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The term "martensite" usually refers to a form of steel with a distinctive atomic structure created through a process called martensitic transformation. Martensite is very hard, meaning that it won't dent or scratch easily; this makes it a popular choice for tools, such as hammers and chisels, as well as swords. It is brittle, however, so it will break rather than bend when put under too much pressure. Martensite is made from austenite, a solid solution of iron with a small amount of carbon in it.
Austenite has a particular crystalline structure known as face-centered cubic (FCC). This means that each cubic unit has a lattice point in the center of each side as well as at each corner; with the lattice points connected, the crystal would look like a square box with an X on each side. This type of steel begins to form at temperatures of about 1,350°F (732°C). Austenite can hold more carbon than other forms of iron. If allowed to cool naturally, austenite turns into ferrite (alpha iron or pure iron) and cementite (iron carbide).
Martensitic transformation occurs when the austenite is rapidly cooled in a process known as quenching. The rapid drop in temperature traps the carbon atoms inside the crystal structures of the iron atoms. This causes the crystals to change from FCC to body-centered tetragonal (BCT); the crystals are stretched so that they are square on each end but longer on the sides (like a shoe box), and the lattice points that were in the center of each face are now joined together at one point in the center of the crystal. This new structure is what greatly increases the hardness of the steel.
The resulting martensitic steel is extremely hard, meaning that it won't scratch, but very brittle, so it will break under stress. To address this weakness, martensite is heated in a process called tempering, which causes the martensite to transform partially into ferrite and cementite. This tempered steel is not quite as hard, but becomes tougher (less likely to break) and more malleable, and thus better suited for industrial use.
Tempered martensite's hardness makes it a good material for tool steels, since resistance to abrasion and deformation is important in such applications. It is a common component in machine parts and forging dies. Tempered steels containing silicon are often used for spring steel, which can be used to make springs, musical instrument strings, and components on model trains and other toys. Spring steel can be twisted or bent without permanent deformation, making it a good choice for components that require the steel to move repeatedly without degradation.
Stainless steel, which contains chromium as well as iron and carbon, can also be made with a martenistic crystalline structure. This form is less resistant to corrosion than other forms of stainless steel, but it is also stronger and more easily machined in most cases. One method of making it, called precipitation hardening (or age hardening), adds impurities like chromium and nickel during a process of extended heat treatment; precipitation-hardened martensitic stainless steel has even greater strength along with high corrosion resistance. Such steel is often used in military and aerospace applications.
The martensitic transformation is the best-known example of displacive transformation, a type of phase change in which the atoms of a material move short distances in unison rather than diffusing individually over longer distances. A phase change occurs when a substance changes from one state, like a solid, to another, like a liquid. Because they are so well known as a type of displacive transformation, the terms "martensite" or "martensitic" are sometimes used in a broader sense to describe any material produced by displacive transformation.