Friction stir welding (FSW) was performed under the two welding conditions (rotation speed–traveling speed) of 150 rpm–100 mm/min and 200 rpm–400
mm/min using 6 massÿNi–0.63 massÿC steels. The slightly lower peak welding temperature and significantly higher cooling rate was obtained under the
condition of 200 rpm–400 mm/min. Texture analysis for retained austenite and martensite revealed that the parent austenite had simple shear texture, which
lead to the formation of {110}<111> texture in transformed martensite. Moreover, material flow behavior as a function of distance from the top surface in the stir
zone was analyzed based on textures obtained by EBSD measurement. A concentric material flow, which has smaller radius as the far from the tool shoulder,
was formed under the condition of 150 rpm–100 mm/min. On the other hand, a heterogeneous material flow with the center shifted to the retreating side was formed under the condition of 200 rpm– 400 mm/min. In addition,
a different vertical component of material flow was predicted to occur under each welding conditions.