Steels_ Metallurgy and Applications, Third Edition

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