Cast iron usually refers to grey cast iron, but identifies a large group of ferrous alloys, which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured due to its carbide impurities which allow cracks to pass straight through. Grey cast iron is named after its grey fractured surface, which occurs because the graphitic flakes deflect a passing crack and initiate countless new cracks as the material breaks.
Iron (Fe) accounts for more than 95 %wt of the alloy material, while the main alloying elements are carbon (C) and silicon (Si). The amount of carbon in cast irons is 2.1-4 %wt. Cast irons contain appreciable amounts of silicon, normally 1-3 %wt, and consequently these alloys should be considered ternary Fe-C-Si alloys. Despite this, the principles of cast iron solidification are understood from the binary iron-carbon phase diagram, where the eutectic point lies at 1154 °C and 4.3 wt% carbon. Since cast iron has nearly this composition, its melting temperature of 1150 to 1200 °C is about 300 °C lower than the melting point of pure iron.
Cast iron tends to be brittle, except for malleable cast irons. With its low melting point, good fluidity, castability, excellent machinability and wear resistance, cast irons have become an engineering material with a wide range of applications, including pipes, machine and car parts.
Iron is most commonly melted in a small blast furnace known as a cupola (see blast furnace for more details). After melting is complete, the molten iron is removed or ladled from the forehearth of the blast furnace. This process was devised by the Chinese, whose innovative ideas revolutionized the field of metallurgy. Previously, iron was melted in an air furnace, which is a type of reverberatory furnace.
Silicon is essential to making grey cast iron as opposed to white cast iron. When silicon is alloyed with ferrite and carbon in amounts of about 2 percent, the carbide of iron becomes unstable. Silicon causes the carbon to rapidly come out of solution as graphite, leaving a matrix of relatively pure, soft iron. Weak bonding between planes of graphite lead to a high activation energy for growth in that direction, resulting in thin, round flakes. This structure has several useful properties.
The metal expands slightly on solidifying as the graphite precipitates, resulting in sharp castings. The graphite content also offers good corrosion resistance.
Graphite acts as a lubricant, improving wear resistance. The exceptionally high speed of sound in graphite gives cast iron a much higher thermal conductivity. Since ferrite is so different in this respect (having heavier atoms, bonded much less tightly) phonons tend to scatter at the interface between the two materials. In practical terms, this means that cast iron tends to “damp” mechanical vibrations (including sound), which can help machinery to run more smoothly.
All of the properties listed in the paragraph above ease the machining of grey cast iron. The sharp edges of graphite flakes also tend to concentrate stress, allowing cracks to form much more easily, so that material can be removed much more efficiently.
With a lower silicon content and faster cooling, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe3C, rather than graphite. The cementite which precipitates from the melt forms as relatively large particles, usually in a eutectic mixture where the other phase is austenite (which on cooling might transform to martensite). These eutectic carbides are much too large to provide precipitation hardening (as in some steels, where cementite precipitates might inhibit plastic deformation by impeding the movement of dislocations through the 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 of toughness. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as a cermet. 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 and volute) of slurry pumps, shell liners and lifter bars in ball mills and autogenous grinding mills, balls and rings in coal pulverisers and (conceivably?) balls for rolling-element bearings and the teeth of a backhoe's digging bucket (although the latter two applications would normally use high quality wrought high-carbon martensitic steels and cast medium-carbon martensitic steels respectively).
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 a “chilled casting”, has the benefits of a hard surface and a somewhat tougher interior.
White cast iron can also be made by using a high percentage of chromium in the iron; Cr is a strong carbide-forming element, so at high enough percentages of chrome, the precipitation of graphite out of the iron is suppressed. High-chrome white iron alloys allow massive castings (for example, a 10-tonne impeller) to be sand cast, i.e., a high cooling rate is not required, as well as providing impressive abrasion resistance.
Malleable iron starts as a white iron casting, that is then heat treated at about 900 °C. Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes. Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack or phonon. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by grey cast iron. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable iron, since it is made from white cast iron.
A more recent development is nodular or ductile cast iron. Tiny amounts of magnesium or cerium added to these alloys slow down 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.
