Comparing Fracture Toughness Transition Curves for Girth Welds in X70 Pipe

Abstract

Most materials for offshore applications are tested for brittle fracture resistance at a single temperature related to the minimum design temperature and by a single fracture test method. It is much rarer to perform tests at multiple temperatures to compare the fracture performance across a range of temperatures and testing methods. EWI recently compared the fracture toughness transition behaviors for an X70 steel pipe across Charpy V-notch (CVN), single-edge notched bending (SENB) crack-tip open displacement (CTOD), and single-edge notched tension (SENT) CTOD test geometries. This showed variability of the material behavior better described by the inhomogeneous behavior models considered for welded joints. It also suggested the possibility that near the ductile-to-brittle transition temperature, the toughness under SENT CTOD may be higher than for SENB CTOD testing where the failure mode is brittle fracture. The testing methods used full-size CVN and nearly full-thickness CTOD specimens in bending, as limited by the pipe curvature of the 219-mm diameter pipe with 35.4-mm wall thickness. The SENT CTOD specimens were pre-cracked in bending with the same dimensions as the SENB specimens, but are then cut down to place the pre-cracked crack tip at approximately one quarter of the thickness through the resulting specimen. This modification places the tip in the higher constraint region for the tension test. Girth welds in the same X70 pipe were prepared using a pulsed GMAW process with ER80S-D2 welding wire. Similar testing was performed with weld centerline notches for the CVN and CTOD specimens. The transition behavior was related between the three testing methods for the weld centerline at the mid-wall of the pipe thickness. Using representative values equivalent to the minimum of three tests, the SENT values were 4.8 to 4.9 times the values for the SENB tests.