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cast iron:see ironiron,
metallic chemical element; symbol Fe [Lat. ferrum]; at. no. 26; at. wt. 55.845; m.p. about 1,535°C;; b.p. about 2,750°C;; sp. gr. 7.87 at 20°C;; valence +2, +3, +4, or +6. Iron is biologically significant.
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an iron alloy containing carbon, usually in amounts greater than 2 percent, and a number of impurities—silicon, manganese, phosphorus, and sulfur—and, sometimes, alloying elements; it hardens, forming a eutectic.
Cast iron is one of the most important primary products of ferrous metallurgy (seeFERROUS METALLURGY and BLAST-FURNACE PRODUCTION), used for reconversion in the production of steel and as a component of the charge used for remelting into iron castings. One of the major construction materials, it is also used as a casting alloy. The widespread use of cast iron in machine building is based on its good casting and strength properties; some cast irons are only slightly inferior in strength to carbon steels (seeINOCULATED CAST IRON). Cast iron accounts for about 75 percent of all metal used in modern machine building. The USSR is the world’s leading producer of cast iron (1976).
History. The first information about cast iron dates from the sixth century B.C. Cast iron containing up to 7 percent phosphorus and having a low melting point was produced in China from high-phosphorus iron ores and was used for casting various items. Cast iron was also known to Greek and Roman metallurgists of the fourth and fifth centuries B.C.
The production of cast iron in Western Europe dates to the 14th century, when the first blast furnaces (bloomery furnaces) for smelting cast iron from ores appeared (seeMETALLURGY). The cast iron produced was used either for conversion into steel in bloomeries (seeBLOOMERY CONVERSION) or for the production of various structural elements, such as pillars, and weapons, such as cannons and cannon balls. In Russia, the production of cast iron dates to the 16th century. It steadily expanded, and by the time of the reign of Peter I, Russia exceeded all other countries in the production of cast iron, only to lag behind the Western European countries a century later. The development of cupola furnaces (seeFURNACE, CUPOLA) in the second half of the 18th century led to the separation of casting shops from blast furnace shops, that is, initiated independent iron casting (at machine-building plants). Malleable cast iron was first produced in the early 19th century.
The alloying of cast iron dates to the second quarter of the 20th century (seeALLOY CAST IRON). Alloying made it possible to greatly improve the properties of cast iron and produce special cast irons resistant to wear, corrosion, and heat. The development of methods of producing inoculated cast iron also dates to this period. Inoculated cast iron with inclusions of spheres of graphite instead of the usual graphite flakes was obtained at the end of the 1940’s; such cast iron, called ductile cast iron, has considerably higher strength (σb), up to 500 meganewtons/m2 (50 kilograms-force/mm2) in the cast state and 1,200 meganewtons/m2 (120 kilograms-force/mm2) after heat treatment. A type of cast iron known as high-duty cast iron, possessing good mechanical properties despite the presence of graphite flakes, was obtained in the 1960’s in electric furnaces from steel scrap with the addition of a carburizing agent (seeIRON-CARBON ALLOYS).
Classification and properties. Cast iron obtained in blast furnaces is characterized either as basic pig iron, used for conversion into steel, or foundry pig iron, which serves as a basic component of the charge in iron casting.
Up until the 1970’s, specular cast iron, which contains 10 to 25 percent manganese and is used as a deoxidizing agent in the production of steel and some special cast irons, was sometimes produced in blast furnaces. Natural alloyed cast irons are produced from iron ores containing chromium, nickel, titanium, and other alloying elements. Various criteria are used at iron foundries in classifying cast iron used in the production of casts. Cast iron is classified as gray, white, or mottled according to the extent of graphitization, which determines the type of fracture. Cast irons can also be classified on the basis of the graphite inclusions, which can be flaked, spheroidal (ductile cast iron), vermicular, or in the form of rosettes (malleable cast iron). Depending on the nature of the metal base, cast iron can be classified as pearlitic, ferritic, pearlitic-ferritic, austenitic, bainitic, or martensitic. On the basis of use, a distinction is made between structural and special-purpose cast irons, and on the basis of chemical composition, between alloy and unalloyed cast irons.
