cr0264_09 - 9 Inclusions and Inclusion Modification 9.1...

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9 ©2001 CRC Press LLC Inclusions and Inclusion Modification 9.1 INTRODUCTION 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, 1 which has 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 clean steel. However, no steel can be totally free from inclusions. The number of inclusions has been variously estimated to range between 10 10 and 10 15 per tonne 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 cleanliness. 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. 9.2 INFLUENCE OF INCLUSIONS ON THE MECHANICAL PROPERTIES OF STEEL Discussions on this subject are available from many sources. Only a few are referenced here. 1–6 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
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©2001 CRC Press LLC augment it. If the local stress becomes high, then microcracks develop. The propagation of micro- cracks leads to fracture. Investigations have established that only large inclusions are capable of doing this kind of damage, and this led Kiessling 1 to develop the idea of critical size . In practice,
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