energy-intensive process of cementation made steel prohibitively expensive for such large-scale uses.
The mass-production of cheap steel only became possible after the introduction of the Bessemer process, named after its brilliant inventor, the British metallurgist Sir Henry Bessemer (1813-1898). Bessemer reasoned that carbon in molten pig iron unites readily with oxygen, so a strong blast of air through molten pig iron should convert the pig iron into steel by reducing its carbon content. In 1856 Bessemer designed what he called a converter, a large, pear-shaped receptacle with holes at the bottom to allow the injection of compressed air. Bessemer filled it with molten pig iron, blew compressed air through the molten metal, and found that the pig iron was indeed emptied of carbon and silicon in just a few minutes; moreover, instead of freezing up from the blast of cold air, the metal became even hotter and so remained molten. Subsequent experimentation by another British inventor, Robert Mushet, showed that the air blast actually removed too much carbon and left too much oxygen behind in the molten metal. This made necessary the addition of a compound of iron, carbon, and manganese called spiegeleisen (or spiegel for short): the manganese removes the oxygen in the form of manganese oxide, which passes into the slag, and the carbon remains behind, converting the molten iron into steel. (Ferromanganese serves a similar purpose.) The blast of air through the molten pig iron, followed by the addition of a small quantity of molten spiegel, thus converts the whole large mass of molten pig iron into steel in just minutes, without the need for any additional fuel (as contrasted with the days, and tons of extra fuel and labor, required for puddling and cementation).