Martensite is a very
hard form of
steel
Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
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.
Properties
Martensite is formed in
carbon steels by the rapid cooling (
quenching) of the
austenite form of
iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form
cementite
Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. It is a hard, b ...
(Fe
3C). Austenite is gamma-phase iron (γ-Fe), a solid solution of iron and
alloying elements. As a result of the quenching, the
face-centered cubic austenite transforms to a highly strained
body-centered tetragonal form called martensite that is
supersaturated with
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
. The shear deformations that result produce a large number of dislocations, which is a primary strengthening mechanism of steels. The highest hardness of a
pearlitic steel is 400
Brinell, whereas martensite can achieve 700 Brinell.
The martensitic
reaction begins during cooling when the
austenite reaches the martensite start temperature (M
s), 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
f 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 °C and ending at 450 °C takes place in 0.7 seconds (a rate of 430 °C/s) no pearlite will form, and the steel will be martensitic with small amounts of retained austenite.
For steel with 0–0.6% carbon, the martensite has the appearance of
lath and is called lath martensite. For steel with greater than 1% carbon, it will form a plate-like structure called plate martensite. Between those two percentages, the physical appearance of the grains is a mix of the two. The strength of the martensite is reduced as the amount of retained austenite grows. If the cooling rate is slower than the critical cooling rate, some amount of pearlite will form, starting at the grain boundaries where it will grow into the grains until the M
s temperature is reached, then the remaining austenite transforms into martensite at about half the speed of sound in steel.
In certain
alloy steels, martensite can be formed by working the steel at M
s temperature by quenching to below M
s and then working by plastic deformations to reductions of cross section area between 20% and 40% of the original. The process produces dislocation densities up to 10
13/cm
2. The great number of dislocations, combined with precipitates that originate and pin the dislocations in place, produces a very hard steel. This property is frequently used in toughened ceramics like
yttria-stabilized zirconia and in special steels like
TRIP steels. Thus, martensite can be thermally induced or stress induced.
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.
Martensite has a lower density than austenite, so that the martensitic transformation results in a relative change of volume.
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
A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. ...
; too little leaves it soft.
See also
*
Eutectic
*
Eutectoid
*
Ferrite (iron)
*
Maraging steel
*
Spring steel
*
Tool steel
Tool steel is any of various carbon steels and alloy steels that are particularly well-suited to be made into tools and tooling, including cutting tools, dies, hand tools, knives, and others. Their suitability comes from their distinctive ...
References
External links
Comprehensive resources on martensite from the University of CambridgeYouTube Lecture by Prof. HDKH Bhadeshia , from the University of CambridgeMetallurgy for the Non-Metallurgist from the American Society for MetalsPTCLab---Capable of calculating martensite crystallography with single shear or double shear theoryNew book for free download, on Theory of Transformations in Steels, the University of Cambridge{{Authority control
Ceramic materials
Steels