Cast iron

Cast iron usually refers to grey iron, but also identifies a large group of ferrous alloys, which solidify with a eutectic. The colour 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% by weight (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 to 4 wt%. Cast irons contain appreciable amounts of silicon, normally 1 to 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 1,154 °C (2,109 °F) and 4.3 wt% carbon. Since cast iron has nearly this composition, its melting temperature of 1,150 to 1,200 °C (2,100 to 2,190 °F) is about 300 °C (572 °F) 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, resistance to deformation, and wear resistance, cast irons have become an engineering material with a wide range of applications, including pipes, machine and automotive industry parts, such as cylinder heads (declining usage), cylinder blocks, and gearbox cases (declining usage). It is resistant to destruction and weakening by oxidisation (rust).

Production

Cast iron is made by remelting pig iron, often along with substantial quantities of scrap iron and scrap steel, and taking various steps to remove undesirable contaminants such as phosphorus and sulfur. Depending on the application, carbon and silicon content are reduced to the desired levels, which may be anywhere from 2 to 3.5% and 1 to 3% respectively. Other elements are then added to the melt before the final form is produced by casting.^{[citation needed]}

Iron is sometimes melted in a special type of blast furnace known as a cupola, but more often melted in electric induction furnaces.^{[citation needed]} After melting is complete, the molten iron is poured into a holding furnace or ladle.

Types

Alloying elements

Cast iron's properties are changed by adding various alloying elements, or alloyants. Next to carbon, silicon is the most important alloyant because it forces carbon out of solution. Instead the carbon forms graphite which results in a softer iron, reduces shrinkage, lowers strength, and decreases density. Sulfur, when added, forms iron sulfide, which prevents the formation of graphite and increases hardness. The problem with sulfur is that it makes molten cast iron sluggish, which causes short run defects. To counter the effects of sulfur manganese is added because the two form into manganese sulfide instead of iron sulfide. The manganese sulfide is lighter than the melt so it tends to float out of the melt and into the slag. The amount of manganese required to neutralize sulfur is 1.7×sulfur content+0.3%. If more than this amount of manganese is added then manganese carbide forms which increase hardness and chilling, except in grey iron, where up to 1% of manganese increases strength and density.^[1]

Nickel is one of the most common alloyants because it refines the pearlite and graphite structure, improves toughness, and evens out hardness differences between section thicknesses. Chromium is added in small amounts to the ladle to reduce free graphite, produce chill, and because it is a powerful carbide stabilizer; nickel is often added in conjunction. A small amount of tin 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 to 2.5%, to decrease chill, refine graphite, and increase fluidity. Molybdenum is added on the order of 0.3 to 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. 0.15 to 0.5% vanadium are added to cast iron to stabilize cementite, increase hardness, and increase resistance to wear and heat. 0.1 to 0.3% zirconium helps to form graphite, deoxidize, and increase fluidity.^[1]

In malleable iron melts bismuth is added, on the scale of 0.002 to 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.^[1]

Grey cast iron

Grey cast iron is characterized 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 use cast material base on weight. Most cast irons have a chemical composition of 2.5 to 4.0% carbon, 1 to 3% silicon, and the remainder is iron. Grey cast iron has less tensile strength and shock resistance than steel, however its compressive strength is comparable to low and medium carbon steel.

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White cast iron

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 the teeth of a backhoe's digging bucket (although cast medium-carbon martensitic steel is more common for this application).

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 (Cr) 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 cast iron

Malleable iron starts as a white iron casting, that is then heat treated at about 900 °C (1,650 °F). 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.

Ductile 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.

Table of comparative qualities of cast irons

Comparative qualities of cast irons^[2]
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
Grey cast iron (ASTM A48)	C 3.4, Si 1.8, Mn 0.5	Cast	—	25	0.5	180	Engine cylinder blocks, flywheels, gears, machine-tool bases
White cast iron	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, camshafts, 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	High strength applications
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

Historical uses

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A cast iron wagon wheel

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. Henry VIII initiated the casting of cannon in England. Soon English iron workers using blast 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. 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.

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.

Cast iron bridges

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 a new bridge carrying the Chester and Holyhead Railway across the River Dee in Chester collapsed in May 1847, less than a year after it was opened. This 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. The centres of the beams were put into bending, with the lower edge in tension, where cast iron is very weak. The best way of using cast iron was by using arches, so that all the material is in compression, where it is very strong. 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. Crucial lugs for holding tie bars and struts had been cast integral with the columns, and they failed during the early stages of the accident. In addition, the bolt holes were also cast and not drilled, so that all the tension from the tie bars was placed on a corner, 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 the Norwood Junction rail accident of 1891. Thousands of cast iron rail under-bridges were eventually replaced by steel equivalents.

Original Tay Bridge from the north

Original Tay Bridge from the north
Fallen Tay Bridge from the north
The iron bridge over the River Severn at Coalbrookdale, England
The Eglinton Tournament Bridge, North Ayrshire, Scotland, built from cast iron
The Pontcysyllte Aqueduct, Llangollen, Wales, viewed from the ground

Buildings

Cast iron columns enabled architects to build tall buildings without the enormously thick walls required to construct masonry buildings of any height. This 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 support 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.

Textile mills

Another important use was in textile mills. The air in these contained flammable fibres from the cotton, hemp, or wool 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. This replaced flammable wood. The first such building was at Ditherington in Shrewsbury, Shropshire. Many other warehouses were built using cast iron columns and beams, although there were many collapses owing to faulty designs, flawed beams or overloading.

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.

References

^ ^a ^b ^c Gillespie, LaRoux K. (1988), Troubleshooting manufacturing processes (4th ed.), SME, p. 4-4, ISBN 9780872633261.
^ Lyons, William C. and Plisga, Gary J. (eds.) Standard Handbook of Petroleum & Natural Gas Engineering, Elsevier, 2006

External links

[gillespie-1] Gillespie, LaRoux K. (1988), Troubleshooting manufacturing processes (4th ed.), SME, p. 4-4, ISBN 9780872633261.

[2] Lyons, William C. and Plisga, Gary J. (eds.) Standard Handbook of Petroleum & Natural Gas Engineering, Elsevier, 2006

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