THE SPHEROIDIZATION OF CEMENTITE IN A MEDIUM CARBON-STEEL BY MEANS OF SUBCRITICAL AND INTERCRITICAL ANNEALING

The spheroidization of cementite at subcritical and intercritical temperatures was studied quantitatively by evaluation of a shape factor for the cementite particles. It was found that the degree of spheroidization, as determined by the values of the shape factor, was markedly accelerated by treatments consisting in either (1) subcritical annealing after cold deformation or (2) intercritical annealing followed by subcritical annealing. Nevertheless, some differences were found among the microstructures resulting from the above treatments, which were related to the mechanisms of cementite formation in each of them. Also, the kinetics of austenitization during intercritical annealing was found to be accelerated by previous deformation, and the analysis of the formation of austenite at intercritical temperatures in terms of an Avrami equation was consistent with an "effective" saturation of ferrite-pearlite boundaries with austenite nuclei, followed by a planar mode of growth into the pearlite nodules.