Page 46 - Corrosion Engineering Principles and Practice

C o r r o s i o n B a s i c s 27

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FIGURE 2.5 Visual description of various grain ASTM grain sizes.

Grain size has an important effect on physical properties. For
service at ordinary temperatures it is generally considered that fine-
grained steels give a better combination of strength and toughness,
whereas coarse-grained steels have better machinability. The Hall-
Petch relation in Eq. (2.2) describes the relation between the grain size
and the yield strength of a material. This relation indicates that the
smaller the grain size of a metal, the stronger it is and the higher its
yield strength.
k y
σ = σ + (2.2)
y
0
d
where k is a constant specific to a material, σ is another constant
0
y
related to the starting stress for dislocation movement, d is the grain
diameter, and σ is the yield strength.
y
In addition to the influence of impurities, inclusions, and cold
work, grain boundaries and differences in grain orientation may also
result in significantly different electrochemical reactivity in many
metals and alloys. When an alloy is cooled somewhat rapidly, the
grain boundaries may end up having a composition significantly
different than the grain core. This variation in chemical composition
from the interior to the exterior of a grain is referred to as
microsegregation or coring. The dendrites quite visible in Fig. 2.6(a)
are typical to the microsegregation associated with nonequilibrium
solidification. The same alloy, that is, 70-Ni 30-Cu (N04400) (also
known as Monel 400), would normally have in its hot rolled form the
crystalline grain structure revealed in Fig. 2.6(b).
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