The Structure and Rate of Formation of Pearlite

Abstract

This lecture is an account of a study of the austenite: pearlite transformation in simple-carbon eutec-toid steels in terms of the rate of formation of nuclei and the rate of growth of these nuclei. The austenitizing reaction is considered from this point of view; the structure and the mode of formation of pearlite are described, and the results of the application of the electron microscope to the study of the structure of steel announced; and data are given on the rate of nucleation and rate of growth of pearlite correlated with austenitizing treatment and the degrees of austenite heterogeneity, which this determines. Those who have thought about the matter seem generally in agreement that the professor's chief duty, apart from his primary duty of the training of students, is to add to what is known concerning the basic principles of his subject-those fundamental principles that underlie the more immediate practical engineering applications. Thinking on this, it seemed to me that when a professor is called upon to deliver the Campbell Lecture, he could not do better than to try to interest his audience in an account of the development of the basic science in some part of the field of metallurgy, provided that his subject pertains to at least one of the practical interests of his hearers. And it seemed not inappropriate to select the subject of pearlite, for pearlite is of general metallurgical interest and some new facts about it have been unearthed in recent years. It was not long after 1886 when Sorby (1) first viewed pearlite under the microscope, Fig. 1, that Osmond (2), following the work of Tschernoff (3), determined the critical points in steel as a function of the carbon content and as a function of the rate of heating and cooling, and it was very shortly thereafter that metallurgists observed that the critical temperature A 1 is associated with the formation of pearlite. It had been stated by Arnold and McWilliam (4) and by Benedicks (5) as early as the year 1905 that pearlite forms by a process of nucleation and growth. Howe and Levy in 1916 (6) speculated upon how the rapid formation of pearlite at low temperatures may at least in part be determined by an increase in the rate of nucleation. The work of Davenport and Bain in 1930 (7) on the isothermal reaction curve and the derived S-curve served to emphasize with great clarity the fact that the formation of pearlite is a process of nucleation and growth of pearlite nodules, and served to est…