Martensite

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Named for the German metallurgist Adolph Martens, Martensite is the hardened phase of steel that is obtained by cooling Austenite fast enough to trap carbon atoms within the cubic iron matrix distorting it into a body centered tetragonal structure.

Since its formation is accomplished by quick cooling, avoiding the formation of pearlite, it is a sheer dependent diffusionless transformation. Free of diffusion processes, it has the same composition as the parent austenite

Under the microscope in cross section it appears, and is often described, as acicular or needle like, but in 3 dimensions is actually either lathe or plate in structure. Alloys with less than .6 percent carbon form lathe martensite. Alloys with more than 1 percent carbon form plate martensite and alloys with from .6 to 1 percent carbon form mixtures of the two in varying degrees.

The temperature at which martensite begins to form in an alloy is given the designation Ms (martensite start) the temperature at which martensite is done forming is Mf . Formation occurs in "packets", with lathe martensite, as the crystalline structure begins with single plane forming across the grain with many subsequent branches nucleating from the central one in a parallel "fern' or "feather" like configuration. Plate martensites form in larger plates that have a tendency to approach others at varying angles (irrational habit planes). Points of great stress are created where the plates intersect, causing plate martensite to be more brittle.








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