Austenite, ferrite, pearlite, bainite, martensite

Austenite

Definition: A solid solution of carbon and alloying elements dissolved in γ-Fe, retaining the faCE-centered cubic lattice of γ-Fe.

Characteristics: Austenite is the microstructure of ordinary Steel at high temperatures, existing within a specific temperature and composition range. Some quenched steels retain some austenite to room temperature; this austenite is called retained austenite. Austenite generally consists of equiaxed polygonal grains with twins within the grains. At the immediate end of the heating transformation, the austenite grains are relatively small, with irregular arc-shaped grain boundaries. After a period of heating or holding, the grains will grow, and the grain boundaries may become straighter. In the iron-carbon phase diagram, austenite is a high-temperature phase, existing above the critical point A1 temperature, formed by the reverse eutectoid transformation of pearlite. When enough chemical elements that expand the austenite phase region are added to the steel, such as Ni and Mn, austenite can be stabilized at room temperature, as in austenitic steel.

Ferrite

Definition: A solid solution of carbon and alloying elements dissolved in α-Fe. Characteristics: In hypoeutectoid steel, slowly cooling ferrite is massive with relatively smooth grain boundaries. When the carbon content approaches the eutectoid composition, ferrite precipitates along the grain boundaries.

Cementite

Definition: A compound formed by carbon and iron. Characteristics: Cementite is not easily corroded by nitric acid-alcohol solutions and appears bright white under a microscope, but it is corroded by alkaline sodium picrate, appearing black under a microscope. Cementite has many microstructures; in steel and cast iron, it coexists with other phases as platy, granular, network, or lamellar structures. In liquid iron-carbon alloys, the cementite that crystallizes first (primary cementite) is blocky with soft corners. Eutectic cementite has a skeletal structure. Carbides precipitated along the Acm line during the cooling of hypereutectoid steel (secondary cementite) form a network, while eutectoid cementite is lamellar. When iron-carbon alloys cool below Ar1, cementite precipitates from ferrite (tertiary cementite), appearing as discontinuous thin plates on the secondary cementite or at grain boundaries.

Pearlite

Definition: A mechanical mixture of ferrite and cementite formed by the eutectoid reaction in iron-carbon alloys. Characteristics: The interlamellar distance of pearlite depends on the degree of supercooling during austenite decomposition. The greater the supercooling, the smaller the interlamellar distance of the formed pearlite. Pearlite lamellar layers formed between Ar1 and 650℃ are relatively thick. Under a metallographic microscope at magnification of 400x or higher, parallel broad strips of ferrite and thin strips of cementite can be distinguished; this is called coarse pearlite or lamellar pearlite, or simply pearlite. Pearlite formed at 650–600℃, when magnified 500 times under a metallurgical microscope, shows only a black line on the cementite layer; the lamellar structure can only be distinguished at 1000x magnification. This is called sorbite. Pearlite formed at 600–550℃, when magnified 500 times under a metallurgical microscope, cannot show pearlite lamellars; only black, spherical structures are visible. The lamellar structure can only be distinguished at 10,000x magnification under an electron microscope. This is called troostite.

Upper bainite

Definition: A mixture of supersaturated acicular ferrite and cementite, with cementite interspersed within ferrite needles. Characteristics: A phase transformation product of supercooled austenite at intermediate temperatures (350~550℃). Its typical morphology is a bundle of roughly parallel ferrite laths with a 6~8° orientation difference, with short carbide rods or small plates distributed between each lath along the long axis of the laths. Typical upper bainite is feather-like, with the grain boundaries as the axis of symmetry. Due to different orientations, the feathers can be symmetrical or asymmetrical. Ferrite feathers can be needle-like, dot-like, or blocky. In high-carbon, high-alloy steel, the needle-like feathers are not clearly visible; in medium-carbon, medium-alloy steel, the needle-like feathers are more clearly visible; in low-carbon, low-alloy steel, the feathers are very clear, and the needles are coarse. During the transformation, upper bainite first forms at the grain boundaries and grows into the grains, without penetrating the grains.

Lower bainite

Definition: Same as above, but with cementite interspersed within ferrite needles. Characteristics: A transformation product of supercooled austenite at 350℃~Ms. Its typical morphology is biconvex lenticular ferrite containing supersaturated carbon, with unidirectionally arranged carbide lamellae distributed within it; within the grains, it appears needle-like, the needles do not cross, but can intersect. Unlike tempered martensite, which has distinct layers, lower bainite has a uniform color. The carbide particles in lower bainite are coarser than those in tempered martensite, making it more susceptible to corrosion and turning black, while tempered martensite is lighter in color and less susceptible to corrosion. High-carbon high-alloy steel has a higher carbide dispersion and finer needles than low-carbon low-alloy steel.

