©2001 CRC Press LLC
Inclusions and Inclusion
Inclusions are nonmetallic particles embedded in the matrix of metals and alloys. In this chapter,
we are speciﬁcally concerned with inclusions in steel. In view of the inﬂuence of inclusions on the
properties of steel, extensive investigations have been and are being carried out. A vast body of
literature is available, as will be evident from the classic book by Kiessling and Lange,
a comprehensive presentation of the structure, properties, and origin of a wide variety of inclusions
found in steel. In this chapter, an attempt is made to outline the salient features of the theory as
well as important ﬁndings and conclusions for a general comprehension of the subject.
As a generalization, inclusions have been found to be harmful to the mechanical properties and
corrosion resistance of steel. This is more so for high-strength steels for critical applications. As a
result, there is a move to produce
However, no steel can be totally free from inclusions.
The number of inclusions has been variously estimated to range between 10
of steel. Again, the yardstick for cleanliness depends on how one assesses it. For example, most
of the inclusions are submicroscopic. Therefore, a microscopic examination cannot faithfully assess
The above considerations lead to the conclusion that cleanliness is a vague and relative term.
Which steel is clean and which steel is dirty can be determined only when we know the intended
applications and consequent property requirements, after which we can understand the correspond-
ing limiting size, frequency of occurrence, and properties of inclusions. Therefore, it is necessary
to have a broad understanding of how inclusions affect the properties of steel. Herein, we restrict
ourselves to mechanical properties only.
INFLUENCE OF INCLUSIONS ON THE MECHANICAL
PROPERTIES OF STEEL
Discussions on this subject are available from many sources. Only a few are referenced here.
The properties that are adversely affected are fracture toughness, impact properties, fatigue strength,
and hot workability. The factors responsible for these may be classiﬁed as follows:
1. Geometrical factors: size, shape (may be designated as the ratio of major axis to minor
axis), size distribution, and total volume fraction of inclusions
2. Property factors: deformability and modulus of elasticity at various temperatures, coef-
ﬁcient of thermal expansion
From a fundamental point of view, an inclusion/matrix interface has a mismatch. This causes
local stress concentration around it. Application of external forces during working or service can