Cast iron is a class ofiron–carbonalloys with a carbon content of more than 2% and silicon content around 1–3%.[1] Its usefulness derives from its relatively low melting temperature. The alloying elements determine the form in which its carbon appears:white cast iron has its carbon combined into aniron carbide namedcementite, which is very hard, but brittle, as it allows cracks to pass straight through;grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks, andductile cast iron has spherical graphite "nodules" which stop the crack from further progressing.
Carbon (C), ranging from 1.8 to 4 wt%, andsilicon (Si), 1–3 wt%, are the main alloying elements of cast iron. Iron alloys with lower carbon content are known assteel.
The earliest cast-iron artifacts date to the 8th century BC, and were discovered byarchaeologists in what is nowJiangsu, China. Cast iron was used inancient China to mass-produce weaponry for warfare, as well as agriculture and architecture.[2] During the 15th century AD, cast iron became utilized forcannons andshot inBurgundy, France, and in England during theReformation. The amounts of cast iron used for cannons required large-scale production.[3] The first cast-iron bridge was built during the 1770s byAbraham Darby III, and is known asthe Iron Bridge inShropshire, England. Cast iron was also usedin the construction of buildings.
Cast iron is made frompig iron, which is the product of meltingiron ore in ablast furnace. Cast iron can be made directly from the molten pig iron or by re-melting pig iron,[4] often along with substantial quantities of iron, steel, limestone, carbon (coke) and taking various steps to remove undesirable contaminants.Phosphorus andsulfur may be burnt out of the molten iron, but this also burns out the carbon, which must be replaced. Depending on the application, carbon and silicon content are adjusted to the desired levels, which may be anywhere from 2–3.5% and 1–3%, respectively. If desired, other elements are then added to the melt before the final form is produced bycasting.[citation needed]
In-stream inoculation addition while molten cast iron is poured to a green sand mold in a foundry
Cast iron is sometimes melted in a special type of blast furnace known as acupola, but in modern applications, it is more often melted in electricinduction furnaces or electric arc furnaces.[5] After melting is complete, the molten cast iron is poured into a holding furnace or ladle.[citation needed]
Cast iron's properties are changed by adding various alloying elements, oralloyants. Next tocarbon,silicon is the most important alloyant because it forces carbon out of solution. A low percentage of silicon allows carbon to remain in solution, forming iron carbide and producing white cast iron. A high percentage of silicon forces carbon out of solution, forming graphite and producing grey cast iron. Other alloying agents,manganese,chromium,molybdenum,titanium, andvanadium counteract silicon, and promote the retention of carbon and the formation of those carbides. Nickel and copper increase strength and machinability, but do not change the amount of graphite formed. Carbon asgraphite produces a softer iron, reduces shrinkage, lowers strength, and decreases density.Sulfur, largely a contaminant when present, formsiron sulfide, which prevents the formation of graphite and increaseshardness. Sulfur makes molten cast iron viscous, which causes defects. To counter the effects of sulfur,manganese is added, because the two form intomanganese sulfide instead of iron sulfide. The manganese sulfide is lighter than the melt, so it tends to float out of the melt and into theslag. The amount of manganese required to neutralize sulfur is 1.7 × sulfur content + 0.3%. If more than this amount of manganese is added, thenmanganese carbide forms, which increases hardness andchilling, except in grey iron, where up to 1% of manganese increases strength and density.[6]
Nickel is one of the most common alloying elements, because it refines thepearlite and graphite structures, improves toughness, and evens out hardness differences between section thicknesses.Chromium is added in small amounts to reduce free graphite, produce chill, and because it is a powerfulcarbide stabilizer; nickel is often added in conjunction. A small amount oftin can be added as a substitute for 0.5% chromium.Copper is added in the ladle or in the furnace, on the order of 0.5–2.5%, to decrease chill, refine graphite, and increase fluidity.Molybdenum is added on the order of 0.3–1% to increase chill and refine the graphite and pearlite structure; it is often added in conjunction with nickel, copper, and chromium to form high strength irons.Titanium is added as a degasser and deoxidizer, but it also increases fluidity.Vanadium at 0.15–0.5% is added to cast iron to stabilize cementite, increase hardness, and increase resistance towear and heat.Zirconium at 0.1–0.3% helps to form graphite, deoxidize, and increase fluidity.[6]
