Metallurgical Evaluation of Hard Spots in X52 Pipe and Associated Cracking

Abstract Hard spots in pipelines are defects that originate during the pipe manufacturing process. These are areas of elevated hardness relative to the bulk material and are susceptible to cracking; thereby posing a threat to pipeline integrity. A metallurgical examination of three X52 pipe cutouts was conducted. Per information provided by the operator, the pipe had been installed in 1957 and contained several features suspected to be hard spots, which had been detected during ILI runs. Two of the cutouts also contained longitudinal cracks. Metallurgical evaluation concluded that the observed cracks resulted from near-neutral pH stress corrosion cracking. The cracks initiated within these hard spots, propagated along a predominantly transgranular path, exhibited a distinct thumbnail shape, and were covered with a dark iron oxide film. The hard spots likely formed due to rapid cooling from the AC3 temperature (~1340°F) during the pipe manufacturing process. The microstructure within the hard spots consisted of either fully transformed martensite or a mixture of martensite and pro-eutectoid/primary ferrite. The bulk microstructure of the pipe consisted of ferrite and pearlite. The tensile properties and the chemical composition of the pipe complied with the requirements per API 5LX Grade X52.