Transition from carbon clusters to ε, θ-carbides in a quenched and aged low-carbon ferritic steel

Carbon redistribution is one of the fundamental issues in quenched and aged carbon steels, which results in carbon clustering and carbide precipitation. Carbon clusters and carbides act as obstacles for dislocations, which have been known closely correlated with strengths of steels. In this study, conventional TEM, atomically-resolved TEM and In-situ heating TEM observations were carried out to understand carbon redistribution in a low-carbon ferritic steel aged at 473 K. Some ε-carbides were uniformly precipitated in the matrix, and the others were heterogeneously precipitated along dislocations. These ε-carbides were coexisted with carbon clusters with c/a ratio of 1.1, assumed to be the formation of Zener-ordered Fe ₈C. Moreover, θ-carbides were found precipitated from ε-carbides via dissolution of ε-carbides, confirmed by In-situ heating TEM experiments. These results suggested that Zener ordering should be the fundamental step for the following carbide precipitation sequences in the steel aged at 473 K.