In malleable iron melts,bismuth is added at 0.002–0.01% to increase how much silicon can be added. In white iron,boron is added to aid in the production of malleable iron; it also reduces the coarsening effect of bismuth.[6]
Grey cast iron is characterised by its graphitic microstructure, which causes fractures of the material to have a grey appearance. It is the most commonly used cast iron and the most widely used cast material based on weight. Most cast irons have a chemical composition of 2.5–4.0% carbon, 1–3% silicon, and the remainder iron. Grey cast iron has lesstensile strength andshock resistance than steel, but itscompressive strength is comparable to low- and medium-carbon steel. These mechanical properties are controlled by the size and shape of the graphite flakes present in the microstructure and can be characterised according to the guidelines given by theASTM.[7]
White cast iron displays white fractured surfaces due to the presence of an iron carbide precipitate called cementite. With a lower silicon content (graphitizing agent) and faster cooling rate, the carbon in white cast iron precipitates out of the melt as themetastable phasecementite, Fe3C, rather than graphite. The cementite which precipitates from the melt forms as relatively large particles. As the iron carbide precipitates out, it withdraws carbon from the original melt, moving the mixture toward one that is closer toeutectic, and the remaining phase is the lower iron-carbonaustenite (which on cooling might transform tomartensite). These eutectic carbides are much too large to provide the benefit of what is called precipitation hardening (as in some steels, where much smaller cementite precipitates might inhibit [plastic deformation] by impeding the movement ofdislocations through the pure iron ferrite matrix). Rather, they increase the bulk hardness of the cast iron simply by virtue of their own very high hardness and their substantial volume fraction, such that the bulk hardness can be approximated by a rule of mixtures. In any case, they offer hardness at the expense oftoughness. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as acermet. White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as the wear surfaces (impeller andvolute) ofslurry pumps, shell liners andlifter bars inball mills andautogenous grinding mills, balls and rings incoal pulverisers.
Cross section of chilled cast-iron roll
It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron. The resulting casting, called achilled casting, has the benefits of a hard surface with a somewhat tougher interior.[citation needed]
High-chromium white iron alloys allow massive castings (for example, a 10-tonne impeller) to be sand cast, as the chromium reduces cooling rate required to produce carbides through the greater thicknesses of material. Chromium also produces carbides with impressive abrasion resistance.[8] These high-chromium alloys attribute their superior hardness to the presence of chromium carbides. The main form of these carbides are the eutectic or primary M7C3 carbides, where "M" represents iron or chromium and can vary depending on the alloy's composition. The eutectic carbides form as bundles of hollow hexagonal rods and grow perpendicular to the hexagonal basal plane. The hardness of these carbides are within the range of 1500-1800HV.[9]
Malleable iron starts as a white iron casting that is thenheat treated for a day or two at about 950 °C (1,740 °F) and then cooled over a day or two. As a result, the carbon in iron carbide transforms into graphite and ferrite plus carbon. The slow process allows thesurface tension to form the graphite into spheroidal particles rather than flakes. Due to their loweraspect ratio, the spheroids are relatively short and far from one another, and have a lowercross section vis-a-vis a propagating crack orphonon. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems found in grey cast iron. In general, the properties of malleable cast iron are more like those ofmild steel. There is a limit to how large a part can be cast in malleable iron, as it is made from white cast iron.[citation needed]
