Fracture Toughness of X70 Pipeline Steel

The fracture toughness of X70 pipeline steel was investigated experimentally by 3-point bend fracture toughness test for samples with different thickness and initiate crack length at - 60 °C , -20 °C and 20 °C . It was discovered that the failure mode can be ductile or brittle depending on test temperature; The delamination cracks are found on all fracture surfaces; The size and amount of delamination cracks are related to sample thickness; The location of delamination cracks related to initiate crack length and test temperature; For brittle fracture at lower temperature, the delamination cracks are produced near main crack roots and the width and length of delamination cracks are small, thus the effect of delamination cracks upon effective thickness can be neglected; For the ductile fracture at higher temperature, the delamination cracks are produced at the tips of the main cracks, and the width and length of delamination crack are large, then delamination cracks decrease the effective thickness. Because of delamination cracking, the mechanical behavior of X70 pipeline steel is different from common steels, and the test methods and safety assessment for common steels are not suitable for X70 pipeline steel. The comprehensive effect of wall thickness, defect size and service temperature of pipeline must be taken into account.