Local cementite cracking induced by heterogeneous plastic deformation in lamellar pearlite

To fully understand the fracture mechanism of pearlitic steel, the effects of lamellar alignment on both cementite cracking and plastic deformation behavior were investigated in two pearlitic materials with lamellar and spheroidized cementite phases. Digital image correlation revealed that local strain develops heterogeneously in the lamellar pearlite structure, but homogeneously in the spheroidized structure. The heterogeneous local strain distribution tends to coincide better with a pearlite colony than with a pearlite block. This finding suggests that the plastic deformation behavior of pearlite is strongly affected by the alignment of the ferrite/cementite lamellae. A detailed crystallography-based analysis revealed that the ferrite matrix was significantly plastically deformed in colonies in which the lamellae were aligned at approximately 45° relative to the direction of applied tension, regardless of the limited deformation of the cementite phase; this caused high degrees of strain in such colonies. In contrast, plastic deformation of the ferrite matrix was restricted by the lamellar cementite in colonies in which the lamellae were aligned parallel to the tensile direction. As a result, the lamellar cementite experienced plastic deformation simultaneously with the ferrite matrix as the applied strain was increased. This simultaneous plastic deformation caused shear deformation in lamellar structures on identical slip systems, by which the ductile fracture associated with cementite cracking occurred.