Microstructural Evolution of C-Mn Dual Phase Steel During Intermediate Quenching: A Comparative Study of Subcritical, Intercritical, and Suppercritical Annealing

Martensitic microstructure, due to its numerous nucleation sites for austenite and its fibrous distribution, is considered as the initial microstructure in the dual-phase steel fabrication process, specifically during intermediate quenching. In the present study, the microstructural evolution of a carbon-manganese steel during intermediate quenching at three temperature ranges (upper-critical (Ac₃<T), inter-critical (Ac₁<T<Ac₃) and sub-critical (T<Ac₁)) was studied using optical microscopy, scanning electron microscopy, and X-ray diffraction analysis. Hardness measurements indicated that the sub-critical annealing process, which is the high-temperature tempering process, consists of two stages with different softening rates. The first stage is associated with the removal of carbon from the lattice structure to form carbides, and the second stage is accompanied by the coarsening of carbides. Annealing martensitic microstructure at intercritical temperatures also occurs in three stages: The first stage is the tempering of martensite, the second stage is the nucleation and growth of austenite in tempered martensite with the dissolution of carbides, and the third stage is the coarsening of the two-phase microstructure, which is associated with a reduction in hardness. An increase in temperature to upper-critical temperatures also causes the appearance of these three stages, but due to the accelerated diffusion at high temperatures, these stages are shortened and merged together.