Steel and Steel Alloys
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Steel and Steel Alloys By the mid-1800s, wrought iron and cast iron were the most commonly used materials for technology and industry. Though wrought and cast iron were stronger than other metals, neither was strong or versatile enough for the many new structures, machines, or tools required for the world's growing industrial base. It had been known for centuries that alloying iron with small amounts of carbon (between 0.3 and 1%) made the resulting metal—steel—very hard and strong, if the metal were cooled rapidly from a high temperature by, for example, quenching in water. Steel was recognized as a superior metal for weapons and tools that needed to be strong, resilient, and capable of retaining a keen cutting edge. Unfortunately, until the mid-19th century, making steel was a costly and time-consuming process. As described in last month's Historical Note, early efforts included a cementation process (sealing wrought iron with charcoal in clay containers and then heating it for days), a layering process (heating alternating layers of high- and low-carbon iron, then working the mass together), and a crucible process (melting layered or cemented steel in a crucible to increase the homogeneity of the metal). In the mid1800s, two new methods—the Bessemer process and the open-hearth process— revolutionized steelmaking and made steel available in sufficient quantities for an explosion in industrialization, bridge building, and railroad construction. The first major breakthrough occurred in England in 1856, when Henry Bessemer worked on improving metal cast for cannon. Knowing that impurities are removed from iron by oxidation, he realized that the necessary oxygen could come from the air itself if he blasted air through molten pig iron. When Bessemer tried this idea, he found that it not only removed the impurities (such as carbon, silicon, and manganese), but also heated the metal to a much higher temperature, rather than cooling and hardening it, as he had expected. The extra heat came from energy released by the oxidation of the impurities. Such a high temperature can be maintained long enough to burn away all contaminants, leaving pure molten iron to which carbon and other alloying elements can be reintroduced to forge the best steel. The same idea was independ-
MRS BULLETIN/JULY 1993
ently developed by William Kelly in Kentucky in the United States. Kelly received an American patent for his process in 1857, while Bessemer had received an English patent a year earlier. Shortly after the introduction of the Bessemer process, German-born English metallurgists William and Friedrich Siemens developed a regenerative-heating process, the "open hearth" process, which preheated air through the furnace before blowing it back into the combustion chamber, thereby greatly increasing the temperature. From 1857 to 1865, the Siemens brothers used their furnace to fuse pig iron and iron ore; the oxygen in iron ore burns away the excess carbon in pig iron, resulting in a far superior alloy. Concurrently, in France, Emile and Pier
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