Effect of Grain Size on Properties — Coarse-Grained Steel Which statement best describes the behavior of coarse-grained steel with respect to toughness and heat-treatment distortion?

Introduction / Context:
Grain size is a fundamental microstructural parameter that influences strength, toughness, and dimensional stability during heat treatment. Coarse grains typically arise from excessive austenitizing temperatures or prolonged soaking. Understanding how coarse grains degrade toughness and complicate quenching responses is crucial in materials and manufacturing engineering.

Given Data / Assumptions:

• Comparing coarse-grained versus fine-grained steels of similar composition.
• Considering typical hardening and tempering cycles.
• Focus on toughness (impact resistance) and dimensional stability/distortion during heat treatment.

Concept / Approach:
The Hall–Petch relationship links yield strength and toughness to grain size: finer grains obstruct crack propagation and dislocation motion, improving both strength and toughness. Coarse grains lower impact energy and increase notch sensitivity. During quenching, coarse-grained steels exhibit nonuniform transformation and higher thermal stresses, aggravating distortion and risk of cracking, especially in complex geometries.

Step-by-Step Solution:
Recognize that coarse grains reduce area for crack deflection → toughness decreases.Austenite-to-martensite transformations become more uneven with coarse grains and larger prior-austenite packets → distortion risk increases.Therefore, the most accurate statement: less tough and greater tendency to distort during heat treatment.Fine-grain practice (lower austenitizing temperature, controlled time, Al/Nb grain refiners) is preferred when toughness and dimensional control are critical.

Verification / Alternative check:
Impact (Charpy) data and quench distortion studies consistently indicate improved toughness and reduced distortion in fine-grained steels compared with coarse-grained counterparts.

Why Other Options Are Wrong:

• Claims of higher ductility with coarse grains overlook increased notch sensitivity and lower impact energy.
• “Less tendency to distort” contradicts quench-stress behavior in coarse-grained structures.

Common Pitfalls:
Equating tensile ductility measured at room temperature with toughness under dynamic loading; they are related but not identical. Grain size strongly affects impact behavior and heat-treatment stability.

Final Answer:
is less tough and has a greater tendency to distort during heat treatment