This orders gray iron into classes which correspond with its minimum tensile strength in thousands of pounds per square inch (ksi) e.g. In the United States, the most commonly used classification for gray iron is ASTM International standard A48. Grey iron also has very good damping capacity and hence it is often used as the base for machine tool mountings. The presence of graphite flakes makes the grey iron easily machinable as they tend to crack easily across the graphite flakes. The tips of the flakes act as preexisting notches at which stresses concentrate and it therefore behaves in a brittle manner. The graphite has no appreciable strength, so they can be treated as voids. In two dimensions, as a polished surface, the graphite flakes appear as fine lines. The graphite takes on the shape of a three-dimensional flake. Rapid cooling partly or completely suppresses graphitization and leads to the formation of cementite, which is called white iron. To achieve a fully ferritic matrix the alloy must be annealed. A moderate cooling rate forms a more pearlitic matrix, while a fast cooling rate forms a more ferritic matrix. Another factor affecting graphitization is the solidification rate the slower the rate, the greater the time for the carbon to diffuse and accumulate into graphite. Silicon is important for 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 at 3% silicon almost no carbon is held in chemical form as iron carbide. Graphite may occupy 6 to 10% of the volume of grey iron. Structure Ī typical chemical composition to obtain a graphitic microstructure is 2.5 to 4.0% carbon and 1 to 3% silicon by weight. Deutsche Bahn for example had replaced grey iron brakes on 53,000 of its freight cars (85% of their fleet) with newer, quieter models by 2019-in part to comply with a law that came into force in December 2020. Its former widespread use on brakes in freight trains has been greatly reduced in the European Union over concerns regarding noise pollution. Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors. It is used for housings where the stiffness of the component is more important than its tensile strength, such as internal combustion engine cylinder blocks, pump housings, valve bodies, electrical boxes, and decorative castings. It is the most common cast iron and the most widely used cast material based on weight. It is named after the gray color of the fracture it forms, which is due to the presence of graphite. Gray iron, or grey cast iron, is a type of cast iron that has a graphitic microstructure.
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