Improved microstructure and mechanical properties of high strength pipeline steel joints via modulating rotation rate of friction stir welding

Abstract

Here, high-strength pipeline steel was friction stir welded (FSW) at various rotation rates of 600, 400, and 300 rpm, and microstructure-property relationships of the joints were comparatively studied. At 600 rpm, coarse granular bainite (GB) appeared in the nugget zone (NZ) and relatively fine GB appeared in the inside HAZ (IHAZ), whereas in the outside HAZ (OHAZ), only coarse polygonal ferrite (PF) was observed. As the rotation rate decreased, the microstructure of each subzone in the joint was gradually refined. At 300 rpm, the NZ contained a fine multiphase of lath bainite (LB), GB, and ferrite, and the IHAZ and OHAZ contained a fine dual-phase microstructure of GB and PF, and fine PF, respectively. Excellent tensile strength of 575 MPa and total elongation of 28.5% were achieved in the joint at 300 rpm, reaching 97.1 and 83.8% of the base metal (BM), respectively. Furthermore, at 300 rpm, the highest toughness (215 J) of up to 107.5% of the BM was obtained in the NZ, which was attributed to the highest fraction of high-angle boundaries, the generation of fine soft-phase ferrite, and high ratio of island martensite-austenite constituents.