A novel observation of Widmanstätten ferrite and pearlite by thermo-mechanical processing of interstitial-free steel M. Sinha, S. Yadav, S. Ghosh Materials and Manufacturing Processes, 2022 In the present work, interstitial-free steel is rapidly cooled from the austenitic state, followed by isothermal holding at 630°C for 5 minutes in a thermo-mechanical simulator (Gleeble 3800®). The heat-treatment provides massive ferrite, Widmanstätten ferrite and pearlite, as the triplex-phase mixture. Massive ferrite, as the early decomposed product of austenite, arises by rapid cooling. Subsequent formation of Widmanstätten ferrite and pearlite during the isothermal holding is elusive in steel. The phase transformations account for a dramatic improvement in tensile strength by two-fold with a negligible reduction in elongation. Morphologically, massive ferrite is granular and contains recovered sub-grain boundaries inside the phase. The pearlite nucleates within a lath-shaped grain, unconventionally. Due to the low-temperature phase transformation (630°C) and a ppm level carbon in the alloy, the observed pearlitic structure is either underdeveloped or fragmented, which promotes cracks against the stronger Widmanstätten ferrite at an advancing front of the crack tip.
Recrystallization in commercial grade interstitial-free steel, discussing criticality of martensite and massive ferrite nucleation along with mechanical property S. Yadav, A. Kamal, M. Sinha, S. Ghosh Journal of Materials Research and Technology, 2021 Interstitial-free steel is very soft and ductile, with a fully ferritic microstructure at room temperature. In order to address this issue in the present work, the cold-rolling increases tensile strength vividly by strain hardening. The heat-treatment thereafter optimizes strength-ductility, with an excellent combination, through fine recrystallized grains. The warm-rolled sample provides relatively an inferior property by partial recrystallization of the sample. Irrespective of unannihilated dislocations, both the samples after the recrystallization treatment in the austenitic domain at 925 °C, were water quenched in a laboratory scale to explore phase transformations additionally in the system. The microstructural characterization reveals that the nucleation of massive ferrite under rapid cooling requires austenite conditioning, similar to martensite in the alloy, constrained by strained lattice. The outcome fundamentally argues against the notion that the accumulation of crystal defects/dislocations promotes an extra driving force for a reconstructive phase transformation leading to massive ferrite.
