AbstractHigh-quality, defect-free welds were successfully produced in S235 and S355 structural steels by friction stir welding. To explore the potential advantages injoining structural steels by FSW, the present study focuses on basic characterization of friction stir welded S235JR and S235JR plates. A remarkably fine-grained microstructure was observed in the stir zone for both materials by optical microscopy. Hardness measurements across the specific zones of the cross-section of the welds indicated higher values compared with base materials ones. The objective of this work was to demonstrate the feasibility of friction stir welding (FSW) for joining of structural steel.