Grain size in steels — effect of cooling from critical temperatures Slow cooling of steel from the __________ tends to produce a coarse-grained structure. Fill in the blank.

Introduction / Context:
Grain size strongly influences strength and toughness in steels. Processing history—particularly the temperatures reached and the cooling rates from those temperatures—controls the final grain size and mechanical properties.

Given Data / Assumptions:

• Critical temperatures: lower critical (Ac1) and upper critical (Ac3 or Acm).
• Slow cooling from high austenitising temperatures tends to promote coarser products.
• No microalloying or special grain-refining practice is assumed.

Concept / Approach:
Austenite grain growth accelerates at temperatures well above the upper critical point. If a steel is heated to or above the upper critical and then cooled slowly, the austenite grains may become large and transform into coarse pearlite/ferrite, yielding a coarse-grained structure. Finer grains are obtained by controlled austenitising at appropriate temperatures and faster cooling (or normalising) to refine pearlite spacing and grain size.

Step-by-Step Solution:
Recognise that coarse grains are associated with high austenitising temperatures and slow cools.The upper critical point marks the start of full austenitisation for hypoeutectoid steels.Hence, slow cooling from the upper critical region produces coarser microstructures than from the lower critical.

Verification / Alternative check:
Heat-treatment guides emphasise normalising (air cooling from above Ac3) to refine grains compared to furnace cooling, which tends to produce coarser structures.

Why Other Options Are Wrong:

• Lower critical point: below full austenitisation; effects on grain growth are less pronounced.
• Temper range or martensite start: related to tempering/hardening, not primary grain coarsening via austenite growth.
• Room temperature: not a heat-treatment control temperature.

Common Pitfalls:
Assuming any slow cooling causes coarse grains; the prior austenite grain size set above the upper critical is the key determinant.

Final Answer:
upper critical point