Anisotropic strain distribution and damage initiation in banded ferrite-pearlite microstructures

Microsegregation occurs during most industrial casting processes and, after hot rolling of hypo-eutectoid steels, often leads to the formation of banded ferrite-pearlite microstructure. Although ferrite-pearlite banding is reported to reduce mechanical properties such as ductility and impact toughness and to introduce anisotropy, these adverse effects cannot be attributed unambiguously to the banding itself as elongated manganese sulfide inclusions concurrently promote anisotropic damage and fracture. This study investigates the origin of mechanical anisotropy and its relationship to local strain distribution and damage initiation in banded ferrite-pearlite steels through a combined experimental and computational approach. Spatially resolved strain fields of banded 20MnCrS5 and 20MoCrS4 were measured by digital image correlation during micro-tensile testing and in-situ scanning electron microscopy (SEM), respectively. Micro tensile tests were performed both parallel (RD) and perpendicular (TD) to the banding direction and revealed significant mechanical anisotropy: RD exhibiting ~ 60 MPa higher ultimate tensile strength and ~10% greater elongation than TD. The results show that the strength anisotropy originates from fundamentally different phase-specific deformation mechanisms: Under TD loading, entire ferrite bands deform independently in a serial, layer-wise manner with minimal pearlite contribution. RD includes multiple shear bands that interconnect ferrite grains by bypassing or traversing pearlite regions, thereby forcing the harder pearlite phase to actively contribute to load bearing and microstructural stabilization. Representative volume element simulations confirmed these experimental observations and additionally revealed strain hotspots that were correlated with damage initiation sites observed through SEM, thereby establishing a direct link between local strain concentration and microstructural damage.