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.

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.

Related Topics:
Graphite - Activation energy

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The metal expands slightly on solidifying as the graphite precipitates, resulting in sharp castings. The graphite content also offers good corrosion resistance.

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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.

Related Topics:
Speed of sound - Thermal conductivity - Phonon - Sound

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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.

Related Topics:
Machining - Concentrate stress

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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.

Related Topics:
Tensile strength - Shock resistance

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Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cookware. Cast-iron pots and pans are durable and provide even heating, but require seasoning—impregnation of the cooking surface with oil—to prevent rusting and to create a non-stick surface. Modern cast-iron pans are available pre-seasoned, but during cleaning one must take care not to scrub off this protective layer. One should never use soap to clean cast iron cookware; it will immediately remove the oil from it and 'unseason' the pan.

Related Topics:
Specific heat capacity - Cookware - Seasoning - Rusting

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