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Subject: Hook Crack, Pipes The repercussion of 11.02.2008 17.55These defects in the longitudinal weld occur during manufacture of the pipe, when inclusions at the plate edge are turned out of the plane of the steel during the welding process.... It is the turning out of the metal at the weld that gives the characteristic “hook” or “J” shape to the crack. Исправьте, пожалуйста, если я где-то ошибся. Заранее спасибо! |
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http://www.kiefner.com/downloads/ERW.PDF page 5: The use of cleaner steels (i.e., with greatly reduced sulfur contents) has virtually eliminated the risks of hook cracks and grooving corrosion. The precursors for hook cracks are non-metallic inclusions, primarily manganese sulfide “stringers”. These flattened, non-metallic inclusions are formed during hot rolling of plate or skelp. In general, they reduce the ductile toughness of the steel even in their normal position (i.e., layers interspersed between the rollingelongated grain structure of the steel). In this position, they can cause poor through-thickness properties that inherently reduce ductile- fracture tearing resistance but not necessarily the yield or tensile strength of the material. Near an ERW bondline, however, these weak layers become reoriented such that they are subjected to tensile hoop stress when the pipe is pressurized. The layers may be of sufficient extent or so closely associated that the resulting planes of weakness separate, forming J-shaped (i.e., hook) cracks that curve from being parallel to the plate surfaces near mid-wall to being nearly parallel to the ERW bondline at the OD or ID surface. These cracks can be up to 50 percent of the wall thickness in depth and up to several inches in length. |
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