Issue 35

X.C. Arnoult et alii, Frattura ed Integrità Strutturale, 35 (2016) 509-522; DOI: 10.3221/IGF-ESIS.35.57 517 microstructure between steel A, B and C did not influence the CTOA properties but only the fracture surface. In this case, the factor promoting the delamination cracks is microstructure composed of hard and soft phases and a high intensity of plane {100}. Steels Microstructure {100} intensity parallel to RD CTOA A Bainite 1.5 12 to 18° B and C Ferrite and bainite- martensitic 1.6 to 1.7 11 to 18° D Deformed ferrite bainite-martensite More than 2.5 <10° Table 4 : Relationship between CTOA, the intensity of the {100} texture parallel to the rolling direction and the microstructure [8]. To understand the role of elongated grains, Yan et al [6,7] used the as-rolled steel and subjected it to the triple-oil-quench heat treatment, thus the elongated grain microstructure changed to equiaxed grain microstructure. Charpy test conducted at -30°C indicated that the samples with equiaxed grains exhibited no delamination cracks on the fracture surface. This result signifies that the presence of elongated grains in the microstructure are a necessary but not sufficient condition. For the authors, the presence of precipitates is assumed to be one of the causes of the delamination cracks. The precipitates have been formed by tempering the HSLA steel at temperatures in the range between 500 to 650°C. Then the elongated grains and aligned precipitates seem together responsible for the delamination cracking process in this steel. For steels having microstructural bands [9], bands could be an important cause of delamination occurrence. Figure 9 shows a significant difference in the crystallography of areas designated X and Y located on opposite sides of the delamination crack. Crystal face orientation is (101) for the area X and (111) for the area Y and a clear difference is apparent in the grain orientation spread map. The different orientations may create a weakness between the bands and allow the development of a delamination crack. Figure 9 : Delamination crack in a L-T orientation Charpy sample broken at -40°C: top: SEM picture, bottom: orientation maps and grain orientation spread maps with the key in degrees [9].