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Carbon steel is steel whose main alloying constituent is carbon, with iron making up the remainder, and other elements present in quantities too small to affect the properties. The only other alloying elements allowed in plain-carbon steel are manganese (1.65% max), silicon (0.60% max), and copper (0.60% max); there is no specified minimum content of these or any other alloying metal.

Steel with a low carbon content has properties similar to iron, soft but easily formed. As the carbon content rises, the metal becomes harder and stronger but less ductile and more difficult to weld. Carbon content as an alloying metal influences the yield strength of steel because carbon mixes readily with it and reinforces this iron base, which results in a hardening effect on the iron. Higher carbon content lowers steel’s melting point and its temperature resistance in general.

Due to the lack of other constituent alloys, carbon steel is also more inexpensive than other steels, such as stainless steel or chrome-moly (low alloy) steel, though also has a lower resistance to corrosion. Though worn out quickly, it has a much lower replacement cost than other steels, such that the decreased life span is offset by this decreased replacement cost. As a result, carbon steel is used in many applications across a myriad of industries, from petrochemical to waste water. For example, carbon steel accounts for 85% of all steel used in the United States.

There are two basic categories of carbon steel: mild and low carbon steels, and high carbon steels.

Mild steel is the most common form of steel as its price is relatively low while it provides material properties that are acceptable for many applications. Low carbon steel contains approximately 0.05–0.15% Carbon, while mild steel has a range of 0.16–0.29% Carbon, making them neither brittle nor ductile. Mild steel has a relatively low tensile strength, but it is cheap and malleable, and surface hardness can be increased through carburizing. It is often used when large amounts of steel are needed, such as structural steel.

Carbon steels that can successfully undergo heat-treatment have a carbon content in the range of 0.30–1.70% by weight. These types of carbon steel are referred to as high yield carbon steels. The heat treatment on these metals is further developed during the forging process, resulting in a change to the mechanical properties of the steel, most notably ductility, hardness, yield strength, and impact resistance; these steels are therefore generally harder with higher yield strength. As trace impurities of various other elements can have a significant effect on the quality of the resulting steel, they are therefore avoided.

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