Østby, Erling (SINTEF Materials and Chemistry) | Nyhus, Bård (SINTEF Materials and Chemistry) | Sandvik, Andreas (StatoilHydro ASA) | Levold, Erik (StatoilHydro ASA) | Thaulow, Christian (Norwegian University of Science and Technology)
In this paper the results from SENT testing of two different welding procedures using an X65 base material is presented. The first welding procedure yields close to evenmatch conditions, whereas the second welding procedure gives 10-15% overmatch compared to the base material. Both defects lying in the weld metal and on the fusion line are investigated. It is observed that the ductile tearing resistances in both weld metals are significantly lower than for the base material. The resistance curves measured for the fusion line defects are more similar to the base material curve, however, slightly different crack growth is obtained depending on which side of the defect the measurements are performed. The crack driving force and strain capacity are on average higher in the overmatch specimens. However, a significant scatter is observed, especially for the weld metal defects. For the fusion line defects the scatter is smaller. For the material systems investigated the strain capacity will on average not depend strongly on the crack position.
Defects may limit the tensile strain capacity of pipelines. Such defects are mainly found in relation to girth welds. Mismatching in weld metal (WM) stress-strain properties compared to the base material will lead to a modification of the crack driving force as a function of the applied strain. It is common practice to specify overmatch conditions in the weld metal in order to shield or reduce the deformation in this region. However, overmatch can be difficult to obtain in some cases (e.g. for very high strength steels). Another aspect is related to the larger scatter in material properties usually found in weld metals. Also, the ductile crack growth resistance will in many cases differ between the weld metal and the base material. Although not without exceptions, the metallurgical conditions in the weld metal will usually lead to a reduced crack growth resistance compared to the base material of the pipe.