Question

At what temperature range is δ iron stable?

(a) Up to 908^oC

(b) 908-1388^oC

(c) 1388-1535^oC

(d) 1535-1800^oC

I have been asked this question in an internship interview.

This interesting question is from Iron-Carbon System and their Alloys in section Constitution of Alloys and Phase Diagrams of Engineering Material & Metallurgy

Answer 1

Correct answer is (c) 1388-1535^oC

To explain: Pure iron exists in three allotropic phases of α iron, ϒ iron, and δ iron. δ iron is a body-centered cubic that is stable around 1388-1535^oC. α iron and δ iron are stable up to 908^oC and between 908-1535^oC respectively.