Developed in 1948,nodular orductile cast iron has its graphite in the form of very tiny nodules with the graphite in the form of concentric layers forming the nodules. As a result, the properties of ductile cast iron are that of a spongy steel without the stress concentration effects that flakes of graphite would produce. The carbon percentage present is 3-4% and percentage of silicon is 1.8-2.8%.Tiny amounts of 0.02 to 0.1%magnesium, and only 0.02 to 0.04%cerium added to these alloys slow the growth of graphite precipitates by bonding to the edges of the graphite planes. Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies. The properties are similar to malleable iron, but parts can be cast with larger sections.[citation needed]
Cast iron andwrought iron can be produced unintentionally when smelting copper using iron ore as a flux.[11]: 47–48
The earliest cast-iron artifacts date to the 8th century BC, and were discovered by archaeologists in what is now modernLuhe County, Jiangsu in China during theWarring States period. This is based on an analysis of the artifact's microstructures.[2]
Because cast iron is comparatively brittle, it is not suitable for purposes where a sharp edge or flexibility is required. It is strong under compression, but not under tension. Cast iron was invented in China in the 8th century BC and poured into molds to make ploughshares and pots as well as weapons and pagodas.[12] Although steel was more desirable, cast iron was cheaper and thus was more commonly used for implements in ancient China, while wrought iron or steel was used for weapons.[2] The Chinese developed a method ofannealing cast iron by keeping hot castings in an oxidizing atmosphere for a week or longer in order to burn off some carbon near the surface in order to keep the surface layer from being too brittle.[13]: 43
Deep within theCongo region of the Central African forest, blacksmiths invented sophisticated furnaces capable of high temperatures over 1000 years ago. There are countless examples of welding, soldering, and cast iron created in crucibles and poured into molds. These techniques were employed for the use of composite tools and weapons with cast iron or steel blades and soft, flexible wrought iron interiors. Iron wire was also produced. Numerous testimonies were made by early European missionaries of theLuba people pouring cast iron into molds to make hoes.[14] Metallographic analysis of Luba artifacts also indicates the use of cast iron.[15]
The technology of cast iron was transferred to the West from China.[16]Al-Qazvini in the 13th century and other travellers subsequently noted an iron industry in theAlburz Mountains to the south of theCaspian Sea. This is close to thesilk route, thus the use of cast-iron technology being derived from China is conceivable.[16] Upon its introduction to the West in the 15th century it was used forcannon andshot.Henry VIII (reigned 1509–1547) initiated the casting of cannon in England. Soon, English iron workers usingblast furnaces developed the technique of producing cast-iron cannons, which, while heavier than the prevailing bronze cannons, were much cheaper and enabled England to arm her navy better.
Cast-iron pots were made at many English blast furnaces at the time. In 1707,Abraham Darby patented a new method of making pots (and kettles) thinner and hence cheaper than those made by traditional methods. This meant that hisCoalbrookdale furnaces became dominant as suppliers of pots, an activity in which they were joined in the 1720s and 1730s by a small number of othercoke-fired blast furnaces.
Application of the steam engine to power blast bellows (indirectly by pumping water to a waterwheel) in Britain, beginning in 1743 and increasing in the 1750s, was a key factor in increasing the production of cast iron, which surged in the following decades. In addition to overcoming the limitation on water power, the steam-pumped-water powered blast gave higher furnace temperatures which allowed the use of higher lime ratios, enabling the conversion from charcoal (supplies of wood for which were inadequate) to coke.[17]: 122
Theironmasters of theWeald continued producing cast irons until the 1760s, and armament was one of the main uses of irons after theRestoration.
The best way of using cast iron for bridge construction was by usingarches, so that all the material is in compression. Cast iron, again like masonry, is very strong in compression. Wrought iron, like most other kinds of iron and indeed like most metals in general, is strong in tension, and also tough – resistant to fracturing. The relationship between wrought iron and cast iron, for structural purposes, may be thought of as analogous to the relationship between wood and stone.
