Steel Hardness — Primary Compositional Dependence For plain steels, which factor most strongly governs the achievable hardness after heat treatment (all else equal)?

Introduction / Context:
Hardness in steels reflects microstructures such as martensite, bainite, and tempered martensite. While many variables influence hardness, the carbon content is the dominant compositional parameter for plain (unalloyed) steels because it controls the maximum martensite hardness achievable after quenching and tempering.

Given Data / Assumptions:

• Plain carbon steels without strong carbide-forming alloying additions.
• Similar heat-treatment schedules and section sizes.
• Comparison at room temperature after appropriate quench/temper.

Concept / Approach:
Carbon stabilizes austenite at high temperature and, upon rapid quenching, is trapped in supersaturated body-centered tetragonal martensite. The lattice distortion increases hardness markedly with carbon percentage up to about 0.6–0.8% C, beyond which additional benefits plateau and brittleness rises. Cementite content is a consequence of carbon level and heat history, while alloying elements and manufacturing route certainly matter, but — in the absence of major alloying — carbon content remains the first-order driver.

Step-by-Step Solution:
Hold process variables constant to isolate composition effects.Relate carbon% to martensite hardness: higher C → higher tetragonality → higher hardness.Conclude that hardness scales primarily with carbon content in plain steels.

Verification / Alternative check:
Jominy end-quench and hardness vs. carbon% curves show strong positive correlation up to medium-carbon levels.

Why Other Options Are Wrong:
Cementite amount (option a) is derivative of C% and heat treatment; not the independent primary lever.Alloying elements (option c) influence hardenability and secondary hardening but the question targets primary dependence for plain steels.Manufacturing method (option d) influences cleanliness and grain size but not as directly as C%.

Common Pitfalls:
Confusing hardenability (depth of hardening) with hardness; alloying chiefly affects the former.

Final Answer:
amount of carbon it contains