Constituents of hardened steel after quenching Identify the essential (dominant) microconstituent present in hardened steel immediately after quenching from the austenitic region.

Introduction / Context:
Hardening of steel is achieved by heating into the austenite region and then quenching rapidly to transform austenite into a hard, supersaturated structure. Recognizing the resulting microconstituent is central to heat-treatment design and property control.

Given Data / Assumptions:

• Steel is heated above the critical temperature to form austenite.
• Quenching is sufficiently rapid to avoid diffusion-controlled transformations.
• Carbon content is sufficient to form martensite upon quench.

Concept / Approach:
Under rapid cooling, the austenite-to-pearlite or bainite transformations (which require diffusion) are suppressed. Instead, a diffusionless shear transformation produces martensite: a supersaturated, body-centered tetragonal structure with very high hardness. Some retained austenite may remain depending on composition and quench severity, but the essential constituent responsible for hardness is martensite.

Step-by-Step Solution:
Heat steel to austenitize (single phase austenite).Quench rapidly to bypass the nose of the time–temperature–transformation curve.Form martensite by shear without carbon diffusion.Therefore, hardened steel contains predominantly martensite (with possible retained austenite).

Verification / Alternative check:
Hardness measurements (e.g., HRC 60+) and characteristic needle-like microstructure confirm martensite. Tempering reduces brittleness by controlled precipitation and stress relief, transforming martensite toward tempered structures.

Why Other Options Are Wrong:
Pearlite forms under slower cooling; austenite is the high-temperature phase not stable at room temperature; troostite and sorbite are tempered microstructures (historical terms) formed after tempering, not as-quenched hardening.

Common Pitfalls:
Confusing bainite or tempered martensite with as-quenched martensite; the question explicitly asks about hardened steel immediately after quenching.

Final Answer:
martensite