In-situ and interrupted cyclic three-point bending tests in SEM of two pearlitic steel microstructures: Lamellar and partially spheroidized

Lamellar and partially spheroidized pearlitic microstructures were lab-manufactured via, respectively, stress relief treatment at 400 (SR400) and 700 °C (SR700) for 60 s. The performance of both microstructures under four successive three-point bending (TPB) tests was investigated via in-situ secondary electron imaging in scanning electron microscopy. Additionally, consecutive interruptions after each TPB were performed for electron backscatter diffraction (EBSD) analyses. Transmission-EBSD and scanning transmission electron microscopy were employed in the starting specimens: SR400 and SR700. Initially, SR400 presented many shear bands, while SR700 depicted small recrystallized grain zones with coarse spheroidized cementite. Additionally, Bagaryatskii and Isaichev cementite/ferrite orientation relationships were found for SR400 and SR700, respectively. During TPB deformation, SR400 exhibited high strength and stable mechanical behavior, while SR700 displayed a softening phenomenon and earlier failure. Moreover, the EBSD analyses indicated that strains were concentrated in the shear bands of SR400 and the small recrystallized grain zones of SR700 during TPB. The small recrystallized grain zones with coarse spheroidized cementite were related to promoting the earlier damage and, consequently, failure of SR700. Finally, the more significant presence of sheared areas and elongated dimples on the SR400's fractured surface demonstrated its higher capacity to support plastic deformation under TPB than SR700.