OBSERVATIONS OF DUCTILE FRACTURE PROCESSES AND CRITERIA OF VOID INITIATION IN SPHEROIDIZED AND FERRITE/PEARLITE STEELS.

Abstract

Two commercial 0. 11 and 0. 40% carbon steels and three vacuum melted steels with 0. 39, 0. 58, and 0. 91% C were used in the investigations. Transmission electron microscopy was employed to observe the dislocation structures in and around spheroidite and pearlite at the stage prior to void initiation. Stages of growth and linkage of voids were pursued by means of scanning electron microscopy. In the vicinity of spheroidite and pearlite, dislocation tangling and cell formation occur as deformation proceeds. A few dislocations were observed in cementite both in spheroidite and pearlite at strains immediately before void initiation. Combining these observations with previously reported results, criteria of void initiation are discussed. In spheroidized steels, stress predicted by J. C. Fisher-E. W. Hart-R. H. Pry model is exerted on spheroidite, and void initiation occurs when the stress reaches the fracture stress of cementite. In ferrite/pearlite steels, FHP stress applied to pearlite induces slip in pearlitic ferrite, and secondary stress concentration when applied to cementite plates. This secondary stress can be predicted by the G. S. Ansell-F. V. Lenel model and void initiation occurs when the secondary stress exceeds the fracture stress of cementite.