EBSD analysis of microstructures and mechanical properties of softened zones in X60 reeled-pipeline welded joint after cyclic plastic deformation

Abstract

The purpose of this study was to evaluate the microstructural evolution of softened zones after cyclic plastic deformation (CPD) and the effect of CPD on the mechanical properties of X60 reeled-pipeline welded joints. The results showed that the existence of softened zone caused the deformation to be mainly concentrated in this area during CPD process. The minimum hardness of softened zone was 18 HV less than the base metal (BM). As the global strain of the welded joints increased from the initial state without CPD to 5% strain, the kernel average misorientation of inter-critical HAZ related to the geometrically necessary dislocation slightly increased from 0.66° to 0.87°, resulting in a slight rise in hardness. Despite the presence of softened zones in HAZ, the location of tensile fracture was at BM, which was attributed to the combined effect of the work-hardening effect in softened zones, the groove type, and the constraining effect of the welded joint, regardless of the CPD strain levels. The decrease in ductility of welded joints was due to the stress concentration caused by dislocation pile-ups during the CPD process and then leading to the early formation of microvoids.