Steels_ Metallurgy and Applications, Third Edition

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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:



  1. The presence of sigma phase at about 50% Cr.

  2. 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
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