TABLE OF CONTENTS Pearlite is probably the most familiar microstructural feature in the whole science of metallography (Figs 3.14a, b). It was discovered by Sorby over 120 years ago, who correctly assumed it to be a lamellar mixture of iron and iron carbide. Pearlite is a very common constituent of a wide variety of steels, where it provides a substantial contribution to strength, so it is not surprising that this phase has received intensive study. Lamellar eutectoid structures of this type are widespread in metallurgy, and frequently pearlite is used as a generic term to describe them. These structures have much in common with the cellular precipitation reactions. Both types of reaction occur by nucleation and growth (Fig. 3.14a), and rely on diffusion. Pearlite nuclei occur on austenite grain boundaries, but it is clear that they can also be associated with both pro-eutectoid ferrite and cementite. In commercial steels, pearlite nodules can nucleate on inclusions.
The idealized view of pearlite is a hemispherical nodule nucleated at an austenite grain boundary, and growing gradually into one austenite grain (Fig. 3.15). Apart from examining possible sites for nucleation, the following information is needed:
how the lamellae increase in number,
the crystallographic relationships between the phases,
the nature of the pearlite/austenite interface,
the rate-controlling process.
