The Bessemer process, developed and patented by Sir Henry Bessemer in 1856, was the first industrial method capable of mass-producing steel at a low cost. This innovation transformed steel from a prohibitively expensive material into an affordable commodity, powering the Industrial Age. It enabled the construction of modern infrastructure, such as skyscrapers, bridges, and extensive railroad networks, which were impossible with weaker wrought iron. The Bessemer process is no longer in use today, having been entirely superseded by more advanced technologies.
How the Bessemer Process Worked
The underlying principle of the Bessemer process involved purifying molten pig iron by blowing air directly through it. This purification occurred inside a large, pear-shaped vessel called the Bessemer converter, which could be tilted for charging and pouring. When the air blast was forced through the molten iron, the oxygen reacted with impurities such as silicon, manganese, and excess carbon.
The oxidation of these elements generated intense, exothermic heat, keeping the iron molten without needing an external fuel source. These chemical reactions converted the brittle, high-carbon pig iron into workable, low-carbon steel within a remarkably short time, typically 10 to 20 minutes per batch. This speed and efficiency dramatically reduced the cost of steel from roughly £40 per ton to about £6 or £7 per ton.
Technical Limitations and Obsolescence
The original Bessemer process suffered from several technical limitations that ultimately led to its decline. The most significant issue was its inability to effectively remove phosphorus, a common impurity in many iron ores. When phosphorus remained in the steel, it caused the metal to become brittle and prone to cracking, especially at cold temperatures. This flaw limited the process’s wide applicability, as only specific, low-phosphorus iron ores could be used.
Furthermore, the incredible speed of the conversion left little time for precise chemical analysis or adjustments to the molten metal’s composition. The process offered limited control over the final carbon content, resulting in steel of inconsistent quality. The use of air, which contains about 78% nitrogen, also introduced nitrogen into the steel, an element that negatively affected the metal’s mechanical properties.
A temporary solution, known as the Thomas-Gilchrist process, was developed by lining the converter with basic materials like dolomite or limestone to help remove phosphorus. However, even this modified “basic Bessemer” process could not match the quality and control required by modern industry. The inherent speed and the introduction of nitrogen from the air blast remained major limitations, paving the way for a more controlled method.
The Basic Oxygen Steelmaking Revolution
The modern steel industry is dominated by the Basic Oxygen Steelmaking (BOS) process. Commercialized in the 1950s, BOS utilizes a vessel known as the basic oxygen furnace (BOF), which closely resembles the original Bessemer converter. The primary difference is that BOS blows pure oxygen, rather than air, onto the surface of the molten pig iron at supersonic speeds through a water-cooled lance.
The use of pure oxygen avoids introducing nitrogen into the steel, solving a major quality issue of the Bessemer method. This directed, high-purity oxygen blast offers far greater temperature and chemical control, allowing for the precise removal of impurities like phosphorus and carbon. BOS can convert a batch of molten iron into high-quality steel in less than 40 minutes, achieving speeds similar to Bessemer but with superior control over the final product’s chemistry.
The Basic Oxygen Steelmaking process accounts for more than half of the world’s steel production today, particularly for primary steel made from raw materials. The other major contemporary method is the Electric Arc Furnace (EAF), which primarily melts and recycles scrap steel using electrical energy. Both BOS and EAF offer the precision, quality, and material flexibility that the Bessemer process could not provide for the modern era.