Metallurgy of steels and cast irons: During heating, the “transformation range” in ferrous alloys refers to the temperature interval over which which phase forms progressively before full conversion?

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:

  • Slow to moderate heating rates allowing near-equilibrium behavior.
  • Carbon steel or cast iron compositions.
  • Focus on heating (not quenching).


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:

Identify the critical heating temperatures Ac1 and Ac3/Acm.Recognize that austenite appears first at Ac1 and grows with increasing T.Complete transformation to austenite by Ac3 (or Acm).Conclude that the “transformation range” during heating is the austenite formation interval.Select “austenite.”


Verification / Alternative check:
Metallographic studies show dissolution of pearlite/cementite into uniform austenite across the critical range on heating.


Why Other Options Are Wrong:

  • Cementite and pearlite: Pre-existing phases that dissolve; they do not newly form on heating across the critical range.
  • Martensite: A diffusionless product of rapid cooling, not heating.


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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