Microstructural characteristics and mechanical heterogeneity of underwater wet friction taper plug welded joints for low-alloy pipeline steel

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 Widmanstätten 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, Widmanstätten ferrite and banding ferrite defect. The microstructural inhomogeneity results in mechanical heterogeneity.