Influence of morphology and structural size on the fracture behavior of a nanostructured pearlitic steel

Subjecting pearlitic steels to severe plastic deformation is known to transform the original colony structure to a nanostructured pearlite with the microstructural constituents aligned parallel to the deformation direction. Besides a huge increase in strength due to an enormous reduction of the interlamellar distance, other mechanical properties such as fracture toughness are deteriorated especially when the crack propagation is parallel to the shear deformed structure. Post-deformation heat-treatments up to 600 °C were applied to modify the oriented nanostructured pearlite after applying high pressure torsion to a micro-duplex structure of ultrafine grained ferrite with spherical cementite dispersoids. Already moderate annealing led to a pronounced increase of the fracture toughness accompanied only by a slight drop in strength. Nevertheless, at those low annealing temperatures the deformation structure was partially present which crucially influenced the crack propagation behavior and thereby the mechanical anisotropy. By raising the annealing temperature it was possible to produce a fully spheroidized microstructure and in further consequence mechanical isotropy was achieved. The decrease in strength due to microstructural coarsening is balanced by a remarkable gain in fracture toughness.