DEFLECTED STRESS CORROSION CRACKS IN THE PIPELINE STEEL

摘要

This research reports a special case of stress corrosion cracks (SCC) in the pipeline steels that had propagated in the direction deviated from the pipe radial direction. It was characterized that the cracks were intergranular in nature with relatively wide crack crevice. Most of crack being characterized consisted of two segments: a crack segment near pipe surface that is normal to the axis of hoop stress, and the subsequent segment that is inclined to the axis of the hoop stress. The segment near the surface was usually less than 1.5 mm long, and the inclined one was up to 10 mm in length. The angle of the inclined segment was dominantly in the range of 30° to 60°. To understand the mechanisms related to the deflected crack growth, the microstructure of the pipeline steels was studied. It was found that The pipeline steel is characterized with a sandwich-like microstructure, for which it is harder at the surface (～ 1.5 mm thick) and progressively softer towards the center of the wall. This particular structure might have caused a complex loading condition to the pipe wall material such that yielding of the soft material become possible, particularly when crack has propagated into the soft region of the pipe wall. As a result, corrosion attack may take place in a direction consistent with the maximum shear stress, and cracking preceded by the concurrent interaction between corrosion attack and mechanical damage.