CONICET | Buscador de Institutos y Recursos Humanos

Some failures by crack propagation along Heat Affected Zone of (in-field) girth welds have taken place in recently constructed pipelines that transport natural gas and natural gas liquids from the Amazon basin, over deep Forest and the Andes. Girth welds are SMAW with cellulosic consumables for the root, fill and cap.Previous studies showed failures were related to events during early service, mainly soil movement. Although most crack propagation characteristics were reproduced on previous tests, the reproduction of three main characteristics could not be fully accomplished: a) subcritical crack propagation in HAZ, b) straight throughthe-thickness fracture propagation and c) staged cracking. The results of these studies suggested that the first characteristic was associated with a very slow strain rate, the second characteristic was related to biaxial loading and the third one, to inhomogeneties on the HAZ. Tensile tests were carried out at slow strain rates (0,03 mm/min or lower) and standard strain rates (2mm/min) on specimens under different biaxiality conditions created by notches. Results up to now are: 1. Pure uniaxial testing specimens showed completely ductile tearing when tested at standard strain rate. 2. Equivalent specimens failed by crack propagation when tested at very slow strain rates. 3. Direction of crack propagation in uniaxial testing followed a path within the HAZ and was not straight through the thickness. 4. API X70 strength varies with loading speed when it is very low, about 10-4 /s. 5. Strength variation rate is different for the different regions of the welded joint, being the HAZ(containing fine and coarse grained regions) the most susceptible region to this change. 6. Mid-thickness segregation of these steels proved to have an important effect on fractographic characteristics of most tests, being a barrier for crack propagation.

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