Effect of long-term service on the tensile properties and capability of pipeline steel 17GS to resist cleavage fracture

A transit oil pipeline of diameter 720 mm and 334 km in length of steel 17GS after 30 years of service was twice non-destructively tested using intelligent 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 the archive material. Metallographic and microprobe analyses revealed no significant difference between the archive and 'aged' materials. 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 minimum brittle fracture stress, R-MC and the 'toughness index' K-T were determined. It has been found that a long period of operation neither affects the values of R-MC nor significantly influences the hardening exponent. The use of the local approach to fracture has shown that an increase in yield stress is the main cause for the decrease in toughness after long-term service. It is shown that a minor (15%) increase in the yield stress results in a rise (by 40degrees) in Charpy transition temperature and may cause a twofold reduction in fracture toughness, K-IC. (C) 2004 Elsevier Ltd. All rights reserved.