Enhancing Strength and Ductility in the Nugget Zone of Friction Stir Welded X80 Pipeline Steel via Applying Cooling Medium

Fusion welding easily causes microstructural coarsening and tempering softening in the heat-affected zone (HAZ) of high-strength pipeline steel joints, which considerably deteriorates the strength and toughness. Here, X80 pipeline steel was subjected to friction stir welding (FSW), and external cooling was used to tailor the microstructure to optimize the strength–ductility combination of the nugget zone (NZ). Coarse granular bainite (GB) appeared at air cooling, whereas a fine ferrite/martensite microstructure was achieved at solid CO2 cooling. The highest ratio of high-angle boundaries was obtained at solid CO2 cooling because the variants were evenly distributed within the four close-packed (CP) groups. The low yield strength (YS) of 595 MPa was obtained in the NZ under air cooling, whereas a high YS of 755 MPa was achieved in the NZ under solid CO2 cooling due to dislocation strengthening and fine-grain strengthening. Furthermore, an ultra-high tensile strength of 910 MPa and utilizable elongation of 15% were obtained in the NZ under solid CO2 cooling, which was attributed to the fine effective grains and ferrite/martensite microstructure facilitating a ductile fracture.