L51675 Effects of Weldment Property Variations on the Behavior of Line Pipe

A steel weldment is a composite of zones or layers of different microstructures that possess different material properties. The zones include the base metal (or the sections of pipe), the weld metal, and a complex heat-affected zone (HAZ) of base metal that has been exposed to a variety of thermal cycles resulting in varying microstructures. The material properties of primary concern with respect to the mechanical behavior of the pipe are the stress-strain response (the constitutive properties) and the resistance to initiation and propagation of cracks or tears in the presence of a crack, notch, or other stress concentrator (the fracture toughness properties).

Most of the experimental data on the behavior of welds with significant discontinuities were obtained from test specimens with surface or through-thickness notches or cracks. These data typically show an increase in load or nominal ductility for overmatched welds and a decrease in load or nominal ductility for undermatched welds. However, there are cases where the presence of a soft zone may enhance the nominal ductility and cases where overmatched welds will decrease the nominal ductility. The latter is especially likely in a girth weld with a circumferential crack in the HAZ.