Long-term service effect on the toughness of pipeline steel 17GS

A transit oil pipeline, 720 mm in diameter, 334 km in length, and made of steel 17GS, after 30 years of service was twice non-destructively tested with intelligent Pipeline Inspection Gauges (PIGs). Two segments of this pipeline were cut in order to investigate the steel structure, composition, and mechanical properties and to compare these properties with those of `archive' material. Metallographic and microprobe analyses revealed no significant difference between the `archive' and `aged' materials. Specimens for testing tensile mechanical properties, for fatigue-crack growth rate measurements, and for a Charpy test were used. All those specimens were fabricated from both the base and weld metal in longitudinal and circumferential directions. At room temperature, the ultimate strength of the aged steel was found to be 5% higher than that of the `archive' material, whereas the yield point was 18% higher for the aged material. Charpy energy changed more significantly; decreasing by approximately 40% at room temperature. In addition to conventional mechanical properties, new characteristics such as `microcleavage stress' R MC (minimum brittle fracture stress) and the toughness index K T were determined. It has been found that a long period of operation does not affect the values of the steel microcleavage stress R MC and does not influence the hardening exponent significantly. The use of the local approach to fracture has shown that an increase in yield stress is the main cause of the decrease in its toughness after long-term service. It is shown that a minor (15%) increase in the yield stress results in a rise (by 40°) in the Charpy transition temperature and may cause a twofold reduction in the fracture toughness K IC.