DIFFERENT MICROSTRUCTURES IN THE HAZ OF DOUBLE SUBMERGED ARC WELDED PIPELINES AND HOW THEY INFLUENCE THE FATIGUE CRACK GROWTH

摘要

To meet the cost and weight reduction requirements in welded structures, the application of high-strength low-alloy (HSLA) steels also increases in the field of pipeline engineering. However, in comparison to low-strength steels these materials are more susceptible to notches or initial cracks. Therefore, an extensive investigation of fatigue crack growth behaviour in the different microstructural zones of the heat-affected zone (HAZ) is necessary for an appropriate lifetime evaluation of welded pipes. For this purpose, fracture mechanics parameters like thresholds and crack growth rates in the three characteristic microstructural zones of the HAZ as well as in the base material are characterized. The different microstructures are reproduced with a Gleeble thermal simulator system using the same heating curves as measured previously during the welding process of a double submerged arc welded pipeline. The effect of varying stress ratios R on the crack growth threshold and the crack growth curves is also assessed. This knowledge about the mean stress sensitivity is needed for estimating the influence of residual stresses stemming from different manufacturing processes like welding and post-welding treatments, which is essential for a reliable, damage tolerant application of HSLA steels in pipelines.