Grain growth in steels — effect of heating to the coarsening temperature When a medium-carbon steel is heated up to the coarsening temperature range, what happens to its austenite grain size?

Introduction / Context:
Grain size exerts a powerful effect on strength and toughness via the Hall–Petch relationship. Understanding how heat exposure influences austenite grain growth helps prevent coarse-grained microstructures that embrittle steels and degrade fatigue performance.

Given Data / Assumptions:

• Medium-carbon steel (about 0.3–0.6% C).
• Heating through full austenitizing into the coarsening (grain growth) regime.
• Normal atmosphere without strong grain growth inhibitors.

Concept / Approach:
At sufficiently high austenitizing temperatures and times, boundary mobility increases and pinning effects (from precipitates such as AlN, TiN, or fine carbides) are reduced, leading to rapid grain coarsening. Excessive grain growth reduces yield strength and toughness and can promote quench cracking and nonuniform transformation on cooling.

Step-by-Step Solution:

Verification / Alternative check:
Metallographic comparison of specimens held at progressively higher austenitizing temperatures shows marked growth in prior-austenite grain size beyond the normalizing window.

Why Other Options Are Wrong:

• No change or minimum: contradicts the definition of the coarsening regime.
• Decrease or increase-then-decrease: does not match typical grain growth kinetics in the absence of abnormal secondary recrystallization phenomena.

Common Pitfalls:
Overheating during hardening; insufficient alloying with nitride-formers to control grain size; misinterpreting fine martensite packets as fine prior grains.

Final Answer: