Gray Iron
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Gray iron, or grey iron, is a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite.It is the most common cast iron and the most widely used cast material based on weight.
It is used for housings where tensile strength is non-critical, such as internal combustion engine cylinder blocks, pump housings, valve bodies, electrical boxes, and decorative castings. Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors.
Structure:
A typical chemical composition to obtain a graphitic microstructure is 2.5 to 4.0% carbon and 1 to 3% silicon. Silicon is important to making grey iron as opposed to white cast iron, because silicon is a graphite stabilizing element in cast iron, which means it helps the alloy produce graphite instead of iron carbides. Another factor affecting graphitization is the solidification rate; the slower the rate, the greater the tendency for graphite to form. A moderate cooling rate forms a more pearlitic matrix, while a slow cooling rate forms a more ferritic matrix. To achieve a fully ferritic matrix the alloy must be annealed. Rapid cooling partly or completely suppresses graphitization and leads to formation of cementite, which is called white iron.
The graphite takes on the shape of a three dimensional flake. In two dimensions, as a polished surface will appear under a microscope, the graphite flakes appear as fine lines. The graphite has no appreciable strength, so they can be treated as voids. The tips of the flakes act as preexisting notches; therefore, it is brittle.
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