The microstructure formed by the eutectoid reaction in steel, with alternating lamellae of ferrite and cementite. The faster the cooling, the finer the lamellar spacing and the harder the pearlite.
Japanese name: パーライト
Pearlite appears in 5 practice questions across 2 topics, including Phase diagrams and the lever rule, Heat treatment and metallic materials.
When austenite of eutectoid composition transforms slightly below the A1 point, pearlite forms mainly from the austenite grain boundaries. Ferrite and cementite grow simultaneously side by side, and just ahead of the growth front, carbon diffuses from the ferrite side to the cementite side and is handed over. A region in which the lamellae share an orientation is called a colony, and many colonies with different orientations form within one austenite grain. The name comes from the pearl-like luster of a polished and etched surface.
The lamellar spacing of pearlite becomes finer the lower the transformation temperature (the greater the undercooling), roughly in inverse proportion to the undercooling, because carbon has to diffuse a shorter distance and more nuclei form. The finer the spacing, the shorter the distance dislocations can travel in the ferrite, so yield strength and hardness rise; a relation in which they are proportional to the inverse square root of the spacing is widely used. Furnace cooling gives coarse pearlite and air cooling (normalizing) gives fine pearlite, so strength changes even at the same carbon content.
Transformation at temperatures below the pearlite range but above the range where martensite forms produces bainite, in which cementite is dispersed not as lamellae but as fine particles or rods in or between the ferrite. When pearlite is held for a long time just below the A1 point, the cementite becomes rounded to lower its surface energy, giving a spheroidized structure. Spheroidized steel is the softest possible at a given carbon content and is easy to machine and cold-work, so this treatment is used on bearing and tool steels before machining.
High-carbon steel wire rod is transformed isothermally to fine pearlite in a lead bath or similar at about 500–550 °C (patenting) and is then heavily cold-drawn. Drawing makes the lamellar spacing even finer, and piano wire, bridge suspension cables and tire reinforcing cords reach tensile strengths well above 2 GPa. Railway rails also use wear-resistant pearlitic steel, and some have heads heat-treated to fine pearlite. Pearlite is a long-established microstructure, but it remains one of the mainstays of high-strength materials today.
Answer: A. At about 0.76 wt% C and 727 °C, γ → α + Fe₃C occurs and pearlite forms
The eutectoid point of the Fe-Fe₃C system is at about 0.76 wt% C and 727 °C, where austenite (γ) transforms into pearlite, a lamellar structure of ferrite (α) and cementite (Fe₃C). The reaction at about 4.3 wt% C and 1147 °C is the eutectic reaction involving the liquid, and the structure it produces is ledeburite, not pearlite. 0.022 wt% C is the maximum solubility of carbon in α at 727 °C, and 912 °C is the γ → α transformation temperature of pure iron. Bainite does not appear on the equilibrium phase diagram; it is a nonequilibrium structure that forms from undercooled austenite in an intermediate temperature range. Steels with less carbon than the eutectoid composition are called hypoeutectoid, and those with more are called hypereutectoid.
Answer: C. About 0.49
Proeutectoid ferrite is the ferrite already formed in the α + γ region just above 727 °C, and all the remaining γ becomes pearlite. From the lever rule, W_proeutectoid α = (0.76 − 0.40)/(0.76 − 0.022) = 0.36/0.738 ≈ 0.49. About 0.51 is the fraction of the remaining pearlite, the mistake of taking the opposite distance. About 0.94 is the fraction of all ferrite treating the steel as the two phases α + Fe₃C, (6.70 − 0.40)/(6.70 − 0.022), which includes the ferrite inside the pearlite as well. The area ratio of proeutectoid ferrite to pearlite in a micrograph is a clue for estimating the carbon content of a hypoeutectoid steel.
Pearlite is a near-equilibrium structure in which carbon has diffused and the steel has separated into ferrite and cementite. Martensite is a non-equilibrium structure formed when quenching prevents diffusion and the steel transforms by shear while retaining carbon in supersaturation; it is far harder and more brittle.
Etching with nital or a similar reagent turns the lamella boundaries into grooves that scatter light. At low magnification the fine lamellae cannot be resolved, so the whole area looks dark. The regions that look white are ferrite or proeutectoid cementite.
Lower the transformation temperature (cool faster). Normalizing with air cooling and patenting with isothermal transformation around 500 °C give fine pearlite. If cooling is too fast, however, bainite or martensite forms.
The eutectoid point of the Fe-Fe₃C system is at about 0.76 wt% C and 727 °C, where austenite (γ) transforms into pearlite, a lamellar structure of ferrite (α) and cementite (Fe₃C).
Which statement about the eutectoid point of the Fe-Fe₃C ph…Proeutectoid ferrite is the ferrite already formed in the α + γ region just above 727 °C, and all the remaining γ becomes pearlite.
A 0.40 wt% C hypoeutectoid steel is cooled under equilibriu…On reaching 727 °C, the remaining austenite (about 0.76 wt% C) becomes pearlite through the eutectoid reaction, so the room-temperature microstructure is proeutectoid cementite and pearlite.
When a 1.2 wt% C hypereutectoid steel is slowly cooled from…Pearlite and bainite are the structures that separate into ferrite and cementite with carbon diffusion, and it is pearlite that forms more readily the slower the cooling.
Which statement about the martensitic transformation in ste…Alloying elements such as Mn, Cr and Mo retard the pearlite and bainite transformations of austenite and move the nose of the TTT and CCT diagrams to longer times.
Adding alloying elements such as Mn, Cr and Mo to steel imp…Martensite Eutectic reaction Lever rule Eutectoid reaction Hardenability Gibbs phase rule