Formation of abnormal structures and their effects on the ductility of eutectoid steel

The formation of abnormal structures and their effects on reduction of area (RA) were investigated in eutectoid steels transformed at different temperatures ranging from 560 A degrees C-650 A degrees C. The occurrence of abnormal structures, such as upper bainite, degenerate pearlite, free ferrite, and grain boundary cementite, was confirmed. The volume fraction of upper bainite and degenerate pearlite decreased on increasing the transformation temperature, while the amount of free ferrite increased. As the transformation temperature increased, RA increased, reached a maximum, and then decreased, while the tensile strength continuously decreased. The crack formations during the tensile test could be classified into three types: tearing, shear cracking, and void formation/ coalescence. The decrease of the ductility at low transformation temperatures was attributed to the increased amount of upper bainite and degenerate pearlite, since the formation of cracks occurred by tearing interfaces or by void formation at abnormal structures during the tensile test. Meanwhile, the decrease in RA at high transformation temperatures was attributed to the occurrence of shear cracking rather than the presence of abnormal structures.