Difficulty: Easy
Correct Answer: austenite
Explanation:
Introduction / Context:
Heat treatment relies on phase transformations governed by the iron–carbon diagram. On heating hypoeutectoid or hypereutectoid steels, there is a range—not a single point—over which transformations occur. Knowing which phase appears across this range under heating is foundational for processes like austenitizing, normalizing, and hardening.
Given Data / Assumptions:
Concept / Approach:
As temperature rises through the critical range, ferrite and carbides (cementite/pearlite) transform to austenite (γ-Fe). The transformation begins at Ac1 and completes at Ac3 (or Acm for hypereutectoid steels). Martensite forms on rapid cooling (not heating), while pearlite and cementite are constituents of the starting microstructure that dissolve into austenite in this range.
Step-by-Step Solution:
Verification / Alternative check:
Metallographic studies show dissolution of pearlite/cementite into uniform austenite across the critical range on heating.
Why Other Options Are Wrong:
Common Pitfalls:
Confusing Ac temperatures (on heating) with Ar temperatures (on cooling), or using Ms/Mf associated with martensitic transformation instead.
Final Answer:
austenite
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