On the use of ultrasonic spectral analysis for the characterization of artificially degraded API 5L X52 steel pipeline welded joints

In this paper, artificial aging of welded joints of an API 5L X52 steel pipeline was studied using ultrasonic spectral analysis. The joints were welded using submerged arc welding process and isothermally aged at 250 °C for different time intervals. Ultrasonic measurements were carried out using a high precision water immersion ultrasonic scanner which produces C-scan maps of the tested weld samples. The results of these measurements show that ultrasonic velocity was not significantly affected by aging time; only small variations were detected associated with the degradation of strength and hardness properties. On the other hand, ultrasonic attenuation measurements were more sensitive to aging in both the weld pool and base metal. An increase in the attenuation coefficient during early aging time was linked to the precipitation of fine nanocarbides and an increase in ferrite grain size. After 200 h of aging time, the attenuation coefficient decreased monotonically, this was found to be related to the nanoparticle coarsening and ferrite grain growth after long periods of over-aging time. This phenomenon was larger in the weld pool than in the base metal due to a higher increase in carbide precipitations and their lower coarsening. These results could lead to the development of a non-destructive testing method for monitoring degradation of welded pipelines having extensive years of service under over-aging conditions.