Fatigue life assessment of low carbon API 5L X52 pipeline steels retired from long-term service

The fatigue lifetime of two API 5L X52 steels retired from long-term service was investigated. Both steels presented pearlite fragmentation, fine precipitate particles and spheroidized cementite at the grain boundaries, which is associated to microstructural degradation due to natural aging. Neither tensile strength nor hardness properties have significant variations respect to the design specifications; however, lower ductility was observed in the most degraded steel. Fatigue char-acterization was carried out by axial force-controlled tests, finding that the fatigue life is 77 % lower in the steel with the lowest ductility and highest microstructural degradation. Results indicate that the fatigue lifetime of a ferrite-pearlite microstructure is more affected by the microstructural degradation due to long-term service of the steel, in spite of traditional consid-erations as non-metallic inclusion or grain size. Hence, the natural aging effect is a main factor to be considered in the fatigue life of long-term exposure low carbon steels.