Stainless steels 295
general corrosion
intergranular corrosion
pitting corrosion
stress corrosion
However, whereas these types of corrosion will be discussed at a later stage, the
authors are conscious of the fact that the treatment of corrosion in this text is
superficial and reflects their limited knowledge of the subject.
Composition-structure relationships
Iron-chromium alloys
The simplest stainless steels consist of iron-chromium alloys but in fact the
binary iron-chromium system can give rise to a wide variety of microstructures
with markedly different mechanical properties. The Fe-Cr equilibrium diagram
is shown in Figure 4.1 and is characterized by two distinctive features, namely:
- The presence of sigma phase at about 50% Cr.
- The restricted austenite phase field, often called the gamma-loop.
Sigma phase is an intermetallic compound, which is hard and brittle and can
be produced in alloys containing substantially less than 50% Cr. It also has an
adverse effect on the corrosion resistance of stainless steels and therefore care
should be taken to avoid extended exposure in the temperature range 750-820~
which favours its formation.
1600 Liquid
.~ 14oo
2
~^1200
C1.
E
~ooo
800 -
600
0
I i
_ ~7+a a(8)
/~Sig'ma
/ phase
sigma
t
I, J J I= i !,,I I
10 20 30 40 50 60 70
Fo % Chromium
1800
I .... I I
so 90 loo
Figure 4.1 Fe-Cr equilibrium diagram