Enhancing strength and plasticity in the nugget zone of friction stir welded high-strength pipeline steel via rotating magnetic field assist

In this work, a rotating magnetic field (RMF) was used to assist the friction stir welding (FSW) of high-strength pipeline steel to improve forming quality and enhance the strength and plasticity of the nugget zone (NZ). The study highlighted a key novelty: achieving a defect-free NZ with RMF-assisted low heat input parameters, whereas in conventional FSW, low heat input typically leads to macroscopic defects. The ferritic dynamic recrystallization (DRX) in the NZ with RMF (NZRMF) caused a significant grain refinement. Furthermore, the unique formation of a large amount of retained austenite, which is rarely observed in the NZ of FSW pipeline steel, was obtained in the NZRMF along with nano-sized precipitates. A superior ultimate tensile strength (UTS) of 664 MPa and uniform elongation (UE) of 9.2% were achieved in the NZRMF due to a persistent high strain hardening rate (SHR), reaching 108.5% and 161.4% of the base metal (BM) values, respectively. Multiple strain-hardening mechanisms were revealed. Fine recrystallized ferrite grains effectively restricted dislocation motion during plastic deformation, and the pinning effect of precipitates and transformation-induced plasticity (TRIP) of austenite further enhanced dislocation multiplication, resulting in a persistent high SHR.