Metallographic comparison to tomography of inclined SCC cracks

Abstract

X-ray microtomography has been used in the past to analyse Stress Corrosion Cracking (SCC) in austenitic stainless steels and aluminium alloys. These studies were carried out in order to determine the depth of these cracks where the cracks grew directly into the material at an angle normal to the free surface. In some rare cases, SCC in gas transmission pipelines initially grows normal to the free surface, but then changes direction away from the normal, growing at an angle of up to 60 degrees from the normal. This can lead to a complex crack interaction problem underneath the pipe surface which could result in unexpected crack growth rates. Previous work found that it was feasible to use x-ray microtomography to determine crack morphologies and interactions for inclined high pH SCC cracks in an Australian gas transmission pipeline. X-ray tomographs were able to show the complex three dimensional SCC crack paths, in addition to some interactions between different cracks. This paper presents the comparison between the SCC crack path determined from the 3D tomographic model and the actual path as determined by metallographic sections in a carbon steel gas pipeline.