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Question: What are the chief impurities in cast iron. How do these impurities affect the properties of iron
Cast iron, an alloy of iron, contains various impurities that can significantly alter its properties. The main impurities in cast iron are: 1. Carbon: One of the main impurities, usually present in amounts of 2-4%. Carbon impacts the hardness and brittleness of the cast iron. Lower the carbon content, the harder and more brittle the iron becomes. When present as free carbon or graphite, it imparts softness and improves machinability. 2. Silicon: Usually present in amounts of 1-3%. Silicon influences the form in which carbon is present. Silicon encourages the formation of graphite, leading to an increase in toughness and ductility. 3. Sulfur: Found in small amounts, typically up to 0.12%. Sulfur forms iron sulfide with iron, which has a lower melting point than iron. This low melting point causes a weakening of the iron at high temperatures, thereby reducing its heat resistance. Sulfur can also increase the brittleness of the cast iron. 4. Phosphorus: Usually present in amounts of up to 1%. Phosphorus enhances the fluidity of cast iron which improves casting. However, it generally reduces tensile strength and toughness, therefore, its presence is often undesired. 5. Manganese: Present in small amounts, it tends to form manganese sulfide rather than iron sulfide, thereby reducing the harmful effects of sulfur. It also stabilizes the pearlite structure of the metal. The properties of the cast iron are highly dependent on the levels and types of impurities present. Certain impurities can be advantageous, enhancing certain properties, like machinability and fluidity. On the other hand, the presence of excessive levels or certain impurities can negatively impact properties like strength and ductility.
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