Cast iron
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224975876
2008-07-11T07:33:45Z
Peterlewis
813120
[[WP:UNDO|Undid]] revision 224969416 by [[Special:Contributions/220.238.166.95|220.238.166.95]] ([[User talk:220.238.166.95|talk]])
{{For|cookware|Cast iron cookware}}
{{Steels}}
[[Image:Phase diag iron carbon.PNG|thumb|right|'''Iron-Cementite''' meta-stable diagram.]]
'''Cast iron''' usually refers to [[Gray iron|grey cast iron]], but identifies a large group of [[ferrous]] [[alloy]]s, 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 compound|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 [[ductility|brittle]], except for [[malleable iron|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.
== 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]].
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 [[ladle (metallurgy)|ladled]] from the forehearth of the [[blast furnace]]. This process was devised by the [[China|Chinese]], whose innovative ideas revolutionized the field of [[metallurgy]]. Previously, iron was melted in an air furnace, which is a type of [[reverberatory furnace]].
==Varieties of cast iron==
===Grey cast iron===
[[Image:Cast_fe_and_cu_dwv_piping.jpg|thumb|right|Cast iron drain, waste and vent piping in a [[Canada|Canadian]] timber-frame building in [[Mission, British Columbia]] in the 1980s.]]
{{main|Grey iron}}
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) [[phonon]]s 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 [[stress concentration|concentrate stress]], allowing cracks to form much more easily, so that material can be removed much more efficiently.
Easier initiation of cracks can be a drawback once an item is finished, however: grey cast iron has less [[tensile strength]] and [[toughness|shock resistance]] than steel. It is also difficult to weld.
Grey cast iron's high thermal conductivity and [[specific heat capacity]] are often exploited to make [[cast iron cookware]] and [[disc brake]] rotors.
===Other cast iron alloys===
[[Image:Yuan Dynasty - waterwheels and smelting.png|thumb|left|Furnace bellows operated by [[waterwheel]]s, from the ''Nong Shu'', by [[Wang Zhen (official)|Wang Zhen]], 1313 AD, during the Chinese [[Yuan Dynasty]].]]
With a lower silicon content and faster cooling, the carbon in '''white cast iron''' precipitates out of the melt as the [[metastable]] phase [[cementite]], Fe<sub>3</sub>C, 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 [[dislocation]]s 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 (disambiguation)|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 bearing]]s 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 treatment|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 (geometry)|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 iron|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.
{| class="wikitable"
|+'''Comparative qualities of cast irons'''<ref>Lyons, William C. and Plisga, Gary J. (eds.) ''Standard Handbook of Petroleum & Natural Gas Engineering'', Elsevier, 2006</ref>
|-
! Name
! Nominal composition [% by weight]
! Form and condition
! Yield strength <nowiki>[</nowiki>[[Pounds per square inch#ksi and kip|ksi]] (0.2% offset)]
! Tensile strength [ksi]
! Elongation [% (in 2 inches)]
! Hardness <nowiki>[</nowiki>[[Brinell scale]]<nowiki>]</nowiki>
! Uses
|-
| Cast grey iron ([[ASTM International|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
|}
==Historical uses==
[[Image:Ww1.JPG|right|thumb|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. In [[England]], the [[ironmaster]]s of the [[Weald]] continued producing these until the [[1760s]], and this was the main function of the [[iron]] industry there after the [[English Restoration|Restoration]], though probably only a minor part of the industry there earlier.
Cast iron pots were made at many [[England|English]] [[blast furnace]]s at that period. In 1707, [[Abraham Darby I|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 (fuel)|coke]]-fired blast furnaces.
The development of the [[Newcomen engine|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 (industrialist)|John Wilkinson]], who amongst other things cast the cylinders for many of [[James Watt]]'s improved [[Watt steam engine|steam engine]]s until the establishment of the [[Soho Foundry]] in [[1795]].
===Cast iron bridges===
[[Image:Eglinton Castle & Tournament Bridge 1884.jpg|thumb|left|The [[Eglinton Tournament Bridge]], [[North Ayrshire]], [[Scotland]], built from cast iron.]]
[[Image:Ironbridge 6.jpg|right|thumb|[[The Iron Bridge]] over the [[River Severn]] at [[Coalbrookdale]], [[England]]]]
The major use of cast iron for structural purposes began in the late [[1770s]] when [[Abraham Darby III]] built the [[The Iron Bridge|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]].
[[Image:WalesC0047.jpg|thumb|The [[Pontcysyllte Aqueduct]], [[Llangollen]], [[Wales]], viewed from the ground]]
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.
===Textile mills===
Another important use was in [[textile mill]]s. The air in these contained flammable fibres from the [[cotton]], [[hemp]], or [[wool]] being spun. As a result, [[textile mill]]s 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 [[foundry|foundries]] producing machinery, not only for industry but also [[agriculture]].
==See also==
*[[Cast-iron architecture]]
*[[Cast iron cookware]]
*[[Mechanical bank]]
*[[Sand casting]]
==References==
* John Gloag and Derek Bridgwater, ''A History of Cast Iron in Architecture'', Allen and Unwin, London (1948)
* Peter R Lewis, ''Beautiful Railway Bridge of the Silvery Tay: Reinvestigating the Tay Bridge Disaster of 1879'', Tempus (2004) ISBN 07524 3160 9
* Peter R Lewis, ''Disaster on the Dee: Robert Stephenson's Nemesis of 1847'', Tempus (2007) ISBN 0 7524 4266 2
* George Laird, Richard Gundlach and Klaus Röhrig, ''Abrasion-Resistant Cast Iron Handbook'', ASM International (2000) ISBN 0-87433-224-9
{{reflist}}
== External links ==
* [http://www.msm.cam.ac.uk/phase-trans/2001/adi/cast.iron.html Metallurgy of Cast Irons, Cambridge University]
* [http://materials.open.ac.uk/about_us/isr1707lowres.pdf Paper on the Tay bridge disaster]
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