Steels_ Metallurgy and Applications, Third Edition

296 Steels: Metallurgy and Applications

From a commercial standpoint, the area of the Fe-Cr diagram of greatest
importance is that containing up to about 25% Cr, and a simplified illustration
of that region for alloys containing 0.1% C is shown in Figure 4.2. Because
chromium is a mild carbide former, many types of stainless steel are solution
treated at temperatures significantly higher than those used for low-alloy steels
in order to dissolve the chromium carbides. A solution treatment temperature of
10500C is typical of a variety of stainless steel grades and this will be used as
a reference temperature in relation to the microstructure at high temperature. As
illustrated in Figure 4.2, 0.1% C steels can accommodate up to about 13.5% Cr
at 10500C and still remain austenitic with a face-centred-cubic structure. As the
chromium content of the steels is increased within this range, the hardenability
also increases very substantially such that large section sizes can be through-
hardened to martensite on cooling to room temperature. For example, a steel
containing 12% Cr and 0.12% C will form martensite at the centre of a 100 mm
bar on air cooling from 10500C and the limiting section can be increased to about
500 mm by oil quenching from this temperature. It should also be noted that
the Ms-Mf transformation range is depressed significantly with large additions
of chromium. However, for most commercial grades of 11-13% Cr steels, the
transformation range is above room temperature and therefore the formation of
retained austenite is not a major problem.
As the chromium content is increased above about 13%, a significant change
takes place in the microstructure at 1050~ as the single-phase austenite region
gives way to the duplex austenite plus ferdte phase field. Whereas the austenite
in this region behaves in a similar way to that within the gamma-loop, i.e. it
transforms to martensite on cooling to room temperature, the ferrite formed at

1500

1300 X

~D z..

E ~D

I-.-

1100

o~(~)

900

0

Fe

I I I I, I

5 10 15 20 25

% Chromium

Figure 4.2 Fe-Cr alloys containing O. 1% C