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The microstructural characteristics of underwater wet friction taper plug welded joints for API X52 pipeline steel were investigated and tensile strength and facture toughness of the welded joints were experimentally evaluated. The microstructure of welded joints is considerably inhomogeneous. The tensile strength of welded joints shows heterogeneity in thickness direction. All of the tensile specimens taken from upper side of welded joints fracture at base material, while those taken from the bottom are broken at bonding interface or base material. The fracture toughness of welded joints deteriorates seriously, as compared with base material. The welded joints with notches in weld center have higher fracture toughness than those in bonding zone and the axial force has few influences on fracture toughness of welded joints. The bonding zone between plug and base plate should play a role as fissure defect for the whole welded joint, considering that it is tens of microns and much softer than its vicinity. In addition, the coarse grain, quenched martensite and Widmanstatten ferrite in weld region should also be responsible for the reduction of fracture toughness.

Underwater wet friction taper plug welding experiments have been performed on X52 pipeline steel with 6500–7500 rpm rotational speeds at 30–45 kN axial forces, and the microstructural characteristics and mechanical heterogeneity of defect-free friction taper plug welded joints are discussed thoroughly. It is found that the microstructure of welded joint is remarkably inhomogeneous and very different from the base metal. The welded joint has dramatically coarse grains and is dominantly characterized by a mixture of quenched martensite, upper bainite and various types of ferrites including Widmanstatten ferrite. Unlike the traditional solid-state friction welding processes, the relative homogeneous microstructure with fine and equiaxed grains cannot be obtained in the friction taper plug welding process. The axial force has a greater impact on microstructure of welded joint as compared to rotational speed. The hardness profiles measured in cross-section of welded joints are severely non-uniform, ranging from 200 to 400 HV1, due to the inhomogeneity of microstructure. The impact absorbed energy of welded joint with V-notch in the bonding zone was considerably lower than that of base material (about only 20% of parent metal) because of the local obviously coarse grain, Widmanstatten ferrite and banding ferrite defect. The microstructural inhomogeneity results in mechanical heterogeneity.