Microstructure, crystallography, and toughness in nugget zone of friction stir welded high-strength pipeline steel

Herein, we address the scientific gap in the relationship among the microstructure, crystallography, and toughness in the nugget zone (NZ) of friction stir welded X80 pipeline steel at various heat input parameters of 300, 400, and 600 rpm. The decreased heat input results in refined prior austenite accompanied with an increase in the Z-parameter. Lath bainite (LB) and granular bainite (GB) appear at a high heat input and a bulk of GB appears at a medium heat input, whereas at a low heat input, a complex phase of LB, GB, and ferrite is obtained. The highest ratio of the bainite packet boundaries was identified at a low heat input, which is attributed to the existence of three Bain groups. The strongest shear texture was detected in the NZ at a medium heat input. An excellent toughness of up to 92.5% of that of the base metal was achieved at a low heat input owing to the highest ratio of packet boundaries, finest martensite-austenite constituents, and soft second phase of ferrite.