Cast-iron beam bridges were used widely by the early railways, such as the Water Street Bridge in 1830 at theManchester terminus of theLiverpool and Manchester Railway, but problems with its use became all too apparent when a new bridge carrying theChester and Holyhead Railway across theRiver Dee inChester collapsed killing five people in May 1847, less than a year after it was opened. TheDee bridge disaster was caused by excessive loading at the centre of the beam by a passing train, and many similar bridges had to be demolished and rebuilt, often inwrought iron. The bridge had been badly designed, being trussed with wrought iron straps, which were wrongly thought to reinforce the structure. The centres of the beams were put into bending, with the lower edge in tension, where cast iron, likemasonry, is very weak.
Nevertheless, cast iron continued to be used in inappropriate structural ways, until theTay Rail Bridge disaster of 1879 cast serious doubt on the use of the material. Crucial lugs for holding tie bars and struts in the Tay Bridge had been cast integral with the columns, and they failed in the early stages of the accident. In addition, the bolt holes were also cast and not drilled. Thus, because of casting's draft angle, the tension from the tie bars was placed on the hole's edge rather than being spread over the length of the hole. The replacement bridge was built in wrought iron and steel.
Further bridge collapses occurred, however, culminating in theNorwood Junction rail accident of 1891. Thousands of cast-iron railunderbridges were eventually replaced by steel equivalents by 1900 owing to the widespread concern about cast iron under bridges on the rail network in Britain.
Cast-ironcolumns, pioneered in mill buildings, enabled architects to build multi-storey buildings without the enormously thick walls required for masonry buildings of any height. They also opened up floor spaces in factories, and sight lines in churches and auditoriums. By the mid 19th century, cast iron columns were common in warehouse and industrial buildings, combined with wrought or cast iron beams, eventually leading to the development of steel-framed skyscrapers. Cast iron was also used sometimes for decorative facades, especially in the United States, and theSoho district of New York has numerous examples. It was also used occasionally for complete prefabricated buildings, such as the historicIron Building inWatervliet, New York.[citation needed]
Another important use was intextile mills. The air in the mills contained flammable fibres from the cotton,hemp, orwool being spun. As a result, textile mills had an alarming propensity to burn down. The solution was to build them completely of non-combustible materials, and it was found convenient to provide the building with an iron frame, largely of cast iron, replacing flammable wood. The first such building was atDitherington inShrewsbury, Shropshire.[18] Many other warehouses were built using cast-iron columns and beams, although faulty designs, flawed beams or overloading sometimes caused building collapses and structural failures.[citation needed]
During the Industrial Revolution, cast iron was also widely used for frame and other fixed parts of machinery, including spinning and later weaving machines in textile mills. Cast iron became widely used, and many towns hadfoundries producing industrial and agricultural machinery.[19]
^Zeytin, Havva (2011). "Effect of Boron and Heat Treatment on Mechanical Properties of White Cast Iron for Mining Application".Journal of Iron and Steel Research, International.18 (11):31–39.doi:10.1016/S1006-706X(11)60114-3.S2CID137453839.
^Lyons, William C. and Plisga, Gary J. (eds.)Standard Handbook of Petroleum & Natural Gas Engineering, Elsevier, 2006
^Tylecote, R. F. (1992).A History of Metallurgy, Second Edition. London: Maney Publishing, for the Institute of Materials.ISBN978-0901462886.
^Wagner, Donald B. (May 2008).Science and Civilisation in China: Volume 5, Chemistry and Chemical Technology, Part 11, Ferrous Metallurgy. Cambridge University Press. pp. 159–169.ISBN978-0-521-87566-0.
^Temple, Robert (1986).The Genius of China: 3000 years of science, discovery and invention. New York: Simon and Schuster. Based on the works of Joseph Needham>
^Childs (1991). "Style, technology and iron furnace in Bantu-speaking Africa".Of Anthropological Archaeology.10: 332-359.doi:10.1016/0278-4165(91)90006-J.
^abWagner, Donald B. (2008).Science and Civilisation in China: 5. Chemistry and Chemical Technology: part 11 Ferrous Metallurgy. Cambridge University Press, pp. 349–51.
^Tylecote, R. F. (1992).A History of Metallurgy, Second Edition. London: Maney Publishing, for the Institute of Materials.ISBN978-0901462886.
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