GRAY CAST IRON. Gray cast iron, or simply gray iron, the most common form of cast iron, used in machine building, sanitary engineering, and building construction, has inclusions of graphite in the form of flakes. Items made of gray iron are characterized by resistance to the effect of external stress concentrators under cyclical loading and by a higher vibration absorption coefficient (two to four times greater than steel). An important structural feature of gray iron is its ratio of creep limit to tensile strength, which is higher than for steel. The presence of graphite improves the lubrication conditions upon friction, which improves the antifriction properties.
The properties of gray iron depend on the structure of the metal base and on the type, size, amount, and nature of distribution of the graphite inclusions. Pearlitic gray cast iron has high strength and is used for cylinders, bushings, and other stressed parts of engines and engine blocks. Gray iron with a ferritepearlite metal base is used for less critical parts.
WHITE CAST IRON. White cast iron, or simply white iron, is an alloy in which the excess carbon not found in the solid iron solution is present in a bound state in the form of the iron carbide Fe3C (cementite) or special carbides (in alloy cast irons). The crystallization of white iron occurs in a metastable system with the formation of cementite and pearlite. Because of its poor mechanical properties and brittleness, white iron has limited use for simple articles subjected to conditions of high abrasive wear. The alloying of white iron with carbide-forming elements, such as chromium, tungsten, and molybdenum, enhances its resistance to wear.
MOTTLED CAST IRON. In mottled cast iron, or mottled iron, part of the carbon is in the free state in the form of graphite and part is in the bound state in the form of carbides. Mottled iron is used for the production of items operating under conditions of dry friction (brake shoes) and wear-resistant parts, such as roller, paper-making, and flour-milling shafts.
MALLEABLE CAST IRON. Malleable cast iron is made as a white-iron casting and then subjected to annealing in which graphitization takes place, which results in the decomposition of the cementite and the formation of flaked graphite. Malleable iron is more resistant to shocks and cuts than steel and performs satisfactorily at low temperatures. Its mechanical properties are a function of the structure of the metal base and the amount and compactness of the graphite inclusions. Depending on the type of thermal treatment, malleable iron may have a ferrite, ferritepearlite, or pearlite metal base. Malleable iron in a matrix of granular pearlite has the best properties and may be used instead of cast or hammered steel. Ferritic malleable iron is used when high ductility is required. To accelerate the graphitization process upon heat treatment, malleable iron is inoculated with tellurium, boron, or magnesium.
Malleable iron is used mainly in the production of automobiles, tractors, and agricultural machinery. There is a trend, especially in automobile production, to replace malleable iron with ductile iron, with spheroidal graphite, in order to provide stronger casts, decrease production time, and improve the final product.
DUCTILE CAST IRON. Ductile iron (also called nodular iron or spheroidal graphite iron) is characterized by spheroidal or nearly spheroidal graphite inclusions, produced by the inoculation of liquid cast iron with magnesium, cerium, yttrium, calcium, or some other element in the pure form or as a component of an alloy. Spheroidal graphite least weakens the metal matrix, which leads to considerable improvement in the mechanical properties of cast iron with pearlite or bainite structure, whose properties approach those of carbon steels. High ductility is observed for a pure ferrite matrix in the cast or heat-treated states.
Ductile iron has good casting and technical properties, such as flowability, low linear shrinkage, and good machinability by cutting, but it is similar to steel with respect to concentrated volumetric shrinkage. It is used to replace cast and hammered steel parts, such as the crankshafts of engines and compressors, as well as items made of malleable iron or ordinary gray iron.
Ductile iron with vermicular graphite inclusions, which appear as thickened, bent plates with twisted edges under a microscope, occupies an intermediate position between cast iron with spheroidal graphite and cast iron with graphite flakes. It has good technical properties and exhibits low volumetric shrinkage and high heat conductivity, which is almost the same as that of gray iron. It is used in the construction of diesel engines and other types of machinery.
ALLOY CAST IRONS. Alloying elements, such as nickel, chromium, copper, aluminum, titanium, tungsten, vanadium, and molybdenum, are added to cast iron to improve its strength and operational characteristics or to impart special properties, such as nonmagnetism, high-temperature strength, and resistance to wear, heat, and corrosion. Manganese in amounts greater than 2 percent and silicon in amounts greater than 4 percent are also considered as alloying elements. Alloy cast irons can be classified according to the content of the major alloying elements, for example, chromium, nickel, or aluminum cast iron. A distinction is made between low-alloy cast irons, with the content of alloying elements totaling less than 2.5 percent, medium-alloy cast irons, with 2.5 to 10 percent alloying elements, and high-alloy cast irons, with more than 10 percent alloying elements. Low-alloy cast irons are in a matrix of pearlite or bainite, medium-alloy cast irons are usually in a matrix of martensite, and high-alloy cast irons are primarily in a matrix of austenite or ferrite.
