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Material properties Iron-carbon phase diagram, showing the conditions necessary to form different phases Iron, like most metals, is found in the Earth’s crust only in the form of an ore, ie. combined with other elements such as oxygen or sulfur. Typical iron-containing minerals include Fe2O3he form of iron oxide found as the mineral hematite, and FeS2yrite (fool’s gold). Iron is extracted from ore by removing oxygen and combining the ore with a preferred chemical partner such as carbon.
This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at approximately 250 C (482 F) and copper, which melts at approximately 1,000 C (1,830 F). In comparison, cast iron melts at approximately 1,370 C (2,500 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 rapidly beyond 800 C, it is important that smelting take place in a low-oxygen environment. Unlike copper and tin, liquid iron dissolves carbon quite readily. Smelting 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. 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 austenite more chemically stable, chromium increases hardness and melting temperature, and vanadium also increases hardness while reducing the effects of metal fatigue.
To prevent corrosion, at least 11% chromium is added to steel so that a hard oxide Fender® Special Edition American Standard Telecaster® – Daphne Blue forms on the metal surface; this is known as stainless steel. Tungsten interferes with the formation of cementite, allowing martensite to form with 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 ore during processing. The density of steel varies based on the alloying constituents, but usually ranges between 7.75 and 8.05 g/cm3 (0.2800.291 lb/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 different properties. Understanding such properties is essential to making quality steel. At room temperature, the most stable form of iron is the body-centered cubic (BCC) structure -ferrite. It is a fairly soft metallic material that can dissolve only a small concentration of carbon, no more than 0.021 wt% at 723 C (1,333 F), and only 0.005% at 0 C (32 F). If the steel contains more than 0.021% carbon then it transforms into a face-centered cubic (FCC) structure, called austenite or -iron. It is also soft and metallic but can dissolve considerably more carbon, as much as 2.1% carbon at 1,148 C (2,098 F)), which reflects the upper carbon content of steel. When steels with less than 0.8% 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 of Fe3C. At the eutectoid, 0.8% carbon, the cooled structure takes the form of pearlite, named after its resemblance to mother of pearl. For steels that have more than 0.8% 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 approximately 0.2% carbon it takes an 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 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 water

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