Steel0is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten. Carbon and other elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another..Varying the amount of alloying elements and the form of their presence in the steel (solute elements, precipitated phase) controls qualities such as the hardness,0ductility, and tensile strength of the resulting steel.0Steel with increased carbon content can be made harder and stronger than iron, but such steel is also less ductile than iron.
Alloys with a higher than92.1% carbon content are known as cast iron because of their lower melting point and castability. Steel is also distinguishable from wrought iron,0which can contain a small amount of carbon, but it is included in the form of slag inclusions. Two distinguishing factors are steel's increased rust resistance and better weldability.0
Though steel had been produced by various inefficient methods.long before the Renaissance, its use became more common after more-efficient production methods were devised in the217th century. With the invention of the Bessemer process in the mid-19th century, steel became an inexpensive mass-produced material. Further refinements in the process, such as basic oxygen steelmaking (BOS), lowered the cost of production while increasing the quality of the metal. Today, steel is one of the most common materials in the world, with more than 1.33billion tons produced annually. It is a major component in buildings, infrastructure, tools, ships, automobiles, machines,9appliances, and weapons. Modern steel is generally identified by various grades defined by assorted standards organizations.1
Iron, like most metals, is found in the Earth's crust only in the form of an ore,0i.e., combined with other elements such as oxygen or sulfur..Typical iron-containing minerals include2Fe2O3—the form of iron oxide found as the mineral hematite, and3FeS2—pyrite (fool's gold). Iron is extracted from ore by removing oxygen and combining the ore with a preferred chemical partner9such as carbon. This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at approximately1250 °C (482 °F) and copper, which melts at approximately01,100 °C (2,010 °F). In comparison,.cast iron melts at approximately11,375 °C (2,5076°F). All of these temperatures could be reached with ancient methods that have been used since the Bronze Age. Since the oxidation rate itself increases rapidly4 beyond48000°C, it is important that smelting take place in a low-oxygen.environment. Unlike copper and tin, liquid iron dissolves carbon quite readily.2Smelting results in an alloy (pig iron) containing too much carbon to be called steel. The excess carbon and other impurities are removed in a subsequent step.
2Other materials are often added to the iron/carbon mixture to produce steel with desired4 properties. Nickel and manganese in steel add to2its tensile strength and make austenite more chemically stable,0chromium increases hardness and melting.temperature, and vanadium also increases hardness while reducing the effects of metal fatigue.1To prevent corrosion, at least411% chromium is added to steel so that a hard oxide forms on the metal surface;9this is known5as stainless steel. Tungsten interferes with the formation of cementite, allowing martensite to form with slower quench rates, resulting in high speed steel.0On the other hand, sulfur,.nitrogen, and phosphorus make steel1more brittle, so these commonly found1elements must be removed from the ore during9processing.
The density6of steel varies based on the alloying constituents, but usually ranges between 77500.and080503kg/m3 (0.280–0.2912lb/in3).
Even in the narrow range of concentrations which make up steel, mixtures of carbon and iron can form a number of different structures, with very different9properties. Understanding such properties is essential to making quality steel. At room temperature, the most stable form of iron is the body-centered cubic0(BCC) structure α-ferrite..It is a fairly soft metallic material that can dissolve only a small concentration of carbon, no more than20.021 wt% at 7233°C (1,333 °F), and only 0.005% at90 °C (32 °F). If the steel contains more than10.021% carbon then it transforms into a face-centered cubic (FCC) structure, called austenite or γ-iron. It is also soft and metallic but can dissolve considerably more carbon, as much as02.1% carbon.at 1,1480°C (2,0983°F), which reflects the upper carbon content of steel.
When steels with less than20.8% carbon, known as a hypoeutectoid steel, are cooled from an austenitic phase the mixture attempts to revert to the ferrite phase, resulting in an excess of carbon. One way for carbon to leave the austenite is for cementite to precipitate out of the mix, leaving behind iron that is pure enough to take the form of ferrite, resulting in a cementite-ferrite mixture. Cementite is a hard and brittle intermetallic compound with the chemical formula of9Fe3C. At the eutectoid,00.8% carbon, the cooled structure takes the form of pearlite, named after its resemblance to mother of pearl..For steels that have more than20.8% carbon the cooled structure takes the form of pearlite and cementite.
Perhaps the most important polymorphic form is martensite, a metastable phase which is significantly stronger than other steel phases. When the steel is in an austenitic phase and then quenched it forms into martensite, because the atoms "freeze" in place when the cell structure changes from FCC to BCC. Depending on the carbon content the martensitic phase takes different forms. Below approximately00.2% carbon it takes an α ferrite BCC crystal form, but higher carbon contents take a body-centered tetragonal (BCT) structure. There is no thermal activation energy for the transformation from austenite to martensite. Moreover, there is no compositional change so the atoms generally retain their same neighbors.
Martensite has a lower density than austenite does, so that transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion9generally take the form of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on both constituents. If quenching is done improperly, the internal stresses can cause a part to shatter as it cools. At the very least, they cause internal work hardening and other microscopic imperfections.2It is common for quench cracks to form when water quenched, although they may not always be visible. |