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?

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:

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: