The Bessemer Process

Prior to 1856, there was no easy way to control the carbon level in iron so as to manufacture steel efficiently and cheaply. Yet beginning in the 1830s, the growth of railroads created a huge market for steel. The first railroads ran on wrought iron rails, which were too soft to be durable (not enough carbon). On some busy stretches, the wrought iron rails had to be replaced every six to eight weeks. The mass production of cheap steel only became possible after the introduction of the Bessemer process, named after its inventor British metallurgist Sir Henry Bessemer (1813-1898). Bessemer recognized that a strong blast of air through molten pig iron should convert pig iron into steel by reducing its carbon content. That’s because oxygen in the air will “attract” the carbon in the molten pig iron. (Regular cast iron has a higher level of carbon, making it brittle.)

In 1865, Bessemer designed a converter, a large, pear-shaped receptacle with holes on the bottom, from where air would be injected. He found that blowing compressed air through the molten metal not only removed the carbon, but also made the molten metal even hotter and so remained molten. It was discovered that actually too much carbon was removed, so the addition of molten manganese, known as spiegel, after the blast of air was necessary. Thus it was possible to convert a whole mass of molten pig iron into steel in just minutes.

The one weakness of the Bessemer process was that it required low-phosphorus iron. (Phosphorus makes steel very brittle.) Iron from the Crown Point-Moriah area fit the bill. In 1876, however, it was noted that adding a chemically basic material, like limestone, would remove phosphorus from the iron ore. Now practically any type of iron ore from almost anywhere could be used. By 1884, steel rails had replaced iron ones.

Reader Interactions