Effect of Carbon Concentration in Austenite on Cementite Morphology in Pearlite

Abstract

To understand the formation mechanism of degenerate pearlite, the effect of carbon concentration on the cementite morphology in pearlite was investigated in hypoeutectoid C–Mn steels with fully pearlite and ferrite–pearlite duplex microstructures. The carbon concentration in untransformed austenite was enriched by the precipitation of proeutectoid ferrite during isothermal holding after austenitization and could be controlled based on local equilibrium theory. Consequently, it was found that the morphology of cementite in the pearlite formed by the decomposition of the untransformed austenite continuously changed from lamellar to fine rod or spherical shapes by decreasing the carbon concentration. The critical carbon concentration for the cementite morphology transition was evaluated at approximately 0.42 mass% at 773 K. The ferrite growth rate increases with decreasing carbon concentration in austenite, which leads to non-cooperative growth between ferrite and cementite in the eutectoid reaction, resulting in the formation of degenerate pearlite. The critical carbon concentration and its temperature dependence for lamellar and degenerate pearlite transition can be estimated by a simple competition model of growth kinetics between ferrite and pearlite formations. In addition, it was found that the softening of degenerate pearlite during annealing after the decomposition was much faster than that of lamellar pearlite because the constrictions of cementite lamella do not exist for Ostwald ripening.

SEM images showing pearlite transformed from untransformed austenite in (a) 0.25C, (b) 0.39C (c) 0.42C, (d) 0.44C, and (e) 0.54 materials isothermally transformed at 773 K. Images (f), (g), and (h) are the magnified images corresponding to the white square in (a), (b), and (c), respectively.