Strain localization and delamination mechanism of cold-drawn pearlitic steel wires during torsion

Highlights: • Grain curling during drawing generates longitudinal grooves on the wire surface. • Grain orientation under grooves favors shear bands during torsion. • Strain and strain rate in shear bands give rise to nanograin formation. • Crack propagates through the nanograined microstructure leading to fracture. • Crack instability causes wire delamination. Pearlitic steel wires are cold-drawn in order to attain high strength from the alignment of the pearlite colonies along the wire axis, as well as achieve a considerable reduction in the thickness of the ferrite lamellae. However, this high level of stress, in addition to surface defects and residual stresses, drastically decreases the strain ductility in tension and often in torsion. A significant limitation in torsion is the nucleation and growth of delamination cracks which propagate along the wire. Although this fracture phenomenon has long been studied, its origin and the underlying mechanisms remain debatable. This paper presents new microstructure investigations of drawn wires during torsion. The stages of initiation and propagation are defined towards a chronology of the development phases of delamination cracks based on the study of the microstructure of cold-drawn pearlitic steel wires before and after torsion. The curling of the grains leads to the creation of long grooves on the surface of the wire. These grooves increase stress concentration during twisting, thus localizing the formation of shear bands. Deformation and strain rate are so high in these bands that nanograins (10–30 nm) are formed. The delamination then appears to be mainly due to the localization of the single-shear deformation along the wire axis with mainly intergranular crack propagation.