Abstract
An investigation was made of the ductile fracture processes both in spheroidized and ferrite/pearlite steels.
Transmission electron microscopic study was performed 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 were discussed. In spheroidized steels, stress predicted by Fisher-Hart-Pry (FHP) 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 slips in pearlitic ferrite and secondary stress concentration to cementite plate. This secondary stress can be predicted by Ansell-Lenel model and void initiation occurs when the secondary stress overcomes the fracture stress of cementite.
