Steel classification — An Fe–0.4% C alloy is best described as which type of steel?

Introduction:
Carbon content determines many microstructural features of plain-carbon steels. The eutectoid composition of iron–carbon is about 0.76–0.8% C. Steels below this are hypo-eutectoid; above are hyper-eutectoid. Recognizing where 0.4% C falls helps in anticipating phases and properties after heat treatment.

Given Data / Assumptions:

• Fe–C phase diagram with eutectoid near 0.76–0.8% C at ~727°C.
• Alloy has 0.4% C.
• Plain-carbon context (no high alloying elements implied).

Concept / Approach:

At 0.4% C, the steel is below the eutectoid composition, so it is hypo-eutectoid. On slow cooling, a mixture of proeutectoid ferrite plus pearlite forms. This composition offers a balance of strength and ductility and is common for structural and mechanical components.

Step-by-Step Solution:

Verification / Alternative check:

Phase diagram readings confirm the proeutectoid ferrite fraction increases as carbon content decreases below eutectoid, consistent with 0.4% C steels.

Why Other Options Are Wrong:

A: Cast irons have >2% C typically. C/D require 0.8% C or higher for hyper/eutectoid; E is a stainless steel family defined by high Cr and austenite stabilization, not by 0.4% C alone.

Common Pitfalls:

Equating mechanical properties purely with carbon without considering processing; heat treatment and grain size also matter.

Final Answer:

hypo-eutectoid steel