Granular Bainite

Definition: A multiphase microstructure characterized by numerous small islands distributed within large or strip-shaped ferrite; a transformation product of supercooled austenite at the uppermost part of the bainite transformation temperature region. When it is first formed, it consists of blocky ferrite formed by the merging of strip ferrite and small island-shaped carbon-rich austenite. During the subsequent cooling process, the carbon-rich austenite may be completely retained as retained austenite; it may also be partially or completely decomposed into a mixture of ferrite and cementite (pearlite or bainite); most likely, it will partially transform into martensite and partially remain to form a two-phase mixture, called M-A structure.

Carbide-free bainite

Definition: A microstructure composed of lath-like ferrite single phases, also known as ferritic bainite. Characteristics: Formation temperature is at the uppermost part of the bainite transformation temperature range. Carbon-rich austenite forms between the lath ferrite particles, and this austenite undergoes a similar transformation during subsequent cooling. Carbide-free bainite is generally found in low-carbon steels, but it also readily forms in steels with high silicon and aluminum content.

Martensite

Definition: A supersaturated solid solution of carbon in α-Fe. Characteristics: Lath martensite consists of fine martensite strips of roughly the same size arranged in parallel orientations, forming martensite bundles or martensite domains. There is a large orientation difference between domains; several domains with different orientations can form within a single original austenite grain. Due to the high formation temperature of lath martensite, self-tempering inevitably occurs during cooling, precipitating carbides within the formed martensite, making it susceptible to corrosion and darkening.

Acicular martensite, also known as plate martensite or high-carbon martensite, is characterized by the following: the first martensite plate forming within an austenite grain is relatively large, often penetrating the entire grain, dividing the austenite grain and limiting the size of subsequently formed martensite. Therefore, plate martensite varies in size and has an irregular distribution. Acicular martensite forms in a specific orientation. A ridge is present within the martensite needles; the higher the carbon content, the more pronounced the ridge, and the sharper the martensite. White retained austenite is also present between the martensite particles.

Leedsburite

Definition: A eutectic mixture of austenite and cementite. Characteristics: Dendritic austenite is distributed in a cementite matrix.

Tempered martensite

Definition: A mixture of extremely fine transition carbides and supersaturated (low carbon content) α-phase obtained from the decomposition of martensite. Characteristics: It is formed by tempering martensite at 150–250°C. This structure is highly susceptible to corrosion. Under an optical microscope, it appears as a dark, needle-like structure (maintaining the orientation of quenched martensite), very similar to lower bainite. Only under a high-powered electron microscope can extremely fine carbide particles be seen.

Tempered Troostite

Definition: A mixture of carbides and α-phase. Characteristics: It is formed by tempering martensite at medium temperature (350-500℃). Its microstructure is characterized by extremely fine granular carbides distributed within a ferrite matrix. The needle-like morphology gradually disappears but is still faintly visible. The carbides are indistinguishable under an optical microscope; only a dark structure is observed. Only under an electron microscope can the two phases be clearly distinguished, and the carbide particles show significant growth.

Tempered Sorbite

Definition: A ferrite matrix with uniformly distributed carbide particles. Characteristics: It is formed by tempering martensite at high temperature (500-650℃). Its microstructure is characterized by a multiphase structure consisting of equiaxed ferrite and fine-grained carbides. The traces of martensite plates have disappeared, and the morphology of cementite is relatively clear, but still difficult to distinguish under an optical microscope. Under an electron microscope, the cementite particles are larger.

Globular Pearlite

Definition: Composed of ferrite and granular carbides. Characteristics: Obtained through spheroidizing annealing, cementite is distributed in spheroidal shapes on a ferrite matrix. The size of the cementite spheroids depends on the spheroidizing annealing process, especially the cooling rate. Globular pearlite can be divided into four types: coarse spheroids, spheroids, fine spheroids, and dot-like pearlite.

Widmanstätten Structure

Definition: If the austenite grains are relatively coarse and the cooling rate is suitable, the proeutectoid phase may exist in a needle-like (lamellar) form mixed with lamellar pearlite, called Widmanstätten structure.

Characteristics: In hypoeutectoid steel, the morphology of ferrite in Widmanstätten structure includes lamellar, feather-like, or triangular shapes. Coarse ferrite is distributed in parallel or triangular patterns. It appears at austenite grain boundaries and grows into the grains. In hypereutectoid steel, Widmanstätten cementite has a needle-like or rod-like morphology and appears inside the austenite grains.