Pearlite is a mechanical mixture of ferrite and cementite. According to the morphology of cementite, pearlite can be classified into lamellar pearlite and granular pearlite.
Lamellar Pearlite
Lamellar pearlite consists of alternating layers of ferrite and cementite. Several roughly parallel ferrite and cementite layers form a pearlite colony or pearlite cluster. Within a single austenite grain, multiple pearlite clusters can form.
The distance between two adjacent cementite (or ferrite) layers in a pearlite colony is called the interlamellar spacing of pearlite, denoted as S0. It is a primary indicator for evaluating the coarseness or fineness of pearlite. The interlamellar spacing is mainly determined by the undercooling during pearlite formation, i.e., the formation temperature of pearlite, and is largely independent of the austenite grain size or uniformity. The greater the undercooling, the lower the pearlite formation temperature, and the smaller the interlamellar spacing.
Based on the interlamellar spacing, pearlite can be divided into three types. The generally referred lamellar pearlite is that whose ferrite and cementite layers can be clearly distinguished under an optical microscope, with an interlamellar spacing of approximately 450 ~ 150 nm, forming in the temperature range around A1 ~ 650℃. Pearlite formed at a lower temperature, in the range of 650 ~ 600℃, has a smaller interlamellar spacing of about 150 ~ 80 nm, and its lamellar structure can only be distinguished under a high-power optical microscope (magnification 800 ~ 1500×). This fine lamellar pearlite is called sorbitic pearlite. Pearlite formed at an even lower temperature, in the range of 600 ~ 550℃, has an extremely fine interlamellar spacing of about 80 ~ 30 nm. Its lamellar features cannot be observed under an optical microscope and can only be seen with an electron microscope. This extremely fine pearlite is called troostite.
Pearlite, sorbite, and troostite all belong to the pearlite-type microstructure. Their essence is the same—they are all mechanical mixtures of ferrite and cementite in alternating layers. The distinctions are relative and primarily concern the differences in interlamellar spacing.
The mechanical properties of lamellar pearlite are mainly determined by the interlamellar spacing and the diameter of pearlite clusters. The smaller the cluster diameter and interlamellar spacing, the higher the strength and hardness of the steel. Smaller cluster size and spacing increase the interface area, which hinders dislocation movement and enhances resistance to plastic deformation, thereby improving strength and hardness. Reducing the interlamellar spacing can also improve ductility. When the cementite layers are very thin, they can slide under applied stress, producing plastic deformation or bending. Moreover, when the spacing is small, the lamellar cementite in pearlite is discontinuous, and the ferrite layers are not fully separated by cementite, which also enhances ductility.
If pearlite in steel forms during continuous cooling, the interlamellar spacing of the transformed product is uneven. Pearlite formed at higher temperatures has larger spacing, and that formed at lower temperatures has smaller spacing. This uneven spacing leads to non-uniform plastic deformation under stress, causing stress concentration and reducing both strength and ductility.
Granular Pearlite
In high-carbon steels used in industry, there is often a microstructure in which granular cementite is distributed in a ferrite matrix; this structure is called granular pearlite. Granular pearlite is generally obtained through spheroidizing annealing or by quenching followed by medium- or high-temperature tempering.
The mechanical properties of granular pearlite mainly depend on the size, shape, and distribution of cementite particles. In general, for a given steel composition, the finer the cementite particles and the more interfaces they form, the higher the hardness and strength. Carbon particles that are closer to equiaxed shape and more uniformly distributed improve the steel' s toughness.
Under the same composition, granular pearlite has slightly lower hardness than lamellar pearlite but better ductility. Its lower hardness is due to the smaller interface area between ferrite and cementite compared with lamellar pearlite. Its superior ductility results from the continuous ferrite matrix and dispersed granular cementite, which provides less obstruction to dislocation motion.
Under the same hardness, granular pearlite exhibits better tensile properties than lamellar pearlite. Therefore, many critical machine components are heat-treated to obtain tempered sorbite structures with granular carbides. Granular pearlite also offers better machinability, cold forming performance, and quenching characteristics.
Post time: Nov-21-2025