CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 2
Chromium in excess of 12 wt % by weight is required to impart “stainless” characteristics
of iron alloys. Thus, all stainless steels contain large amount of chromium from 12% to a
maximum of 25 %. Chromium as major alloying element stabilizes the body-centred cubic (bcc)
phase of iron, therefore with increasing chromium content the low temperature and high
temperature and ferrite field expands. As the ferrite field expands the austenite field contracts,
and produces what is often called as the gamma () loop. However, the addition of carbon to the
Fe-Cr binary alloy extends the gamma loop to higher chromium content, and also widens the
(alpha plus gamma) phase field, and above 0.40 wt % C, the steel can be made fully austenitic if
cooled directly from the gamma loop region. Moreover, nickel with a face-centred-cubic structure
favours the formation of fcc austenite solid solution, and the stability range of austenite phase
expands at the expanse of bcc -ferrite phase. The Fe-Cr-Ni ternary phase diagram is shown in
Fig. 2.3 [33].
Fig. 2.3: Fe-Cr-Ni ternary phase diagram [33]
Thus, if nickel is added to a low carbon-Fe-18 wt % Cr alloy, the gamma phase field
expands, and at about 8 wt % Ni, the gamma phase persist at room temperature leading to familiar
group of austenitic stainless steel based upon 18 Cr-8 Ni (wt %). This particular composition