Characterization and Morphological Analysis of Pearlite in an Eutectoid Steel

Pearlite is a lamellar product of eutectoid decomposition, which may form in steels and non-ferrous alloys during transformations under isothermal or continuous-cooling (1,2). A pearlite nodule is composed of multiple colonies; each colony has parallel lamellae, which are orientated differently with respect to lamellae in adjacent colonies. This also exhibits a wide range of interlamellar spacings in different colonies because of the intersection of pearlite colonies at different angles with the polishing plane. The interlamellar spacing is reflected by the diffusion kinetics at the transformation front and is a sensitive parameter which, in a particular steel, is larger as the transformation temperature increases (3). Mehl and co-workers (3) demonstrated that the spacing decreased as the degree of undercooling, ∆T, below the eutectoid temperature, increased. Zener (4) provided the first theoretical analysis of these observations, which allows to calculate the interlamellar spacing of pearlite as a function of undercooling. Pearlite transformation in steels is reconstructive and known to show a constant growth rate (5). The growth rate of pearlite is believed to be controlled by either volume diffusion of carbon (4,6) or by boundary diffusion of substitutional alloying elements (7). In the present work, three different morphologies of pearlite were formed isothermally at three temperatures in an eutectoid steel finding out, as Mehl et al. and Zener reported (3,4), that the pearlite is finer as the formation temperature decreases. Moreover, the interlamellar spacings were calculated using the theoretical method proposed by Zener and Hillert (4,6,7). Experimental results suggest that the growth of pearlite is controlled mainly by volume diffusion of carbon in austenite in the temperature range studied in this steel.