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Martensite is a very hard form of steel crystalline structure. It is named after German metallurgist Adolf Martens . By analogy the term can also refer to any crystal structure that is formed by diffusionless transformation .
As a result of the transformation, the microconstituents, pearlite and bainite, form; pearlite forms at higher temperatures and bainite at lower. TTT diagram of isothermal transformations of a hypoeutectoid carbon steel; showing the main components obtained when cooling the steel and its relation with the Fe-C phase diagram of carbon steels.
Tempering reduces the hardness in the martensite by transforming it into various forms of tempered martensite. Tempering is a heat treatment technique applied to ferrous alloys , such as steel or cast iron , to achieve greater toughness by decreasing the hardness of the alloy.
In the martempering process, austenitized metal part is immersed in a bath at a temperature just above the martensite start temperature (Ms). By using interrupted quenching, the cooling is stopped at a point above the martensite transformation region to ensure sufficient time for the center to cool to the same temperature as the surface.
The term "martensite" was originally coined to describe the rigid and finely dispersed constituent that emerges in steels subjected to rapid cooling. Subsequent investigations revealed that materials beyond ferrous alloys, such as non-ferrous alloys and ceramics, can also undergo diffusionless transformations.
A deeper eutectic or more rapid cooling will result in finer lamellae; as the size of an individual lamellum approaches zero, the system will instead retain its high-temperature structure. Two common cases of this include cooling a liquid to form an amorphous solid, and cooling eutectoid austenite to form martensite.
[9] [10] The alloy composition, and the high cooling rate of quenching enable the formation of martensite. Untempered martensite is low in toughness and therefore brittle.Tempered martensite gives steel good hardness and high toughness as can be seen below, and is largely used for medical surgical instruments, such as scalpels, razors, and ...
This phase is called martensite, and is extremely hard due to a combined effect of the distorted crystal structure and the extreme solid solution strengthening, both mechanisms of which resist slip dislocation. All hardening mechanisms introduce crystal lattice defects that act as barriers to dislocation slip.