Cast iron is melted in furnaces usually in half ton measures. The metal melted usually consists of discs and drums and its properties are changed by adding an inoculant. This alters the characteristics of the metal to various grades and between grey and SG iron.
|Name||Nominal composition [% by weight]||Form and condition||Yield strength [ksi (0.2% offset)]||Tensile strength [ksi]||Elongation [% (in 2 inches)]||Hardness [Brinell scale]||Uses|
|Cast grey iron (ASTM A48)||C 3.4, Si 1.8, Mn 0.5||Cast||—||25||0.5||180||Engine blocks, fly-wheels, gears, machine-tool bases|
|White||C 3.4, Si 0.7, Mn 0.6||Cast (as cast)||—||25||0||450||Bearing surfaces|
|Malleable iron (ASTM A47)||C 2.5, Si 1.0, Mn 0.55||Cast (annealed)||33||52||12||130||Axle bearings, track wheels, automotive crankshafts|
|Ductile or nodular iron||C 3.4, P 0.1, Mn 0.4, Ni 1.0, Mg 0.06||Cast||53||70||18||170||Gears, cams, crankshafts|
|Ductile or nodular iron (ASTM A339)||—||Cast (quench tempered)||108||135||5||310||—|
|Ni-hard type 2||C 2.7, Si 0.6, Mn 0.5, Ni 4.5, Cr 2.0||Sand-cast||—||55||–||550||Strength|
|Ni-resist type 2||C 3.0, Si 2.0, Mn 1.0, Ni 20.0, Cr 2.5||Cast||—||27||2||140||Resistance to heat and corrosion|
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 first invented in China (see also: Du Shi), and poured into molds to make weapons and figurines. Historically, its earliest uses included cannon and shot. In England, the ironmasters of the Weald continued producing these until the 1760s, and this was the main function of the iron industry there after the Restoration, though probably only a minor part of the industry there earlier.
Cast iron pots were made at many English blast furnaces at that period. In 1707, Abraham Darby patented a method of making pots (and kettles) thinner and hence cheaper than his rivals could. This meant that his Coalbrookdale Furnaces became dominant as suppliers of pots, an activity in which they were joined in the 1720s and 1730s by a small number of other coke-fired blast furnaces.
The development of the steam engine by Thomas Newcomen provided a further market for cast iron, since this was considerably cheaper than the brass of which the engine cylinders were originally made. A great exponent of cast iron was John Wilkinson, who amongst other things cast the cylinders for many of James Watt's improved steam engines until the establishment of the Soho Foundry in 1795.
The major use of cast iron for structural purposes began in the late 1770s when Abraham Darby III built the Iron Bridge, although short beams had been used prior to the bridge, such as in the blast furnaces at Coalbrookdale. This was followed by others, including Thomas Paine, who patented one; cast iron bridges became common as the Industrial Revolution gathered pace. Thomas Telford adopted the material for his bridge upstream at Buildwas, and then for a canal trough aqueduct at Longdon-on-Tern on the Shrewsbury Canal.
It was followed by the Chirk Aqueduct and the Pontcysyllte Aqueduct, both of which remain in use following recent restorations. Cast iron beam bridges were used widely by the early railways, such as the Water street bridge at the Manchester terminus of the Liverpool and Manchester Railway. However, problems arose when such a bridge collapsed shortly after opening in 1846. The Dee 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 in wrought iron. The bridge had been under-designed, being trussed with wrought iron straps, which were wrongly thought to reinforce the structure. Nevertheless, cast iron continued to be used for structural support, until the Tay Rail Bridge disaster of 1879 created a crisis of confidence in the material. Further bridge collapses occurred, however, culminating in the Norwood Junction rail accident of 1891. Thousands of cast iron rail under-bridges were eventually replaced by steel equivalents.
Cast iron columns enabled architects to build tall buildings without the enormously thick walls required to construct masonry buildings of any height. It also allowed tall buildings to have large windows, in large cities, manufacturing buildings and early department stores were built with cast iron columns to allow daylight to enter. Examples can be seen in New York City's SoHo Cast Iron Historic District. Architects also liked cast iron because slender cast iron columns could suppport the weight that would require thick masonry columns or piers, opening up floor space in practical building like factories, and sight lines in houses of worship and auditoriums.
During the Industrial Revolution, cast iron was also widely used for the frame and other fixed parts of machinery, including spinning and later weaving machinery in the textile mills. Cast iron became a widespread material, and many towns had foundries producing machinery, not only for industry but also agriculture.