Transition from carbon clusters to epsilon, theta-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 epsilon-carbides were uniformly precipitated in the matrix, and the others were heterogeneously precipitated along dislocations. These epsilon-carbides were coexisted with carbon clusters with c/a ratio of 1.1, assumed to be the formation of Zener-ordered Fe8C. Moreover, theta-carbides were found precipitated from epsilon-carbides via dissolution of epsilon-carbides, confirmed by In-situ heating TEM experiments. These results sug-gested that Zener ordering should be the fundamental step for the following carbide precipitation sequences in the steel aged at 473 K.