Cast irons with 5 to 7 percent silicon (Silals) are used as heat-resistant materials. Cast irons with 12 to 18 percent silicon (Ferrosilides) exhibit high corrosion resistance in solutions of salts, acids (except hydrochloric acid), and alkalies. Such cast irons alloyed with molybdenum (antichlor) are highly resistant to hydrochloric acid. Of all known cast irons, cast iron with 19 to 25 percent aluminum (Chugal’, or aluminum cast iron) has the highest heat resistance in the air and in sulfur-containing media. The most common wear-resistant cast irons are alloyed with up to 2.5 percent chromium and up to 6 percent nickel (Nikhard cast irons). Austenitic nickel cast irons alloyed with manganese, copper, and chromium (Ni-resist alloys) are used as corrosion-resistant and high-temperature strong materials.
Marking of cast irons. The system of marking cast irons adopted in the USSR uses letters and numbers. The letters indicate the principal intended use of the iron product: P for basic iron, used in oxygen-converter and open-hearth production, and L for foundry iron, used for iron casting. Foundry coke iron is indicated by LK, and iron produced with charcoal, by LD. The silicon content decreases with increasing number; for example, LK5 contains less silicon than LK4. Each brand of cast iron, depending on its content of manganese, phosphorus, and sulfur, is placed, correspondingly, in a group, class, and category. Cast irons for foundry production, as a rule, are designated by letters indicating the principal type or intended use of the cast iron: SCh indicates gray iron, VCh indicates ductile iron, and KCh indicates malleable iron. Antifriction cast irons are designated by the letter A placed at the beginning, for example, ASCh, AVCh, and AKCh.
The numbers in the brands of unalloyed cast irons indicate the mechanical properties of the cast irons. Standardized tensile and bending strengths are given for gray irons in kilograms-force/mm2, for example, SCh21-40. For ductile and malleable irons, the numbers indicate the tensile strength (in kilograms-force/mm2) and relative elongation (in percent), for example, VCh60-2.
A brand of alloy cast iron is designated by a letter indicating the alloying elements and a number directly after each letter indicating the average content of the given alloying element. When the content of the alloying element is less than 1.0 percent, the numbers after the corresponding letter are omitted. The arbitrary system of symbols for the chemical elements is the same as for steel (seeSTEEL). An example of an alloy cast iron is ChN19Kh3, which is a cast iron containing about 19 percent nickel and about 3 percent chromium. If spheroidal graphite is required for an alloy cast iron, the letter Sh is placed at the end, for example, ChN19Kh3Sh.
Cast iron in art. Cast iron has been used in artistic ironwork since the Middle Ages; for example, a remarkable sculpture, now lost, of a lion weighing 100 tons was cast in China in the tenth century A.D. Cast iron sculpture appeared in Germany in the 15th century and subsequently in other European countries; it appeared in Russia at the end of the 17th century. The production of objects from cast iron became widespread (park sculpture, memorials, fences, walls, and garden furniture). In the 20th century, the casting of objects from cast iron is nearly as widespread as casting from bronze, since cast iron objects, although more massive and characterized by the expressiveness of the heavy weight and muted tones, are less costly to produce.
Cast iron is used for various purposes in architecture (beginning in the late 18th century). The use of cast iron structural elements was characteristic of architecture of the 19th century (the age of cast iron).
REFERENCESGirshovich, N. G. Chugunnoe lit’e. Leningrad-Moscow, 1949.
Girshovich, N. G. Kristallizatsiia i svoislva chuguna v otlivkakh. Moscow-Leningrad, 1966.
Bunin, K. P., Ia. N. Malinochka, and Iu. N. Taran. Osnovy metallografii chuguna. Moscow, 1969.
B. S. MIL’MAN, E. V. KOVALEVICH, and V. T. SOLENKOV