An eutectoid steel consists of:
🔬 Understanding the Eutectoid Point
The Iron-Carbon phase diagram, which maps the different structures of steel at various temperatures and carbon contents, has a very special point called the eutectoid point. This point is crucial for understanding the microstructure of steel.
Eutectoid Steel: A steel with a specific carbon content of approximately 0.77% (often rounded to 0.8%). At this exact composition, when the steel is cooled slowly from a high temperature, the single-phase austenite transforms directly into a single, two-phased microstructure at a constant temperature (723°C).
⚖️ Detailed Analysis of Steel Microstructures
The final room-temperature microstructure of a slowly cooled steel depends entirely on its carbon content relative to the eutectoid point.
(a) wholly pearlite
Why it's correct: The unique microstructure formed from the eutectoid reaction is called pearlite. Pearlite is not a single phase, but a lamellar (layered) mixture of two phases: soft, ductile ferrite (pure iron) and hard, brittle cementite (iron carbide). Because eutectoid steel has the perfect carbon balance, the entire austenite structure transforms into 100% pearlite upon slow cooling.
(b) wholly austenite
Why it's incorrect: Austenite is the high-temperature, single-phase crystal structure of steel (Face-Centered Cubic). It is only stable above the critical temperature of 723°C. At room temperature, it will have transformed into other structures.
(c) pearlite and ferrite
Why it's incorrect: This is the microstructure of hypoeutectoid steel (carbon content < 0.77%). This steel has "excess" iron that doesn't fit into the pearlite structure. This excess iron forms separate grains of ferrite alongside the pearlite.
(d) pearlite and cementite
Why it's incorrect: This is the microstructure of hypereutectoid steel (carbon content > 0.77%). This steel has "excess" carbon. This excess carbon forms a network of cementite around the pearlite grains.
💡 Study Tips for Steel Microstructures
- Eutectoid is "Just Right": Think of the eutectoid point like the Goldilocks of steel—the carbon content is "just right" to form 100% pearlite.
- Hypo = Less Carbon, More Ferrite: "Hypo" means under or less. Hypoeutectoid steel has less carbon than the eutectoid point, so it has extra, soft ferrite.
- Hyper = More Carbon, More Cementite: "Hyper" means over or more. Hypereutectoid steel has more carbon than the eutectoid point, so it has extra, hard cementite.