Microstructure evolution in pearlitic steels during wire drawing

Abstract

Major processes affecting microstructure of a drawn pearlitic wire including lamellae thinning, changes in inter-lamellar interface and metallographic and crystallographic texture, plastic flow localization, and dynamic strain aging were characterized. Heavily drawn pearlite represents a nano-composite with thickness of ferrite and cementite lamellae decreasing during wire drawing. Volume fraction of inter-phase interfaces is comparable with that of pearlitic cementite and they are associated with high elastic stresses. Stretching and rotation of pearlite colonies result in their alignment with the wire axis. This is accompanied by increase in a local ductility at the true strain below from 1.5 to 2 and then decrease at higher strain levels. Development of a strong crystallographic texture causes anisotropy in mechanical properties. Targeted observations of plastic flow at the same region showed two systems of localized shear bands and provided information on their development. A dramatic decrease in elongation to failure in wires after drawing is linked to the existence of the localized shear bands. Dynamic strain aging increases strength and degrades ductility of drawn wires.