Steel is an alloy of iron and carbon, and sometimes other elements. Because of its high tensile strength and low cost, it is a major component used in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons. Iron is the base metal of steel. Iron is able to take on two crystalline forms (allotropic forms), body-centered cubic (BCC) and face-centered cubic (FCC), depending on its temperature. In the BCC form iron has a density of 7.87 g/cm3 , lower than pure iron’s 7.874 g/cm3 . The FCC form of iron has a higher melting point than BCC iron; at 1,538 °C it melts only slightly above chromium’s 1,483 °C melting point. Steel castings are usually worked hot by forging or machining; they are then heat treated as necessary to achieve desired properties.
While the term “steel” originally referred only to iron alloys with carbon, the usage of the word has evolved to include steel produced without any added carbon, including types such as stainless steel and tool steel. Various grades of carbon steel are distinguished according to their carbon content:
The large quantity of industrial waste generated by manufacturing processes based on Carbon steels presents an important environmental problem. Recycling represents one solution but not all types of waste can be recycled due to their chemical composition or mechanical properties.:37–38
Carbon steels with low carbon content (< 0.20 wt%) are called soft steels because they can easily be cold-formed into shapes such as wires.:142 They cannot be heat treated for hardening purposes because they lack enoughcarbon for carburizing or forming martensite (a very hard microstructure).:159
On the other hand, steels with high levels of carbon (> 0.60 wt%) are known as hard steels because they resist deformation at room temperature due to the formationof martensite.:160