Mechanical properties anisotropy in X80 line pipes

The anisotropy of tensile properties and impact toughness of X80 ferrite-bainite pipeline steels was investigated. The lowest strength values were found in longitudinal (L) direction, medium - in the direction of wall thickness (Z-direction), the highest - in transverse (T) direction. The anisotropy of the tensile properties is low and does not change significantly with the test temperature. The maximum variation in yield stress and tensile stress do not exceed the ranges of 100 MPa and 50 MPa respectively. The anisotropy in toughness is more clearly pronounced. Impact toughness in both T and L directions is more than 250 J/cm2 at minus 20°C while in Z-direction it is less than 30 J/cm2 at the same temperature. Low Z-direction toughness determines the susceptibility of steel to splitting during ductile fracture propagation (formation of brittle cracks parallel to the rolling plane of the plate). Microstructure and crystallographic texture analyses showed that the susceptibility to splitting is controlled by a number of factors like the predominant orientation of cleavage crystallographic planes parallel to the rolling plane, morphology of the microstructure elements, and the distribution of "second" phases.