Martensite, named after the German metallurgist Adolf Martens, most commonly refers to a very
hard form of steel crystalline structure, but it can also refer to any crystal structure
that is formed by diffusionless transformation. It includes a class of hard minerals occurring
as lath- or plate-shaped crystal grains. When viewed in cross section, the lenticular crystal
grains are sometimes incorrectly described as acicular.
Properties Martensite is formed in carbon steels by the
rapid cooling of austenite 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. As a
result, the face centered cubic austenite transforms to a highly strained body centered
cubic form of ferrite that is supersaturated with carbon. The shear deformations that result
produce large numbers 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 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 Mf is reached, at which time the
transformation is completed. For a eutectoid steel, 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 to 13% retained austenite at 0.95% C and 30–47% for a 1.4% carbon
steels. 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 no pearlite will form and the steel will be martensitic with small amounts
of retained austenite. For steel 0-0.6% carbon the martensite has the appearance of lath,
and is called lath martensite. For steel 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 at rates of cm/s until the Ms 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 also be formed by the working and hence deformation of the
steel at temperature, in its austenitic form, by quenching to below Ms and then working
by plastic deformations to reductions of area to 40% or as little 20% of the original. The
process produces dislocation densities up to 1013/cm2. The great number of dislocations,
combined with precipitates that originate and pin the dislocations in place, produces
a very hard steel. Thus, martensite can be thermally induced or stress induced.
One of the differences between the two phases is that martensite has a body-centered tetragonal
crystal structure, whereas austenite has a face-centered cubic structure. The transition
between these two structures requires very little thermal activation energy because it
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 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 phenomenon 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 right 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.
See also Eutectic
Eutectoid Ferrite
Maraging steel Spring steel
Tool steel References
External links Comprehensive resources on martensite, 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