Fracture toughness in the circumferential–longitudinal and circumferential–radial directions of longitudinal weld API 5L X52 pipeline using standard C(T) and nonstandard curved SE(B) specimens

Abstract

In this work the fracture toughness of a longitudinal submerged arc-welding (SAW) seam weld API 5L X52 pipeline of 36 in. diameter and 1 in. thickness, was evaluated and compared in the circumferential–longitudinal (CL) and circumferential-radial (CR) directions. These tests were evaluated by means of compact tension C(T) specimens and nonstandard curved SE(B) specimens, respectively. The K function of the nonstandard curved SE(B) specimens was experimentally calibrated using the compliance experimental technique. The crack initiating notch was aligned in the longitudinal weld direction so the crack propagated through the deposited metal zone for both specimens. The laboratory tests were performed at room temperature. The K\(_{IC}\) average value in the CL direction was \(75.4 \pm 3.22\,\hbox {MPa}\sqrt{\hbox {m}}\), and for the CR direction was \(56.3 \pm 2.9\,\hbox {MPa}\sqrt{\hbox {m}}\). The results showed a large difference of K\(_{IC}\) values for each direction analyzed, which was attributed to the anisotropic properties of the weld metal, particularly in the CR direction, where the predominance of acicular ferrite grains (fragile zones) and the porosity provided a favorable crack propagation direction for separation of the fracture plane, resulting in low fracture toughness values in this direction. Finally the fractographic analysis showed that the deposited metal zone consisted of a mixture of acicular ferrite and ferrite grains, presenting a ductile behavior with dimple coalescence in the CL direction and a fragile behavior exhibiting cleavage regions that corresponded to acicular ferrite grains in the CR direction.