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). more...

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-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.

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