Definition

Noun: A type of carbon steel with a specific composition of approximately 0.8% carbon by weight. This precise composition corresponds to the eutectoid point in the iron-carbon phase diagram, meaning that upon slow cooling from a high temperature, it transforms entirely into a microstructure called pearlite, without any primary ferrite or cementite forming first.

Usage

This term is used specifically in the fields of metallurgy and materials science to classify and describe steels based on their carbon content and resulting microstructure. - Eutectoid steel is a fundamental reference point in the study of steel microstructures. - The properties of a plain carbon steel change significantly as its composition approaches the eutectoid steel composition.

Advanced Usage

• "Hypoeutectoid steel": A steel with less than the eutectoid carbon content (below ~0.8% C). Upon cooling, it forms a microstructure of primary ferrite and pearlite.
• "Hypereutectoid steel": A steel with more than the eutectoid carbon content (above ~0.8% C). Upon cooling, it forms a microstructure of primary cementite and pearlite.
• The term is often used when discussing phase transformations, heat treatment processes (like annealing), and the resulting mechanical properties of steel.

Variants and Related Words

• Pearlite (n): The lamellar microstructure of alternating layers of ferrite and cementite that forms when eutectoid steel is cooled slowly. It is the defining microstructure of this steel.
• Eutectoid point (n): The specific composition and temperature on a phase diagram where a single solid phase transforms directly into two other solid phases upon cooling. For the iron-carbon system, this is at about 0.8% carbon and 727°C (1341°F).
• Eutectoid reaction (n): The phase transformation where a single solid phase (austenite) cools to form a mixture of two other solid phases (ferrite and cementite in the form of pearlite).

Synonyms

• Pearlitic steel: A descriptive synonym emphasizing the resulting microstructure. All eutectoid steel is pearlitic when slowly cooled, but not all pearlitic steel is necessarily of the exact eutectoid composition.
• 0.8% carbon steel: A compositional synonym, though the exact eutectoid point is often cited as 0.76-0.8% carbon.

Related Concepts

• Austenite: The high-temperature face-centered cubic (FCC) phase of iron that transforms into pearlite in eutectoid steel.
• Ferrite: The soft, body-centered cubic (BCC) iron phase present in pearlite.
• Cementite: The hard, iron carbide (Fe₃C) phase present in pearlite.
• Phase Diagram: The graphical representation (like the Iron-Carbon diagram) used to predict the phases present in an alloy at different temperatures and compositions.

Noun

1. a steel that contains 0.9% carbon (the eutectic point); a carbon steel with 0.9% carbon is pure pearlite