The technology and science of metallic materials. Metallurgy as a branch of engineering is concerned with the production of metals and alloys, their adaptation to use, and their performance in service. As a science, metallurgy is concerned with the chemical reactions involved in the processes by which metals are produced and the chemical, physical, and mechanical behavior of metallic materials. Metallurgy is one of the major subdivisions of materials science and engineering, along with ceramics, polymers, glasses, electronic materials, carbon materials, nanomaterials, and composite materials.
Metallurgy has played an important role in the history of civilization. Metals were first produced more than 6000 years ago. Because only a few metals, principally gold, silver, copper, and meteoric iron, occur in the uncombined state in nature, and then only in small quantities, primitive metallurgists had to discover ways of extracting metals from their ores. Fairly large-scale production of some metals was carried out with technical competence in early Near Eastern and Mediterranean civilizations and in the Middle Ages in central and northern Europe. Basic metallurgical skills were also developed in other parts of the world.
The winning of metals would have been of little value without the ability to work them. Great craftsmanship in metalworking developed in early times: the objects produced included jewelry, large ornamental and ceremonial objects, tools, and weapons. It may be noted that almost all early materials and techniques that later had important useful applications were discovered and first used in the decorative arts. In the Middle Ages metal working was in the hands of individual or groups of craft workers. The scale and capabilities of metal working developed with growth of industrial organizations. Today's metallurgical plants supply metals and alloys to the manufacturing and construction industries in a variety of forms, such as beams, plates, sheets, bars, wire, and castings. Technologies such as communications, nuclear power, and space exploration continue to demand new techniques of metal production and processing.
The field of metallurgy may be divided into process metallurgy (production metallurgy, extractive metallurgy) and physical metallurgy. According to another system of classification, metallurgy comprises chemical metallurgy, mechanical metallurgy (metal processing and mechanical behavior in service), and physical metallurgy. The more common division into process metallurgy and physical metallurgy, which is adopted in this article, classifies metal processing as a part of process metallurgy and the mechanical behavior of metals as a part of physical metallurgy.
Process metallurgy, the science and technology used in the production of metals, employs some of the same unit operations and unit processes as chemical engineering. These operations and processes are carried out with ores, concentrates, scrap metals, fuels, fluxes, slags, solvents, and electrolytes. Different metals require different combinations of operations and processes, but typically the production of a metal involves two major steps. The first is the production of an impure metal from ore minerals, commonly oxides or sulfides, and the second is the refining of the reduced impure metal, for example, by selective oxidation of impurities or by electrolysis. Process metallurgy is continually challenged by the demand for metals which have not been produced previously or are difficult to produce; by the depletion of the deposits of the richer and more easily processed ores of the traditional metals; and by the need for metals of greater purity and higher quality. The mining of leaner ores has greatly enhanced the importance of ore dressing methods for enriching raw materials for metal production. Several nonferrous metals are commonly produced from concentrates. Iron ores are also treated by ore dressing. See also: Electrometallurgy; Hydrometallurgy; Nonferrous pyrometallurgy; Ore dressing; Pyrometallurgy
Process metallurgy today mainly involves large-scale operations. A single blast furnace can produce crude iron at the rate of 3000–11,000 tons (2700–9900 metric tons) per day. A basic oxygen furnace for steelmaking can consume 800 tons (720 metric tons) of pure oxygen together with required amounts of crude iron and scrap to produce 12,000 tons (11,000 metric tons) of steel per day. Advanced methods of process analysis and control have been applied to such processing systems. The application of vacuum to extraction and refining processes, the leaching of low-grade ores for the extraction of metals, the use of electrochemical reduction cells, and the refining of reactive metals by processing through the vapor state are other important developments. See also: Iron metallurgy; Steel manufacture
Because the production of metals employs many different chemical reactions, process metallurgy has been closely associated with inorganic chemistry. Techniques for analyzing ores and metallurgical products originated several centuries ago and represented an early stage of analytical chemistry. Application of physical chemistry to equilibria and kinetics of metallurgical reactions has led to great progress in metallurgical chemistry.
Physical metallurgy investigates the effects of composition and treatment on the structure of metals and the relations of the structure to the properties of metals. Physical metallurgy is also concerned with the engineering applications of scientific principles to the fabrication, mechanical treatment, heat treatment, and service behavior of metals. See also: Alloy; Heat treatment (metallurgy)
The structure of metals consists of their crystal structure, which is investigated by x-ray, electron, and neutron diffraction, their microstructure, which is the subject of metallography, and their macrostructure. Crystal imperfections provide mechanisms for processes occurring in solid metals; for example, the movement of dislocations results in plastic deformation. Crystal imperfections are investigated by x-ray diffraction and metallographic methods, especially electron microscopy. The microstructure is determined by the constituent phases and the geometrical arrangement of the microcrystals (grains) formed by those phases. Macrostructure is important in industrial metals. It involves chemical and physical inhomogeneities on a scale larger than microscopic. Examples are flow lines in steel forgings and blowholes in castings. See also: Crystal structure; Metallography; X-ray diffraction
Phase transformations occurring in the solid state underlie many heat-treatment operations. The thermodynamics and kinetics of these transformations are a major concern of physical metallurgy. Physical metallurgy also investigates changes in the structure and properties resulting from mechanical working of metals. See also: Plastic deformation of metals
The composition of metallic objects is often characterized by the presence of impurities, nonuniform distribution (segregation) of solute elements and nonmetallic inclusions, especially in steel. These phenomena, which originate in the production process, can have important effects on the properties of metals and alloys. They illustrate the close relation between process and physical metallurgy.
The applications for which a metal is intended determine the properties that are of practical interest. For use in machinery and construction, mechanical properties including deformation and fracture behavior are of greatest importance. In transportation equipment the strength-to-weight ratio deserves special consideration. In other applications, electrical or thermal conductivity or magnetic properties may be decisive. Resistance to corrosion is a common requirement and accounts for another close link between metallurgy and chemistry, especially electrochemistry and surface chemistry. See also: Corrosion; Mechanical properties of metal
An old distinction between ferrous and nonferrous metallurgy, although still of some practical significance, is no longer considered fundamental. Certain general principles have become established and apply to all metals. In some respects, however, there are great differences between metals which has led to specialization in research and industrial practice. On the other hand, an underlying structural science is developing to bring all materials—metallic and other inorganic as well as organic materials—within a unified framework.
Metallurgy occupies a position at the juncture of physics, chemistry, mechanical engineering, and chemical engineering. It also borders electrical, civil, aeronautical, and nuclear engineering. Metallurgical knowledge can be relevant to fields that are as removed from engineering as archeology, crime investigation, and orthopedic surgery. In the field of materials science and engineering, metallurgy takes its place as one of the oldest and most highly developed disciplines.
The area of concern of metallurgy widened as problems of materials availability were recognized and focused attention to the need for new production processes and the recycling of secondary metals. Environmental considerations require new technology for pollution abatement. Energy conservation favors more efficient processes in metals production. Conservation of materials also calls for more effective utilization of metals and the substitution of more plentiful for scarce metals.
For more information on metallurgy and some associated techniques see articles on individual metals and their metallurgy.