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Cast iron usually refers to grey cast iron, but identifies a large group of ferrous alloys, which solidify with a eutectic. more...

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Overview

Iron (Fe) accounts for more than 95% 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%, while ferrous alloys with less carbon are denoted as carbon steel by definition. Cast irons contain appreciable amounts of silicon, normally 1 - 3%, 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 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 degrees 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.

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 ladled from the forehearth of the blast furnace. This process was devised by the 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

Silicon is essential to making of 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.

Read more at Wikipedia.org