Material properties::Steel


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Material properties

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Iron-carbon phase diagram, showing the conditions necessary to form different phases

Iron is commonly found in the Earth's crust in the form of an ore, usually an iron oxide, such as magnetite, hematite etc. Iron is extracted from iron ore by removing the oxygen through combination with a preferred chemical partner such as carbon that is lost to the atmosphere as carbon dioxide. This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at approximately {{safesubst:#invoke:convert|convert}} and copper, which melts at approximately {{safesubst:#invoke:convert|convert}}. In comparison, cast iron melts at approximately {{safesubst:#invoke:convert|convert}}.<ref name="Smelting">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> Small quantities of iron were smelted in ancient times, in the solid state, by heating the ore buried in a charcoal fire and welding the metal together with a hammer, squeezing out the impurities. With care, the carbon content could be controlled by moving it around in the fire.

All of these temperatures could be reached with ancient methods that have been used since the Bronze Age. Since the oxidation rate of iron increases rapidly beyond {{safesubst:#invoke:convert|convert}}, it is important that smelting take place in a low-oxygen environment. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily. Smelting, using carbon to reduce iron oxides, results in an alloy (pig iron) that retains too much carbon to be called steel.<ref name="Smelting"/> The excess carbon and other impurities are removed in a subsequent step.

Other materials are often added to the iron/carbon mixture to produce steel with desired properties. Nickel and manganese in steel add to its tensile strength and make the austenite form of the iron-carbon solution more stable, chromium increases hardness and melting temperature, and vanadium also increases hardness while making it less prone to metal fatigue.<ref name=materialsengineer>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

To inhibit corrosion, at least 11% chromium is added to steel so that a hard oxide forms on the metal surface; this is known as stainless steel. Tungsten interferes with the formation of cementite, allowing martensite to preferentially form at slower quench rates, resulting in high speed steel. On the other hand, sulfur, nitrogen, and phosphorus make steel more brittle, so these commonly found elements must be removed from the steel melt during processing.<ref name="materialsengineer"/>

The density of steel varies based on the alloying constituents but usually ranges between {{safesubst:#invoke:convert|convert}}, or {{safesubst:#invoke:convert|convert}}.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Even in a narrow range of concentrations of mixtures of carbon and iron that make a steel, a number of different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel. At room temperature, the most stable form of pure iron is the body-centered cubic (BCC) structure called ferrite or α-iron. It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0.005% at {{safesubst:#invoke:convert|convert}} and 0.021 wt% at {{safesubst:#invoke:convert|convert}}. At 910 °C pure iron transforms into a face-centered cubic (FCC) structure, called austenite or γ-iron. The FCC structure of austenite can dissolve considerably more carbon, as much as 2.1%<ref>Sources differ on this value so it has been rounded to 2.1%, however the exact value is rather academic because plain-carbon steel is very rarely made with this level of carbon. See:

  • {{#invoke:Footnotes|harvard_citation_no_bracket}}—2.08%.
  • {{#invoke:Footnotes|harvard_citation_no_bracket}}—2.11%.
  • {{#invoke:Footnotes|harvard_citation_no_bracket}}—2.14%.</ref> (38 times that of ferrite) carbon at {{safesubst:#invoke:convert|convert}}, which reflects the upper carbon content of steel, beyond which is cast iron.<ref>{{#invoke:Footnotes|harvard_citation_no_bracket}}.</ref>

When steels with less than 0.8% carbon (known as a hypoeutectoid steel), are cooled, the austenitic phase (FCC) of the mixture attempts to revert to the ferrite phase (BCC). The carbon no longer fits within the FCC structure, resulting in an excess of carbon. One way for carbon to leave the austenite is for it to precipitate out of solution as cementite, leaving behind a surrounding phase of BCC iron that is low enough in carbon to take the form of ferrite, resulting in a ferrite matrix with cementite inclusions. Cementite is a hard and brittle intermetallic compound with the chemical formula of Fe3C. At the eutectoid, 0.8% carbon, the cooled structure takes the form of pearlite, named for its resemblance to mother of pearl. On a larger scale, it appears as a lamellar structure of ferrite and cementite. For steels that have more than 0.8% carbon, the cooled structure takes the form of pearlite and cementite.<ref>{{#invoke:Footnotes|harvard_citation_no_bracket}}.</ref>

Perhaps the most important polymorphic form of steel is martensite, a metastable phase that is significantly stronger than other steel phases. When the steel is in an austenitic phase and then quenched rapidly, it forms into martensite, as the atoms "freeze" in place when the cell structure changes from FCC to a distorted form of BCC as the atoms do not have time enough to migrate and form the cementite compound. Depending on the carbon content, the martensitic phase takes different forms. Below approximately 0.2% carbon, it takes on a ferrite BCC crystal form, but at higher carbon content it takes 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.<ref name="smith&hashemi">{{#invoke:Footnotes|harvard_citation_no_bracket}}.</ref>

Martensite has a lower density than does austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally 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. It is common for quench cracks to form when steel is water quenched, although they may not always be visible.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Heat treatment


There are many types of heat treating processes available to steel. The most common are annealing, quenching, and tempering. Annealing is the process of heating the steel to a sufficiently high temperature to soften it. This process goes through three phases: recovery, recrystallization, and grain growth. The temperature required to anneal steel depends on the type of annealing to be achieved and the constituents of the alloy.<ref>{{#invoke:Footnotes|harvard_citation_no_bracket}}.</ref>

Quenching and tempering first involves heating the steel to the austenite phase then quenching it in water or oil. This rapid cooling results in a hard but brittle martensitic structure.<ref name="smith&hashemi"/> The steel is then tempered, which is just a specialized type of annealing, to reduce brittleness. In this application the annealing (tempering) process transforms some of the martensite into cementite, or spheroidite and hence reduces the internal stresses and defects. The result is a more ductile and fracture-resistant steel.<ref>{{#invoke:Footnotes|harvard_citation_no_bracket}}.</ref>

Steel sections
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Material properties
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