Deformation and Fracture Behaviors of Spheroidized Pearlitic Steel under Tensile Loading

Abstract

The deformation and fracture behavior in spheroidized pearlitic steels were investigated using digital image correlation and the replica methods, and the origin of the inhomogeneous strain distribution and its effect on fracture were discussed. The cementite roundness increased while the area fraction within a colony decreased with increasing spheroidization time. Many coarse cementites were observed on the colony and block boundaries, which explains the decrease in the cementite area fraction within a colony. The strength–ductility balance deteriorated with cementite spheroidization. The inhomogeneous strain distribution in a unit of the colony was introduced by the tensile deformation in the spheroidized pearlitic steels. The numerous voids or cracks detected at low- and high-angle boundaries inside the cementite and in ferrite at the ferrite/cementite boundary tended to nucleate from the high-strain region. The deformability of the colony depended on the progress of cementite spheroidization, and the crystallographic orientation relationship between ferrite and cementite could affect the ease of cementite spheroidization in a colony. The strain gradient between the soft ferrite and hard cementite phases induced void or crack nucleation around the coarse cementite at the colony or block boundary; hence, voids or cracks tended to nucleate from the high-strain region. It can be concluded that inhomogeneous cementite spheroidization results in an inhomogeneous strain distribution, which causes the preferential nucleation of voids or cracks in the high-strain colony.