Avaliação da tenacidade à fratura e do efeito da microtextura e mesotextura na curva de transição dúctil frágil do aço API 5L X70

Abstract

In the present study was investigated the API 5L X70 steel mechanical behavior through tensile and instrumented impact tests. Results from the instrumented Charpy impact test have allowed quantify the forces, energies and deflections of the specimens. It was used to obtain the fracture mechanics parameters such as the dynamic fracture toughness JId, the stress intensity factor for dynamic loading KId, as also range the ductile brittle transition temperature. Simultaneously was evaluated the mechanisms that induce to the delamination phenomenon as well as the investigation of their influence on the absorbed energy during the impact test. It was also analyzed the microtexture and mesotexture of different samples from the brittle, ductile and mixed regions from the ductile brittle transition temperature curve. Results have shown that the longitudinal direction was slightly superior mechanical properties when compared to the transverse direction. The dynamic fracture toughness JId was higher for the L-T specimen orientation when compared to the T-L orientation and it tends to increase with increasing temperature for both directions. The stress intensity factor values for dynamic loading was close to in both L-T and T-L orientations. The microtexture has revealed that deformation process from the impact test preserves some of its characteristics from the prior processing such as phase transformations and recrystallization components. While the mesotexture feature has shown higher energy values in the ductile brittle transition temperature curve, it is resulted from the formation of deformation substructure. Results obtained by scanning electron microscopy and light microscopy have shown that the main factors that may be contributing to the formation of crack-divider type delaminations are banded microstructure and elongated ferrite grains. While the microtexture from the edges delamination has shown that propagation mechanism is intergranular. Taylor factor mapping has shown that the mechanism responsible for the formation of delamination is accompanied by low plastic deformation.