thumb|200px|Martensite in AISI 4140 steel

thumb|200px|0.35% carbon steel, water-quenched from 870 °C

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.

The martensitic reaction begins during cooling when the austenite reaches the martensite start temperature (M<sub>s</sub>), and the parent austenite becomes mechanically unstable. As the sample is quenched, an increasingly large percentage of the austenite transforms to martensite until the lower transformation temperature M<sub>f</sub> is reached, at which time the transformation is completed.

For a eutectoid steel (0.76% C), between 6 and 10% of austenite, called retained austenite, will remain. The percentage of retained austenite increases from insignificant for less than 0.6% C steel, to 13% retained austenite at 0.95% C and 30–47% retained austenite for a 1.4% carbon steel. A very rapid quench is essential to create martensite. For a eutectoid carbon steel of thin section, if the quench starting at 750&nbsp;°C and ending at 450&nbsp;°C takes place in 0.7 seconds (a rate of 430&nbsp;°C/s) no pearlite will form, and the steel will be martensitic with small amounts of retained austenite.

The growth of martensite phase requires very little thermal activation energy because the process is a diffusionless transformation, which results in the subtle but rapid rearrangement of atomic positions, and has been known to occur even at cryogenic temperatures. Of considerably greater importance than the volume change is the shear strain, which has a magnitude of about 0.26 and which determines the shape of the plates of martensite.

Martensite is not shown in the equilibrium phase diagram of the iron-carbon system because it is not an equilibrium phase. Equilibrium phases form by slow cooling rates that allow sufficient time for diffusion, whereas martensite is usually formed by very high cooling rates. Since chemical processes (the attainment of equilibrium) accelerate at higher temperature, martensite is easily destroyed by the application of heat. This process is called tempering. In some alloys, the effect is reduced by adding elements such as tungsten that interfere with cementite nucleation, but more often than not, the nucleation is allowed to proceed to relieve stresses. Since quenching can be difficult to control, many steels are quenched to produce an overabundance of martensite, then tempered to gradually reduce its concentration until the preferred structure for the intended application is achieved. The needle-like microstructure of martensite leads to brittle behavior of the material. Too much martensite leaves steel brittle; too little leaves it soft.

A strain-induced martensitic transformation commonly takes place from face-centered cubic (FCC) austenite to body-centered cubic (BCC) or body-centered tetragonal (BCT) structures in steels. However, a unique martensitic transformation from a disordered BCC parent phase to an FCC product phase was also observed in steels and high-entropy alloys.

See also

  • Eutectic
  • Eutectoid
  • Ferrite (iron)
  • Maraging steel
  • Spring steel
  • Tool steel

References

  • Comprehensive resources on martensite from the University of Cambridge
  • YouTube Lecture by Prof. HDKH Bhadeshia, from the University of Cambridge
  • Metallurgy for the Non-Metallurgist from the American Society for Metals
  • PTCLab---Capable of calculating martensite crystallography with single shear or double shear theory
  • New book for free download, on Theory of Transformations in Steels, the University of Cambridge