Carbon redistribution in quenched and tempered lath martensite

Tempering-induced carbon redistribution in martensitic steels from clustering to carbide precipitation is relevant to various steel processing operations and resulting property combinations. In low carbon steels with martensite start temperatures well above room temperature, carbon redistribution is already initiated during quenching. The extent of autotempering varies locally across the as-quenched microstructure, even within a single prior austenite grain, depending on the sequential transformation of martensite laths. To overcome this practical limitation in the study of carbon redistribution, we systematically track the tempering-induced microstructure evolution within a single martensite lath. This is achieved through interrupted tempering treatments and microstructure analyses by electron channeling contrast imaging and atom probe tomography, both correlated with crystallographic orientation information from electron backscatter diffraction. The results show plate-shaped carbon clusters parallel to {100} martensite lattice planes in the as-quenched state, where the maximum carbon compositions remain below epsilon- or eta-carbide compositions. The driving force for initial cluster formation is discussed in relation to spinodal decomposition - analogous to room temperature aged Fe-Ni-C alloys after martensitic transformation during cryogenic quenching. Upon tempering, direct evidence for cluster dissolution and simultaneous cementite nucleation at carbon-rich dislocation cores is obtained. A crucial role is ascribed to the dislocation network which serves as a carbon diffusion path from clusters to cementite nucleation sites. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.