Fractographic Features of Long Term Operated Gas Pipeline Steels Fracture Under Impact Loading

Pipelines during their service life subjected to operational degradation, i.e. their mechanical characteristics worsened with time. Pronounced texture of pipe steels associated with their manufacturing process revealed itself in an essential difference in impact toughness determined for specimens cut in mutually perpendicular directions with respect to the pipe axis. Higher KCV values for longitudinal specimens as compared with transverse ones were explained by the difference in a length of perlite grain strips separated by ferrite grains in specimens of different orientation. A role of hydrogen, absorbed my metal during its operation, in steel degradation was discussed.
The main fractographic peculiarity for the operated steels comparing to the steels in the initial state is the appearance of delamination on the fracture surfaces, which are oriented in the rolling direction. Correlation was found for the tested steels between fractographic sings of their embrittlement due to operational degradation and their loss of brittle fracture resistance. It is concluded that a decrease of impact toughness caused by long term operation of pipeline steels is definitely concerned with the amount of delamination on the fracture surfaces.