The earliest evidence of steel production is from sub-Saharan Africa in the first millennium BC. The Haya people, from an area now in north western Tanzania, developed a technology for producing high-grade carbon steel that was in use until the 1950s.
The Hayas used a cone-shaped clay kiln to heat the iron ore, which was pre-roasted to increase its carbon content. Air was pumped into the base of the kiln, passing through the furnace through clay pipes, before arriving inside the kiln. This preheated air allowed very high temperatures to be achieved inside the kiln, improving fuel efficiency as well as the quality of the iron.
In Europe before Henry Bessemer invented his Converter in 1856, steel had been an expensive, high-quality metal made by artisans. It was used mainly for tools and weapons while engineers generally preferred wrought and cast iron. The Bessemer Converter had a huge impact on the use of steel. Its cheap, easily produced, medium-quality steel quickly replaced iron for all large-scale engineering work and transformed both industry and society in
Europe and the United States. When cheap steel products were imported into Africa in the early twentieth century, it also spelled the end of the Hayas' steel industry.
The Bessemer method converts molten pig iron into steel in a squat, egg-shaped bowl. Blasts of cold air are blown through the liquid metal via perforations in the bowl's base. The air burns off excess carbon and other impurities. When the iron is nearly purified, more carbon might be added. Finally the bowl is tilted down and the newly made steel poured into a ladle ready to cast into ingot moulds.
The Bessemer process was simple but the visual effect was extraordinary. In 1893, an over-excited journalist at McClure's Magazine wrote:
In America the introduction of the Bessemer Converter had an enormous impact on an expanding society. The availability of cheap, durable steel rails allowed pioneers to build over 30,000 miles of railroad and colonise the western part of the country.
The early period of this expansion was satisfied by Britain's steel railmakers, particularly those in Sheffield, which remained the centre of the trade until 1870. The steel industry, along with coal, textiles and iron, had enabled Britain to build its Empire. But by the end of the nineteenth century the industry was strongest in the United States, whose steel producers increased efficiency and dropped prices.
As well as railways, the Bessemer Converter was instrumental in the production of armaments. Guns, cannons and warships became relatively inexpensive to produce, boosting America's military power.
After 1900 the Bessemer process was rapidly replaced by an alternative method, the Siemens-Martin or open hearth process. This allowed precise control of temperatures resulting in better quality steel.
Steel frames were used to build towering skyscrapers as the rural population moved to the booming American cities. New sheet steel led to the development of myriad consumer products including washing machines, electric ovens, refrigerators and, perhaps most importantly of all, the automobile. By 1914 automobile manufacturers had become the leading consumers of steel.
The last Bessemer Converter in North America went out of commission in the 1960s.