CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
Pure iron upon heating experiences two changes in crystal structure before it melts. At
room temperature the stable form is called as ferrite (-iron) has body centred cubic (bcc) crystal
structure. Ferrite experiences a polymorphic phase transformation to austenite (-iron) at 912 °C
which has a face centred cubic (fcc) crystal structure. The austenite phase persists up to 1394 °C,
and at this temperature the austenite reverts back to the bcc phase known as -ferrite. Carbon as an
interstitial element forms solid solution with both and ferrite, and also with -austenite as
indicated by , and phase field in Fig. 2.1. Moreover, the much larger phase field of -iron
compared to that of -iron reflects the greater solubility of carbon in -iron, with a maximum
value of 2.08 wt% at 1147 °C. Austenite is characterized as non-magnetic, high ductility, rapid
work hardening rates, and excellent toughness.
However, the iron-carbon equilibrium diagram as shown in Fig. 2.1 is modified by the
addition of alloying elements such as chromium (Cr), Nickel (Ni), and other minor alloying
elements, resulting in poly-component systems. Fig. 2.2 shows the Fe-Cr equilibrium phase
diagram [15].
Fig. 2.2: Fe-Cr equilibrium phase diagram [15]