Steel is iron carbonized. It is of several sorts, each possessing special properties and being made by a particular process; is extremely elastic; and has greater tensile strength than any other metal. No law of connection between its composition and its properties has been discovered, and carbon's form in steel remains undetermined. The term formerly designated a carbonized iron that would harden when heated to a certain, temperature and then suddenly cooled; but only carbonized iron produced by modern methods is now considered steel. Iron is used in the industrial arts as cast-iron, wrought iron and steel, and each has marked physical properties that fit each for special purposes.

Cast iron is produced by heating iron-ores to a high temperature in a blast-furnace in connection with various fluxes, particularly limestone. The molten metal contains considerable carbon dissolved and, usually, silicon, phosphorus and sulphur. The amount of carbon in cast iron varies from 1.5% to 4.5%. Cast iron can be melted and cast in molds. It is only slightly malleable, except when specially treated, is rigid and has great resistance to crushing, but is more or less brittle and not adapted to tensile strains. Wrought iron is nearly pure iron, and in modern metallurgical processes is usually obtained by removing the carbon and other impurities from cast iron. The common method of making wrought iron is that of puddling. This consists of melting cast iron in the chamber of a flame-furnace (see Furnace) and in removing the carbon, phosphorus etc. by stirring the molten metal and, sometimes, by adding oxidizing materials. Wrought iron is very malleable, can be welded, has a high tensile strength and shows a fibrous rather than a crystalline structure at a break. Steel has been made since very early times by a process called cementation. This consists of taking a high grade of wrought iron rolled into bars, putting these in fireclay boxes with finely divided charcoal and heating them from three to six days. This recarbonizes the iron, and produces cement or blister-steel. Bars of blister-steel may be welded together (then called shear-steel), or they may be melted in crucibles and cast, and are then called crucible-steel. High-grade tool and spring steel is made by this process, while the cheaper open-hearth and pneumatic processes are used in the manufacture of the great quantities of steel used in engineering construction-work.

The pneumatic process, universally called the Bessemer process, was invented by William Kelly (q. v.) of Pittsburgh in 1847, and ranks among the most important of industrial achievements. The discovery was an entirely new idea, revolutionized steel-making, and supplied the most useful and precious of metals cheaply as well as abundantly. Without such steel modern railways and steamships and much machinery would be impossible. Years before Bessemer of England experimented in iron, steamers on the Ohio had boilers of iron refined by Kelly's process. This consists of burning pigiron's impurities away by blowing fine aircurrents (whence the term pneumatic) through the melted metal and pouring back the needed amount of carbon. Bessemer discovered the process in 1855, and invented machinery that made it a commercial success. Mushet perfected the process chemically. The operation is performed in a pear-shaped pot, the converter, commonly 15 feet high by eight across, which since 1858 has but slightly changed its form. It is made of boiler-plate and lined with refractory material. When the iron turns white, it has become steel, and is poured out and worked. Bessemer steel lacks sufficient hardness for every purpose. Steel for hammers, knives, springs and a thousand finer articles, must be made by slower and more careful methods. Hence it is manufactured on the open hearths of flame or reverbatory furnaces by melting cast-iron, scrap-iron and wrought-iron together. In reliability and uniformness openhearth steel surpasses bessemer, but bessemer is the steel for engineers and builders.

Within 15 years steel has largely superseded brick, stone and wood in bridges and great buildings, ships being built of steel almost exclusively. Pittsburgh began about 1885 to produce beams, girders and other structural shapes; in 1892 production by automatic machinery displaced production by men; and American cities have since been rebuilt in steel. Barrels, bathtubs and furniture are now made of steel, as are cars, cotton-bands, roadways, sleepers and ties. Corrugated sheet-steel replaces wooden siding in buildings; expanded steel, a mesh of ribboney laths; ornamental steel-ceilings, plaster. Steel-frame churches have been erected, and New York's subway is a steel-tube.

No industry equals that of iron and steel in numbers, organization and wealth. America in its 5,370 establishments, produces nearly half the steel of the world. Pittsburgh makes more iron and steel than Great Britain, Chicago ranking second and making a third of our rails. The total value of the industry, whose movement is toward the lakes, is two billion dollars.

Though steelmaking has been revolutionized, vaster changes impend. The rolling-mill will become even more automatic. The important problems are the improvement of the quality of steel and the protection of workmen. The Bessemer converter is being replaced by the open hearth. A process has been invented, the duplex, that combines the pneumatic and openhearth methods. Less and less coal is needed in ironmaking, while oil, gas and electricity supply additional fuels or new processes and means. Highclass steel said to equal crucible-steel has been made directly from iron ore by an electric process. Finally, since dry air produces a hotter fire, with less fuel, than damp air, a process has been invented that takes the moisture, no small item, from air blown into the furnace, and the process increases production nearly a fifth.