Effect of Long-Term Operation on the Physical and Mechanical Properties and the Fracture Mechanisms of X70 Pipeline Steels

The mechanical and physical properties of X70 pipeline steels produced in France and Japan are comprehensively studied using tensile and impact bending tests, analysis of the fracture reliefs of specimens, and the estimation of acoustic emission parameters, the attenuation coefficient of longitudinal ultrasonic waves, and the residual magnetic field intensity. A correlation between the mechanical and physical characteristics during tensile tests is revealed. After long-term operation, the yield strength and the ultimate tensile strength are shown to decrease and the plasticity and impact characteristics are shown to increase in comparison with the initial values from pipelines sertificates for both studied steels. The predominant mechanism of these changes is associated with the presence of a large number of delaminations, which cause fracture energy dissipation and hide the true picture of changes in the estimated strength characteristics.