Microstructure and Mechanical Properties of X80 Pipeline Steel Joints by Friction Stir Welding Under Various Cooling Conditions

X80 pipeline steel plates were friction stir welded (FSW) under air, water, liquid CO2 + water, and liquid CO2 cooling conditions, producing defect-free welds. The microstructural evolution and mechanical properties of these FSW joints were studied. Coarse granular bainite was observed in the nugget zone (NZ) under air cooling, and lath bainite and lath martensite increased significantly as the cooling medium temperature reduced. In particular, under the liquid CO2 cooling condition, a dual phase structure of lath martensite and fine ferrite appeared in the NZ. Compared to the case under air cooling, a strong shear texture was identified in the NZs under other rapid cooling conditions, because the partial deformation at elevated temperature was retained through higher cooling rates. Under liquid CO2 cooling, the highest transverse tensile strength and elongation of the joint reached 92% and 82% of those of the basal metal (BM), respectively, due to the weak tempering softening. A maximum impact energy of up to 93% of that of the BM was obtained in the NZ under liquid CO2 cooling, which was attributed to the operation of the dual phase of lath martensite and fine ferrite.