Mechanistic Investigation of Deflected Stress Corrosion Cracking in Pipeline Steels

Metallurgical examination of pipe from a hydrotest failure site revealed axially aligned SCC cracks. All SCC cracks were located in a layer (ca. 2.5mm) from the surface. The deflected SCC cracks changed direction at a depth of 200 – 600μm from the outer surface with an initial straight section perpendicular to the outer surface. The deflection angles were in the range of 30 – 50° with respect to the direction perpendicular to the outer surface. Statistically more than 50% of all the cracks with a straight length in the range of 200 – 600μm were deflected. Crack branching was present during the propagation of cracks at the deflection point and/or at the crack tip. The branched cracks were roughly 90° to each other.
To understand the cause of the deflected SCC cracks, detailed investigations were conducted on the morphology of SCC cracks, initiation and propagation of SCC cracks, microstructure of the steel, and microhardness characterization. It is revealed that there were no unusual changes in microhardness and microstructural characterization along the pipe wall thickness. A residual shear stress is believed that may have been established at the surface region of 200 – 600μm, thus promoting the deflection of the SCC cracks.