A reaction in which two solid phases form simultaneously from one solid phase on cooling. In steel, γ → α + Fe₃C occurs at 0.76 wt% C and 727 °C, producing pearlite.
Japanese name: 共析反応
Eutectoid reaction appears in 2 practice questions across 1 topic, including Phase diagrams and the lever rule.
A eutectoid reaction is one in which one solid phase splits simultaneously into two solid phases on cooling, and it appears on a phase diagram as a horizontal line where three phases coexist (zero degrees of freedom). In steel, at about 0.76 wt% C and 727 °C, austenite (γ) splits into ferrite (α, about 0.022 wt% C) and cementite (Fe₃C, 6.70 wt% C). This temperature is the A1 point, the reference temperature for heat treatment of steel. Besides steel, eutectoid reactions also occur in alloys such as the Cu–Al system (aluminum bronze).
Unlike a eutectic reaction from a liquid, a eutectoid reaction proceeds by diffusion in the solid, so it takes time. The transformation temperature therefore differs between heating and cooling: Ac1 on heating is above 727 °C, and Ar1 on cooling is below it. The lever rule gives the mass fraction of ferrite in pearlite as (6.70 − 0.76)/(6.70 − 0.022) ≈ 0.89. Because the densities of the two phases are similar, the thickness ratio of the ferrite and cementite layers is about 8:1.
In hypoeutectoid steels, with less carbon than the eutectoid composition, proeutectoid ferrite forms first and the remaining austenite becomes pearlite at 727 °C. In hypereutectoid steels, with more carbon, proeutectoid cementite precipitates as a network at the grain boundaries. For a slowly cooled hypoeutectoid steel with a pearlite area fraction f, the carbon content can be estimated as about 0.022 + 0.74f wt%, about 0.39 wt% if half the structure is pearlite. A micrograph thus gives a rough idea of a steel's carbon content.
The 727 °C and 0.76 wt% C on the phase diagram are equilibrium values for the pure Fe–C system. Alloying elements shift the eutectoid point: Mn and Ni lower the eutectoid temperature, while Cr, Si, Mo, W and Ti raise it. Most alloying elements also reduce the eutectoid carbon content. With fast cooling, transformation occurs below the A1 point, giving pearlite with a finer lamellar spacing, and with still faster cooling bainite or martensite forms. Practical heat-treatment conditions are set by combining the phase diagram with TTT or CCT diagrams.
Answer: C. L + α → β
A peritectic reaction is one in which, on cooling, the liquid L reacts with a previously formed solid α to produce a different solid β: L + α → β. The name comes from the new β phase forming around the α grains as if wrapping them. In the Fe-C system δ + L → γ occurs at about 1495 °C, and peritectics are common in the Cu-Zn and Cu-Sn systems. L → α + β is the eutectic reaction, in which two solids form simultaneously from the liquid, and γ → α + β is the eutectoid reaction, in which two solids form from a solid (such as γ → α + Fe₃C in the Fe-C system). In a peritectic reaction, the β product coats the α and hinders diffusion, so in real solidification the reaction does not go to completion and α tends to remain.
Answer: B. Proeutectoid cementite and pearlite
In a hypereutectoid steel, with more carbon than the eutectoid composition of 0.76 wt% C, crossing the Acm line during cooling first precipitates proeutectoid cementite as a network along the austenite grain boundaries. 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. Proeutectoid ferrite and pearlite is the microstructure of a hypoeutectoid steel, and pearlite alone forms in a eutectoid steel. Because a grain-boundary cementite network causes brittleness, tool steels and similar grades are given a spheroidizing anneal to make the cementite globular.
All are used, and the difference comes from the source of the phase diagram data. Different assessments of the thermodynamic data shift the value slightly, so in textbooks and problems, calculate with the value given there.
It is the temperature of the eutectoid reaction in iron and steel (727 °C in equilibrium), below which austenite cannot exist stably. Tempering is done below the A1 point, while quenching and normalizing are done after heating above the A1 or A3 point.
No, it is a microconstituent in which two phases, ferrite and cementite, alternate in layers. When counting phases with the phase rule or the lever rule, pearlite is treated as two phases.
L → α + β is the eutectic reaction, in which two solids form simultaneously from the liquid, and γ → α + β is the eutectoid reaction, in which two solids form from a solid (such as γ → α + Fe₃C in the Fe-C system).
Which equation represents a peritectic reaction? Write it i…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…