Characterization of age hardening mechanism of low-temperature aged low-carbon steel by transmission electron microscopy

Low-temperature aging treatment at 323 K results in the dramatical increase in hardness in low-carbon ferritic steels quenched from 983 K, possibly caused by carbon clusters and/or fine ε-carbides. In this study, transmission electron microscopy (TEM) analysis was carried out to characterize the change of the microstructure during the low-temperature aging treatment. Until the early stage of the peak hardness, the carbon clusters were formed homogeneously with zig-zag structures. At the latter stage of the peak hardness, it was found that the ε-carbides were partially precipitated within the carbon clusters, which suggested that the carbon clusters might have acted as the precursors of ε-carbides. In-situ tensile TEM observations showed that dislocation motions were free-glide type, and carbon clusters and fine-carbides interacted with dislocations via cutting-type. Dislocation interaction force was also evaluated, which suggested that the lattice misfit played as important role of the interaction mechanism. [Display omitted] •Low-temperature aging treatment resulted in the formation of carbon clusters, followed by partial precipitations of fine ε-carbides.•Carbon clusters interacted with dislocations via cutting mechanism.•The interaction mechanism between fine ε-carbides and dislocations transferred from cutting-type to Orowan-type as the carbides grew larger.•The lattice misfit between carbon clusters and dislocations played an important role of the low-temperature age hardening behavior.