Reference Library: Cast iron
'Cast iron' usually refers to grey cast iron, but can mean any of a group of iron-based alloys containing more than 2% carbon (alloys with less carbon are carbon steel by definition). It 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, which weaken the material. Carbon and silicon content are reduced to the desired levels, which may be anywhere from 2% to 3.5% for carbon and 1% to 3% for silicon depending on the application. Other elements are then added to the melt before the final form is produced by casting.
The iron is today melted in a kind of small blast furnace known as a cupola (see blast furnace for more details). This is thought to have been devised by the late 18th century ironmaster John Wilkinson. Previously, the iron was melted in an air furnace, a variety of reverberatory furnace. Alternatively, molten iron was tapped from (or ladled from the forehearth of a blast furnace.
The iron-carbon eutectic point lies at 1403 kelvins and 4.3 mass % carbon. Since cast iron has nearly this composition, its melting temperature of 1420 to 1470 K is about 300 K lower than the melting point of pure iron. Cast iron tends to be brittle, unless the name of the particular alloy suggests otherwise. The color of a fracture surface can be used to identify an alloy: carbide impurities allow cracks to pass straight through, resulting in a smooth, '"white"' surface, while graphite flakes deflect a passing crack and initiate countless new cracks as the material breaks, resulting in a rough surface that appears 'grey'.
Grey cast iron
Silicon is essential to making of 'grey cast iron' as opposed to white cast iron. 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.
Easier initiation of cracks can be a drawback once an item is finished, however: grey cast iron has less tensile strength and 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.
Other cast iron alloys
With a lower silicon content and faster cooling, the carbon in 'white cast iron' precipitates out of the melt as the metastable phase cementite, FeC, rather than graphite. These precipitates inhibit plastic deformation by impeding the movement of dislocations through the ferrite matrix, offering 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 most uses, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as balls for rolling-element bearings, the wear surfaces (impeller and volute) of slurry pumps and the teeth of a backhoe's digging bucket.
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.
Recycling of Cast Iron
For purposes of recycling, cast iron is classified into two types. One is HMS 1, which means Heavy Melting Scrap grade 1,and HMS 2, which means Heavy Melting Scrap grade 2.
Because cast iron is comparatively brittle, it is unsuitable for purposes where a sharp edge or flexibility is required. It is strong under compression, but not under tension. 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 use of cast iron for structural purposes only began in the late 1770s when Abraham Darby III built the Iron Bridge. This was followed by others, but not initially in great numbers. 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
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.
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