Pearlite is a two-phased, lamellar structure composed of alternating layers
of ferrite and cementite that occurs in some steels and cast irons. In fact, the
lamellar appearance is misleading since the individual lamellae within a colony
are connected in three dimensions; a single colony is therefore an
interpenetrating bicrystal of ferrite and cementite. In an iron-carbon alloy,
during slow cooling pearlite forms by a eutectoid reaction as austenite cools
below 727 °C. Pearlite is a microstructure occurring in many common
grades of steels. The eutectoid composition of austenite
is approximately 0.76% carbon; steel with less carbon content will contain a
corresponding proportion of relatively pure ferrite crystallites that do not
participate in the eutectoid reaction and cannot transform into pearlite.
Likewise steels with higher carbon contents will form cementite before
reaching the eutectoid point. The proportion of ferrite and cementite
forming above the eutectoid point can be calculated from the iron/iron—carbide
equilibrium phase diagram using the lever rule.
Steels with pearlitic or near-pearlitic microstructure can be drawn into thin
wires. Such wires, often bundled into ropes, are commercially used as piano
wires, ropes for suspension bridges, and as steel cord for tire reinforcement.
High degrees of wire drawing leads to pearlitic wires with yield strengths of
several Giga Pascals. It makes pearlite one of the strongest structural bulk
materials on earth. Some hypereutectoid pearlitic steel wires, when cold wire
drawn to true strains above 5, can even show a maximal tensile strength above 6
GPa. Although pearlite is used in many engineering applications, the origin of
its extreme strength is not well understood. It has been recently shown
that cold wire drawing not only strengthens pearlite by refining the
lamellae structure, but also simultaneously causes partial chemical
decomposition of cementite, associated with an increased carbon content of the
ferrite phase, and even a structural transition from crystalline to amorphous
cementite. The deformation-induced decomposition and microstructural change
of cementite is closely related to several other phenomena such as a strong
redistribution of carbon and other alloy elements like Si and Mn in both the
cementite and the ferrite phase; a variation of the deformation
accommodation at the phase interfaces due to a change in the carbon
concentration gradient at the interfaces; and mechanical alloying.
Pearlite was first identified by Henry Clifton Sorby and initially named
sorbite, however the similarity of microstructure to nacre and especially
the optical effect caused by the scale of the structure made the alternative
name more popular. Bainite is a similar structure with
lamellae much smaller than the wavelength of visible light and thus
lacks this pearlescent appearance. It is prepared by more rapid cooling. Unlike
pearlite, whose formation involves the diffusion of all atoms, bainite grows by
a displacive transformation mechanism. Eutectoid steel
Eutectoid steel can in principle be transformed completely into pearlite;
hypoeutectoid steels can also be completely pearlitic if transformed at a
temperature below the normal eutectoid. Pearlite can be hard and strong but is
not particularly tough. It can be wear resistant because of a strong lamellar
network of ferrite and cementite. Examples of applications include cutting
tools, high strength wires, knives, chisels, and nails.
References Further reading
Comprehensive information on pearlite Introduction to Physical metallurgy by
Sidney H. Avner, second edition, McGraw hill publications.
Steels: Processing, Structure, and Performance, Chapter 15 High-Carbon
Steels: Fully Pearlitic Microstructures and Applications by George Krauss, 2005
Edition, ASM International.