Allotropic forms of iron — temperature range for delta iron Over which temperature range in pure iron does delta iron exist as the stable allotropic form?
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A400°C to 600°C
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B600°C to 900°C
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C900°C to 1400°C
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D1400°C to 1530°C
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E1530°C to 1600°C
Answer
Correct Answer: 1400°C to 1530°C
Explanation
Introduction / Context:Iron exhibits multiple allotropic forms with different crystal structures as temperature changes. These transitions underpin phase diagrams, heat-treatment schedules, and transformation behavior in steels.Given Data / Assumptions:
- Pure iron in equilibrium (no significant alloying elements).
- Transition temperatures near the melting point are considered.
Concept / Approach:Delta iron (δ-Fe) has a body-centered cubic (BCC) structure and is stable from just below the melting point down to the gamma-to-delta transition. Its approximate stability range is from about 1394°C (often rounded to 1400°C) up to the melting point near 1536–1538°C (commonly simplified as 1530°C in many texts). Below this, gamma iron (austenite, FCC) is stable down to about 912°C, and alpha iron (ferrite, BCC) is stable at lower temperatures.
Step-by-Step Solution:
Recall approximate critical temperatures for pure iron: α↔γ around 912°C; γ↔δ around 1394°C; melting around 1536°C.Identify the δ range: from ~1400°C to ~1530°C (rounded textbook values).Select the option matching this interval.Verification / Alternative check:Phase diagrams of Fe confirm δ-Fe occupies the narrow high-temperature band just below the melting point.
Why Other Options Are Wrong:
- Lower ranges correspond to α or γ fields, not δ.
- 1530°C to 1600°C extends above melting and is not meaningful for solid iron.
Common Pitfalls:Confusing δ-Fe with α-Fe because both are BCC; forgetting that δ occurs only near melting.
Final Answer:
1400°C to 1530°C