Question

Eutectoid steel contains

• A. 0.008% C
• B. 2.1% C
• C. 0.18% C
• D. 0.8% C

Hint:

Memorize the key compositions from the Iron-Carbon diagram:

• Eutectoid (steel): 0.8% C

• Eutectic (cast iron): 4.3% C

• Max solubility of C in Ferrite: 0.022% C

• Max solubility of C in Austenite: 2.11% C

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The question asks for the carbon content of a eutectoid steel. This refers to a specific point on the Iron-Iron Carbide (\(Fe-Fe_3C\)) phase diagram known as the eutectoid point.

Step 2: Key Formula or Approach:
The eutectoid reaction in the \(Fe-Fe_3C\) system is: \[ \text{Austenite (\(\gamma\))} \xrightarrow[\text{cooling}]{\text{723°C}} \text{Ferrite (\(\alpha\))} + \text{Cementite (\(Fe_3C\))} \] The resulting lamellar microstructure of ferrite and cementite is called pearlite. This reaction occurs at a single temperature (723°C) and a specific, fixed carbon concentration.

Step 3: Detailed Explanation:
By definition, a eutectoid steel is a steel that has the exact carbon concentration of the eutectoid point. On the \(Fe-Fe_3C\) phase diagram, this point occurs at approximately 0.76% to 0.8% carbon by weight. For most engineering and academic purposes, this value is commonly rounded to

0.8% C.
Let's review the other options:

• 0.008% C: This is near the maximum solubility of carbon in ferrite at room temperature.

• 2.1% C: This value (often cited as 2.11% or 2.14%) is the conventional dividing line between steels and cast irons. It represents the maximum solubility of carbon in austenite.

• 0.18% C: This represents a low-carbon, hypo-eutectoid steel.
Therefore, 0.8% C is the correct carbon content for a eutectoid steel.

Step 4: Final Answer:
A eutectoid steel is defined as a steel having the eutectoid composition, which is approximately 0.8% carbon.