Study of partition to non-partition transition of austenite growth along pearlite lamellae in near-eutectoid Fe-C-Mn alloy

The austenite formation from lamellar pearlite was studied in a near-eutectoid Fe-0.6mass%C-2mass%Mn alloy with focus on the transition between partitioned and non-partitioned growth of austenite. The austenite was nucleated at pearlite colony boundaries and grew along the lamellae of pearlite. The isothermal austenite transformation curves revealed that the transition from slow to fast growth of austenite occurred over a narrow temperature range (720-725 degrees C). The ghost pearlite which had formed above the transition temperature transformed to a very fine mixture of martensite (previously pearlitic ferrite) and austenite (pearlitic cementite) upon cooling to room temperature. According to thermodynamic analysis which takes into account carbon transport from theta/alpha and theta/gamma interfaces toward gamma/alpha interfaces the transition temperature appears to be closer to experiment and significantly (ca. 20 degrees C) lower than that previously proposed assuming that the austenite was nucleated at the theta/alpha interface. Phase field simulation of the carbon and Mn distributions around the tip of a cementite lamella also seems to indicate the change of diffusivity which plays a leading role in the growth of austenite. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.