Microstructural Evolution and Phase Transformations in Plain Carbon Steel under Cyclic Heat Treatment

Cyclic heat treatment (CHT), involving repeated austenitization and rapid cooling cycles, substantially alters the microstructure of plain carbon steels. In this study, a low-carbon steel was subjected to multiple short-duration thermal cycles above the austenitizing temperature. Initial ferrite–pearlite microstructures gradually transformed into a fine-grained martensitic–bainitic matrix with nano-scale cementite particles as cycle count increased. Each cycle imparted defects and stored energy that drive recrystallization and grain refinement, shrinking average grain size from tens of microns down to a few microns. Optical and electron microscopy revealed the breakdown of pearlite lamellae, progressive bainite formation, and eventual predominance of lath martensite. X-ray diffraction confirmed the diminishing ferrite/pearlite peaks and the rise of martensite/bainite phases. These microstructural changes explain the concomitant rise in hardness and strength (roughly two-fold) observed in similar steels. In summary, cyclic thermal processing is an effective tool for engineering ultra-fine microstructures in carbon steels, trading off some ductility for greatly enhanced strength and hardness.