Cracking of underground welded steel pipes caused by HAZ sensitization

Low alloy steel welded pipes buried in the ground were sent for failure analysis investigation. Failure of steel pipes was not caused by tensile ductile overload but resulted from low ductility fracture in the area of the weld, which also contains multiple intergranular secondary cracks. The failure is most probably attributed to intergranular cracking initiating from the outer surface in the weld heat affected zone and propagated through the wall thickness. Random surface cracks or folds were found around the pipe. In some cases cracks are emanating from the tip of these discontinuities. Chemical analysis, visual inspection, optical microscopy and SEM/EDS analysis were used as the principal analytical techniques for the failure investigation.

► Low ductility fracture of welded pipes during service.
► Investigation of failure mechanism using macro- and microfractography.
► Metallographic evaluation of transverse sections close to the fracture area.
► Evidence of multiple secondary cracks at the HAZ area following intergranular mode.
► Presence of Zn in the interior of the cracks manifested that HAZ sensitization and cracking occurred prior to galvanizing process.