Using Fe-Fe3C phase diagram (see below), determine at what temperature iron chan

Question

Using Fe-Fe3C phase diagram (see below), determine at what temperature iron changes its crystal structure from BCC to FCC. Composition (at% C) 10 15 20 25 1600 1538 C 1493C 1400 2500 1394C 1200 1147 C 2.14 4.30 y. Austenite 1000 912 C 800 1500 27 C 0.76 0.022 , Ferrite 600 ? + Fe3C Cementite (FegC) 1000 400 6.70 (Fe) Composition (wt% C) IE9.21 The iron-iron carbide phase diagram. [Adapted from Binary Alloy Phase Diagrams, 2nd edition, Vo, T. B. Massalski (Editor-in-Chief), 1990. Reprinted by permission of ASM International, Materials Park, OH.

Explanation / Answer

=> BCC ?-ferriteTransforms to FCC ?-austenite at 912 °C.

=>FCC Fe Transforms to BCC ?-ferrite at 1395 °C

Phases in Fe–Fe3C Phase Diagram:

?-ferrite - solid solution of C in BCC Fe

• Stable form of iron at room temperature.

• The maximum solubility of C is 0.022 wt%

• Transforms to FCC ?-austenite at 912 °C

?-austenite - solid solution of C in FCC Fe

• The maximum solubility of C is 2.14 wt %.

• FCC Fe Transforms to BCC ?-ferrite at 1395 °C

• Is not stable below the eutectic temperature

(727 ° C) unless cooled rapidly (Chapter 10)

?-ferrite solid solution of C in BCC Fe

• The same structure as ?-ferrite

• Stable only at high T, above 1394 °C

• Melts at 1538 °C

Fe3C (iron carbide or cementite)

• This intermetallic compound is metastable, it

remains as a compound indefinitely at room T, but

decomposes (very slowly, within several years)

into ?-Fe and C (graphite) at 650 - 700 °C

Fe-C liquid solution

• C is an interstitial impurity in Fe. It forms a solid solution

with ?, ?, ? phases of iron

• Maximum solubility in BCC ?-ferrite is limited (max.

0.022 wt% at 727 °C) - BCC has relatively small interstitial

positions

• Maximum solubility in FCC austenite is 2.14 wt% at 1147

°C - FCC has larger interstitial positions.

thank you.....

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